-
Article history: The effects of three freezing methods, air
blast freezing (ABF), immersion freezing (IF) and
ultrasound-assisted immersion freezing (UIF), on quality and
microstructure of lotus roots
)
assiste par ultrasons ;
Qualite ; Microstructure
Lotus (Nelumbo nucifera Gaertn), an aquatic perennial from
Nelumbonaceae family, is an important economic plant
widely cultivated in the Orient. The lotus root is used as a
ess, attractive white
root is considered to
be rich in dietary fiber, vitamins, phenolic compounds, and
antioxidants (Man et al., 2012; Xing et al., 2010). In such
context, lotus root is used as food as well as traditional
* Corresponding author. School of Food Science and Technology,
Jiangnan University, 214122 Wuxi, Jiangsu Province, China. Tel.: 86
(0)510 85917089; fax: 86 (0)510 5807976.
Available online at www.sciencedirect.com
ScienceDirect
e:
i n t e rn a t i o n a l j o u r n a l o f r e f r i g e r a t i
o n 5 2 ( 2 0 1 5 ) 5 9e6 5E-mail address: [email protected] (M.
Zhang).1. Introductionpopular vegetable because of its crispn
color and abundant nutrients. The lotusmicrostructure de racine
de lotus (Nelumbo nucifera
Mots cles : Racine de lotus ; Congelation par air force ;
Congelation par immersion ; Congelation par immersionEffets de
diverses methodes de congelation sur la qualite et laReceived 18
October 2014
Received in revised form
16 December 2014
Accepted 22 December 2014
Available online 30 December 2014
Keywords:
Lotus root
Air blast freezing
Immersion freezing
Ultrasound-assisted immersion
freezing
Quality
Microstructurehttp://dx.doi.org/10.1016/j.ijrefrig.2014.12.0150140-7007/
2014 Elsevier Ltd and IIR. All rigwere investigated. The parameters
used to evaluate the freezing methods effect were the
freezing time, color, firmness, drip loss, vitamin C and
microstructure of the final frozen
products. The results showed that the UIF products had several
advantages in terms of the
freezing time, color, firmness and drip loss over ABF and IF. No
significant difference
(p > 0.05) of vitamin C content was observed between the ABF
and IF products, while sig-
nificant difference (p < 0.05) of vitamin C was observed
between UIF and ABF/IF products.
ABF caused the largest destruction to the tissue, while the
microstructure of the UIF
products was the best preserved. It is concluded that UIF
processing was a better freezing
method for lotus root with improved quality and less damaged
microstructure than the
two other methods.
2014 Elsevier Ltd and IIR. All rights reserved.a r t i c l e i n
f o a b s t r a c tEffects of different freezing methods on the
qualityand microstructure of lotus (Nelumbo nucifera) root
Jing Tu a, Min Zhang a,*, Baoguo Xu a, Huihua Liu b
a State Key Laboratory of Food Science and Technology, Jiangnan
University, 214122 Wuxi, Jiangsu, Chinab School of Health Sciences,
Federation University Australia, VIC 3353, Australiawww. i ifi i r
.org
journal homepaghts reserved.www.elsevier .com/locate / i j refr
ig
-
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t
i o n 5 2 ( 2 0 1 5 ) 5 9e6 560medicine. However, the shelf-life of
fresh lotus root is very
short since it easily browns and deteriorates during storage
when the peel is damaged.
With the aim to prolong the shelf-life of lotus root,
several
preservation processes have been assayed including bloated
by salt, some vacuum package by plastic film and frozen
lotus
root (Guo, 2008). Among them, freezing is one of the most
important approaches since it not only significantly extends
vegetable shelf-life but also diversifies the offer of foods
for
consumers (Sahari et al., 2004). However, previous studies
had
shown that freezing usually resulted in undesirable
physical,
chemical, and structural changes leading to quality losses
in
color, texture, and nutrition (Alvarez et al., 2005; Koushki
et al., 2013). During the deep freezing process, one of the
most important parameters affecting the microstructure of
the frozen product is the ice crystal size which is directly
related to the freezing rate. A slow freezing gives rise to
the
formation of large ice crystals that irreversibly damages
the
tissues. On the other hand, a high freezing rate leads to
small
ice crystals that contribute to a much preserved structural
quality of the food (Sanz et al., 1999).
