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Received: 6 August 2019 Revised: 21 October 2019 Accepted: 8 November 2019
DOI: 10.1111/1541-4337.12514
COMPREHENSIVE REVIEWS IN FOOD SCIENCE AND FOOD SAFETY
Application of extrusion technology in plant food processing
byproducts: An overview
William Leonard1Pangzhen Zhang1Danyang Ying2Zhongxiang Fang1
1School of Agriculture and Food, The
University of Melbourne, Parkville, Victoria,
Australia
2CSIRO Agriculture & Food, Melbourne,
Victoria, Australia
Correspondence
Zhongxiang Fang, School of Agriculture and
Food, The University of Melbourne, Parkville,
Victoria 3010 Australia.
Email: zhongxiang.fang@unimelb.edu.au
Abstract
The food processing industry generates an immense amount of waste, which leads
to major concerns for its environmental impact. However, most of these wastes, such
as plant-derived byproducts, are still nutritionally adequate for use in food manufac-
turing. Extrusion is one of the most versatile and commercially successful process-
ing technologies, with its widespread applications in the production of pasta, snacks,
crackers, and meat analogues. It allows a high degree of user control over the pro-
cessing parameters that significantly alters the quality of final products. This review
features the past research on manufacture of extruded foods with integration of vari-
ous plant food processing byproducts. The impact of extrusion parameters and adding
various byproducts on the nutritional, physicochemical, sensory, and microbiologi-
cal properties of food products are comprehensively discussed. This paper also pro-
vides fundamental knowledge and practical techniques for food manufacturers and
researchers on the extrusion processing of plant food byproducts, which may increase
economical return to the industry and reduce the environmental impact.
KEYWORDS
extrusion, nutrition, plant food byproduct, processing parameters, sensory property
1INTRODUCTION
Increasing amount of waste is generated from the food
processing industry (Sagar, Pareek, Sharma, Yahia, & Lobo,
2018). This creates a major concern over its management and
disposal, due to the negative impact on the environment and
natural resources (Kummu et al., 2012). However, most plant
food-derived byproducts, such as fruit pomace and oilseed
hulls, are rich in nutrients, and therefore can be utilized to
manufacture value-added foods. Extrusion is currently one
of the most important food processing technologies that
has the potential to be utilized for this purpose. The high-
temperature-short-time (HTST) process employed during
extrusion ensures the product safety without significantly
altering the nutritional value (Arêas, Rocha-Olivieri, &
Marques, 2016). Nevertheless, most extruded products are
nutritionally inadequate due to the domination of starchy
carbohydrates over its composition. Therefore, it allows the
opportunity to integrate plant byproducts in an attempt to
diversify the nutrients content of the extrudates. Past research
has successfully added plant byproducts in extruded snacks
and energy bars, with the main aims of improving nutritional
value and sensory properties (Table 1).
This review first introduces the basic principles of extru-
sion technology, followed by the effects of its processing
parameters on the major nutrients in a food system. In
this paper, the plant food byproducts refer to the parts
that are often discarded after food processing, commonly
fruit pomace, peels, seed hulls, and oilseed cake. Despite
its nutrition-enhancing properties, the presence of plant
byproducts significantly transforms the nutritional compo-
sition and sensory properties of a food system. Thus, its
potential desirable and undesirable impacts over nutritional,
physicochemical, and functional behavior of several extruded
products will be discussed in this review. Additionally, the
effects of both extrusion parameters and incorporation of plant
218 © 2019 Institute of Food Technologists®Compr Rev Food Sci Food Saf. 2020;19:218–246.wileyonlinelibrary.com/journal/crf3
EXTRUSION OF PLANT FOOD BYPRODUCTS…219
TABLE 1 Selected studies in manufacturing extruded plant byproduct foods
Plant
byproduct Final Product
Feed
moisture
(%)
Screw
speed
(rpm)
Tem pe ra ture
(◦C) Notes Reference
Bran and hulls
Buckwheat
bran
Pasta from semolina
and buckwheat
bran flour (25%)
30.5 25 45 • No effect after extrusion on mineral composition and
content (Ca, Cu, Fe, K, Mg, Mn, Zn), protein content,
amino acid profile (except lysine).
Manthey and
Hall (2007)
Corn fiber Corn grit: corn fiber
at ratio
70–100:0–30
extrudates
30 150 90 to 120 • Increasing melt temperature improved physical
characteristics of corn extrudates
• Inclusion of corn fiber reduced expansion ratio, bulk
density and breaking strength
• Higher corn fiber content led to higher phenol
retention, increased antioxidant properties, but
lowered L, b, ΔEvalues
• Extrusion resulted in higher WAI (especially with
added CO2), b, ΔEvalues
Wang and
Ryu
(2013a;
2013b)
Pea hull Corn semolina-pea
hull (20% to 80%)
extrudates
14 to 26 72 80 to 200 • Higher pea hull proportion led to increased density,
reduced radial expansion ratio, poorer texture,
loweredWAIandWSI.
• Higher temperature (120 to 220 ◦C) increased
expansion and lowered density
• Higher moisture content lowered expansion and WSI,
increased density and WAI
Rzedzicki,
Sobota &
Zarzycki
(2003)
Rye bran Blend of corn starch,
rye endosperm
flour and rye bran
(28 and 440 μm;
15% and 30%
level)
17 500 40 to 110 • Addition of bran resulted in increased hardness and
density, and lowered crispiness and expansion
• Reduction in bran particle size enhanced
macrostructural properties, WAI, expansion and
crispiness
• 30%fineryebraninbarrel-waterfeedresultedin
crispiest extrudates
Alam et al.
(2016)
(Continues)
220 EXTRUSION OF PLANT FOOD BYPRODUCTS…
TABLE 1 (Continued)
Plant
byproduct Final Product
Feed
moisture
(%)
Screw
speed
(rpm)
Tem pe ra ture
(◦C) Notes Reference
Sorghum bran Corn flour-sorghum
bran (5% and
10%) tortilla
30 112 60 to 90 • Integration of sorghum bran led to improved
antioxidant activity, higher ferulic, p-coumaric,
diferulic and triferulic acids.
• Extrusion increased the free forms of phenolic acids
Buitimea–
Cantúa
et al.
(2018)
Soybean hull Maize grits-soybean
hull (0% to 40%)
extrudate
16 100 to 200 100 to 200 • Inclusion of soybean hull lowered specific
mechanical energy and sectional expansion
• Higher temperature and levels of hull reduced SME
• Soybean hull can be added up to 30% without
significant difference in sensory evaluation scores
Duarte et al.
(2009)
Wheat bran Corn meal-wheat
bran-milk protein
extrudate
Water
feed:1.02
L/h
300 100 to 125 • Addition of fiber at 125 g/kg resulted in higher
expansion and breaking strength. It also improved
specific mechanical energy and product quality
Onwulata
et al.
(2001)
Peel
Jabuticaba
(Myrciaria
cauliflora)
fruit peel
Corn flour, whole
grain wheat flour,
JPP: jabuticaba
peel powder (up to
10%) extrudate
16 325 75 to 100 ▪JPP at 10% does not considerably affect the hardness
and crispiness of corn flour-wheat flour blends.
▪Presence of JPP significantly improved appearance,
color, aroma and overall impression scores
Oliviera,
Alencar,
and Steel
(2018)
Mango peel Corn-mango peel
(8.34% to 33.33%)
extrudate
15 to 21 76 to 100 75 to 175 ▪Formulations with higher content of mango peel and
moisture, lower temperature and screw speed
produced extrudates with lower expansion and
increased hardness.
▪Post drying applied to the extrudate at 105 ◦Cfor2hr
compensate the mango peel’s lower expansion effect,
by giving a well texture (crispy and no grittiness)
Mazlan et al.
(2019)
(Continues)
EXTRUSION OF PLANT FOOD BYPRODUCTS…221
TABLE 1 (Continued)
Plant
byproduct Final Product
Feed
moisture
(%)
Screw
speed
(rpm)
Tem pe ra ture
(◦C) Notes Reference
Potato peel Muffin (25% PP)
Oatmeal cookies
(10% and 15% PP)
31 to 33.5 300 143 ▪Oatmeal cookies are more flexible to peel
incorporation compared to muffin
▪Addition of peel resulted in decreased muffin volume
and cookie spread
▪Extrusion increased mouthfeel scores in sensory
evaluation of muffins. Also induces formation of
antioxidants and Maillard reaction products, as
shown by lower peroxide value.
Arora and
Camire
(1994)
Pumpkin peel Corn grit – pumpkin
powder (10% to
50% peel)
extrudate
water rate:
0.29
L/h
315 40 to 180 ▪Incorporation of pumpkin flour at 10% resulted in
extrudates with similar properties (bulk density,
expansion ratio) as control.
▪Hardness and color of extruded snacks were
significantly different from control.
Norfezah,
Hardacre
& Brennan
(2011)
Pomace
Apple
pomace
Corn flour-apple
pomace (5 to
10%) blend
10.5 60 to 100 150 to 200 • Maximum screw speeds resulted in higher bulk
density, lower expansion ratio, decreased porosity,
small specific volume, lower moisture, and higher
starch degradation (high final viscosity).
• Higher temperature led to less expanded and lower
quality extruded products.
• Apple pomace, due to its fiber content, significantly
affected radical expansion ratio, texture, acoustic
properties, starch properties and moisture.
O’Shea et al.
(2014)
Banana and
Carambola
(Averrhoa
carambola
L.) pomace
Cereal made from
low-amylose rice,
seeded banana and
carambola
pomace at ratio
60:30:10 to
80:10:10
9 to 21 200 to 300 90 to 130 • Decrease in expansion with higher pomace levels, due
to high fiber content and starch dilution.
Borah, Lata
Mahanta,
and Kalita
(2016)
Blueberry
pomace
White sorghum
flour-Blueberry
pomace (30%)
blend
45 150 to 200 160 to 180 • Significantly higher procyanidin monomer (highest at
180 ◦C and 150 rpm screw speed), dimer and trimer
content at both temperatures and screw speeds, Due
to reduction in polymer contents.
• Extrusion lowers anthocyanin content by 33% to 42%.
Khanal,
Howard,
Brown-
miller, et
al. (2009)
(Continues)
222 EXTRUSION OF PLANT FOOD BYPRODUCTS…
TABLE 1 (Continued)
Plant
byproduct Final Product
Feed
moisture
(%)
Screw
speed
(rpm)
Tem pe ra ture
(◦C) Notes Reference
Carrot
pomace
Corn starch-carrot
pomace (5% to
15%) blend
15 to 30 100 to 250 50 to 140 • Slightly higher expansion at low moisture content and
carrot pomace at 5%. Decreasing trend with added
pomace after that.
• Highest beta-carotene retention rate with carrot
pomace at 5%.
Kaisangsri
et al.
