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Drying Technology
An International Journal
ISSN: 0737-3937 (Print) 1532-2300 (Online) Journal homepage: https://www.tandfonline.com/loi/ldrt20
Walnut structure and its influence on the
hydration and drying characteristics
Chang Chen, Zhang Weipeng, Chandrasekar Venkitasamy, Ragab Khir, Tara
McHugh & Zhongli Pan
To cite this article: Chang Chen, Zhang Weipeng, Chandrasekar Venkitasamy, Ragab Khir, Tara
McHugh & Zhongli Pan (2019): Walnut structure and its influence on the hydration and drying
characteristics, Drying Technology, DOI: 10.1080/07373937.2019.1605610
To link to this article: https://doi.org/10.1080/07373937.2019.1605610
Published online: 24 Apr 2019.
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Walnut structure and its influence on the hydration and drying
characteristics
Chang Chen
a
, Zhang Weipeng
b
, Chandrasekar Venkitasamy
a
, Ragab Khir
a
, Tara McHugh
c
, and
Zhongli Pan
a
a
Department of Biological and Agricultural Engineering, University of California, Davis, Davis, CA, USA;
b
School of Materials and
Mechanical Engineering, Beijing Technology and Business University, Beijing, China;
c
Healthy Processed Foods Research Unit, USDA-
ARS-WRRC, 800 Buchanan St, Albany, CA, USA
ABSTRACT
Fresh harvested walnuts are dehulled, washed, and then dried by hot air (HA) as a continu-
ous process in the industry. The objective of the current work was to study the walnut
structure and investigate its effect on the moisture transfer characteristics during the walnut
soaking and drying processes. Moisture transport pathways into the walnuts were deter-
mined using fluorescence tracer approach, and the hydration kinetics of walnuts under dif-
ferent soaking temperatures (15, 25, and 35 C) was studied using Peleg model. HA drying
experiments in single layer in a self-designed automatic HA dryer at 43 C and air velocity
of 1.41 m/s. The influence of the stem pore (sealed and non-sealed) and the soaking process
(0, 2- and 5-min soaking time) on the walnut drying characteristics were investigated sys-
tematically. The results indicated that both the presence of the stem pore and the soaking
time had significant influence (p<0.05) on the hydration and drying characteristics of wal-
nuts. Moisture absorptions through the stem pore and the shell were equally important dur-
ing the soaking process. Two to five minutes soaking process led to 2–4 h additional drying
time. This study contributed valuable knowledge for the simulation and prediction of mois-
ture transfer characteristics during the walnut soaking and drying processes. The findings
from this study could potentially be applied to the walnut drying industry for more effi-
cient processing
ARTICLE HISTORY
Received 15 October 2018
Revised 5 April 2019
Accepted 7 April 2019
KEYWORDS
Walnut; stem pore;
hydration kinetics; drying
characteristics
1. Introduction
Walnut is an important crop in California and a popu-
lar tree nut due to its high nutritional value and health
benefits.
[1–4]
The bearing acreage and the yield of wal-
nut increased about 22.3 times and 5.7 times, respect-
ively
[5]
since 1920. In 2017, more than 650,000 tons of
walnuts were produced, which represented 99% of the
production in the U.S. and 29% worldwide.
[6]
Fresh walnut has high moisture content (MC) that
can range from 20% to 43% at harvest and thus the
drying process is a necessary processing step for mois-
ture removal and product quality preservation.
[7–9]
In
a typical industrial post-harvest practice, the mechan-
ically dehulled walnuts are soaked in water first for
various times to wash out the dirt from the surface,
and then transferred to fixed bed bins for HA drying
at 43 C
[10]
as a continuous process. Due to the ligno-
cellulosic composition and hygroscopic structure,
[11]
the walnut shell may pick up moisture during the
industrial soaking/washing process. In addition, dur-
ing mechanical harvesting, the connection between
the stem and walnut fruit breaks, which consequently
generate a small pore at the stem location of walnut
shell (referred to as the stem pore in this study).
