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Formulation of zein coatings for apples
(Malus domestica Borkh)
1
Jinhe Bai
a
, Victorine Alleyne
b
, Robert D. Hagenmaier
c
, James P. Mattheis
d
,
Elizabeth A. Baldwin
c,
*
a
Department of Horticultural Sciences, University of Florida, Gainesville, FL 32611, USA
b
Florida Department of Citrus, Citrus Research and Education Center, Lake Alfred, FL 33850, USA
c
Citrus and Subtropical Products Laboratory, US Department of Agriculture, Agricultural Research Service, 600 Ave. S.N.W.,
Winter Haven, FL 33881, USA
d
Tree Fruit Research Laboratory, US Department of Agriculture, Agricultural Research Service, Wenatchee, WA 98801, USA
Received 20 February 2002; accepted 30 August 2002
Abstract
High gloss coatings are used to improve apple fruit (Malus domestica , Borkh) appearance and sales. The industry
standard has been shellac-based formulations, which have problems with whitening, low gas permeability, and
association with non-food uses. Zein, a natural corn protein, was used to formulate alternative, shiny coatings by
dissolving zein in aqueous alcohol with propylene glycol (PG). Gloss levels on ‘Gala’ apple surfaces varied due to zein
and PG content in coating formulations from that of controls to levels observed for shellac-coated fruit. At least 4% (by
weight) PG was necessary for adequate gloss. However, increasing levels of both compounds resulted in increased gloss.
Whitening, which occurred on the coated fruit surface upon wetting, was reduced by decreasing zein content to less than
11%. Permeability to CO
2
,O
2
, and water vapor was strongly dependent on the zein content in the coating. Internal CO
2
and O
2
in zein-coated ‘Gala’ fruit ranged 4 /11 and 19/6 kPa, respectively, by increasing zein content in the coatings.
An optimum formulation with 10% zein and 10% PG was developed, applied to ‘Gala’ apple, and was found to
maintain overall fruit quality comparable to a commercial shellac coating.
#2002 Published by Elsevier Science B.V.
Keywords: Gloss; Whitening; Permeance; Modified internal atmosphere; Volatile; Sensory
* Corresponding author. Tel.:
/1-863-293-4133x120; fax: /1-863-299-8678.
E-mail address: ebaldwin@citrus.usda.gov(E.A. Baldwin).
1
Mention of a trademark or proprietary product is for identification only and does not imply a guarantee or warranty of the
product by the US Department of Agriculture. The US Department of Agriculture prohibits discrimination in all its programs and
activities on the basis of race, color, national origin, gender, religion, age, disability, political beliefs, sexual orientation, and marital or
family status.
Postharvest Biology and Technology 28 (2003) 259 /268
www.elsevier.com/locate/postharvbio
0925-5214/02/$ - see front matter #2002 Published by Elsevier Science B.V.
PII: S 0 9 2 5 - 5 2 1 4 ( 0 2 ) 0 0 1 8 2 - 5
1. Introduction
Shellac and carnauba wax are often used
commercially to coat apples and citrus to improve
appearance by adding gloss, to prevent water loss
that leads to shriveling and subsequent loss of
marketability, and to maintain quality through
delayed ripening and senescence. Unfortunately,
both materials are associated with non-food uses,
and shellac has problems with low gas permeabil-
ity that, although it leads to delayed ripening in
some fruit (Baldwin et al., 1999), can also cause
anaerobic conditions. Shellac also has problems
with whitening, or ‘‘blushing’’ as it is referred to in
the industry, when water condenses on the coated
fruit surface after removal from cold storage.
Nevertheless, shellac is recognized as one of the
most shiny coatings, was found to improve the
appearance and, it is assumed, increased subse-
quent sales of red and green apple cultivars such as
‘Delicious’ and ‘Granny Smith’, respectively (Bai
et al., in press).