At industrial-scale production, the most common used
freezing methods are air blast, plate contact, fluidised-bed
and cryogenic freezing (Lakshmisha et al., 2008; Norton
et al., 2009). Freezing rate achievable by these methods is
limited by the thermal conductivity of foods, which has a
low
value (approximately 0.5e1.5 W m1 K1) (Singh andHeldman, 2009;
Sun and Li, 2003). By directly contacting of
food products with refrigerating medium or refrigerant, The
immersion freezing method offers significant advantages
including high-heat transfer coefficients, good product
quality and energy savings (Delgado et al., 2009). However,
the main disadvantage of immersion freezing method is the
uncontrollable solute uptake from the refrigerated solution
into the product. Fortunately, some approaches are devel-
oped to solve this problem, such as improving the freezing
rate, choosing the suitable solution solute for a particular
product, conducting pre-freezing treatments and so on
(Zorrilla and Rubiolo, 2005; Chourot et al., 2001). In
recent
years, the growth of the frozen food industry has become the
major driven force for the research activities on optimiza-
tion/improvement studies of the existing methods.
Ultrasound-assisted immersion freezing technologies as a
new method is being developed attributed to its promising
positive effects in food processing and preservation (Zheng
and Sun, 2005). Results from a previous research work sug-
gested that a shorter freezing time is required for sample
(apple and potato) subjected to power ultrasound-assisted
immersion freezing compared to immersion freezing
(Delgado et al., 2009; Comandini et al., 2013).
The objective of this study is to investigate the influence
of
different freezing methods on the product quality and
microstructure. Samples are subjected to three different
freezing methods including immersion freezing, ultrasound-
assisted immersion freezing and air blast freezing. The
qual-
ity of the froze product are compared from different aspects
such as the freezing time, color, firmness, drip loss, vitamin
C
amount and finally the microstructure of the final samples
isalso assessed by SEM images.2. Materials and methods
2.1. Materials
Lotus roots (Nelumbo nucifern) were purchased on a com-
mercial farm in Wuxi, Jiangsu, China. Fresh lotus roots were
cut into 2 cm thick slices and immediately kept in pre-made
solution (citric acid~1%, sodium chloride~0.5%, liquid cal-
cium chloride~0.5%) for 20 min (The experimental factors
were the appropriate parameters of preprocessing experiment
which was proved in our previous studies). Then lotus root
slices were blanched with boiling water for 60 s and
followed
by immediate cooling in ice bath. The pretreated samples
were refrigerated at 4 C until frozen by the three
freezingprocesses: conventional air blast freezing, immersion
freezing
and ultrasound-assisted immersion freezing.
2.2. Freezing process
An air blast freezer (Qi Hong refrigeration company,
Jiangsu,
China) was used in the ABF experiments. The ABF process was
carried out at 35 C using an air speed of 3.8 m s1. The
im-mersion freezing (IF) and ultrasound-assisted immersion
freezing (UIF) processes were carried out in an ultrasound-
assisted immersion freezer (Zhejiang Scientific Research In-
strument, Jiangsu, China). The output power of the generator
can be adjusted within the range of 0e300 W.
Unidirectionalultrasound waves were delivered to a freezing medium
in the
tank at the frequency of 30 kHz. A solution of calcium
chloride
and water (29/71, w/w) was used as the freezing medium
operating at 25 C. Each sample was positioned in the centerof
the vessel at a 2.5 cm depth below the freezing solution, in
which power ultrasound was applied intermittently in phase
transition stage for 6 min. The ultrasound power and ultra-
sound duty cycle per minutes can be adjusted during the UIF
process. Thus, five UIF processing conditions (named as
UIF-1
to UIF-5) at various ultrasound power and ultrasound duty
cycle per minutes are studied. Detailed UIF processing con-
dition is shown below: UIF-1 (90 W, 30 s on/30 s off), UIF-2
(150 W, 30 s on/30 s off), UIF-3 (210 W, 30 s on/30 s off),
UIF-4
(150 W, 15 s on/45 s off) and UIF-5 (150 W, 45 s on/15 s
off).