Presence of the stem pore was suspected to affect the
moisture transfer characteristics during both the wal-
nut washing and drying processes, and thus may
influence the drying time.
Hydration kinetics is essential for the quantification
of the moisture absorption into food materials during
soaking process.
[12,13]
An empirical model proposed
by Peleg
[14]
has been widely applied to accurately
model the hydration kinetics of various foods with
porous structure, such as legumes,
[15–18]
, rice
[19]
, and
starchy foods
[20,21]
due to its simplicity. The hydration
kinetics of walnuts, which has not been well
CONTACT Zhongli Pan zlpan@ucdavis.edu Department of Biological and Agricultural Engineering, University of California, Davis, One Shields
Avenue, Davis, CA 95616, USA.
These authors contributed equally to this work.
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/ldrt
ß2019 Taylor & Francis Group, LLC
DRYING TECHNOLOGY
https://doi.org/10.1080/07373937.2019.1605610
understood in the past, was investigated in this study.
The knowledge will be useful to understand the water
transport mechanism and to predict the water absorp-
tion during the soaking/washing process of walnuts.
With the growing walnut production, new drying
technologies are demanded in the industry for higher
processing capacity. In recent years, different technol-
ogies have been studied in lab scale for efficient dry-
ing, such as radio frequency drying,
[7,22,23]
infrared
(IR) drying,
[24,25]
and intermittent oven drying.
[26,27]
Nonetheless, conventional HA drying technology is
still mostly applied in the industry due to processing
capacity and operating cost. However, the drying effi-
ciency of HA drying is relatively low.
[24]
It usually
takes up to 24 h to dry the walnuts to an average 8%
MC (safe storage MC) by HA at around 43 C, par-
ticularly for high initial moisture content (IMC) wal-
nuts.
[28]
Thus, it is necessary to study the influence of
both the stem pore and the soaking process on the
drying characteristics of the walnuts systematically.
Therefore, the current work was aimed to under-
stand the moisture transfer characteristics during the
walnut soaking and drying process. The specific objec-
tives of this study were: 1) to study the water trans-
port pathway into walnuts during the soaking process;
2) to determine the hydration kinetics of walnuts
under different conditions; and 3) to investigate the
influence of the stem pore and the soaking process on
the drying characteristics of walnuts. The findings
from this study will provide important information
for the understanding of the moisture transfer mecha-
nisms during the walnut soaking and drying process,
which could potentially be helpful to guide the
improvement of the drying process.
2. Materials and methods
2.1. Sample preparation
Dehulled walnuts (Juglans regia.) of Howard variety
were used in this study. The samples were collected
from Emerald Farms, Maxwell, CA, USA, during the
harvest season in September and October 2017. Thirty
walnuts were randomly selected to determine the size
of walnuts using the method introduced in a previous
study by Khir et al.,
[9]
as shown by Figure 1. The
dimensions of the principal axes of walnuts were
measured as: length (L): (46.0 ± 3.4) 10 3m
intermediate (D1): (44.0 ± 3.7) 10 3m minor
diameter (D2): (44.0 ± 4.0) 10 3m.
2.2. Determination of MC
Walnuts were placed in aluminum weighing dishes
and weighed first using an electronic balance (Denver
Instrument, Arvada Co., USA) with an accuracy of
0.01 g. Then the walnuts were separated into different
components (shell, kernel and dissepiment) and
weighed individually.
[29]
Dry mass of the different
components of walnut samples was measured after
drying in an air oven at 105 C for 24 h. The MC of
the shell, kernel and dissepiment was determined
using following equation (1):
MCdb
¼WsWsd
ðÞ
þWkWkd
ðÞ
þWde Wded
ðÞ
Wsd þWkd þWded
ðÞ
100%
(1)
Where, w
s
,w
k
, and w
de
are the instantaneous weight
of the shell, kernel and dissepiments, respectively; and
w
sd
,w
kd
, and w
ded
are the dry mass weight of
the samples.