Carnauba wax is a natural plant wax with
generally recognized as safe (GRAS) status
(FDA, 1999), is relatively permeable to gases
and, in microemulsion form, is quite shiny. The
problem with carnauba wax is loss of gloss over
storage time, and due to its relatively high gas
permeability, it does not effectively delay ripening
(Baldwin et al., 1999). However, it is an excellent
barrier to water vapor, and can be combined with
shellac to create a coating of moderate perme-
ability to gases and low permeability to water
vapor. However, the apple industry is concerned
that consumers may object to shellac, which does
not currently have GRAS status.
Zein is a natural protein found in corn kernels.
Zein coatings have been used to coat nuts and
candy for increased gloss, and prevention of
oxidation and development of off-odors (Cosler,
1958). Extracting a zein /lipid mixture from
ground corn costs only 2 /4 US dollars per kg
(Anon., 2001), and is a GRAS substance (FDA,
1999). Zein coatings offer a reasonable alternative
to shellac and carnauba wax.
Surface coatings can decrease fruit peel per-
meance, modify the internal atmosphere, reduce
water loss, and depress fruit respiration rate (Bai et
al., 2002; Banks et al., 1993; Hagenmaier and
Shaw, 1992). Apple anatomy differs by cultivar in
the number of stomates and lenticels which affects
how the fruit react to surface coatings (Bai et al.,
in press). Therefore, selecting coatings with proper
gas permeance for a particular cultivar is impor-
tant (Bai et al., in press). Preliminary research
showed that zein coatings with different zein and
propylene glycol (PG) contents resulted in a wide
range of internal CO
2
and O
2
partial pressures in
coated apple fruit. It will benefit the fruit industry
if alternative coatings to shellac and carnauba
wax, with a wide range of gas permeabilities, were
available to suit cultivar and/or commodity re-
quirements.
Like shellac, zein also has whitening problems
when in contact with water. Preliminary experi-
ments showed that the whitening of zein-coated
fruit was dependent on coating zein and PG
contents.
In this work, an experimental zein coating series
was formulated with different zein and PG con-
tents. Permeability of coatings, internal atmo-
sphere, gloss, and surface whitening were
determined when the formulations were applied
to various surfaces or to apple fruit. Finally, the
overall effect of an optimal zein formulation on
‘Gala’ apple quality was investigated in compar-
ison to commercial shellac- and carnauba wax-
coated fruit.
2. Materials and methods
Zein was obtained from Sigma Chemical Co.
(St. Louis, MO), and defatted by washing with
hexane. The solvent used to dissolve the zein was
35% ethanol plus 35% isopropanol /water solution
(by volume: 36.8 ml of 95% ethanol, 35 ml of 100%
isopropanol, and 28.2 ml of water), except when
described otherwise. This aqueous alcohol combi-
nation had been determined in preliminary work
to be optimum for zein coating performance. PG
was added as a plasticizer after testing of different
plasticizers showed that only PG resulted in glossy
zein-based coatings. A series of 4 /15% zein
combined with 2/16% PG coating formulations
(4/15 g zein and 2/16 g PG to 94/69 g aqueous
J. Bai et al. / Postharvest Biology and Technology 28 (2003) 259 /268260
alcohol) was generated for evaluating gas per-
meance and gloss characteristics when applied to
apples.
To determine coating permeability, a polyethy-
lene film was used as a carrier. Approximately 1 ml
of each coating solution was deposited on the
carrier film and was spread to a smooth level with
a flexible blade. The film was dried for 2/4 weeks
at 50% RH and 23 8C. Film thickness was
measured using a caliper micrometer (Federal
Products Co., Model XLI 20000, Providence, RI)
taking measurements at six locations on the film
and averaging the result. Thickness of the films
ranged from 8 to 40 mm depending on the zein and
PG concentration. O
2
and CO
2
permeance of the
coated and uncoated polyethylene was determined
at 30 8C and 60% RH with an oxygen analyzer
(Ox-Tran 100, Modern Controls, Minneapolis,
MN) and gas chromatograph (HP 5890A, Hew-
lett-Packard, Avondale, PA), respectively. The gas
chromatograph was equipped with a CTR-1
column (1.8 m/3.2 mm) packed with a porous
polymer mixture connected to a thermal conduc-
tivity detector. Permeability was calculated by
dividing the O
2
or CO
2
transmission rate by the
gas partial pressure and multiplying by the film
thickness (Hagenmaier and Shaw, 1991, 1992).