The freezing process was considered as finished when the
temperature at the centre of the sample reached 18 C.Temperature
of the centre of samples wasmonitored using K-
type thermocouples which were connected to a digital ther-
mometer (UT325 thermometer, Uni-Trend Technology
Limited, Dongguan, China). At least four replications were
carried out for each treatment.
2.3. Color measurement
Surface color of lotus roots was measured with a Minolta
spectrophotometer (CR-400, Konica Minolta Sensing, Tokyo,
Japan) using CIE color parameters L* (light/dark), a*
(red/green)
and b* (yellow/blue) values. The whiteness index (WI) was
calculated using the above three values as described by
Rupasinghe et al. (2006), which was calculated using
thefollowing equation:
-
WI 100100 L*2 a*2 b*2
q(1)
2.4. Texture analysis
The firmness of lotus root samples was determined by a
compression test using a texture analyzer (TA-XT plus,
Stable
Micro Systems, Ltd., Surrey, United Kingdom) fitted with a
cylindrical probe (P/2). The pre-speed, test-speed, and
post-
speed were set to be 1.5 mm s1, 1.5 mm s1, and 5 mm
s1,respectively, and the deformation ratio was 60%. The trigger
force was 5 g. The forceetime curve was recorded and
analyzed using the software Texture Exponent 32 (StableMicro
standard ascorbic acid solution consumed in calibration
(mg mL1), m is the sample weight (g).
2.7. Microscopic analysis
Structural observation was carried out using a SEM (SU1510;
Hitachi, Japan) at 10.0 kV. Samples were prepared according
to
the method of Delgado et al. (Delgado and Rubiolo, 2005).
Freeze-drying was the method used for the fresh control and
the frozen samples for removing the water prior to the SEM
observation. Slices of frozen samples were mounted on the
metal stubs with silver conducting paint, and were gold
coated in the same evaporator.
freezing methods and/or parameters.
As can be seen from Table 1, IF showed a shorter freezing
ot amm
sta
, 30
i n t e rn a t i o n a l j o u r n a l o f r e f r i g e r a t i
o n 5 2 ( 2 0 1 5 ) 5 9e6 5 61Systems, Ltd.). The firmness was the
maximum peak value in
the first compressed force.
2.5. Drip loss measurement
Drip loss for different sample during thawing was tested.
Thawing was conducted at 4 C in a thermostaticallycontrolled
refrigerator. Drip loss was measured by weighing
the lotus root sample during thawing (Kidmose and Martens,
1999). The drip loss was calculated as follows:
Drip loss% w0 wtw0
100% (2)
where: w0 and wt are the weights of the lotus root at time
0 and time t during thawing.
2.6. Determination of vitamin C content
The vitamin C content was measured using the oven method
(2,6-dichloroindophenol titration method, GB/T 6195-1986,
National Standard of China). Samples were washed by the
same content of 2% (m/v) oxalic acid. The mashed tissue was
accurately weighed and diluted to 100 mL by 1% oxalic acid.
After filtration, samples were titrated to pink color using
standard 2,6-dichloroindophenol solution. The vitamin C
content of samples is determined by the following equation:
X VTm
100 (3)
where: X is the vitamin C content of sample (mg/100g), V is
the
amount of 2,6-dichloroindophenol consumed (mL), T is the
Table 1 e The time spent on each freezing stage of lotus
rofreezing, IF: immersion freezing, UIF: ultrasound-assisted i
Treatments Precooling stage (s) Phase transition
ABF 170 28b 847 75cIF 95 7a 460 22bUIF-1 87 4a 415 15abUIF-2 87
4a 362 16aUIF-3 92 4a 387 11aUIF-4 95 7a 420 29abUIF-5 92 4a 412
14ab
UIF-1(90 W, 30 s on/30 s off), UIF-2(150 W, 30 s on/30 s off),
UIF-3(210 W
15 s off).The results are mean standard deviation (n 3).Values
with different online letters (a,b,c) in a column are significantly
dtime (95 s) for precooling stage than that of ABF (170 s).