2.3. Determination of water permeation pathway
Walnut samples were randomly selected from the
same batch and divided into experimental group and
control group. The stem pore was sealed with water-
resistant epoxy glue in the experimental group wal-
nuts (to simulate walnuts without the stem pore) and
the control group walnuts were left with no treatment
(to simulate walnuts with the stem pore), as shown in
Figure 2. The average diameter of the stem pore was
around 1 mm, and the geometric diameter of the wal-
nut shell was around 45 mm. Therefore, the area of
the stem pore was relatively small compared to the
shell surface area (<1%). The change of walnut weight
after the sealing process was less than 1%, and the
MC of the glue was neglected. Therefore, the effect of
sealing on the IMC of walnuts was considered to be
Figure 1. Major axial dimensions of walnuts.
[7]
2 C. CHEN ET AL.
negligible. Both groups were placed under room tem-
perature for 30 min to solidify the glue. To evaluate
the change of the pore shape before and after the dry-
ing process, both fresh and dried walnuts were used
in the experiments.
Fluorescence food grade dye with light pink color
(GloMania, USA) was dissolved in water to prepare
the fluorescence tracking solution. The walnuts were
soaked in the fluorescent solution and stirred continu-
ously to simulate the industrial washing process. After
various soaking periods, 5 samples were removed
from the solution, wiped clean with paper tissue, and
placed in cool desiccator for 2 h. The samples were
then cut through by band saw in different directions
(cross-section, alongside the suture and perpendicular
to suture), and the kernel was carefully separated in
halves. A USPAR F15T8 UV light with black filter
was used to activate the fluorescence dye in the wal-
nut. For comparison, the same samples were also
placed under incandescent lamp for imaging. The
water permeation pathways within each individual
walnut was photographed and observed with a digital
camera with 10zoom lens (Canon Ixus 155, USA).
2.4. Determination of hydration kinetics
Hydration experiments were performed using
2000 mL beakers containing 1600 mL of deionized
water, which was maintained at constant temperatures
(15, 25, and 35 C) in a water bath. In each experi-
ment, 20 fresh wet walnuts (sealed and non-sealed)
were randomly selected and submerged in water in
the beaker. The samples were removed from water,
wiped clean with tissue paper and weighed. The
amount of water absorption was calculated based on
the weight change of samples during the soaking pro-
cess. Subsequently, the samples were put back in the
beakers and the water absorption measurement con-
tinued until the relative change in sample weights
were <0.1%. Triplicate measurements were conducted,
and the average moisture absorption was calculated.
The experimental data were fitted by Peleg
model,
[14]
as indicated in Equation (3).
MðtÞ¼M0þt
KiþK2t(2)
Where, (t) is the MC of material at time t (%); M
0
is
the IMC (%); K
1
is the Peleg rate constant (min%
1
);
and K
2
is the Peleg capacity constant (%
1
).
When t!0, Equation (3) became:
MðtÞ¼M0þt
K1
(3)
When t!1,Equation (3) became:
Ms ¼M0þ1
K2
(4)
Where, M
s
is the saturated MC of the material.
2.5. Drying equipment
A laboratory scale HA dryer equipped with automatic
measuring system was designed and developed (Figure
3(A,B)) in this study to conduct the HA drying of
walnuts. A commercial resistance heating element and
an electrical fan (Avantco Instrument CFD 10, CA,
USA) were applied to provide the HA with control-
lable temperature. Two S-beam load cell sensors with
4000 g total capacity and 2 g accuracy (Meacon auto-
mation, Hangzhou, China) were placed horizontally
on top of the drying chamber. A material basket
made of wire mesh was connected with the load cell
beam and was dropped into the drying chamber. The
weight change of walnut samples was automatically
measured during the drying process. The temperature
and relative humidity in the drying chamber were
measured continuously with T-type thermocouples
(Omega, SA1XL-T-120, USA). The data were gath-
ered, monitored and recorded continuously and auto-
matically with a 5 s interval by a touch screen logger
(NT8071iE, Weinview, China).