(2016)
Cherry
pomace
Corn starch-cherry
pomace blend (5%
to 15%)
15.5 150 to 250 140 • The radial expansion ratio increased with 5% pomace
addition compared with control, but decreased
significantly at 15% addition
• Reduction in WAI and WSI (smaller particle size)
with higher pomace level
• No effect on phenolic content after extrusion
• The smallest particle-sized (<125 μm) pomace at the
5% level of inclusion resulted in extrudates with the
largest expansion ratio among all treatments
Wang,
Kowalski,
et al.
(2017)
Cranberry
pomace
Corn starch-
Cranberry pomace
(30% to 50%)
blend
30 150 to 200 150 to 190 • Lower temperature and pomace level led to higher
anthocyanin retention (Highest retention at 150 ◦C
and 30% pomace)
• Extrusion improved flavonol content by 30% to 34%,
ORAC values increased at 150 ◦C and 190 ◦C
• Increased in procyanidin monomer (DP1) and dimer
(DP2), but decreased from DP 3 to 9.
White et al.
(2010)
Grape
pomace
Barley-grape
pomace (2% to
10%) blends
21.66 150 to 200 140 to 160 • Blends of 2% grape pomace extruded at 160 ◦C,
200 rpm and 10% grape pomace extruded at 160 ◦C,
150 rpm had higher preference levels for parameters
of appearance, taste, texture and overall acceptability.
Altan et al.
(2008a)
Olive pomace Rice-oat
flour/maize-oat
flour and olive
pomace (5% to
10%)
20 500 50 to 95 • Olive pomace inclusion improved fiber and
polyphenol quantity of extrudates
• Presence of pomace lowered die pressure, SME and
expansion
• Reduced expansion and higher density from pomace
were more profound in rice-oat flour extrudates
Ying et al.
(2017)
(Continues)
EXTRUSION OF PLANT FOOD BYPRODUCTS…223
TABLE 1 (Continued)
Plant
byproduct Final Product
Feed
moisture
(%)
Screw
speed
(rpm)
Tem pe ra ture
(◦C) Notes Reference
Orange pulp Biscuit-type cookies
from wheat flour
and orange pulp
(5% to 25%)
22 to 38 126 to 194 83 to 167 • No significant difference between control and biscuits
with 15% added pulp for flavour, texture and general
acceptance scores
• Significantly increased moisture content and
hardness, lower expansion ratio, in pulp-added
cookies
Larrea et al.
(2005)
Peach
pomace
Rice flour:peach
pomace at ratio
6 to 12:1
13.5 400 25 to 120 • Addition of peach pomace increased porosity, radial
and overall expansion ratios.
• Reverse trend for apparent and true density, and
breaking strength of extrudates
Sarkar and
Choudhury
(2014)
Pineapple
pomace
Corn flour-pineapple
pomace (10.5%
and 21%) blend
14 to 16 220 140 to 160 • Extruded products with added pomace have lower
expansion and darker color.
• No significant differences between control and 10.5%
added pomace in terms of bulk density, hardness,
WAS and b* values of extruded products. Showing
satisfactory results.
Selani et al.
(2014)
Tomato
pomace
Barley-tomato
pomace (2% to
10%) blends
21 to 22 133 to 217 140 to 160 • Extrudates with 2% and 10% tomato pomace levels
extruded at 160 ◦C and 200 rpm had higher
preference levels for parameters of color, texture,
taste, and overall acceptability
Altan et al.
(2008b)
Defatted seed or seed cake
Almond
powder
Corn flour-defatted
almond powder
(10% to 30%)
puffed snack
12 to 16 120 to 220 140 • High moisture feed and screw rate resulted in
desirable characteristics (high ER, low BD, low
hardness)
• Increased almond powder was followed with
reduction in ER and air cell diameter, increased BD,
hardness and thickness of cell wall
Hashemi
et al.
(2016)
Blackcurrant
(Ribes
nigrum)
seed
Cornmeal-Defatted
blackcurrant seeds
(10% to 50%)
cereal extrudate
14 190 150 to 180 • Addition of seeds resulted in higher vitamin C and
sugar, and lower starch
• Higher WAI and WSI in 10% and 30% added seeds
compared to control
• Higher seed content caused higher density and lower
expansion in extrudates
Gumul,
Ziobro,
Zieba, and
Roj (2013)
(Continues)
224 EXTRUSION OF PLANT FOOD BYPRODUCTS…
TABLE 1 (Continued)
Plant
byproduct Final Product
Feed
moisture
(%)
Screw
speed
(rpm)
Tem pe ra ture
(◦C) Notes Reference
Flaxseed
meal
Cereal bars
containing
extruded FM
(1.36%) and flour
(mainly wheat)
mixes
25 to 40
L/h
water
input
60 129 to 142 • Cereal bar enriched with FM showed improved
protein quality and quantity, dietary fiber and 𝜔6:𝜔3
ratio.
Giacomino
et al.
(2013)
Hempseed
(Cannabis
sativa L.)
cake
Corn grit-hemp cake
(5% to 10% DM)
snack
15 to 25 100 150 to 180 • Higher levels of defatted hempseed cake and moisture
reduced expansion ratio and fracturability, and
increased hardness and bulk density
• Temperature has a significant influence on hardness
and color change
Jozinovic
et al.
(2017)
Hempseed
powder
Energy bar from rice
flour and
hempseed powder
(20% to 40%)
20 200 60 to 130 • Higher inclusion of hemp resulted in reduced
expansion of extrudate
• Higher phenolic and flavonoid content, DPPH radical
scavenging activity and 𝛽-carotene bleaching assays
with higher hemp %.
• Higher moisture absorption in extruded rice/defatted
hemp than extruded rice/whole hemp
Norajit et al.
(2011)
Linseed cake
Rapeseed
cake
Soybean
cake
Sunflower
cake
Fish Pellets
(contains 25%
cake for each
formulation,
fishmeal and
wheat)
30 300 70 to 120 • A replacement of 25% of the reference fishmeal
formulation with oilseed cakes resulted pellets with
similar nutritional profiles to the reference fish feed
but with reduced expansion, increased sedimentation
velocity, lower water stability and abrasion resistance.
Tyapkova
et al.
(2016)
Olive paste Corn flour-olive
paste (4% to 8%)
extrudate
14 to 19 150 to 250 140 to 180 • Higher moisture and paste content, lower temperature
and screw speed, were associated with higher density,
decreased expansion and porosity in extrudates
Bisharat,
Eleni,
Pana-
giotou,
Krokida,
and
Maroulis
(2014)
(Continues)
EXTRUSION OF PLANT FOOD BYPRODUCTS…225
TABLE 1 (Continued)
Plant
byproduct Final Product
Feed
moisture
(%)
Screw
speed
(rpm)
Tem pe ra ture
(◦C) Notes Reference
Sesame oil
cake
Corn grit, containing
semi-defatted
sesame cake (10%
to 20%)
15 333 to 387 Room
Tem-
pera-
ture
• Higher SDSC lowered sectional expansion of
corn extrudates and enhanced compression
force.
• Acceptable sensory properties after
presentation of nutritional benefits, especially
for snack with 20% SDSC
da Graca Costa
do Nascimento
et al. (2012)
Other plant parts or combination
Brewer’s
spent grain
Pasta from yam
starch and BSG
(5% to 15%)
12 100 to 140 100 to 110 • Expansion increased with screw speed and
temperature, but decreased with higher BSG
content
• WAI increased with screw speed and
temperature, particularly at low BSG levels.
WSI increased with higher temperature.
Sobukola,
Babajide, and
Ogunsade
(2013)
Cauliflower
florets,
curd, stem
and leaves
Snack containing
mainly wheat
flour, corn starch
and cauliflower
waste (5% to 20%)
9to11 250 to 350 80 to 120 • Higher dietary fiber, protein content and WAI
with addition of cauliflower
• No significant effect on hardness of
extrudates. Lower expansion indices with
higher cauliflower content.
• Cauliflower can be added up to 10% for
acceptable sensory scores
Stojceska et al.
(2008)
Defatted
soybean
meal,
germinated
brown rice
meal,
mango peel
fiber
Corn grit-mixture of
plant byproduct
(20% total) snack
13 to
18.36
126 to 180 146 to 214 • Addition of byproduct mixture at 20%
increased protein, dietary fiber, polyphenol
content and antioxidant activity
• Reduction in expansion ratio with added meal
due to protein and fiber content
• Extrusion converted insoluble into soluble
fiber, decreased phenolic compound and
antioxidant activity
Korkerd,
Wanlapa,
Puttanlek,
Uttapap, and
Rungsardthong
(2016)
(Continues)
226 EXTRUSION OF PLANT FOOD BYPRODUCTS…
TABLE 1 (Continued)
Plant byproduct Final Product
Feed
moisture
(%)
Screw
speed
(rpm)
Tem pe ra ture
(◦C) Notes Reference
Partially defatted
hazelnut flour,
durum clear
flour, fruit
waste blend
(orange peel,
grape seed,
tomato
pomace)
Rice grit-mixture of
plant byproduct
(total 30%) snack
12 to 18 200 to 280 150 to 175 • Higher PDHF led to increased bulk density
and WSI, but decreased porosity and WAI of
extruded snack.
• Snacks with well expansion and sensory
properties were obtained at low PDHF content.
Yağcı and
Göğüş
(2008)
Sugarbeet pulp Corn grit-sugar beet
pulp (5% to 15%)
extrudates
–100 135 to 170 • Higher SBP resulted in lower expansion and
fracturability, higher hardness and density. But
the addition of pectin (0.5% to 1.0%), to some
extent, reversed this trend
• Extrusion process significantly affected color,
andresultedinhigherWAIandWSI.
• 1% pectin must be added to SBP-(all
levels)-incorporated extrudates for acceptable
sensory scores
Ačkar et al.
(2018)
EXTRUSION OF PLANT FOOD BYPRODUCTS…227
byproducts on the sensory and microbiological properties of
extrudates are elaborated.
2BRIEF PRINCIPLE OF
EXTRUSION TECHNOLOGY
Extrusion is a combined act of mixing, shearing, knead-
ing, cooking, compressing, and forcing a molten material,
under high pressure, through a narrow opening (die) (Fellows,
2009). As the material left the opening, a sudden decrease in
pressure translates water into steam, thus expanding the mate-
rial. The extruded product is often called the extrudate. The
shape of the extrudates generally reflects the shape of the die.
Aside from the physical alteration, the process denatures pro-
tein, solubilizes fiber, gelatinizes starch, and induces cross-
linking of biopolymers. This explains the significant changes
in the functional and chemical properties of the material.
Compared to other heat-involving food processing methods,
extrusion results in a relatively small loss of nutritional value
(Fellows, 2009). Hence, since its introduction in the 20th cen-
tury, it has been widely utilized in the manufacture of cereal
based products, snacks, frankfurters, pasta products, and meat
analogues (Singh, Gamlath, & Wakeling, 2007).