Regarding the time spent in the phase transition stage,
it'sgenerally accepted that minimizing the phase transition
stage
could contribute to better the product quality. Compared to
IF
and UIF, the phase transition time of ABF (847 s) turned out
to
be the longest. This finding was in agreement with previous
studies from Chourot et al. (2003). The UIF-2 and UIF-3 sam-
ples with the phase transition time of 362 s and 387 s,
which
s affected by different freezing methods (ABF: air blastersion
freezing).
ge (s) Subcooling stage (s) Total freezing time (s)
679 37c 1641 69c387 17ab 939 45b405 24ab 908 35b340 10a 785
48a370 32ab 849 70ab365 21ab 850 41ab385 31b 891 88b
s on/30 s off), UIF-4(150 W, 15 s on/45 s off) and UIF-5(150 W,
45 s on/2.8. Statistical analysis
Data were subjected to analysis of variance (ANOVA) and
Duncan's Multiple Range Test (P 0.05) using the SPSS
16statistical software (SPSS, Chicago, Illinois, USA). The data
obtained in this study were reported as mean value standard
deviation (SD) and significant differences between mean
values were determined by Tukey's test.
3. Results and discussion
3.1. Effect of different freezing methods on freezingprocess
The whole freezing process can be divided into three stages:
precooling, phase transition and subcooling stage (Hu et
al.,
2013). The times for each stage of freezing process and
total
freezing time are summarized in Table 1. It revealed that
the
distribution of freezing time for each stagewas affected by
theifferent (P 0.05).
-
showed a significant improvement of the freezing efficiency
(p < 0.05) for IF samples (460 s). This confirmed that
ultrasoundirradiation significantly improved the freezing rate.
Similar
finding was reported in the work of Delgado et al. (2009).
However, UIF-1, UIF-4 and UIF-5 samples showed a lower in-
crease in the freezing efficiency (10%) than that of UIF-2
(21.3%). This phenomenon can be explained as the ultra-
sound irradiation showed promising effect for the enhance-
ment of convective heat transfer rate between samples and
cooling medium (Kiani et al., 2013, 2012).
From the Table 1, the required time for the subcooling
stage of IF samples was 387 s, which was shorter than 679 s
of
ABF sample. It reconfirmed that the air blast freezing
method
was the most time consuming technique. The total freezing
time for UIF process (especially 785 s for UIF-2 sample) is
shorter than that of IF and ABF (939 s and 1641 s for IF and
ABF
samples respectively). This finding clearly showed that
lotus
root freezing improved about 16.4% and 52.2% by ultrasound
irradiation compared to IF and UIF method, respectively.
found for IF and UIF products (p < 0.05), which indicated
that
b* andWI values of thawed lotus roots had a little decrease
but
The texture of many plant foods was determined by the cell
wall composition and contents, which is one of the most
important sensory characteristics determining consumer
preferences (Chiang and Luo, 2007). Fig. 1 shows the
firmness,
values of lotus root (Control: Fresh, ABF: air blast freezing,
IF:eezing).
a* b* WI
0.22 0.14bc 11.49 1.42ef 69.17 1.56c0.72 0.59cd 10.33 0.30cd
65.44 0.28a0.68 0.35cd 10.34 0.69cd 71.87 1.23d1.29 0.90d 12.31
1.59f 71.77 1.41d1.45 0.18d 12.20 0.80ef 70.51 0.60d0.21 0.20bc
11.93 0.63f 72.28 0.45d0.29 0.30bc 12.41 1.49f 71.17 1.26d1.34
0.31d 12.15 0.61a 69.79 0.57cd
0.25 0.43abc 6.10 0.42a 65.87 0.55a0.09 0.86ab 6.00 1.66b 67.23
1.67b0.70 0.31a 8.28 1.81ab 67.25 0.33b0.45 0.48ab 7.38 0.48bc
67.50 0.39b0.72 0.62a 8.87 1.05bc 69.39 0.30c0.90 0.46a 8.87 1.05bc
69.38 0.30c0.38 1.19bc 7.69 0.85ab 67.85 0.57b
Control ABF IF UIF-1 UIF-2 UIF-3 UIF-4 UIF-5600
800
1000
1200
1400
1600
1800
2000
2200
2400
0
5
10
15
20
25
Fig. 1 e The firmness and drip loss of lotus root under
different freezing methods (Control: Fresh, ABF: air blast
freezing, IF: immersion freezing, UIF: ultrasound-assisted
immersion freezing; UIF-1(90 W, 30 s on/30 s off), UIF-
2(150 W, 30 s on/30 s off), UIF-3(210 W, 30 s on/30 s off),
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t
i o n 5 2 ( 2 0 1 5 ) 5 9e6 562Table 2 e Effects of different
freezing methods on the colorimmersion freezing, UIF:
ultrasound-assisted immersion fr
Stages Treatments L*
Control 71.43 1.59cdFreezing ABF 65.99 0.23a
IF 73.62 1.18eUIF-1 74.62 1.01eUIF-2 73.71 0.63eUIF-3 74.99
1.01eUIF-4 74.01 1.06eUIF-5 72.39 0.84d
Thawing ABF 65.43 0.58aIF 68.31 2.68bUIF-1 68.36 0.39bUIF-2
68.36 0.49bUIF-3 70.73 0.61cUIF-4 70.00 0.61cUIF-5 68.81 0.65b3.2.