2.6. Drying experiments
The HA drying experiments were performed at 43 C
and air velocity of 1.41 m/s. The sealed and non-sealed
walnuts of the same IMC were soaked in water for
different time periods (0, 2 and 5 min), weighed and
then uniformly spread on a perforated tray as a single
layer for HA drying. The weight change of the wal-
nuts after soaking was measured by an electronic bal-
ance. The temperature of the shell surface was
Figure 2. Walnut sample A) non-sealed (control) and B) sealed
(experimental).
DRYING TECHNOLOGY 3
measured with T-type thermocouple. The dryer was
warmed up for 30 mins before sample loading to
achieve stable HA temperature and velocity. Triplicate
experiments were conducted and the average MC at
each time points was calculated.
2.7. Statistical analysis
The goodness of model fit was tested using the
adjusted coefficient of determination (R
adj2
). The
influence of the stem pore and temperature on the
hydration kinetics and on the drying characteristics
was studied through two-way analysis of variance.
Multiple comparison test (Tukey test) was performed
at a confident level of 95%. The OriginPro software
(Version 2016, OriginLab Corporation, Northampton,
MA) was used to carry out the statistical analysis.
3. Results and discussion
3.1. Water transport pathway into walnuts
Although the washing time for walnut in the industry
is usually less than 5 min, the soaking time in the cur-
rent work was extended on purpose to allow the
fluorescent water to transfer into the walnuts. Non-
sealed and sealed walnuts were cut alongside the
suture then illuminated under visible light and black-
light, as depicted in Figures 4 and 5. The fluorescence
dye was activated by the ultraviolet and the fluores-
cence pattern was observed. As seen from Figure 4C
and D, after short soaking times (<15 mins), fluores-
cence pattern was hardly observed within the non-
sealed walnut for both wet or dry samples, which sug-
gested that water permeation through the stem pore
was not significant in short time. As the soaking time
increased, the fluorescence pattern appeared at the
stem pore location (left hand side of the figures) and
extended gradually through the dissepiments and the
inner membrane of walnut shell. Such moisture trans-
fer characteristics should be due to the unique struc-
ture of walnuts. According to the Pinney et al.,
[30]
the
dissepiments in walnuts are the remnant of the stem
connected with the plants, which functions as the
nutrient and water supply pathway between the plant
and the fruit during the growth of the walnut fruit.
[31]
The dissepiments, which should have lignocellulosic
composition, may have highly porous structure and
were saturated with water during the fruit growth.
Interestingly, the fluorescence patterns within fresh
wet walnuts and dry walnuts were very similar, which
suggested that the characteristics of the stem pore did
not change significantly after drying.
On the other hand, in the sealed walnut samples
(Figure 5(C,D)), despite the IMC, no obvious fluores-
cence pattern was observed on the kernel or across
the shell throughout the soaking process. This phe-
nomenon may be attributed to the structural charac-
teristics of the walnuts. The walnut shell has dense
structure made of lignocellulosic stone cells, and
micropores with 2–8lm diameter are formed between
the fibrous structures,
[32]
which was much smaller
compared with the size of the stem pore (1 mm). The
resistance of moisture diffusion within porous food
material depends on the pore size and pore struc-
ture.
[33]
Therefore, the permeation rate through the
stem pore should be faster than across the shell.
The results revealed and proposed the possible
water transfer pathway during the walnut washing/
soaking process: water permeated through the stem
pore, moved alongside the inner membrane system
that connected the shell and the dissepiments, as
depicted by the blue arrows in Figure 6. The perme-
ation pathway of water into the walnut during the
Figure 3. A) Schematic diagram and B) photo of the lab-scale HA dryer equipped with automatic weighing and temperature
measuring system.
