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: min@jiangnan.edu.cn (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).

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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