Effect of different freezing methods on the color oflotus root
Color is the primary quality parameter when consumers are
assessing the natural and processed foods. Vegetables are
easy to change color during the preprocessing, freezing,
thawing and frozen storage (Alvarez et al., 2005; Koushki et
al.,
2013). Table 2 shows the effects of different freezing
methods
on the color variations of frozen and thawed lotus root.
During
freezing process, the value of L* and WI ranged from 71.43
to
69.17 in control sample to 65.99 and 64.44 in ABF samples,
which indicated that ABF showed adverse effects on the color
of frozen samples. Conversely, IF resulted in an increase
from
71.43 to 69.17 to 73.63 and 71.87 in L* and WI values, which
confirmed that the IF frozen products become brighter and
whiter when compared with the control samples. No statis-
tically significant differences in the value of L* and WI
wereExplanations as in Table 1.still close to the fresh samples,
which indicated that IF and
UAF had an advantage over ABF on color preservation.
3.3. Effect of different freezing methods on firmness anddrip
loss after thawingultrasound irradiation had little impact on color
of lotus root
samples. After thawing, except for the ABF products, the L*,
a*,
UIF-4(150 W, 15 s on/45 s off) and UIF-5(150 W, 45 s on/15 s
off)). The results are mean standard deviation (n 3).Values with
different superscript letters in a column are
significantly different (P 0.05).
-
trasound irradiation had not significant impact on VC as
read
from the difference of VC amount between UIF samples is
small. As a result, selection of appropriate ultrasonic pro-
cessing parameters can adjust vitamin C retention under
refrigeration.
3.5. Effect of different freezing methods on themicrostructure
of lotus root
Microstructure and their degradation play major roles in
food
quality and are generally under estimated in their
importance
in food quality. To visualize the difference between samples
prepared from different freezing processes, a scanning elec-
tronmicroscopy (SEM) of one representative series of samples
of each group was studied (ABF: 35 C, 3.8 m s1; IF:25 C;UIF:25
C, 150 W, 30 s on/30 s off). It can be seen in the Fig. 3that the
raw lotus root samples (Fig. 3A) showed a partly
damaged cell structure due to ice sublimation during freeze
drying for SEM preparation, but basically remained well
defined and organized individual cells. After the freezing
process, the cells appeared torn and irregular in shape and
some loss of amorphous material and tissue distortion were
observed, comparing Fig. 3AeD. Compared to other freezing
methods, ABF caused the largest destruction of tissue
texture
(Fig. 3B), which manually supported the argument that the
lower the freezing rate usually formed the large and extra-
cellular ice crystals resulting in texture damage, as
reported
UIF-4(150W, 15 s on/45 s off) and UIF-5(150W, 45 s on/15 s
i n t e rn a t i o n a l j o u r n a l o f r e f r i g e r a t i
o n 5 2 ( 2 0 1 5 ) 5 9e6 5 63the index for texture, changes of
lotus roots subjected to
different freezing treatments. Compared to the control sam-
ples, all frozen samples showed a significant decrease in
firmness after thawing, which suggested that crystallization
of ice during freezing process causes cell damage, resulting
in
its texture changed (Sanz et al., 1999). Reduction in
textural
firmness were in the order of UIF-2 < UIF-3< UIF-1 <
UIF-4 < UIF-5 < IF < BAF samples. The ABF samples
firmnessshowed the largest decrease from 1954 to 1093 g. This
was
considered to contribute to the lowest freezing rate which
resulted in the formation of the relatively large and
extracel-
lular ice crystals. Similar results were reported by Chourot
et al. (2003). When compared with the IF samples, a signifi-
cant increase in the firmness after thawing of UIF
sampleswas
found due to its improvement in the freezing rate (as shown
in
Table 1). These results indicated that UIF appeared to be
effective in reducing histological damage and improving the
texture of thawed lotus root.