4 C. CHEN ET AL.
soaking process was observed vividly. The fluoresce
tracing approach developed here has the potential to
be applied for the study of other agricultural com-
modities with similar structure. Further research is
necessary to study the microstructure of the walnut
components (walnut shell, kernel, and dissepiments)
through advanced characterization methods to verify
the findings.
3.2. Determination of hydration kinetics
To determine the hydration kinetics of walnuts, the
MC change during the soaking process was measured.
The hydration curves of non-sealed walnuts at differ-
ent temperatures (15, 25, and 35 C) were shown in
Figure 7A. It was found that the MC of walnuts
increased rapidly at the initial stage of soaking and
leveled off quickly after about 5 min. The high mois-
ture absorption rate at the initial soaking stage may
be attributed to the fast hydration of capillaries and
cavities on the surface of the walnut shell by unbound
moisture. With more moisture absorbed, the shell sur-
face rapidly saturated, the hydration rate decreased
until the MC gradually stabilized.
[17]
It was also found that at higher temperature, both
the initial rate of moisture absorption and the total
moisture absorption amount were increased. The satu-
rated MC increased from 26.5% at 15 C to around
29.1% at 35 C with the same IMC of 22.5%.
According to Saravacos and Maroulis,
[33]
the rate of
water diffusion increases with the increase in tempera-
ture, which may lead to the increase in moisture
absorption rate. Meanwhile, the lignocellulosic struc-
ture in the walnuts might have thermally expanded
due to the enhancement in soaking temperature,
[11]
generating larger void volume in the microstructure,
which increased the water holding capacity in
the walnuts.
To evaluate the role of the stem pore in the water
absorption, the MC change of sealed walnuts was
compared with non-sealed samples. Figures 7B–D
showed the hydration curves of non-sealed and sealed
walnuts at 15, 25, and 35 C. It was found that the
moisture absorption rate and the total amount of
moisture absorption of non-sealed walnuts were
higher than the sealed walnuts, particularly at the ini-
tial stage of soaking process. The results were in
accordance with the water transfer pathway
Figure 4. Water permeation in the non-sealed walnuts under visible light: (A) dry walnuts; (B) wet walnuts; and blacklight: (C) dry
walnuts; (D) wet walnuts.
DRYING TECHNOLOGY 5
determined in the previous section. The MC increase
in the sealed walnuts should represent the amount of
water absorbed on the walnut shell (surface satur-
ation). Comparing the moisture absorption between
the sealed and non-sealed walnuts after the same
soaking time, the difference in MC may represent the
amount of moisture absorbed through the stem pore.
The total amount of moisture absorption in the sealed
walnuts also increased with increase in the soaking
temperature.
The Peleg model was applied to study the hydra-
tion kinetics of walnuts and the fitted parameters
were summarized in Table 1.K
1
(min/%) was a par-
ameter related to mass transfer rate, the lower the K
1
,
Figure 5. Water permeation in the sealed walnuts under visible light: (A) dry walnuts; (B) wet walnuts; and blacklight (C) dry wal-
nuts; (D) wet walnuts.
Figure 6. Water permeation path-way in the walnut depicted by the blue arrows.
6 C. CHEN ET AL.
the higher the initial rate of water absorption,
[14]
as
indicated by Equation (4). Decreased K
1
values with
the increase in temperature suggested an increase in
the initial water absorption rate. Similar phenomenon
was also observed in the hydration of other food
materials.
[12,13,34]
K
2
(%
1
) was a parameter related to
maximum capacity of water absorption, the lower the
K
2
, the higher the water absorption capacity,
[14]
as
explained by Equation (5). The results indicated that
for both sealed and non-sealed walnuts, the initial
hydration rate and saturation MC increased signifi-
cantly (p<0.05) with the soaking tempera-
ture increase.