An extruder consists of one or two screw(s) rotating
in a tightly fitting cylindrical barrel, which is equipped
with a feeder at its inlet end and a die at its discharge
end. Based on screw type, extruders can be grouped into
single-screw extruder (SSE) and twin-screw extruder (TSE)
(Fellows, 2009). Selection for the right extruder depends on
the interests of manufacturer and the requirements of the pro-
cess. SSE is relatively cheaper and simpler to operate com-
pared to TSE. However, only a narrow range of materials with
specific moisture and fat content are appropriate for SSE pro-
cessing. A TSE provides greater flexibility over the range of
products manufactured and the control over process param-
eters, such as pressure and temperature. Unlike SSE, TSE
can process wet, viscous, and powdery ingredients. It is suit-
able to manufacture products with very low (<10%) and high
moisture content, smaller particle size, and higher fat content
(Moscicki, 2016). Nevertheless, TSE has a more complex
design and higher capital cost compared to SSE.
3EFFECTS OF EXTRUSION ON
THE NUTRITIONAL PROPERTIES
OF PLANT FOOD BYPRODUCTS
3.1 Protein
The conditions of extrusion carry a profound effect on the
intrinsic properties of plant protein, which leads to break-
age of existing bonds, cross-linkage with other nutrients,
and formation of new compounds. Depending on the pro-
cess parameters, extrusion can both improve and impair pro-
tein digestibility. Under some circumstances, particularly at
severely high temperature, extrusion denatures protein and
exposes the hydrophobic residues previously hidden inside the
compact structure of protein. This, coupled with formation of
new disulphide and hydrophobic bonds, significantly reduces
protein digestibility (Björck & Asp, 1983).
On the contrary, the majority of studies have demonstrated
enhanced protein digestibility after extrusion (Alonso &
Marzo, 2000; Arêas et al., 2016). While exposing hydropho-
bic residues, denaturation of protein simultaneously provides
higher surface area of protein available for enzymatic diges-
tion. Alonso and Marzo (2000) observed the highest protein
digestibility rate in extruded faba and kidney beans, as com-
pared to other traditional processing methods such as soak-
ing and germination. Protein digestibility, however, appears to
be influenced by the process parameters of extrusion. Zhang,
Liu, Ying, Sanguansri, and Augustin (2017) found a signif-
icantly higher digestibility rate only when the feed moisture
exceeds 30% level in extruded canola meal. In addition, extru-
sion inactivates the antinutritive compounds, such as phytate
and protease inhibitors, which limit nutrient bioavailability.
Amino acid profile analysis assesses how well a process-
ing method retains the original protein quality. Lysine, fol-
lowed by cysteine and arginine, have been shown to be the
most unstable amino acids during extrusion (Björck & Asp,
1983). The process conditions used, particularly temperature
and feed moisture, have the largest influence on amino acid
retention. Singh et al. (2007) observed the minimum lysine
loss at higher (>15%) moisture content and lower tempera-
ture (<180 ◦C). They reasoned that the high temperature used
in extrusion catalyzes Maillard reaction, a chemical reaction
between a reducing sugar and an amino group of amino acid,
including the 𝜀-amino group of lysine, thus exacerbating the
loss of lysine.
Extrusion also leads to noncovalent interactions, cova-
lent cross-linking, and interactions between macronutrients.
These interactions might affect the functional properties of
extruded protein. According to Alonso, Orúe, Zabalza, Grant,
and Marzo (2000), extrusion promotes the formation of disul-
phide bonds and noncovalent interactions, thus the decreased
protein solubility in extruded pea and kidney beans. Similarly,
Beck, Knoerzer, and Arcot (2017) observed impaired solubil-
ity in pea protein concentrate at low feed moisture content
compared to control samples. They found that temperature
and shear stress have a greater impact on solubility than the
feed moisture. During protein denaturation, changes in sur-
face hydrophobicity allow the protein to aggregate and form a
three-dimensional structure with higher water holding capac-
ity and lower solubility. Protein solubility plays a part in emul-
sifying properties of plant protein by accelerating protein dif-
fusion at the interface, which lowers the interfacial tension
228 EXTRUSION OF PLANT FOOD BYPRODUCTS…
(Panyam & Kilara, 1996). Although solubility is often cor-
related to emulsifying capacity, extrusion seems to reorga-
nize the molecular structure of protein in some beans, thereby
improving its ability to stabilize emulsions (Beck et al., 2017;
Silva, Arêas, Silva, & Arêas, 2010).
3.2 Lipid
Lipids are often present at low amounts in the extrusion
of food formulations as it reduces the friction required to
transfer mechanical and heat energy. In other instances, lipid
may act as a plasticizer and provide the adhesive texture
required in some products. Several studies have documented
the effect of extrusion on reducing lipid content. Tumuluru,
Sokhansanj, Bandyopadhyay, and Bawa (2013) associated
lower feed moisture and higher temperature to higher degree
of fat loss in fish and rice flour extrudates. On the other hand,
fat loss increased with dough moisture from 26% to 30%, but
displayed a decreasing trend from 30% to 36% in extrusion of
fatty meal (de Pilli, Giuliani, Carbone, Derossi, & Severini,
2005). The reason behind these conflicting trends is unclear,
though it may relate to other process parameters and viscos-
ity of the mixture. Although both parameters were significant,
Sandrin, Caon, Zibetti, and de Francisco (2018) suggested that
screw speed was the more critical factor than temperature in
explaining the fat loss in extrusion of oat and rice flour.
There are several explanations for the reduction in lipid
content during extrusion. Higher temperature, in combina-
tion to shear stress, melts solid lipids into liquid oil, thus
inducing the migration of oil out of the food system (de Pilli
et al., 2005). Moreover, formation of lipid–starch and lipid–
protein complex may result in lower oil extraction yield from
extruded products. This phenomenon also depends on the pro-
cess parameters of extrusion. Bhatnagar and Hanna (1994)
reported the formation of amylose-lipid complex at medium
temperature (110 to 140 ◦C) and low moisture (<20%) after
examining the effects of extrusion on corn starches. At
low moisture condition, amylose has a more flexible 𝛼-1,4-
glucose linked chain with hydrophobic sites that interact with
the aliphatic tails of lipid, thus forming the V-amylose com-
plex and reducing the lipid content (Arêas et al., 2016).
3.3 Starch
Fruit and vegetable byproducts typically have very low pres-
ence of starch. However, most extruded snacks still require
starch as a major component in extruded food product for-
mulations. This is due to starch’s contribution to a well
expanded and crunchy product texture. During extrusion,
starch undergoes some major changes, most notably gela-
tinization, depolymerization, and dextrinization. The high
temperature used in extrusion disrupts the crystalline struc-
ture of starch and breaks the intermolecular hydrogen bonds.
Thus, the higher availability of hydrogen binding sites, along
with expulsion of amylose molecules, allows starch granules
to absorb moisture and swell (Wang, Fu, et al. 2017). As the
pressure suddenly drops at the die, evaporation of water pro-
vides the expansion and gas holding property that are desir-
able in extruded food products.
The extent of starch transformation is affected by bar-
rel temperature and feed moisture applied during extrusion.
de la Rosa-Millán, Heredia-Olea, Perez-Carrillo, Guajardo-
Flores, and Serna-Saldívar (2019) reported a higher propor-
tion of damaged starch with increasing temperature. The
thermal treatment during thermoplastic extrusion shifts the
starch granule integrity from a highly ordered-crystalline to
disorganized-amorphous state. Additionally, dextrinization, a
process involving hydrolysis and repolymerization of starch
into dextrins and pyrodextrins, may take place under condi-
tions of low feed moisture and high shear stress (Lai & Kokini,
1991).
As reviewed by Singh, Anne Dartois, and Kaur (2010),
extrusion cooking presents the highest starch digestibility
of all listed processing methods, including gamma irradia-
tion, autoclaving, roasting, and toasting. Extrusion promotes
higher starch digestibility by destructing the covalent hydro-
gen bonds and structure of starch granules, thus the less resis-
tance to enzymatic digestion. Starch digestibility was shown
to improve with higher feed moisture and barrel temperature
(Rafiq, Sharma, & Singh, 2017). The high shear stress also
increases the surface area available for hydrolysis. Neverthe-
less, reduction in starch digestibility could also occur after
extrusion due to low moisture of raw materials and formation
of amylose–lipid and protein–starch complexes (Guha, Ali, &
Bhattacharya, 1997).
3.4 Fiber
Fiber is a major component in most plant food byproducts,
particularly from pomace and seed hull group. In humans,
intake of dietary fiber has been linked to reduction in the
risk of cardiovascular diseases and colorectal cancer (Aune
et al., 2011; Threapleton et al., 2013). Mixed findings were
found for the effects of extrusion cooking on total dietary fiber
(TDF) content. Camire, Violette, Dougherty, and &McLaugh-
lin (1997) indicated insignificant difference in the TDF of
extruded potato peel. Another study by Vasanthan, Gaosong,
Yeung, and Li (2002) reported significantly higher TDF after
extrusion of barley flour. On the contrary, Arribas et al. (2017)
showed a reduction in total fiber content after extrusion of
gluten-free snacks derived from pea and rice flour.
The temperature gradient, moisture, and shear applied dur-
ing extrusion significantly affect the quantity and proportion
of dietary fiber (Garcia-Amezquita, Tejada-Ortigoza, Serna-
Saldívar, & Welti-Chanes, 2018). Rashid, Rakha, Anjum,
Ahmed, and Sohail (2015) reported that higher temperature
EXTRUSION OF PLANT FOOD BYPRODUCTS…229
and screw speed resulted in increased TDF content of wheat
bran incorporated extrudate. This may be explained by the
higher soluble dietary fiber (SDF) content and formation
of resistant starch (retrograded amylose: RS3) during stor-
age. Extrusion induces the formation of anhydro-compounds,
which may react with starch in a transglycosylation mech-
anism (formation of glycosidic bonds) to form branched,
amylase-resistant glucans, thus contributing to the higher SDF
content (Vasanthan et al., 2002). Whereas the shear conditions
during extrusion depolymerizes starch and increases the lin-
earity of amylopectin/amylose chains, hence its higher ten-
dency to be retrograded into RS3. Another explanation is the
formation of new covalent bonds with other macronutrients
that resulted in insoluble compounds (Robin, Schuchmann, &
Palzer, 2012). However, Jan, Saxena, and Singh (2017) did
not share the same view, as they reported a significant nega-
tive effect of temperature, moisture, and screw speed on TDF
content. These findings may be attributed to formation of low
molecular weight soluble fibers that are not recovered by alco-
hol precipitation during TDF analysis.