Three to four replicates were made per formula-
tion.
Coating shine in gloss units (GU) was measured
with a reflectance meter (micro-TRI-gloss, BYK
Gardner, Inc., Silver Spring, MD). Coatings were
spread on polished test sheets (Leneta Co., Mah-
wah, NJ) at a thickness of 0.05 mm with a coating
applicator (BYK-Gardner, Columbia, MD).
Coated sheets were dried at 25 8C for 12 h after
which reflectance was measured at an angle of 208.
Two test sheets were coated per formulation and
10 measurements made for each sheet.
Apples were obtained from Washington State.
Non-coated, post-commercial controlled atmo-
sphere stored ‘Gala’ and ‘Delicious’ apples were
obtained from 1999 to 2001 from a commercial
packinghouse, and sent to Winter Haven, FL, in a
refrigerated truck.
Uniform defect-free fruit, weighing 180/200 g
for ‘Gala’ and 200/245 g for ‘Delicious’ apples
were equilibrated at room temperature (25 8C) for
24 h, prior to application of coatings. A formula-
tion, ‘‘zein10’’, with 10% zein plus 10% PG was
used for comparison with two commercial coatings
(EcoScience, Orlando, FL): Natural Shine TM
8000 (carnauba wax) and Apple Wax 55 (shellac)
as well as with non-coated fruits as control.
Coatings were applied using latex gloved hands
at 0.5 ml per fruit (Bai et al., 2002). A pilot-plant
scale conveyor dryer (Central Florida Sales and
Service, Inc., Auburndale, FL) was used to dry
fruit (including controls) at 50 8C for 5 min. Fruit
were held at 20 8C and 50% RH. For non-coated
controls, water was used instead of coating.
Coating gloss on the fruit surface of 4 /5 fruit
per treatment was measured with a reflectance
meter. The reflectance meter was fitted with a
shield having a 19 mm diameter hole (Hagenmaier
and Baker, 1994), and reflectance was measured at
an angle of 608. Ten measurements were made per
fruit.
Coated fruit surface whitening was measured by
incubating the fruit at 50 8C, with at least 98%
RH for 30 min, after which the fruit were moved to
ambient conditions (20 8C, 50% RH) for 30 min
and degree of whitening was measured. A whiten-
ing scale was developed and scored depending on
relative area of discoloration with 5 /70/%of
the surface area discolored; 4/50/70%; 3/10/
50%; 2/5/10%; 1 /5% isolated spots; and 0 /
none.
Flesh firmness was assessed with a penetrometer
(FT 327, McCormick, Facchini, Alfonsine, Italy),
equipped with a 1.1 cm diameter cylindrical
plunger.
Samples for internal gas were obtained from the
core cavity of fruit submerged in tap water
(Alleyne and Hagenmaier, 2000). O
2
and CO
2
partial pressures were determined using a HP
5890A gas chromatograph equipped with a CTR-
1 column (Alltech Associates, Deerfield, IL) con-
sisting of an outer column (1.8 m/6.4 mm)
packed with activated molecular sieve and an
inner column (1.8 m/3.2 mm) packed with a
porous polymer mixture connected to a thermal
conductivity detector. O
2
partial pressure of sam-
ples was not corrected for argon, which is 0.9 kPa
in air. Ethylene was measured with a PE 8500 gas
chromatograph (Perkin /Elmer, Norwalk, CT)
J. Bai et al. / Postharvest Biology and Technology 28 (2003) 259 /268 261
equipped with an activated alumina column and a
flame ionization detector (Baldwin et al., 1995).
Ten individual fruit were used for each gas
measurement. For weight loss determinations, 20
fruit were individually weighed initially and at the
end of the storage period.