To the best of the authors’knowledge, this is the
first study that investigated the hydration kinetics of
walnuts. It could be used to evaluate the influence of
washing/soaking process on the MC of walnuts quan-
titatively. As the typical washing time of walnut in the
industry is usually less than 5 min, it could be calcu-
lated that for walnuts with IMC of 22.7%, the soak-
ing/washing process may lead to 3.3% MC increase
(around 15% relative increase) at 25 C. Meanwhile, it
was observed that the difference of MC between the
sealed and non-sealed walnuts was around 2%, which
should account for the moisture absorption through
the stem pore. The results suggested that both the
stem pore and the shell surface played equally import-
ant role in moisture absorption during the walnut
soaking process, even though the permeation depth of
water across the shell was much slower compared to
through the stem pore according to the previous sec-
tion. The cause of the results could be attributed to
Figure 7. Water absorption curves of (A) non-sealed walnuts at different temperatures and comparison between sealed and non-
sealed walnuts at (B) 15 C; (C) 25 C; and (D) 35 C.
Table 1. Parameters in Peleg’s model for the hydration kinet-
ics of sealed and non-sealed walnuts under different soaking
temperatures.
Condition IMC (%)
Temperature
(C) K
1
(min %
1
)K
2
(%
1
)R
2
Non-sealed 15 0.230 ± 0.019
e
0.245 ± 0.004
D
0.99
22.7 ± 7.8 25 0.157 ± 0.019
c
0.199 ± 0.005
B
0.98
35 0.135 ± 0.012
ab
0.161 ± 0.003
A
0.99
Sealed 15 0.507 ± 0.041
f
0.497 ± 0.009
F
0.99
22.7 ± 7.8 25 0.166 ± 0.019
cd
0.389 ± 0.007
E
0.98
35 0.128 ± 0.015
a
0.231 ± 0.004
C
0.98
Superscript letters indicate that means with same letters designation in
each column are not significant different at p<0.05.
DRYING TECHNOLOGY 7
the much larger surface area of the shell surface than
the stem pore, which allowed equal amount of mois-
ture absorption. The findings may have the potential
to be applied for predicting the moisture absorption
in the walnuts during the industrial soaking/washing
process under different conditions during the har-
vest season.
3.3. Investigation of drying kinetics
As the diffusivity of water vapor in porous structure
is much higher than liquid water,
[35]
it was suspected
that the stem pore may also be an important pathway
of vapor transfer during the walnut drying process. In
addition, the interactive effect of the stem pore and
the soaking process on the drying characteristics of
walnuts has not been investigated systematically. The
influence of operating conditions (presence of the
stem pore, soaking time and IMC) on the MC, drying
time (time to achieve 8% MC), preheating time (time
to achieve 43 C) of walnuts were investigated. As
shown in Table 2, for non-sealed walnuts, 2 min and
5 min soaking lead to 2.3% and 3.0% MC increase,
respectively. On the other hand, only 1.3% and 2%
MC increase was found in the sealed walnuts after the
same soaking times. The results were in accordance
with the findings from the previous section. As for
the drying time, 2–5 min soaking resulted in 2–4 add-
itional hours of HA drying for the non-sealed walnuts,
and 0.6–1.9 hour for the sealed walnuts.
The drying curves, drying rate curves and tempera-
ture curves of the sealed and non-sealed walnuts were
shown in Figures 8–10. A general exponential decay
pattern for the MC changed over drying time was
observed for both groups. The drying rate curves
showed two distinct falling-rate stages, which are the
indicator of increasing heat and moisture transfer
resistance
[36]
during the HA drying process of food
materials, where liquid diffusion should be the domin-
ant moisture transfer mechanism. The higher drying
rate in the first falling-rate stage might be attributed
to the rapid evaporation of the unbound moisture on
the shell surface and within the walnuts. The second
falling-rate stage took place when unbound moisture
did no longer exist in the walnut. The outwards diffu-
sion of bonded moisture from the kernel became the
rate limiting step, and therefore led to lower drying
rates. The MC of walnuts with the open stem pore
was lower than walnuts without the open stem pore
after the same drying time.