Extrusion generally enhances the proportion of SDF. Jing
and Chi (2013) showed a 10% increase in SDF content in
extruded soybean residues compared to control. Likewise,
the SDF content in lupin seed coat rose from <3% to 5%
to 9% after extrusion (Zhong, Fang, Wahlqvist, Hodgson, &
Johnson, 2019). Higher temperature, pressure, screw speed,
and moisture content seem to result in higher SDF content.
However, after a certain parameter point, SDF content seems
stagnant or even decreased. This may be a result of glycosidic
linkage depolymerization in polysaccharides.
The changes in its solubility and structure thus modify the
functional properties of extruded fiber. Water holding capac-
ity (WHC) is often used to determine the interactions between
fiber and water. Compared to non-extruded orange pomace,
Huang and Ma (2016) reported higher WHC after extrusion
by up to 85%. The higher WHC is linked to increased SDF,
such as pectin polysaccharides. In pea hulls, however, a
significant reduction in the WHC and swelling property
was demonstrated after extrusion. Ralet, Valle, and Thibault
(1993) reasoned that the cell walls in some hulls have higher
resistance to heat treatment. Similarly, Zhong et al. (2019)
recorded lower WHC in extruded lupin seed coat, despite
increased SDF. Using light microscopy, Redgwell, Curti,
Robin, Donato, and Pineau (2011) observed a reduction in
cell size and higher wall density in extruded citrus fiber,
which impairs its ability to absorb and retain water. Higher
SDF was also linked to lower oil holding capacity (OHC) in
extruded orange peel fiber (Garcia-Amezquita et al., 2019).
The authors indicated that the OHC depended on the porous
character of fiber, rather than the lipophilic nature of fiber
molecules. Furthermore, Zhang, Bai, and Zhang (2011)
reported enhanced solubility, foaming ability, and apparent
viscosity of extruded oat bran SDF. The same study also
reported higher gelatinization temperature after extrusion,
which was in contrary to the result obtained by Wang, Xia,
Wang, Ali, and Li (2019) with extruded oat powder. These
researches suggest that the functional properties of plant food
fiber after extrusion may improve or deteriorate depending
on the materials and processing parameters.
3.5 Vitamins and minerals
The effects of extrusion on vitamins and minerals vary due
to the wide range of their chemical structures. Vitamins A
and E are the least stable lipid-soluble vitamins, compared to
vitamins D and K (Brennan, Brennan, Derbyshire, & Tiwari,
2011). The same study indicated that low moisture and high
temperature conditions accelerate degradation of ascorbic
acid during extrusion. Dar, Sharma, and Kumar (2014) stud-
ied carrot pomace-based extrudates and found a significant
reduction in 𝛽-carotene and vitamin C content after increasing
the barrel temperature from 110 to 140 ◦C. Similar to vitamin
C, extrusion cooking caused around 63% reduction in total
vitamin E content of buckwheat groats, which is largely con-
tributed from the loss of 𝛾-tocopherol (Zieliński, Michalska,
Piskuła, & Kozłowska, 2006).
Thiamine and riboflavin are among the most extensively
studied B-group vitamins after extrusion cooking. Higher feed
moisture and lower temperature are generally linked to higher
retention of these vitamins (Boyaci et al., 2012). This is due
to the reduction in viscosity at high moisture level, thus facil-
itating a faster material flow and reduces the residence time
in the barrel. However, an extrusion study of triticale by
Beetner, Tsao, Frey, and Lorenz (1976) failed to prove any sig-
nificant effect of moisture on thiamine content. Furthermore,
Bajaj and Singhal (2019) investigated the effects of extrusion
parameters on the vitamin B12 stability in puffed snacks. They
reported a complete degradation of B12 at 194 ◦C, but no
notable effect was observed from changing the feed rate and
screw speed.
Few studies have examined the effects of extrusion on min-
erals due to its stability in most food processing conditions. In
spaghetti with added buckwheat bran flour, Manthey and Hall
(2007) reported no significant effect of extrusion on its min-
eral content. Likewise, Alonso, Rubio, Muzquiz, and Marzo
(2001) showed no difference in the mineral composition of
extruded pea and kidney seed beans, except iron. It is likely
that the increased iron content is derived from the metallic
fragments of the screw. Compared to temperature, high feed
moisture appears to have more influence on iron degradation
(Makowska, Zielinska-Dawidziak, Niedzielski, & Michalak,
2018).
Conflicting findings were observed on the effect of
extrusion on mineral bioavailability. Kivistö, Andersson,
Cederblad, Sandberg, and Sandström (1986) demonstrated
impaired absorption of Zn, P, and Mg after two 4-day intake
230 EXTRUSION OF PLANT FOOD BYPRODUCTS…
periods of phytic acid-containing extruded cereal products
in a human ileostomy study. Lombardi-Boccia, di Lullo, and
Carnovale (1991) recorded increased iron content and signifi-
cant reduction for its dialysability in extruded Italian legumes,
which covers mottled bean, lentil, faba beans, chickpea, and
white bean. On the contrary, extrusion improved apparent
digestibility for Cu, Fe, and P in rats with pea-based diets
(Alonso et al., 2001). The same research also showed a signifi-
cant increase in apparent absorption for Cu, P, Zn, Mg, and Ca
in kidney bean-based diets. They reasoned that thermal treat-
ment inactivates antinutritional factors, which improve min-
eral absorption in the digestive system.
3.6 Polyphenol and antioxidants
Polyphenols are naturally occurring compounds in fruits and
vegetables, with a structure characterized by the attachment
of hydroxyl group to a phenyl ring. They have the ability to
quench reactive oxygen and nitrogen species, thus extinguish
the oxidation cycle. The effect of extrusion parameters on
the total phenolic content (TPC) of plant byproducts has
been documented in previous studies. Altan, McCarthy, and
Maskan (2009) showed a reduction in total polyphenols
and antioxidant activity after extrusion of barley flour and
tomato/grape pomace blend. This reduction was intensified
at higher temperature (160 ◦C) and screw speed (160 rpm).
Similar reduction in TPC was reported in extruded nix-
tamalized corn flour, though extrusion increased the free
forms of phenolic acids, such as ferulic acid and p-coumaric
acid (Buitimea-Cantua et al., 2017; Buitimea-Cantua et al.,
2018). On the other hand, higher temperature was linked
to higher TPC in extruded cereals containing apple pomace
(Leyva-Corral et al., 2016). Increase in phenolic value
could be due to the release of bound polyphenols from cell
wall membrane, while decrease is usually explained by the
structural changes of phenolic acids. Moisture is another
parameter that may significantly affect the retention of
polyphenols during extrusion. High feed moisture induces
decarboxylation and polymerization of polyphenols, leading
to the low polyphenol extraction yield (Chalermchaiwat,
Jangchud, Jangchud, Charunuch, & Prinyawiwatkul, 2015).
Pomace is a solid byproduct after pressing juice or oil from
fruits. Past research has investigated the effect of extrusion
on the rich anthocyanin and flavonoid content in pomace.
Khanal, Howard, Brownmiller, and Prior (2009) indicated
33% to 42% reduction of anthocyanin content in extruded
blueberry pomace. Similar results by the same lead author
were obtained from extruded grape pomace (Khanal, Howard,
& Prior, 2009). The process parameters, particularly barrel
temperature and screw speed seemed to have the largest influ-
ence on the anthocyanin level. After examining the effects of
extrusion on bilberry anthocyanin, Hirth, Leiter, Beck, and
Schuchmann (2014) concluded that the highest anthocyanin
loss occurred at the highest temperature and screw speed, and
the lowest feed moisture and feed rate.
Contrary to anthocyanin, the same study by Khanal,
Howard, and Prior (2009) revealed significantly higher pro-
cyanidin monomer and dimer after extrusion. This result is
congruent to the study by White, Howard, and Prior (2010),
who recorded increase in monomer (DP1) and dimer (DP2)
of procyanidins, but decrease from DP4 to 9, which shows a
significant impact of extrusion on the polyphenolic compo-
sition of cranberry pomace. The authors also recommended
additional treatment to minimize anthocyanin loss. However,
Chaovanalikit, Dougherty, Camire, and Briggs (2003) failed
to observe any difference after attempting to utilize ascor-
bic acid to protect anthocyanin from extrusion conditions.
In some situations, the addition of ascorbic acid accelerates
anthocyanin loss through condensation and formation of poly-
meric pigments.
Antioxidant activity depends on both the quantity and com-
position of the bioactive compounds. Thus, the antioxidant
activity of a product may not be affected by reduction in TPC.
Xu and Chang (2009) subjected pinto and black beans to heat
treatment and found that the overall antioxidant activity can
be explained by different polyphenol composition. In their
study, phenolic acids and flavonols have a large influence on
the overall antioxidant capacity of pinto beans, while antho-
cyanin is the largest contributor to the antioxidant activity of
black beans. In extrusion cooking, antioxidant capacity gen-
erally increases with higher temperature, despite decreased
phenolic or anthocyanin content (White et al., 2010). Com-
pared to freeze-dried samples, Liu et al. (2019) observed
higher oxygen radical absorbance capacity (ORAC) values in
extruded apple pomace, despite the lower count of extractable
polyphenol. These findings may be explained by formation
of bioactive compounds and pigments due to Maillard reac-
tions during extrusion. Moreover, balance between tempera-
ture and moisture seems to affect the compounds produced
from Maillard reaction, thus influencing the antioxidant activ-
ity (Sharma, Gujral, & Singh, 2012).
3.7 Antinutritional factors
The presence of antinutritional factors (ANFs) in plant foods
are known to negatively affect nutrient bioavailability and
human digestive systems. The most common ANFs that
are present in plant food products include phytate, saponin,
oxalate, and trypsin/protease inhibitors. Rathod and Annapure
(2017) demonstrated maximum elimination rate of trypsin
inhibitors, phytic acid, and tannin at the extrusion conditions
of high temperature (180 ◦C) and moisture content (22%)
in lentil-derived noodles. Thermal treatment has been linked
to hydrolysis of phytate, formation of insoluble complexes,
and degradation of ANFs compounds, such as myoinosi-
tol 1,2,3,4,5,6-hexakis dihydrogen phosphate. Kaur, Sharma,
EXTRUSION OF PLANT FOOD BYPRODUCTS…231
Singh, and Dar (2015) also showed the highest removal of
antinutrients at higher moisture (20%), albeit with a moderate
temperature of 140 ◦C in extruded cereal brans. Contrary to
the other studies, Yağcı and Evci (2015) showed no signifi-
cant effect of increasing moisture content on phytate content.
They suggested, instead, to focus on the control of pressure
and duration of extrusion to remove the phytate content. How-
ever, past research on the effect of extrusion pressure on ANFs
were limited.
