- Email: [email protected]
The effects of ultrasound-assisted freezing on the freezing time and quality of broccoli (Brassica oleracea L. var. botrytis L.) during immersion freezing
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 4 1 ( 2 0 1 4 ) 8 2 e9 1
Available online at www.sciencedirect.com
ScienceDirect w w w . i i fi i r . o r g
journal homepage: www.elsevier.com/locate/ijrefrig
The effects of ultrasound-assisted freezing on the freezing time and quality of broccoli (Brassica oleracea L. var. botrytis L.) during immersion freezing Ying Xin a, Min Zhang a,b,*, Benu Adhikari c a
State Key Laboratory of Food Science and Technology, Jiangnan University, 214122 Wuxi, Jiangsu, China School of Food Science and Technology, Jiangnan University, 214122 Wuxi, Jiangsu, China c School of Applied Sciences, RMIT University, City Campus, Melbourne, VIC 3001, Australia b
article info
abstract
Article history:
The application of ultrasound during immersion freezing of broccoli was studied and
Received 3 August 2013
particular attention was given to the effects on freezing time, microstructure, firmness and
Received in revised form
drip loss of broccoli. Broccoli florets were immersion frozen in an ultrasound-assisted freezer
26 December 2013
at two frequencies and four different power levels. The results showed that the total freezing
Accepted 28 December 2013
time and times required for pre-cooling, phase change and sub-cooling stages of broccoli
Available online 8 January 2014
were significantly reduced by the application of ultrasound-assisted freezing (UAF) at 150 (30 kHz) or 175 W (20 kHz) power level and with judicious combination of process parameters
Keywords:
(exposure time, ultrasound irradiation temperature and pulse mode). The microstructure
Ultrasound-assisted freezing
and the firmness of broccoli tissue were better preserved and the drip loss was significantly
Freezing time
reduced by the application of optimized UAF compared to the normal immersion freezing.
Microstructure
These findings indicate that there is a great potential of UAF in immersion freezing of food.
Drip loss
ª 2014 Elsevier Ltd and IIR. All rights reserved.
Firmness Broccoli
Effets de la conge´lation assiste´e par ultrasons sur la vitesse de conge´lation et la qualite´ de broccoli (Brassica oleracea L. var. botrytis L.) pendant la conge´lation par immersion Mots cle´s : Conge´lation assiste´e par ultrasons ; Temps de conge´lation ; Microstructure ; Pertes par e´coulements ; Fermete´ ; Brocoli
* Corresponding author. School of Food Science and Technology, Jiangnan University, 214122 Wuxi, China. Tel./fax: þ86 510 85877225. E-mail address: [email protected] (M. Zhang). 0140-7007/$ e see front matter ª 2014 Elsevier Ltd and IIR. All rights reserved. http://dx.doi.org/10.1016/j.ijrefrig.2013.12.016
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 4 1 ( 2 0 1 4 ) 8 2 e9 1
1.
Introduction
Freezing is one of the most successful methods for long term preservation of perishable food products. This method is also capable of preserving initial sensory characteristics of food products and ensures that the products remain suited for further processing. However, fresh food products having higher water content are more susceptible to the formation of large ice crystals during freezing. The large ice crystals present in the tissues of frozen foods cause mechanical damage and drip loss which results into poor product quality (Ferna´ndez et al., 2006; Wang et al., 2013). It has been shown that the quality of frozen food is closely related to the size and location of ice crystals, which in turn, largely depends on the freezing time or the rate of freezing (Petzold and Aguilera, 2009). Therefore, conscious efforts are made to shorten the freezing time, to increase the freezing rate and to reduce the size of the ice crystals in the frozen products. All of these can be achieved by applying the advanced methods of food freezing, such as high-pressure food freezing, application of ice nucleation active bacteria, dehydrofreezing and ultrasound-assisted freezing (UAF) (Comandini et al., 2013; Ando et al., 2012; Volkert et al., 2012; Zhang et al., 2010). Ultrasound has attracted great interest in recent years and its application in food freezing has shown promising
83
advantages. Power ultrasound has been proven to be effective in the initial nuclei formation process and also in the subsequent crystallization process during freezing (Li and Sun, 2002). The cavitation bubbles produced by ultrasound can serve as nuclei for ice crystal formation once they reach the critical nucleus size. In addition, the strong forces originating from the collapse of cavitation bubbles fragment the dendritic ice crystals into small size. These fragmented crystals then act as new nuclei (Chow et al., 2003, 2005; Saclier et al., 2010). The vigorous collapse of cavitation bubble creates local zone of high pressure (50 MPa or more) for very short time (nano-seconds), which results in high degrees of supercooling. The supercooling can act as a driving force for instantaneous nucleation (Inada et al., 2001a,b; Zhang et al., 2001). Furthermore, power ultrasound has been recognized to enhance convective heat transfer by acoustic cavitation or acoustic streaming (Legay et al., 2011). These two phenomena arise from the propagation of power ultrasonic waves into a fluid. The passage of power ultrasound waves causes macroscopic turbulence, increases the frequency of high speed collision of microscopic particles, reduces the thickness of solideliquid boundary layer and decreases the heat transfer resistance (Kiani et al., 2013a,b; Legay et al., 2011). Therefore, ultrasound-assisted freezing is able to decrease the freezing time, reduce the size of the
Fig. 1 e Schematic diagram and photograph of the ultrasound-assisted freezer.