The effect of the stem pore on the drying charac-
teristics of the walnuts was studied in Figure 8.
Walnuts with the open stem pore dried faster than
walnuts without the pore. As shown in Figure 8A, the
total drying time for sealed walnuts was 2 h longer
than non-sealed walnuts. Accordingly, the drying rate
for non-sealed walnuts was higher than the sealed
walnuts during the whole drying process, particularly
at the first 5 h of drying (Figure 8(B)). However, the
temperature curves of both groups were similar to
each other (Figure 8(C)). The results suggested that
Table 2. Influence of the stem pore and soaking process on the walnut drying characteristics.
Soaking time (min) Stem pore IMC (kg/ kg dry mass)
MC after soaking
(kg/kg dry mass)
Preheating
time (min) Drying time (min)
0 sealed 0.2688 ± 0.0191 0.2688 ± 0.0191 37.0 ± 2.6
a
938.3 ± 4.7
A
0 non-sealed 0.2688 ± 0.0191 0.2688 ± 0.0191 38.0 ± 1.0
a
814.3 ± 7.1
B
2 sealed 0.2688 ± 0.0191 0.2812 ± 0.0238 89.7 ± 2.5
b
977.7 ± 6.7
C
2 non-sealed 0.2688 ± 0.0191 0.2924 ± 0.0187 122.7 ± 2.1
c
1025.0 ± 6.2
D
5 sealed 0.2688 ± 0.0191 0.2886 ± 0.0169 127.8 ± 3.1
d
1055.7 ± 6.5
E
5 non-sealed 0.2688 ± 0.0191 0.2983 ± 0.0211 169.3 ± 4.0
e
1076.0 ± 4.0
F
Superscript letters indicate that means with same letters designation in each column are not significant different at p<0.05.
Figure 8. A) Drying curves; B) Drying rate curves; and C) Temperature curves of sealed and non-sealed walnuts without soaking.
8 C. CHEN ET AL.
the stem pore independently had significant influence
(p<0.05) on the moisture transfer rate during the
drying process but may had little effect on the heat
transfer characteristics.
To study the interactive effect of the stem pore and
the soaking process on the drying characteristics, the
soaked walnuts were used for comparison. Figure 9
(with 2 min soaking) and Figure 10 (with 5 min soak-
ing) show the MC curves, drying rate curves and tem-
perature curves of the walnuts including the soaking
process. Similar to the non-soaked walnuts, the MC of
sealed walnuts was higher than the non-sealed walnuts
at the same drying time. Accordingly, the drying rate
of non-sealed walnuts was higher than sealed walnuts,
particularly at the initial drying stage. However, the
difference of MC between the two groups was less
than the non-soaked walnuts. This may be attributed
to the higher amount of unbound moisture absorbed
on the outer layer of the nuts (which had lower resist-
ance to be removed during the drying process) and
the presence of the stem pore (which facilitated the
moisture removal). A hysteresis in the temperature
curves was observed in Figures 9C and 10C, as it took
longer time for the preheating of non-sealed walnuts
than sealed walnuts. This may be due to the higher
amount of moisture absorption in the non-sealed wal-
nuts during the soaking process, which required more
thermal energy for evaporation. As the soaking time
increase, the MC curves of the two groups became
partly overlapped (Figure 9(A)), indicating the
reduced difference of drying time and drying rate
between the sealed and non-sealed walnuts.
Figure 11 compared the drying curves, drying rate
curves and temperature curves of non-sealed walnuts
after different soaking times. When the soaking time
was longer, the MC of walnuts was also higher after
the same drying time (Figure 11(A)). The hysteresis of
temperature change increased from 123 min to
169 min when the soaking time increased from 2 min
to 5 min (Figure 11(C)), as listed in Table 2.