In combination with moisture and temperature, Mukhopad-
hyay, Sarkar, and Bandyopadhyay (2007) found that the max-
imum tannin reduction occurred at a higher screw speed of
96.8 rpm. Severely high screw speed has also been associ-
ated to higher phytic acid and trypsin inhibitor activity in
breadfruit–corn–soy blend, as evident after elevating screw
speed from 90 to 190 rpm (Nwabueze, 2007). However, other
studies have reported no significant effect of screw speed
on phytic acid content (Ainsworth et al. 2007; Gualberto,
Bergman, Kazemzadeh, & Weber, 1997). From these studies,
it can be deduced that feed moisture, barrel temperature, and
screw speed are the most important factors that determine the
degree of ANFs inactivation in plant foods and plant byprod-
ucts by extrusion processing.
4INTERACTIONS OF EXTRUSION
PARAMETERS AND PLANT
BYPRODUCT FOODS IN AN
EXTRUSION PROCESS
4.1 Specific mechanical energy
Specific mechanical energy (SME) is defined as the mechan-
ical work from the motor that is converted into the heat accu-
mulated in the feed material (Godavarti & Karwe, 1997).
Higher SME suggests more heat will be generated during the
extrusion process. This process parameter affects the physic-
ochemical qualities of extruded products, including expan-
sion, water solubility, density, and texture (Fang, Zhang, &
Wei, 2014). SME may also reflect the degree of degrada-
tion and transformation of compounds, such as starch, dur-
ing extrusion. Past literature has shown the positive corre-
lation between screw speed and SME in extruded products.
Ainsworth et al. (2007) observed significantly higher SME
after increasing screw speed from 100 to 300 rpm. This trend
is not affected by the addition of fiber-rich brewer’s spent
grain (BSG) into the snack. Similarly, higher screw speed
enhances the SME in barley-tomato pomace blends, which is
speculated as a result of greater shear stress (Altan, McCarthy,
& Maskan, 2008b). On the contrary, increasing temperature
has been shown to reduce the input of mechanical energy
due to drop in melt viscosity (Altan et al., 2008b; Duarte,
Carvalho, & Ascheri, 2009). Greater feed moisture also
reduces melt viscosity, thus a lower SME is expected.
Inconsistent effects on SME were reported after the incor-
poration of plant byproducts in extrudate. Replacement with
soybean hull, along with high temperature, reduced the
SME to around 20% the value of maize-only control extru-
dates (Duarte et al., 2009). Onwulata, Konstance, Smith, and
Holsinger (2001) reported insignificant difference in SME
with addition of 50 g/kg wheat bran fiber in extruded corn
products, but inclusion at 125 g/kg resulted in significant
decrease of SME. This research group suggested that the
effect of fiber addition was nonlinear, and was highly depen-
dent on extrusion parameters, such as screw speed. These
observations are in disagreement to the research led by Altan
et al. (2008b), who showed improvement in SME with higher
proportion of tomato pomace in barley flour-based snacks.
The presence of fiber restricts the water available for starch,
thereby the higher melt viscosity leads to greater SME input
in extrudates. Additionally, high protein content may reduce
SME by diluting starch and declining melt viscosity (Meng,
Threinen, Hansen, & Driedger, 2010). Thus, besides the pro-
cess parameters, it can be concluded that SME depends on the
source and chemical composition of the plant food byproducts
before extrusion.
4.2 Die pressure and torque
Die pressure refers to the work required to surmount
the resistance of feed material at the extruder barrel die
(Rauwendaal, 2019). Torque quantifies the force required by
the screw to rotate and propel the feed material out of the
die. When expressed in percentage (%), it compares the actual
and permissible torque, thus its role as a safety indicator
of the extrusion process (Guha et al., 1997). Higher mois-
ture, barrel temperature, and screw speed could minimize the
pressure and torque during extrusion (Mazlan et al., 2019;
Stojceska, Ainsworth, Plunkett, & İbanoğlu, 2009). These
parameters promote the reduction of melt viscosity and
enhance the melt plasticity and the mass flow rate. It is also
possible that the rise in screw speed shortens the fill length
in the barrel, subsequently lowering the shaft load and torque
(Meng et al., 2010).
The presence of plant byproducts in starch-based foods
results in lower extrusion torque and die pressure. Mazlan
et al. (2019) revealed lower torque in 25% mango peel-added
corn extrudates as compared to control. A decreasing trend
in torque and pressure was revealed by Pitts, McCann, Mayo,
Favaro, and Day (2016) with increasing proportion of citrus
fiber in wheat-corn extruded snacks. Similarly, reduced die
pressure was observed with higher tomato pomace level in
barley flour dominated blends (Altan et al., 2008b). It is pro-
posed that fiber possesses a lubricating effect, which reduces
the melt viscosity and die pressure. In addition to fiber,
232 EXTRUSION OF PLANT FOOD BYPRODUCTS…
the presence of fat in plant byproducts may provide similar
lubricant effect, as evident in extrusion of olive pomace and
rice flour blend (Ying et al., 2017).
5EFFECTS OF EXTRUSION ON
THE PHYSICOCHEMICAL AND
FUNCTIONAL PROPERTIES OF
PLANT BYPRODUCT FOODS
5.1 Bulk density and expansion
Bulk density is the ratio of extrudate mass per unit volume,
which represents the extent of porosity that may affect pack-
aging material and design (Rathod & Annapure, 2017). The
bulk density is inversely proportional to the expansion ratio
of a product. The expansion ratio is considerably affected by
moisture content, temperature, and screw speed. Mazlan et al.
(2019) studied the physicochemical properties of corn-mango
peel extrudates and observed the maximum linear expansion
at the lowest moisture level (15.5%) and the highest screw
speed (100 rpm). Similarly, Rzedzicki, Sobota, and Zarzycki
(2004) associated the higher moisture content and lower tem-
perature to the lower expansion ratio in pea hull extrudates.
High moisture materials tend to reduce the viscosity of the
mixture, thus negatively affecting the friction and starch gela-
tinization. This is exacerbated under low screw speed and tem-
perature conditions, which corresponds to the low pressure
inside the barrel. As a consequence, the smaller difference
in pressure between the interior and exterior of the extruder
results in a poorly expanded product.
Some studies, however, reported the opposite effects of
these process parameters on expansion. Hashemi, Mortazavi,
Milani, and Yazdi (2016) observed higher expansion ratios
after increasing the moisture content from 12% to 16% in
defatted almond powder-incorporated snacks. They explained
that the higher feed moisture increased the amount of steam
available to expand. In combination with lower viscosity, the
higher steam availability enhances the number of air cells
with thinner cell walls. We speculated that another possibil-
ity is due to the different protein or fiber composition con-
tributing to these findings. Furthermore, higher screw speed
and temperature is expected to improve expansion by inten-
sifying barrel pressure and starch gelatinization (Liu et al.,
2011). Nevertheless, O’Shea, Arendt, and Gallagher (2014)
demonstrated that severely high screw speed may damage the
hilum (core of starch), thus its ability to carry moisture, com-
mence nucleation, and gelatinization. Above a certain limit,
extreme temperature may promote the collapse of the porous
structure.
Addition of plant byproducts in a starch based extruded
food generally leads to higher bulk density and lower expan-
sion values. In the manufacture of extruded corn grits, Wang
and Ryu (2013a) observed lower expansion index with higher
proportion of corn fiber. The same trend was reported after
inclusion of jatobá flour in extruded cassava starch snacks
(Chang, Silva, Gutkoski, Sebio, & da Silva, 1998). They rea-
soned that fiber hinders expansion by diluting starch content
thus its weaker gelation ability, rupturing the air cell walls
before maximum expansion is reached, and increasing the
mass viscosity. It is also probable that the poor expansion
is due to the weak interaction between starch and insoluble
fiber, and fiber’s capacity to enhance the extensional viscosity
and lower the elasticity. Moreover, van der Sman and Broeze
(2013) noted that insoluble fiber has high hydrophobicity,
which negatively affects water sorption properties.
On the other hand, soluble fiber improves the expansion
properties of extrudates by retaining bubble growth, reduc-
ing melt viscosity, and accelerating steam generation at die.
Enriching corn extrudates with alkaline-soluble bran (64%
soluble fiber) led to similar expansion values to that of control
(no bran addition; Pai, Blake, Hamaker, & Campanella, 2009).
It appears that the low shear viscosity and larger heat/mass
transfer area of cornmeal encourages high vapor diffusion.
Likewise, Ačkar et al. (2018) successfully resolved the poor
expansion issues arising in corn snack products enriched with
byproducts by addition of 1% pectin. The authors empha-
sized pectin’s capacity to interact with starch without nega-
tively affecting the continuous melt structure and rheology,
hence stimulating expansion and bubble formation. Addition-
ally, in a review on fibers in extruded cereals, Robin et al.
(2012) indicated that reducing the size of fiber particle may
promote expansion. Smaller, finer particles allow the develop-
ment of bubbles and has greater water binding capacity due to
the higher number of nucleation sites. Therefore, these find-
ings have provided the opportunities to further investigate the
application of soluble fiber and hydrocolloids to develop plant
byproduct added foods with improved expansion.
Protein affects the expansion of extruded food products
by altering the melt extensibility, water distribution within
matrix, and covalent and nonbonded interactions (Moraru &
Kokini, 2003). This also depends on the plant source and
behavior of the resultant starch–protein network. Chaiyakul,
Jangchud, Jangchud, Wuttijumnong, and Winger (2009) dis-
covered that the presence of wheat gluten protein impairs
the starch polymer’s ability to extend and expand upon
release from the die. Furthermore, swelling of starch becomes
restricted due to formation of three-dimensional gluten pro-
tein network, leading to the lesser expansion. On the con-
trary, the network formed between soy protein and starch pro-
motes expansion of extrudates. Higher expansion ratios and
lower bulk density were reported with increasing proportion
of soy protein isolate (SPI) and higher barrel temperature in
extruded corn starch (Chen et al., 2017). The same findings
were echoed by Camire and King (1991), who included 15%
SPI in cornmeal and reported significant reduction in bulk
EXTRUSION OF PLANT FOOD BYPRODUCTS…233
density. Therefore, our understanding is that the expansion
properties of protein are often neutralized under the presence
of other components of plant food byproduct, such as fiber.
5.2 Water absorption index and water
solubility index
Water absorption index (WAI) and solubility index (WSI)
indicate the ability of an extruded product to interact with
water, which may be used as a predictor of the product’s
behavior after processing (Alam et al., 2016). WAI specifi-
cally reflects the ability of food components, such as starch or
fiber, to bind with water, whereas the WSI indicates the extent
of which soluble compounds are released from macronutri-
ents (Rathod & Annapure, 2017). Moisture level of the orig-
inal ingredients and barrel temperature have a considerable
impact on WAI of extruded foods. Yağcı and Göğüş (2008)
observed a trend of increasing WAI and decreasing WSI with
elevation of moisture from 12% to 18% during extrusion of
byproduct enriched rice grits. Excess of water availability
enhances water absorption and reduces the starch viscosity,
thereby accelerating starch gelatinization with more uniform
mixing and heat distribution during extrusion. Nevertheless,
this finding is in disagreement to that of Chang et al. (1998),
who offered a different explanation that high moisture envi-
ronment acts as a lubricant, which protects starch granules
from breakage and limits starch gelatinization.