Analysis of sugars was accomplished using an
HPLC system (Perkin /Elmer Series 410, Norwalk,
CT) which separated sucrose, glucose, and fruc-
tose for quantification (Baldwin et al., 1991; Bett
et al., 2000). Fruit homogenate with equivalent
water (by volume) was kept at
/20 8C prior to
analysis. Thawed homogenate was added to 80%
ethanol, blended for 30 min, and vacuum-filtered
through Whatman No. 4 filter paper. The resulting
extract was passed through a C-18 Sep Pak
(Waters/Millipore, Milford, MA) and a 0.45 mm
millipore filter. Filtered extract was analyzed using
a Waters Sugar Pak column at 90 8C, with a
mobile phase of 100 mM ethylenediamine tetra-
acetic acid disodium-calcium salt (Ca EDTA),
using a flow rate of 0.5 ml min
1
, and a refractive
index detector (Model LC-50, Perkin /Elmer,
Norwalt, CT).
For titratable acidity (TA) analysis, homoge-
nates were centrifuged at 25,000 /gand the
supernatant titrated to pH 8.1 with 0.1 N
NaOH, and the acidity was calculated as malic
acid on weight basis (gram per kilogram) (Jones
and Scott, 1984).
For volatile analysis (Bai et al., 2002), 50 g apple
slices (core tissue removed) were homogenized
with 25 ml deionized water and 25 ml saturated
NaCl solution. Two milliliters of homogenate were
then placed into a 6 ml vial sealed with a crimp-top
and Teflon/silicone septum, flash-frozen in liquid
nitrogen and stored at
/80 8C prior to analysis.
Sample vials were thawed under running tap
water, heated rapidly to 80 8C, and incubated
for 15 min by a Perkin /Elmer HS-6 headspace
sampler heating block before headspace was
pressurized and injected onto the GC column.
Analysis was carried out using a gas chromato-
graph (Perkin/Elmer Model 8500, Norwalk, CT)
equipped with a 0.53 mm /30 m polar Stabilwax
capillary column (1.0 mm film thickness, Restek,
Bellefonte, PA) and a flame ionization detector.
Oven temperature was held at 40 8C for 6 min,
then raised to 180 8C at a rate of 6 8C min
1
.
Compounds were identified by comparison of
retention times with those of standards and by
enrichment of apple homogenate with authentic
samples. Concentrations were calculated by using
regression equations determined by injecting five
different concentrations of each standard to obtain
a peak height calibration curve as described by
Nisperos-Carriedo et al. (1990). Identification of
volatiles was periodically checked by spiking
homogenate with standards. Volatile components
that are abundant, or that have been reported to
have significance for apple or other fruit flavors
(Mattheis et al., 1995) were analyzed including
ethanol, ethyl acetate, ethyl butyrate, butyl ace-
tate, 2-methylbutyl acetate and hexyl acetate.
Sensory panel analyses for sweetness, acidity,
texture, off-flavor and flavor were conducted by 20
experienced panelists using hedonic scales. Quality
factors were scored on a 9-point scale with 9 /
high intensity or very firm to 1 /low intensity or
soft.
PROC GLM of SAS Version 6.12 (SAS Insti-
tute, Cary, NC) was used for analysis of variance.
Mean separation was determined by the Scheffe’s
test.
3. Results and discussion
3.1. Solubility of zein
Zein is soluble in aqueous alcohol. To formulate
a 10% zein solution, the alcohol:water (v/v)hadto
be 6:4 to 9:1. Aqueous alcohol solutions with
either ethanol, isopropanol, or their mixture were
equally effective as a zein solvent, and did not
affect coating gloss (data not shown). Viscosity
increased with increased zein content, with a 15%
zein formulation being the limit for commercial
coating use. Insolubility of zein in water is due to
its amino acid composition (Reiners et al., 1973).
The advantage of using an alcohol solvent is the
reduction of drying time, but the shortcomings are
increased coating cost and an increase in volatile
organic compounds (VOCs) which can become a
regulatory issue. A commercial water-soluble zein
is sold by Freeman Industries, L.L.C (Tuckahoe,
J. Bai et al. / Postharvest Biology and Technology 28 (2003) 259 /268262
NY) but the gloss and other coating properties are
not compatible for coating apple fruits.