The drip loss of most fruits and vegetables, after thawing,
might involve soluble solids, such as polysaccharides, pro-
teins and a small fraction of the water-soluble vitamins and
minerals. The results of drip loss associated with the lotus
roots subjected to different freezing treatments are pre-
sented in Fig. 1. The UIF process significantly reduced the
thawing drip loss as compared with the two conventional
freezing processes (8.2%~11.3%, 12.9% and 18.5% for UIF, IF
and ABF samples respectively). This could be related to the
ice crystal size during the freezing process. It was
reported
in literature that crystallization damaged the cell
structure
resulting in drip loss (Sanz et al., 1999). Thus, it could
be
concluded that reduced drip loss from UIF samples are
attributed to the relative smaller ice crystals formed
during
the UIF process that resulted in considerably less damage to
lotus roots when compared to that from the conventional
processes. When compared with IF treatment, UIF-1, UIF-4
and UIF-5 treatments had little impact on deceasing the
thawing drip loss (p 0.05) due to its little improvement inthe
freezing rate which directly related to the crystal size
and size distribution (Li et al., 2006). UIF-2 treatment was
selected for the best condition for minimizing the thawing
drip loss.
3.4. Effect of different freezing methods on the vitamin
Ccontent of lotus root
Vitamin C (VC) is an important nutrient component of frozen
fruits and vegetables, and routinely used as an index to
measure processing effects on nutrient retention due to its
lability (Giannakourou and Taoukis, 2003). Fig. 2 showed the
VC content changes of lotus roots subjected to different
freezing treatments. Compared to the control samples, all
frozen samples showed a significant decrease in VC content,
which was because VC was affected by the pretreatment due
to its heat lability. When comparing the VC amount retained
in samples prepreaed from different freezing techniques, ABF
and IF samples showed higher amount of VC than that from
UIF samples. In addition, the effect of ultrasound
irradiation
can be read from Fig. 2. Slight variation in VC amount can
beread from UIF samples with different processing parameters
(especially for UIF-3 sample), this could be attributed to
thefree radicals in sonolysis of water molecules oxidating VC
(O'Donnell et al., 2010) under ultrasound irradiation.
However,due to the low concentration of free radicals generated,
ul-
off)). The results are mean standard deviation (n 3).Values with
different superscript letters in a column are
significantly different (P 0.05).Control ABF IF UIF-1 UIF-2
UIF-3 UIF-4 UIF-50
10
20
30
40
50
abaa
b
d
ab
c c
Fig. 2 e Changes of vitamin C content (%) of lotus root
under different freezing methods (Control: Fresh, ABF: air
blast freezing, IF: immersion freezing, UIF: ultrasound-
assisted immersion freezing; UIF-1(90 W, 30 s on/30 s off),
UIF-2(150W, 30 s on/30 s off), UIF-3(210W, 30 s on/30 s off),by
Sanz et al. (1999). The microstructure of the UIF samples
(Fig. 3D) was less damaged than the microstructure of the IF
-
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t
i o n 5 2 ( 2 0 1 5 ) 5 9e6 564samples (Fig. 3C), which indicated
that the use of power ul-
trasound provides a useful approach to minimize the damage
of cell structure (Deora et al., 2013). These results were
in
agreement with the results of the texture analysis in this
experiment, UIF processing seems to be the best freezing
method to retain the texture the most.