The effect of temperature on WAI fluctuates depending on
the forces that govern the structural and chemical changes
between fiber and starch. Yağcı and Göğüş (2008) suggested
that the WAI increases with higher extrusion temperature in
extruded snacks made from various plant food byproducts.
However, after a certain point, the WAI displayed an opposite
trend due to starch dextrinization and amylose/amylopectin
depolymerization. In contrast, Hashimoto and Grossmann
(2003) demonstrated an initially decreasing trend of WAI of
cassava bran/starch extruded at milder temperature, before
increasing the temperatures to above 180 ◦C. The pair the-
orized that starch degradation occurs at lower temperature,
which impairs its swelling ability. At extremely high tempera-
ture, structural modifications on the fiber component may take
place, therefore, the open structure allows higher water inter-
action and retention. These interactions, however, rely upon
the flexibility exhibited by the fiber surface. Insoluble fibers
tend to form highly structured, low density water layer that is
preferred by hydrophobic molecules, as opposed to the less-
structured, high density water favored by hydrophilic fractions
(Larrea, Chang, & Martinez-Bustos, 2005).
The proportion of soluble and insoluble fiber fraction
in the plant byproduct, along with the component being
replaced, determines the final WAI and WSI of the starch-
based extruded products. Rzedzicki et al. (2004) included 20%
to 80% pea hull and observed poorer water absorption in the
cereal mix extrudates. This can be explained by the dilution
of starch and presence of insoluble fiber. More studies, how-
ever, recorded higher WAI with increasing levels of plant
food byproduct (Ačkar et al., 2018; Hashimoto & Grossmann,
2003; Makowska, Mildner-Szkudlarz, & Obuchowski, 2013).
The higher soluble fiber content, together with breakdown of
covalent and noncovalent bonding induced by extrusion con-
ditions, may promote the higher WAI in extruded products.
After analyzing the soluble fiber fraction, Larrea et al. (2005)
proposed that instead of the main cellulose and rhamnogalac-
turonan chains, extrusion could solubilize the lateral linkages
and neutral pectic compounds. Similar to WAI, mixed results
were found for the impact of various plant byproduct addi-
tion on the WSI values of extruded foods (Ačkar et al., 2018).
Higher WSI is linked to the destruction of starch granules and
the release of low-molecular weight component, while lower
WSI is due to the decrease in starch content and formation
of starch–fiber complex. In addition, the presence of plant
byproduct may affect the way process parameters alters the
WAI of extrudates. Incorporation of partially defatted hazel-
nut flour eliminates the significant effect of moisture content
on WAI of extruded rice grits (Yağcı & Göğüş, 2008). With-
out the presence of brewers spent grain (BSG), higher WAI
was reported with increasing screw speed (Ainsworth et al.,
2007). However, the trend dissipated and even reversed when
BSG was added at 30% level.
5.3 Tex t ure
Texture is a reflection of the structural integrity of extruded
products and acts as one of the most critical factors affecting
consumer acceptability of extruded foods. Its parameters
cover hardness, crispiness, adhesiveness, gumminess, and
springiness. Hardness refers to the amount of force required
by the probe or molar teeth to crush the extrudate. Past
research has shown the inverse relationship between hardness
and expansion indices (Ačkar et al., 2018). Extrudate hardness
is greatly influenced by the feed moisture, temperature, and
screw speed. Higher temperature and screw speed are asso-
ciated with softer extrudates (Altan, McCarthy, & Maskan,
2008a; Dehghan-Shoar, Hardacre, & Brennan, 2010).
Decrease in melt viscosity under high temperature conditions
promote expansion, bubble formation, and less dense product.
Stojceska et al. (2009) recorded significantly enhanced hard-
ness upon increasing the water feed level from 12% to 17% in
BSG and red cabbage added snacks. Water has a plasticizing
effect that reduces starch viscosity and mechanical energy,
thus limiting bubble growth and increases the extrudate
density (Ding, Ainsworth, Tucker, & Marson, 2005). For
the same reason, integration of fiber and protein-rich plant
byproducts generally result in dense, hard extrudates due to
dilution of starch, premature air cell rupture, and thinning of
cell wall. Nevertheless, reducing the fiber particle size of feed
234 EXTRUSION OF PLANT FOOD BYPRODUCTS…
material may lead to crispier and softer extrudate. Alam et al.
(2016) extruded rye bran and suggested that fine particles
could promote greater expansion by allowing continuity of
fiber–starch matrix, increasing number of nucleation sites,
greater foam expansion and diluting insoluble fiber content.
Crispiness is a characteristic textural parameter for snack
products, and it reflects the density of cell structure. Like
hardness, crispiness is affected by several extrusion process
parameters. Improved crispiness was observed in barley–
grape pomace blends with gradually increasing tempera-
ture (Altan et al., 2008a). Similarly, Geetha, Mishra, and
Srivastav (2014) studied the extrusion of kodo millet-chickpea
blend and reported the maximum crispiness at a combined
high temperature and screw speeds. They stated that the
increased screw speed facilitated bubble formation and con-
version of molten starch into a continuous foam, while the
high temperature reduced the thickness of cell wall and
expands the air cell radius. These results, however, are con-
flicting to the study of carrot pomace-based extrudates by Dar
et al. (2014), who recorded less crispy snacks at higher tem-
perature due to the larger air pockets and decrease in number
of thin cell walls. The presence of fiber and protein in plant
byproducts may also contribute to the low crispiness due to
their negative effects on expansion.
Past research has studied the relationship between textural
attributes and the acoustic properties of extruded snacks. In a
review of acoustic research, Duizer (2001) explored two main
approaches in the study of sound texture, that is: (1) to deci-
pher the contribution of sound on the sensation of crispiness
and crunchiness, and (2) to translate sound records of masti-
cation into quantitative measure of crisp, crackle, and crunch.
They showed a high positive correlation value of 0.922 with
crispiness and crunchiness sound level in twists and chips.
Crispy extrudates are known for its porous, less dense struc-
ture, and lower hardness, thus a greater sound loudness from
mastication is expected. As mechanical failure often occurs in
food manufacturing, integration of acoustic studies is encour-
aged to develop extruded products with desired crispiness.
Saeleaw, Dürrschmid, and Schleining (2012) studied rye-
based extruded snack and revealed a positive correlation
between instrumental and some sensory parameters, such as
hardness, crunchy sound intensity, mean, and maximum force.
The team also noted the significant impact of barrel temper-
ature and feed moisture on the sound emission, texture and
cellular structure of the extrudate. On the contrary, instru-
mental hardness was poorly correlated (r=–0.21) to acoustic
hardness in pea-fortified snacks, along with the low correla-
tion (RV =0.47) between mechanical and acoustic measure-
ments (Philipp, Buckow, Silcock, & Oey, 2017). Mechanical
measurements are provided by the texture analyzer machine
or equipment, whereas acoustic properties are based on the
sound generated during breakage of product. Therefore, rather
than for hardness analysis, this study implies that acoustic
measurements may provide more valuable information on the
crispiness and fracture properties of the extruded products.
Studies assessing the impact of process parameters on other
textural properties of plant byproduct incorporated extrudates
are still scarce. Liu, Hsieh, Heymann, and Huff (2000) stud-
ied the extrusion of oat–corn puff and showed a significant
effect of moisture content and screw speed on the extru-
dates’ springiness, gumminess, and cohesiveness. Specifi-
cally, higher screw speed was linked to increased gummi-
ness and lower chewiness and springiness. In buckwheat
precooked pasta, higher feed moisture was associated to
lower adhesiveness (Wójtowicz, 2012). However, the same
team found different trends in extruded breakfast cereals,
of which the maximum adhesion value was reached at 20%
moisture level, before gradually decreasing above that point
(Wójtowicz et al., 2015). Moreover, Sobota, Rzedzicki, Zarzy-
cki, & Kuzawinska (2015) indicated a trend of decreasing
springiness and greater stickiness and chewiness with higher
replacement with wheat bran in pasta production. This may be
attributed to the high water binding capacity of dietary fiber
in the wheat bran.
Microstructure examination allows researchers to gain
understanding on the cell size, thickness of cell wall and
void between cells in a food system. It can be carried out
using scanning electron microscopy (SEM) and corresponds
to expansion and density indices. Increasing screw speed and
feed moisture facilitates starch gelatinization, which leads
to the observed thinner cell walls and larger air cell size
(Figure 1). It is also probable that the softer extrudates is due
to the thinning of starch layer that encapsulates the air cells,
therefore the less energy required to chew the snack. Dar et al.
(2014) investigated the effects of increasing temperature from
110 to 140 ◦C on the microstructure of carrot pomace added
extrudates. Initially, only fractures and small gap between
fibers were present, which may be due to expulsion of water
from destruction of protein or carbohydrate matrix. With ele-
vation in temperature, the fiber was enlarged and large vac-
uoles were appeared, indicating the porous and spongy struc-
ture of the extrudate. Greater proportion of defatted or whole
hemp powder resulted in higher number of smaller size pores
(Norajit, Gu, & Ryu, 2011). Nevertheless, Kaisangsri et al.
(2016) added 5% carrot pomace and recorded a smoother sur-
face texture compared to control. This suggests that contrary
to common explanation, fiber at low level may play an active
role in interacting with starch to form an adhesive and well
expanded matrix.
5.4 Color
Color is one of the most important visual properties that deter-
mines the acceptability of a food product. It is highly depen-
dent upon the raw materials and process parameters of extru-
sion. Ilo and Berghofer (1999) examined the kinetics of color
EXTRUSION OF PLANT FOOD BYPRODUCTS…235
FIGURE 1 Effect of different combinations of screw speed and feed moisture on the microstructure of extruded snack with partially defatted
almond powder: corn flour ratio of 20:80. For each block (a–d), magnification was done at 80×(left) and 500×(right) (Hashemi et al., 2016; with
permission)
change and concluded that Hunter Lvalue (brightness) is the
most reliable indicator of changes during extrusion cooking.
High temperature, in particular, significantly affects the light-
ness of extrudates by inducing Maillard reaction, carameliza-
tion and pigment degradation. In a study to develop fiber-
rich cereals from banana and carambola pomace, Borah, Lata
Mahanta & Kalita (2016) reported lower Lvalue with increase
in barrel temperature. Correspondingly, after increasing the
temperature up to 120 ◦C, Wang and Ryu (2013b) found a
decrease in Land bvalues (yellowness) of both CO2and non-
CO2injected extrudates. It appears that the rate of nonenzy-
matic browning reaction between protein and sugar in the feed
material is accelerated under high temperature.