Interestingly, it was shown Figure 11B that the initial
drying rate of “2 min soaking”walnuts was the high-
est, followed by “5 min soaking”and “0 min soaking”
groups. After 3 hours of drying, the drying rates for
the three groups were close to each other until the
end of the drying process. The magnitude of the dry-
ing rate should represent the effect of both the mois-
ture transfer and heat transfer rate, which were
dependent on the MC, thermo-physical properties and
the structure of the material. More unbound moisture
Figure 9. A) Drying curves; B) Drying rate curves; and C) Temperature curves of sealed and non-sealed walnuts after
2 min soaking.
Figure 10. A) Drying curves; B) Drying rate curves; and C) Temperature curves of sealed and non-sealed walnuts after
5 min soaking.
DRYING TECHNOLOGY 9
was absorbed in the walnuts after longer soaking time
and appeared as higher IMC, which led to higher dry-
ing rate. On the other hand, since the specific heat of
water is much higher than the dry mass of the wal-
nut,
[11]
the higher moisture absorbed resulted in slow
temperature increase in the walnuts, which reduced
the drying rate at the initial stage. It was suggested
that the presence of the stem pore not only facilitated
the moisture absorption during the soaking process,
but also benefited the moisture removal during the
drying process.
As observed in Figure 11A, the distance between
the MC curves of “5 min soaking”and “2min
soaking”groups were much smaller than the distance
between the “2 min soaking”and the “0 min soaking”
groups. On the other hand, when the stem pore was
sealed, the MC of walnuts and the drying time also
increased with the soaking time (Figure 12(A)).
However, the distance between the drying curves of
‘5 min soaking’and ‘2 min soaking’groups was higher
than the distance between the ‘2 min soaking’and the
‘0 min soaking’group. Therefore, the drying rate
curves represented the interactive effect between the
soaking time and the stem pore, particularly at initial
stage. Both the stem pore and the soaking time had
had significant influence (p<0.05) on the drying
characteristics of walnuts by HA.
In summary, the industrial soaking/washing process
may result in higher MC the walnuts and longer dry-
ing time. Walnuts with the open stem pore at harvest
tended to absorb more moisture during the soaking/
washing process, which resulted in prolonged drying
time (2–4 hours). In comparison, walnuts without the
open stem pore would take 0.5–1 hour longer drying
time due to the washing process. The findings from
this study suggested that although the soaking/wash-
ing process is necessary in the industry, the washing
time could be controlled to avoid over-washing. For
walnuts with the open stem pore at harvest, the wash-
ing time should be minimized to avoid additional dry-
ing time caused by the moisture absorption, which
will be helpful for the improvement of drying effi-
ciency. Meanwhile, it was found that the soaking/
washing process may also increase the preheating time
of walnuts. Therefore, future research should be per-
formed to explore the application of high temperature
treatment at the early drying stage to improve the
heat transfer and enhance the drying rate of walnuts.
Figure 12. A) Drying curves; B) Drying rate curves; and C) Temperature curves of sealed walnuts without soaking and after 2 min,
5 min soaking.
Figure 11. A) Drying curves; B) Drying rate curves; and C) Temperature curves of non-sealed walnuts without soaking and after
2 min, 5 min soaking.
10 C. CHEN ET AL.
4. Conclusions
In the current study, the structure of walnut compo-
nents was carefully characterized. Hydration experi-
ments were conducted to simulate the industrial
soaking/washing process of walnuts. A fluorescence
tracking approach was developed to study the perme-
ation pathway of moisture during the walnut soaking
process. The hydration kinetics of walnuts was studied
for the first time, which was affected significantly (p
<0.05) by both the presence of the stem pore and
the soaking temperature. The influence of the stem
pore and the soaking time on the drying time and
drying rate of walnuts were quantified systematically.
This study contributed new knowledge for the under-
standing of moisture characteristics of walnuts during
the soaking/washing and HA drying processes. The
outcomes have practical value in the walnut process-
ing industry for the optimization of soaking/washing
time and improvement of drying efficiency.
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