4. Conclusions
The effect of three freezing techniques, air blast freezing,
immersion freezing and ultrasound-assisted immersion
freezing, on quality and microstructure of lotus root has
been comprehensive studied. Freezing time, color, drip loss,
texture, vitamin C content and microstructure were
measured. ABF was the most time consuming and uneco-
nomic method, on the other hand, ultrasound-assisted im-
mersion freezing at 150 W and 30 s intervals shortened the
freezing time by approx. 17% accordingly. IF and UIF method
exhibited better color retaining ability than ABF method,
while ABF and IF method resulted in higher amount of VC
retained than that from UIF method. The clearest advantage
of UIF method is the fast freezing rate it offered. With a
fast
freezing rate from UIF method, the UIF samples showed an
improved firmness, reduced drip loss and less damaged
microstructure.
Fig. 3 e SEM images of the frozen lotus roots. (A) raw lotus
roo
immersion frozen lotus roots (25 C), (D) ultrasound-assisted
imAcknowledgments
This work was financially supported by the National
Scientific
& Technological Supporting Project of China (Contract
No.
2012BAD27B03-3) and the National Science Foundation of
China (Contract No. 21176104).
r e f e r e n c e s
Alvarez, M.D., Fernandez, C., Canet, W., 2005. Effect of
freezing/thawing conditions and long-term frozen storage on
thequality of mashed potatoes. J. Sci. Food Agric. 85,
2327e2340.
Chiang, P.Y., Luo, Y., 2007. Effects of pressurized cooking on
therelationship between the chemical compositions and
texturechanges of lotus root (Nelumbo nucifera Gaertn.). Food
Chem.105, 480e484.
Chourot, J.M., Lauwers, J., Massoji, N., Lucas, T., 2001.
Behaviourof green beans during the immersion chilling and freezing.
Int.J. Food Sci. Tech. 36, 179e187.
Chourot, J.M., Macchi, H., Fournaison, L., Guilpart, J.,
2003.Technical and economical model for the freezing costcomparison
of immersion, cryomechanical and air blastfreezing processes.
Energy Convers. Manage. 44, 559e571.
Comandini, P., Blanda, G., Soto-Caballero, M.C., Sala,
V.,Tylewicz, U., Mujica-Paz, H., Valdez, F.A., Gallina, T.T.,
2013.
ts, (B) air blast frozen lotus roots (35 C, 3.8 m s1),
(C)mersion frozen lotus roots (25 C, 150W, 30 s on/30 s off).
-
Effects of power ultrasound on immersion freezingparameters of
potatoes. Innov. Food Sci. Emerg. 1, 1e6.
Delgado, A.E., Rubiolo, A.C., 2005. Microstructural changes
instrawberry after freezing and thawing processes. Food Sci.Tech.
38, 135e142.
Delgado, A.E., Zheng, L.Y., Sun, D.W., 2009. Influence
ofultrasound on freezing rate of immersion-frozen apples.
FoodBioprocess Technol. 2, 263e270.
Deora, N.S., Misra, N.N., Deswal, A., Mishra, H.N., Cullen,
P.J.,Tiwari, B.K., 2013. Ultrasound for improved crystallisation
infood processing. Food Eng. Rev. 5, 36e44.
Giannakourou, M.C., Taoukis, P.S., 2003. Kinetic modelling
ofvitamin C loss in frozen green vegetables under variablestorage
conditions. Food Chem. 83, 33e41.
Guo, H.B., 2008. Cultivation of lotus (Nelumbo nucifera
Gaertn.ssp. nucifera) and its utilization in China. Genet. Resour.
CropEvol. 56 (3), 323e330.
Hu, S.Q., Liu, G., Li, L., Li, Z.X., Hou, Y.i, 2013. An
improvement inthe immersion freezing process for frozen dough
viaultrasound irradiation. J. Food Eng. 114, 22e28.
Kiani, H., Sun, D.W., Zhang, Z.H., 2013. Effects of
processingparameters on the convective heat transfer rate
duringultrasound assisted low temperature immersion treatment ofa
stationary sphere. J. Food Eng. 115, 384e390.
Kiani, H., Sun, D.W., Zhang, Z., 2012. The effect of
ultrasoundirradiation on the convective heat transfer rate
duringimmersion cooling of a stationary sphere. Ultrason.
Li, H., Guo, Z., Liu, Y., 2006. The application of power
ultrasoundto reaction crystallization. Ultrason. Sonochem. 13,
359e363.