3.2. Gloss
Combinations of zein and PG contents in the
aqueous alcohol solutions affected the gloss of
coatings. PG was added as a plasticizer, without
which zein coatings are very brittle. PG (2%)
combined with any amount of zein did not result
in gloss differences on ‘Gala’ apples compared
with control. Gloss increased only when PG
concentrations were increased from 2 to 6%, after
which increasing PG did not result in significant
increases in gloss (Fig. 1). However, gloss in-
creased along with increasing zein content up to
15% zein (Fig. 1,Table 1), above which viscosity
became a problem.
Gloss of non-coated ‘Gala’ was 7.3 GU, which
was higher than that of ‘Delicious’ which was 4.1
GU. To the human eye, ‘Gala’ apples appear to
have a natural gloss, but ‘Delicious’ fruit havevery
low gloss. This is one reason that has led the apple
industry, generally, to apply a shiny coating to red
‘Delicious’ apples. When formulating zein coat-
ings, alteration of zein and/or PG content resulted
in a wide range of gloss options. Mixtures of less
than 10% zein and 4% PG resulted in coatings of
low gloss that would not be commercially accep-
table to the apple industry, but might be useful for
other types of fruit where a more natural appear-
ance is preferred.
3.3. Whitening
Zein content was the key factor for whitening of
zein-coated fruit. When zein content was higher
than 11%, the degree of whitening was rated higher
than 1.3, indicating that, most of the fruit ex-
hibited white spots (scale of 1), and quite a number
of them exhibited whitening on over 5 /10% of the
fruit surface area (Fig. 2). The discolored area
increased with increasing zein content. Lower than
4% PG in coating formulations further increased
whitening. Therefore, the combination with less
than 11% zein and more than 4% PG is desirable
for minimal discoloration.
Whitening on shellac-coated apples has been
recognized (Baldwin, 1994). Whitening spots on
‘Delicious’ apples were exhibited when the shellac
content was higher than 4%, and the degree of
Fig. 1. Effect of zein and PG contents on gloss of ‘Gala’ apple.
Non-coated control was 7.3 GU (n/100).
Table 1
Gloss of zein coatings on test sheets and on ‘Delicious’ apple
Combination (wt.%) Gloss (GU)
Zein PG Test sheet
a
‘Delicious’ apple
b
15 15 16.5 a
c
11.2 a
15 4 3.3 e /
12 8 10.2 c 7.8 b
10 8 9.1 c 6.9 bc
10 4 2.8 e 5.6 d
8 15 12.1 b 6.8 bc
8 8 8.7 cd 6.3 c
4 15 8.5 d 4.4 d
4 4 2.3 e /
0 0 2.4 e 4.1 d
a
Three test sheets were coated at a thickness of 0.05 mm and
incubated at 20 8Covernight and then 10 measurements for
each sheet were made at an angle of 208.
b
Five fruit were coated and stored at 20 8C for 2 days and
then 10 measurements for each fruit were made at an angle of
608. The fruit were stored in CA for 6 months before being
used.
c
Mean values (n/30 for sheet and n/50 for apple) in same
column that are not followed by the same letter are significantly
different (PB/0.05).
J. Bai et al. / Postharvest Biology and Technology 28 (2003) 259 /268 263
whitening was increased by increasing shellac
contents in a carnauba /shellac emulsion coating
with a 20% total solids (unpublished data). A
starch-based coating (Bai et al., 2002) also showed
whitening when coated on ‘Delicious’ apple (un-
published data). However, no whitening occurred
on carnauba- and candelilla waxes-coated apples.
This indicates that whitening can be somewhat
controlled by regulating type and ratio of material
in the coating formulation, ingredients, and the
content.