Besides temperature, the color of extruded products is
also affected by feed moisture. Selani et al. (2014) showed
decrease in lightness and redness (avalue) with higher
moisture content in pineapple pomace added extrudates.
The authors stated that besides high temperature, Maillard
reaction favors low moisture environment. This conclusion
is further supported by Yağcı and Göğüş (2008) who noted
a generally higher redness with decrease in feed moisture.
However, Stojceska et al. (2009) offered another perspective
and suggested that the effect of feed moisture is dependent
on the feed material. For instance, with the raise in moisture
level, increase in redness was detected for WRC (wheat
flour +red cabbage) and CRC (corn starch +red cabbage)
samples, but the opposite trend was displayed for CBSG
(corn starch +BSG). Moreover, increase in screw speed was
linked to lighter color extrudates (Borah et al. 2016), though
Ainsworth et al. (2007) showed no significant effect of higher
screw speed on the Lvalues of BSG-added snacks.
The presence of plant byproduct has a differing conse-
quence on the color parameters of starch based extrudates.
Addition of grape pomace in barley-based extrudates led
to lower Lvalue, but higher a,b, and ΔE(color change)
values (Altan et al., 2008a). In another tomato pomace study,
the same team observed a higher intensity of darkness and
redness, which can be attributed to the lycopene pigment in
tomato (Altan et al., 2008b). On the other hand, Stojceska,
Ainsworth, Plunkett, İbanoğlu, and İbanoğlu (2008) found
a significant negative correlation (r ←0.6) between the
proportion of cauliflower trimmings and all color parame-
ters (lightness, redness, yellowness) tested in wheat-based
expanded snacks.
Furthermore, 10% addition of jabuticaba (Myrciaria
cauliflora) peel powder lowered the redness of extruded
236 EXTRUSION OF PLANT FOOD BYPRODUCTS…
FIGURE 2 Typical pasting profile and terms
used for pasting characteristics determination
cereals, which may be due to degradation of anthocyanin dur-
ing extrusion, whereas inclusion of pumpkin peel increased
the redness in corn grit extrudates (Norfezah, Hardacre &
Brennan, 2011). Therefore, it can be deduced that due to the
distinct intrinsic nature (including pigment) and food system,
each plant byproduct influences the color of extrudates in a
different manner.
5.5 Lipid oxidation
Lipid oxidation negatively affects product acceptance due to
its relationship with rancidity, off-flavors and off-odors. Prod-
ucts rich in unsaturated fatty acid is particularly susceptible to
rancidity, as the presence of double bonds weaken the bond
dissociation energy that holds the hydrogen atoms. During
extrusion cooking, feed moisture and temperature appears to
have the greatest effect on lipid oxidation. Imran et al. (2015)
observed a trend of increasing peroxide value and free fatty
acids with higher temperature and moisture during storage of
extruded full-fat flaxseed meal. Lower moisture content was
linked to longer storage stability in extruded bran (Moisio
et al., 2015). The team demonstrated that low water avail-
ability accelerated the release of volatile Maillard products,
which may possess high antioxidant capacity. By increasing
the particle size, they also found a higher retention of tocol
and suppression of furfural, thence the greater lipid stability.
However, Leyva-Corral et al. (2016) reported that only tem-
perature has a significant effect over lipid oxidation values
in pomace-added extruded cereals. High temperature exacer-
bates rancidity by breakage of carbon–hydrogen bonds, thus
the possibility to initiate free radical chain reaction. Neverthe-
less, adding antioxidant rich materials, such as potato peel and
wheat bran, may improve the lipid stability of extruded food
products (Camire, Dougherty, & Briggs, 2005).
5.6 Pasting and viscoelastic properties
The pasting characteristic of extruded products is determined
based on the pasting temperature (PT), peak viscosity (PV),
and final viscosity (FV), breakdown viscosity (BV), and set-
back viscosity (SV) (Dalbhagat, Mahato, & Mishra, 2019;
Figure 2). In the food industry, these characteristics are
often quantified by the rapid viscoelastic analyzer. Like other
physicochemical properties, it is easily altered by the change
in extrusion parameters. Pasting temperature is the lowest
temperature to initiate swelling and gelatinization of starch.
Peak viscosity indicates the degree of starch gelatinization.
Final viscosity refers to the viscosity measurement at the end
of the testing period. Breakdown viscosity signifies the abil-
ity of paste to resist thermal and mechanical stress. Whereas
the ability of paste to recover its viscosity during cooling
is termed setback viscosity. This also reflects the propen-
sity for starch retrogradation, a process characterized by re-
association of amylose and amylopectin chains. High extru-
sion temperature results in lower peak viscosity values, which
is expected due to the destruction of starch granule structure
by heat (Tacer-Caba, Nilufer-Erdil, Boyacioglu, & Ng, 2014).
Similarly, Wang, Fu, et al. (2017) found significantly lower
PV, BV, FV, and SV values in extruded rice starch as com-
pared to non-extruded samples.
Aside from temperature, it appears that feed moisture and
screw speed may influence the pasting profiles of extru-
dates. da Silva, Ascheri, and Ascheri (2016) showed lower
PV, BV, FV, and SV with increasing feed moisture in brown
EXTRUSION OF PLANT FOOD BYPRODUCTS…237
rice and corn meal blend pasta. Higher water availability
allows starch to swell, thus the lower viscosity values. Further-
more, decrease in BV has been correlated to higher swelling
capacity of starch (Kong, Zhu, Sui, & Bao, 2015). On the
other hand, Sayanjali et al. (2017) demonstrated maximum
peak viscosity with higher feed moisture and lower screw
speed in oat fiber. They also reported the highest FV in non-
extruded oat, compared to extruded samples with all com-
binations of process parameters. These differences may be
linked to the different process parameters and feed materials
used.
The presence of plant byproduct lowers the paste viscosity
profile of extruded products. Duarte et al. (2009) revealed
lower BV and SV with inclusion of soybean fibers. Incor-
poration of rice bran promotes the reduction of PV, BV, FV,
and SV in extruded rice starch (Wang, Fu, et al. 2017). Like-
wise, a gradual decrease in PV and FV was exhibited after
including up to 15% wheat bran in extruded cereal products
(Brennan, Merts, Monro, Woolnough, & Brennan, 2008).
Tacer-Caba et al. (2014) added grape extract powder and
reported diminishing peak viscosity. Fiber may compete
with starch for free water, thus decreasing the occurrence
of gelatinization and PV. However, some soluble fiber-
rich compounds, such as gums, may improve viscosity by
interacting with swollen starch granules (Weber, Clerici,
Collares-Queiroz, & Chang, 2009). Another explanation is
that the reduction in PV is caused by formation of amylose-
lipid complexes during extrusion, thereby limiting amylose
leaching and swelling within starch granules (Wang, Fu, et al.
2017). Additionally, protein content in plant byproducts has
been negatively correlated to peak viscosity (Sharma, Singh,
Hussain, & Sharma, 2017).
In relation to pasting profiles, several studies have investi-
gated the impact of integrating fiber and protein-rich ingredi-
ents on the viscoelastic properties of extrudates. Based on the
storage (G’) and loss moduli (G’’) parameters, Baek, Kim,
and Lee (2014) revealed the lower rigidity and hardness of
rice-based extrudates in the presence of 20% to 60% corn bran.
This is further supported by the higher tan 𝛿, a measure dis-
playing the magnitude of viscoelasticity, in bran-added sam-
ples (Figure 3). The authors argued that corn bran disrupts the
three-dimensional starch network in paste, thus the reduced
pasting parameters. Correspondingly, lower G’ and G’’ values
were reported after addition of inulin fiber into snack dough
(Peressini, Foschia, Tubaro, & Sensidoni, 2015). It appears
that inulin interacts poorly with water and starch. In contrast
to fiber, protein may lower the viscous and elastic behavior
of pastes. Ortiz, Martín-Martínez, and Padilla (2008) stud-
ied rice starch–soy protein isolate blends and observed an
increase in G’ value with protein content above 50%. For-
mation of intermolecular disulfide bonds and protein disper-
sion at high temperature may incite the assembly of three-
dimensional gel matrix.
5.7 Thermal properties
Thermal properties of extrudates are typically evaluated in
terms of onset melt temperature, peak temperature, and
enthalpy (ΔH). The onset and peak temperatures indicate the
amount of energy required to commence the gelatinization
process (Cleary & Brennan, 2006). Melting enthalpy (ΔH)
is defined as the energy needed to break the hydrogen bonds
present on the junction zones connecting the biopolymers.
It also reflects the extent of starch gelatinization, density
of junction zones and cross-linking between macronutrients
(Vaikousi, Biliaderis, & Izydorczyk, 2004). Wang, Fu, et al.
(2017) presented significantly lower ΔH during extrusion of
rice starch, which was attributed to its gelatinization. High
extrusion temperature facilitates starch gelatinization by dis-
rupting the structure of starch granules and allowing the entry
of moisture. Addition of fiber-rich stabilized rice bran (SRB)
at 10% level (w/w) further intensified the reduction in ΔH
value and extent (%) of gelatinization. It is probable that the
SRB may interact with amylose and alter the extrudate’s crys-
talline order to promote starch gelatinization. On the con-
trary, a raise in ΔH was observed during extrusion of soluble
dietary fiber derived from oat bran (Zhang et al., 2011). Pos-
sible explanations include reduction in specific surface area
of extruded samples and a shifting that favors larger particle
size distribution after extrusion.
Increasing proportion of fiber-rich material in a starch
based extruded food is associated with poorer gelatinization
properties. Altan et al. (2008b) demonstrated significantly
lower enthalpy change after integrating up to 12.7% tomato
and grape pomace into barley flour compared to the whole
barley flour sample. Similarly, inclusion of 10% barley 𝛽-
glucan (BBG) fiber into pasta did not affect the onset temper-
ature, but reduced ΔH value considerably (Cleary & Brennan,
2006). They postulated that several polysaccharides of BBG
may limit the movement of water, thus restricting swelling and
gelatinization of starch granules.