Man, J.M., Cai, J.W., Cai, C.H., Xu, B., Huai, H.Y., Wei, C.X.,
2012.Comparison of physicochemical properties of starches fromseed
and rhizome of lotus. Carbohydr. Polym. 88, 676e683.
Norton, T., Delgado, A., Hogan, E., Grace, P., Sun, D.W.,
2009.Simulation of high pressure freezing processes by
enthalpymethod. J. Food Eng 91, 260e268.
O'Donnell, C.P., Tiwari, B.K., Bourkec, P., Cullenc, P.J., 2010.
Effectof ultrasonic processing on food enzymes of
industrialimportance. Trends Food Sci. Tech. 21, 358e367.
Rupasinghe, H.P.V., Boulter-Bitzer, J., Ahn, T., Odumeru, J.A.,
2006.Vanillin inhibits pathogenic and spoilage microorganismsin
vitro and aerobic microbial growth in fresh-cut apples. FoodRes.
Int. 39, 575e580.
Sahari, M.A., Boostani, F.M., Hamidi, E.Z., 2004. Effect of
lowtemperature on the ascorbic acid content and
qualitycharacteristics of frozen strawberry. Food Chem. 86,
357e363.
Sanz, P.D., Elvira, C., Martinob, M., Zaritzkyb, N., Oteroa,
L.,Carrascoa, J.A., 1999. Freezing rate simulation as an aid
toreducing crystallization damage in foods. Meat Sci.
52,275e278.
Singh, R.P., Heldman, D.R., 2009. Introduction to
FoodEngineering, Fourth ed. Academic Press, Oxford, UK.
Sun, D.W., Li, B., 2003. Microstructural change of potato
tissuefrozen by ultrasound-assisted immersion freezing. J. Food
Eng.57, 337e345.
Xing, Y.G., Li, X.H., Xu, Q.L., Jiang, Y.H., Yun, J., Li, W.L.,
2010.
i n t e rn a t i o n a l j o u r n a l o f r e f r i g e r a t i
o n 5 2 ( 2 0 1 5 ) 5 9e6 5 65Sonochem. 19, 1238e1245.Kidmose, U.,
Martens, H.J., 1999. Changes in texture,
microstructure and nutritional quality of carrot slices
duringblanching and freezing. J. Sci. Food Agric. 79,
1747e1753.
Koushki, M.R., Mohammadi, M., Javadi, N.H.S., Komeily,
R.,Moslemy, M., Ahmadian, F.S., Zali, H., 2013. The influence
offreezing conditions on the organoleptic attributes of
Iranianleafy vegetable foods. JPS 4, 42e47.
Lakshmisha, I.P., Ravishankar, C.N., Ninan, G., Mohan,
C.O.,Gopal, T.K.S., 2008. Effect of freezing time on the quality
ofIndian Mackerel (Rastrelliger kanagurta) during frozenstorage. J.
Food Sci. 73, 345e353.Effects of chitosan-based coating and
modified atmospherepackaging (MAP) on browning and shelf life of
fresh-cut lotusroot (Nelumbo nucifera Gaerth). Innov. Food Sci.
Emerg. 11,684e689.
Zheng, L.Y., Sun, D.W., 2005. Innovative applications of
powerultrasound during food freezing processesda review. TrendsFood
Sci. Tech. 17, 16e23.
Zorrilla, S.E., Rubiolo, A.C., 2005. Mathematical modeling
forimmersion chilling and freezing of foods. Part I:
modeldevelopment. J. Food Eng 66, 329e338.
Effects of different freezing methods on the quality and
microstructure of lotus (Nelumbo nucifera) root1. Introduction2.
Materials and methods2.1. Materials2.2. Freezing process2.3. Color
measurement2.4. Texture analysis2.5. Drip loss measurement2.6.
Determination of vitamin C content2.7. Microscopic analysis2.8.
Statistical analysis
3. Results and discussion3.1. Effect of different freezing
methods on freezing process3.2. Effect of different freezing
methods on the color of lotus root3.3. Effect of different freezing
methods on firmness and drip loss after thawing3.4. Effect of
different freezing methods on the vitamin C content of lotus
root3.5. Effect of different freezing methods on the microstructure
of lotus root
4. ConclusionsAcknowledgmentsReferences