3.4. Permeability and internal gas
Average permeability of zein coatings was 5.4 /
10
16
mol m
1
s
1
Pa
1
for CO
2
and 1.2/
10
16
mol m
1
s
1
Pa
1
for O
2
(Table 2). Zein
and PG contents had little effect on gas perme-
ability. In comparison, a typical shellac coating
had 1.4/10
16
mol m
1
s
1
Pa
1
for CO
2
and
0.4/10
16
mol m
1
s
1
Pa
1
for O
2
, while a
more permeable carnauba /shellac mixture had
12.1/10
16
mol m
1
s
1
Pa
1
for CO
2
and
2.6/10
16
mol m
1
s
1
Pa
1
for O
2
.
Coatings with low permeability offer more of a
barrier to gas exchange between the fruit internal
atmosphere and the external atmosphere, resulting
in a modified internal fruit atmosphere of rela-
tively high CO
2
and low O
2
. This can benefit fruit
shelf life in the same way that controlled atmo-
sphere or modified atmosphere packaging does.
Appropriate internal low O
2
and high CO
2
partial
pressures can lower respiration rate, maintain flesh
firmness, and retard ripening and senescence (Bai
et al., 2002; Banks et al., 1993). Less modification
of the internal fruit atmosphere gives less benefit in
terms of ripening control, while excessive modifi-
cation can cause anaerobic metabolism (Ueda et
al., 1993; Yearsley et al., 1996).
Contents of zein and PG in coatings affected
internal CO
2
and O
2
partial pressures. Generally,
the zein content correlated with gas modification
Table 2
CO
2
and O
2
permeability of zein and other coatings at 20 8C and 60% RH
Combination (wt.%) Permeability (10
16
mol m
1
s
1
Pa
1
)CO
2
/O
2
Zein PG CO
2
O
2
15 15 4.1 c
a
0.8 c 5.4
15 4 5.4 c 1.4 b 3.9
8 15 4.3 c 1.7 b 2.5
8 8 4.9 c 0.9 bc 5.1
4 15 3.6 c 1.0 bc 3.3
4 4 9.8 b 1.3 bc 7.5
Average of zein coatings 5.4 1.2 4.6
Shellac 1.4 d 0.4 d 4.5
Carnauba/shellac 12.1 a 2.6 a 4.6
a
Mean values (n/3 or 4) in same column that are not followed by the same letter are significantly different (PB/0.05).
Fig. 2. Effect of zein and PG contents in coatings on whitening
of ‘Gala’ apple (n/10).
J. Bai et al. / Postharvest Biology and Technology 28 (2003) 259 /268264
in the fruit (Table 3). Gas exchange between
internal fruit and external atmosphere is by
permeation through cuticle and by diffusion
through pores (Banks et al., 1993). Application
of surface coatings covers the cuticle and may
plug pores on the fruit surface (Banks, 1984;
Banks et al., 1993; Ben-Yehoshua et al., 1985)
depending on viscosity, surface tension, and other
factors. Ratios of permeabilities between CO
2
and O
2
(Table 2) were 2.5 /7.5 for coatings, and
the ratio of diffusion through air between them
was reported to be 0.78 at 0 8C(Weast, 1988).
If the dominant pathway for gas exchange is by
permeation through the peel (i.e. through a
barrier), the ratio of pressure differences across
the peel (
/DPCO2=DPO2)is expected to be smaller
than if pore diffusion (i.e. through air) is the
dominant pathway (Bai et al., in press). Since the
relation between zein content and DPCO2=DPO2was
negatively correlated, it can be inferred that the
higher zein content tended to result in more
blocked pores. Uncoated fruit, for example, had
the highest pressure difference value (
/DPCO2=DPO2)
of 4.44 while coatings with 15% zein had values
less than 1.0 (Table 3). The total partial pressures
of O
2
and CO
2
were roughly 21/22 kPa for zein
contents of 8% and less, which also indicates
diffusion through pores (Table 3). PG content in
most formulations generally did not affect fruit
internal gas partial pressures, except when zein
was 15% and PG content was 4 /8%. These
formulations led to changes in internal O
2
partial
pressures (Table 3). When PG content was 8 /15%,
the coating tended to block pores in the peel; thus,
the gas exchange between internal and external
atmosphere was more dependent on permeance
than diffusion. In contrast, when PG was 4%, the
gas exchange through the pores in the peel
increased. Banks et al. (1993) reported that coat-
ings mainly exert their effects on peel resistance to
diffusion of gases by blocking a greater or lesser
proportion of the pores on the fruit surface. When
the PG level was 4%, the resulting coating blocked
the pores less than when PG levels in the formula-
tion were higher.