Glass transition temperature (Tg) refers to the point when
the amorphous compound transforms from glassy to rub-
bery/leathery state (Nithya, Saravanan, Mohan, & Alagusun-
daram, 2015). Previous studies have recognized the inverse
relationship between moisture content and Tg(Fan, Mitchell,
& Blanshard, 1996; Robin, Dubois, Curti, Schuchmann, &
Palzer, 2011). Water acts a plasticizer in the food system that
supresses thermal transition. Additionally, Robin et al. (2011)
showed a reduction in Tgfrom 194 to 105 ◦C after incorpora-
tion of wheat bran. Aside from diluting starch, the hydropho-
bic nature of fiber allows more free water, thus the lower glass
transition. This may negatively affect expansion properties of
extrudates due to lower starch viscosity, melt temperature, and
mechanical stress. The result disagrees with the findings by
Nithya et al. (2015) who showed a higher Tgwith addition
of roasted Bengal gram flour in rice flour blend. The team
238 EXTRUSION OF PLANT FOOD BYPRODUCTS…
FIGURE 3 Viscoelastic attributes of corn bran
added rice extrudates in terms of (a) G’ and G’’ and
(b) tan 𝛿(Baek et al., 2014; with permission)
speculated that protein in pulse may limit the movement of
starch polymers. Hence, the presence of protein may negate
the Tgreducing effect from fiber and moisture.
6EFFECTS OF EXTRUSION ON
THE MICROBIOLOGICAL
PROPERTIES OF PLANT
BYPRODUCT FOODS
Microbiological property is a critical aspect in food process-
ing as it determines the safety and shelf-life of a food product.
As extrusion is a HTST process, it can lower the survival rate
of microorganisms. Likimani and Sofos (1990) indicated that
extrusion at the temperature of 80 to 100 ◦C was sufficient to
injure Bacillus globigii spores. Even after 2 weeks of refrig-
erated storage, the counts of Clostridium sporogenes spores
continued to decrease in extruded turkey-corn flour mixture
(Li, Hsieh, Fields, Huff, & Badding, 1993). In a whey protein
study, low moisture (4% to 5%) extrusion cooking resulted
in lower counts of viable Streptococcus thermophilus with-
out impairing the protein’s functional properties (Quéguiner,
Dumay, Cavalier, & Cheftel, 1989). Compared to other cook-
ing methods (stove toasted, microwave toasted, parboiled),
no significant difference was detected in bacterial counts of
extruded rice bran (Oliveira, Bassinello, Lobo, & Rinaldi,
2012), further confirming that extrusion can achieve the same
effect of heat processing in controlling of microorganisms.
EXTRUSION OF PLANT FOOD BYPRODUCTS…239
Suppression of microbial growth appears to be favored
under high temperature and low moisture environment.
Awolu, Oluwaferanmi, Fafowora, and Oseyemi (2015)
reported the lowest total viable count (TVC) under the mini-
mum moisture of 12% and moderate temperature of 80 ◦Cin
extruded snacks made from broken rice, cassava root powder,
and kersting’s groundnut seeds. Salmonella was undetected in
most samples, though the lowest temperature (63 ◦C) led to
its appearance. While increasing barrel temperature promotes
higher fatality of spores, higher screw speed, and shorter res-
idence time seems to present the opposite effect (Likimani &
Sofos, 1990). This finding is contrary to the research by Bulut,
Waites, and Mitchell (1999), who reported a positive, albeit
weak, correlation between screw speed and log reduction of
Microbacterium lacticum. They also proposed that the phys-
ical forces governing extruder die and reverse screw element
may play a more important role than temperature in destruc-
tion of microorganisms.
Moreover, the presence of plant byproducts in food may
reduce the microbial counts in food. Higher dose of carrot
dietary fiber addition was followed by a reduction in lipolytic
microorganisms during the ripening of dry fermented sausage
(Eim, Simal, Rosselló, & Fermenia, 2008). Selani et al. (2014)
suggested that the addition of pineapple pomace may intensify
the acidity of extruded snacks and reduce the TVC. Although
some mould and yeast species may thrive under low pH con-
ditions, survival of common pathogens such as Salmonella is
very low at higher acidity level. More studies, however, are
still required to investigate the effects of including pomace or
other plant derived byproducts on the microbiology and safety
of extrudates.
7EFFECTS OF EXTRUSION ON
THE SENSORY PROPERTIES OF
PLANT BYPRODUCT FOODS
The sensory properties of food products relate to its appear-
ance, texture, aroma, and flavor. Sensory evaluation is carried
out with human participants and predicts the acceptance of
a product before its release to the market. The barrel tem-
perature and feed moisture are considered to have the great-
est influence over the sensory properties of extruded food
products. Appearance is a broad sensory aspect and may
include the color, surface texture, and expansion of the extru-
dates. Chen, Serafin, Pandya, and Daun (1991) studied corn
meal extrudates and recorded poorer appearance scores with
increasing temperature. As appearance is strongly related to
color, this result may be attributed to corn pigment degrada-
tion and nonenzymatic browning reactions (Maillard reaction,
caramelization) that occur under high temperature conditions.
Another reason is that severely high temperature, along with
high feed moisture, have a negative impact on the expansion
index. Additionally, while high temperature results in rougher
extrudate surface, low moisture promotes dough elasticity,
and smoother surface texture. Thus, study on the interaction
between moisture and temperature may be essential to develop
extrudates with an optimal appearance and texture scores.
Due to its close relation between appearance and texture,
several studies have investigated the correlation between sen-
sory score and instrumental data of extrudates. Mendonça,
Grossmann, and Verhé (2000) showed a correlation index of
0.80 for radial expansion and appearance, and 0.78 for specific
volume and palatability, in a study of corn bran in extruded
snacks. The pair also regarded specific volume as the physic-
ochemical attribute with the highest correlation with sensory
analysis scores. This finding is supported by Liu et al. (2000)
who suggested that high bulk density is linked to lower sen-
sory characteristics in extruded oat–corn puff, due to the dry
surface, roughness, and irregular shape.
Unlike other sensory properties, the flavor and aroma of
food products cannot be predicted from instrumental anal-
ysis. In extrusion, flavorings are most commonly evaluated
pre-extrusion and post-extrusion. The advantages and disad-
vantages of these methods have been reviewed by Bhandari,
D’Arcy, and Young (2001). Flavor compounds are often lost
due to harsh extrusion conditions and resultant chemical reac-
tions, such as degradation, cross-linking, and interactions with
other nutrients (Palkert & Fagerson 1980). However, extru-
sion conditions can be manipulated to control these losses.
Higher temperature (range: 100 to 200 ◦C) and low moisture
content (range: 20% to 30%) are associated with higher fla-
vor scores of extrudates among panelists (Chen et al., 1991).
Additionally, Menis, Milani, Jordano, Boscolo, and Conti-
Silva (2013) observed an inverse relationship between fla-
vor acceptance and volatile concentration of flavored corn
grits. They explained that the low moisture improves fla-
vor scores by enhanced material shearing and subsequent
decrease in volatile concentration, particularly ethyl butyrate.
Ethyl butyrate is a naturally occurring ester with a charac-
teristic fruity pineapple smell, but higher concentration of
this compound may not be favorable for consumers. In con-
trast, Conti-Silva, Bastos, and Areas (2012) reported greater
volatile retention and flavor intensity with milder temperature
of 90 ◦C and higher moisture content at 20%. These differ-
ences may take place due to variation in flavorings, extrusion
parameters, and feed materials.
The effect of incorporating plant byproducts on a food’s
sensory attributes varies widely based on the raw materials
and experimental methodology. Inclusion of 20% semi-
defatted sesame cake resulted in the poorest acceptability
scores of corn extrudates (da Graca Costa do Nascimento,
Wanderlei Piler Carvalho, Takeiti, de Grandi Castro Freitas,
& Ramirez Ascheri, 2012). Nevertheless, after informing
the panelists about the health benefits of sesame cake, no
240 EXTRUSION OF PLANT FOOD BYPRODUCTS…
significant difference was observed as compared to 5%
sesame seed added corn extrudates. On the other hand, even
without exposure to any information, Altan et al. (2008a)
demonstrated that replacement with grape pomace at 10%
improved the overall acceptability of barley flour extrudate.
This result is largely contributed by the enhanced desirable
textural (hardness, crispness, brittleness) properties. Larrea
et al. (2005) showed that extruded orange pulp can be added
up to 15% level in biscuits. However, at very high proportion
(25%) of the pulp, the acceptability scores were considerably
lowered. While the appearance score was higher compared to
the control, it appeared that the high fiber content could result
in undesirable darkness, hardness, chewiness, and gumminess
in the product. The presence of pectin in orange pulp may
also compete with starch for free water, therefore the greater
biscuit thickness. Contrary to the studies above, a decreasing
trend in sensory acceptability was reported with higher
integration of soybean hull (Duarte et al., 2009). Identical
trends were displayed by Stojceska et al. (2008) in cauliflower
trimmings added extrudates. In addition to the poor texture, it
seems that flavor resulting from these plant byproducts is still
intolerable for some consumers. Thence, whether integration
of byproducts into extruded products could enhance the
sensory acceptance of the final product highly depends
on the variety of byproducts used and the food matrix
added to.
8CONCLUSION
This review highlights the effect of extrusion on the nutri-
tional, physicochemical, microbiological, and sensory prop-
erties of extruded foods incorporated with plant byproducts.
Despite the in-depth research on protein and fiber, investi-
gation for the impact of extrusion on vitamins and minerals,
along with its bioavailability, is still relatively limited. More-
over, previous tests suggested that extrusion has a comparable
effect as other food processing methods in suppressing
microbial growth. Given the rarity of microbiological studies
involving extruded foods, further investigation in this area
is required. In most of the past studies, very high proportion
of plant byproduct addition in extruded foods was associated
with undesirable physicochemical qualities, including low
expansion, high density, and increased hardness. Contrasting
trends in sensory evaluation scores with greater level of
plant byproduct was also observed, indicating the high
dependency on the type of byproducts added and the food
system. Therefore, additional studies are necessary to develop
methods capable of solving these issues associated with the
inclusion of plant byproducts in extruded foods, including:
(1) optimization of extrusion processing parameters with
desirable physicochemical properties and minimal nutritional
loss, (2) selection of soluble fiber rich materials such as pectin
in extrusion formulation, and (3) intensive research on the
interactions between process parameters and raw materials.
We are optimistic on the application of extrusion technol-
ogy to process value-added foods by incorporating plant
byproducts, which will benefit both food industry and our
environment.
CONFLICTS OF INTEREST
The authors declared no conflicts of interest.
AUTHOR CONTRIBUTIONS
William Leonard drafted the original manuscript. Critical
inputs and corrections were successively provided by Zhongx-
iang Fang, Danyang Ying, and Pangzhen Zhang. Zhongxi-
ang Fang also contributed to the conception and structure
design of the manuscript and finalized the manuscript for
submission.
ORCID
Pangzhen Zhang https://orcid.org/0000-0002-9794-2269
Zhongxiang Fang https://orcid.org/0000-0002-9902-3426
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How to cite this article: Leonard W, Zhang P,
Ying D, Fang Z. Application of extrusion technol-
ogy in plant food processing byproducts: An overview.
Compr Rev Food Sci Food Saf. 2020;19:218–246.
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