Retardation of water loss is another benefit
derived from application of coatings to fruit.
Weight loss of ‘Gala’ apple reached 4.4% at
20 8C in the non-coated control (Table 3). Shrink-
age was observed (shriveling of the peel), and the
fruit were less firm to the touch than they had been
at the start of the experiment. Zein coatings kept
the weight loss to less than 2.8%, with the 15% zein
coating resulting in fruit with weight loss less than
1.9%. No shrinkage was detected for zein-coated
fruit. Hatfield and Knee (1988) and Maguire et al.
Table 3
Internal gases and weight loss of ‘Gala’ apples coated with different zein and PG combinations and stored at 20 8C for 14 days
Combination (wt.%) Internal gases (kPa) /DPCO2=DPO2
/
a
Weight loss (%)
Zein PG CO
2
O
2
15 15 11.0 a
b
6.0 a 0.73 d 1.7 a
15 8 11.4 a 5.6 a 0.74 d 1.7 a
15 4 9.4 a 10.2 b 0.87 c 1.9 a
8 15 4.9 b 17.6 c 1.46 b 2.4 b
8 8 4.3 b 18.3 c 1.59 b 2.6 b
8 4 4.5 b 18.2 c 1.61 b 2.6 b
4 15 4.0 bc 18.5 cd 1.66 b 2.5 b
4 8 3.9 bc 18.7 cd 1.69 b 2.6 b
4 4 3.6 c 18.9 cd 1.75 b 2.8 b
0 0 1.7 d 20.6 d 4.44 a 4.4 c
Fruit had been previously stored in CA for 3 months before being used.
a
/DPCO2and DPO2:CO
2
and O
2
partial pressure differences across the peel, respectively.
b
Mean values (n/10) in same column that are not followed by the same letter are significantly different (PB/0.05).
J. Bai et al. / Postharvest Biology and Technology 28 (2003) 259 /268 265
(2000) reported that even as little as 3.5 /5%
weight loss can lead to shrivel in apples.
3.5. Quality of zein-coated fruit
Based on previous experiments, an optimal
coating of 10% zein and 10% PG (zein10) was
used for investigating the effect of zein coating on
‘Gala’ apple quality compared with commercial
shellac, carnauba, and a non-coated control.
Zein10-coated fruit exhibited modified internal
O
2
and CO
2
partial pressures between that found
for carnauba- and shellac-coated fruit (Table 4)
with 10.6 kPa CO
2
/8.2 kPa O
2
. The zein coating
induced a 17-fold increase in ethanol content
compared with non-coated control. Shellac mod-
ified the internal fruit atmosphere such that
shellac-coated apples had the highest and lowest
CO
2
and O
2
partial pressures, respectively, and
had a similar ethanol content to that of zein-
coated fruit. Carnauba wax-coated apples exhib-
ited the least difference in internal atmosphere
compared with non-coated controls with no sig-
nificant increase in ethanol. Zein10 and shellac
coatings also resulted in increased butyl acetate
and 3-methylbutyl-2-methylbutyrate concentra-
tions, which are reported to have apple-like or
fruit-like aromas. Zein10-coated fruit were rated
Table 4
Effect of coating on internal gases, weight loss, sugar, acidity, volatiles, and sensory quality of ‘Gala’ apples stored at 21 8C for 14 days
N
a
Control Carnauba Zein10 Shellac
Gloss 100 5.2 c
b
7.2 b 8.9 ab 9.5 a
Whitening 10 0 c 0 c 0.9 b 2.6 a
Internal gases (kPa) 10
CO
2
2.6 d 8.8 c 10.6 b 12.3 a
O
2
18.3 a 10.1 b 8.2 c 6.1 d
Weight loss (%) 20 3.0 a 1.8 d 2.3 b 2.0 c
Firmness (N) 20 66 ab 61 b 63 b 73 a
Sugar (g kg
1
)4
Sucrose 0.30 a 0.32 a 0.31 a 0.30 a
Glucose 0.22 a 0.20 a 0.21 a 0.22 a
Fructose 0.68 a 0.63 b 0.68 a 0.67 a
pH 4 4.1 a 4.0 a 4.0 a 4.1 a
Titratable acidity (g kg
1
) 4 2.5 a 2.7 a 2.8 a 2.9 a
Volatiles (mgkg
1
)4
Ethanol 1.12 b 1.94 b 18.21 a 18.76 a
Ethyl acetate 0.05 a 0.03 a 0.03 a 0.05 a
Ethyl propionate 0.01 b 0.03 b 0.11 a 0.12 a
Butyl acetate 0.25 ab 0.22 b 0.29 a 0.28 a
3-Methylbutyl-2-methylbutyrate 0.07 b 0.06 b 0.12 a 0.12 a
Sensory test 20
Sweetness 6.1 a 5.2 b 5.7 a 5.9 a
Acidity 3.7 b 4.4 a 4.2 a 4.2 a
Texture 5.7 a 5.9 a 6.4 a 6.6 a
Off-flavor 2.6 b 4.1 a 2.4 b 3.0 b
Flavor 6.1 a 5.5 b 6.5 a 5.8 ab
Fruit had been previously stored in CA for months before being used.
a
Replicate number.
b
Mean values in same row that are not followed by the same letter are significantly different (PB/0.10 for sensory test (Meilgaard et
al., 1991) and PB
/0.05 for others).
J. Bai et al. / Postharvest Biology and Technology 28 (2003) 259 /268266
highest for flavor and lowest for off-flavor in a
sensory test. Carnauba-coated fruit had lower
levels of the abovevolatiles compared with
zein10- or shellac-coated fruit and scored lowest
for flavor and highest for off-flavor in the sensory
test. Results indicate that the increase in ethanol
levels in zein10-coated fruit did not impair apple
flavor, when other important flavor esters were
simultaneously increased (Table 4).
The major sugar in ‘Gala’ apples was fructose
which was 0.63/0.68 g kg
1
(fresh weight basis),
and then sucrose and glucose with contents of
0.30/0.32 and 2.0/2.2 g kg
1
, respectively, in
non-coated control fruit. Coated fruit had similar
sugar levels and no differences were found due to
treatment except for lower fructose levels in
carnauba-coated fruit, which was confirmed in
the sensory panelists’ ratings for sweetness (Table
4).
Juice pH of ‘Gala’ was 4.0/4.1, and TA was
2.5/2.9 g kg
1
(fresh weight basis, Table 4). No
difference was apparent in pH or TA. Coated
apples tended to have lower pH and higher TA
than non-coated controls, but the differences in
acidity ratings by the sensory panel were signifi-
cant, with coated fruit being rated higher in acid
flavor than controls.
Shellac-coated fruit exhibited the highest flesh
firmness as determined by the penetrometer (Table
4), most likely due to delayed ripening caused by a
modification of the internal atmosphere or due to
less weight (water) loss. However, there were no
significant differences for firmness in the sensory
test. Since control fruit lost more water than
coated fruit, causing shrinkage (Table 3), this
may have falsely elevated the sensory response to
firmness (Table 4).
In summary, an optimum zein formulation was
developed that provided shine, resulted in a
desirable modified atmosphere, and exhibited
minimal whitening when applied to apple fruit
under simulated commercial conditions. Zein10
coating maintained apple quality similar to a
commercial shellac formulation, and extended
apple shelf life compared with non-coated controls
or commercial carnauba wax-coated fruit.
Acknowledgements
Funding for this research was provided by a
grant from the Washington Tree Fruit Research
Commission.
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