84
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 4 1 ( 2 0 1 4 ) 8 2 e9 1
Table 1 e Ultrasound treatments at phase change and sub-cooling stages of broccoli-freezing process. Treatment Control P20-1 P20-2 P20-3 P20-4 P30-1 P30-2 P30-3 P30-4 Control S20-1 S20-2 S20-3 S30-1 S30-2 S30-3
Ultrasound application stage
Frequency (kHz)/ power (W)
Initial temperature at which the ultrasound was applied ( C)
Ultrasound pulse mode (s)
Exposure time (s)
Phase change
e 20/175
e 0 0 0.5 0.5 0 0 0.5 0.5 e 5 5 5 5 5 5
e 60/60 30/60 60/60 30/60 60/60 30/60 60/60 30/60 e 60/60 60/60 30/60 60/60 60/60 30/60
e 120 120 120 120 120 120 120 120 e 120 180 180 120 180 180
30/150
Sub-cooling
e 20/175
30/150
ice crystals, better maintain the cell structure and finally improve the quality of frozen food. The efficacy of ultrasound-assisted freezing depends on the frequency, power level, time of exposure to ultrasound and pulse mode (Comandini et al., 2013; Delgado et al., 2009; Hu et al., 2012; Li and Sun, 2002). It is commonly accepted that the higher the power of the ultrasound, the longer the exposure time, the stronger is the effect of sonication. However, the amount of heat generated due to the application of ultrasound can increase the temperature of the food and refrigerating medium and which reduces the freezing rate (Delgado et al., 2009; Hu et al., 2012; Li and Sun, 2002). Furthermore, the inherent characteristics of food also affect the efficacy of ultrasound-assisted freezing (Hu et al., 2012; Lee et al., 2004; McClements and Gunasekaran, 1997). Although research have been undertaken to study the effect of ultrasoundassisted freezing on a number of food materials such as potato, apple, dough, sucrose solution and agar gel (Comandini et al., 2013; Delgado et al., 2009; Hu et al., 2012; Kiani et al., 2011, 2012; Li and Sun, 2002), the application of ultrasoundassisted freezing on broccoli has not been reported. In this study, we have applied the UAF for freezing of broccoli. The main objective of this work was to quantify the effect of ultrasound frequency and power level on immersion freezing of broccoli, with particular attention to their effect on the freezing time. In order to obtain an appropriate protocol for UAF, ultrasound was applied at different frequencies, different power levels, different exposure times and different pulse modes both at phase change and sub-cooling stages. The microstructure, firmness and drip loss of broccoli when frozen using normal immersion freezing and optimized UAF were also measured in order to assess the effects of UAF on the quality of broccoli.
2.
Materials and methods
2.1.
Materials
Fresh broccoli (Brassica oleracea L. var. botrytis L.) was obtained directly from cultivated area in Wuxi (Jiangsu, China). Broccoli
inflorescences were divided into small florets of about 5 0.5 cm in diameter, cut 2 0.5 cm below the lowest ramification. Then, broccoli florets were blanched in a thermostated water bath (HH-1, Aohua Instrument Inc., Changzhou, China) at 95 1 C for 3 min. After blanching, samples were kept in a refrigerator at a temperature of 8.5 0.5 C for 12 h to achieve uniform initial temperature.
2.2.
Experimental set up
2.2.1.
The ultrasound-assisted freezing system
An ultrasound-assisted freezer (Hechuang Co., China), which operates at 20 and 30 kHz frequency and dissipated powers from 125 to 190 W was used for broccoli immersion freezing (Fig. 1). The coolant (40% calcium chloride solution, 7500 cm3) was cooled at 25 C and was circulated through the immersion cooling bath (25 20 20 cm) (Fig. 1-D) by a refrigeration cycle system (Fig. 1-C). The flow rate of the coolant used for freezing was set to1 L min1. The samples (Fig. 1-H) were suspended into the circulating coolant and the temperature of the samples was recorded by T-type thermocouples (Fig. 1-I) connected to a data acquisition system (Fig. 1-J). The temperature variation at the geometric center of sample was monitored and recorded. The power output from the generator (Fig. 1-F) to the transducers (Fig. 1-E) could be adjusted by the control system of ultrasound (Fig. 1-G) when the UAF was used. And two refrigeration systems (Fig. 1-A, B) were used for cooling.
2.2.2.
Immersion freezing process
Broccoli samples (90.5% w/w moisture content) were removed from the refrigerator and immediately immersed into the coolant (25.0 0.5 C). Although the ultrasound transducers are evenly located at the bottom of the immersion cooling bath, the intensity of the ultrasound waves delivered to the medium is different at different positions in the tank (Kiani et al., 2011). Therefore, for each experimental trial, samples were kept at the same position (10 cm from the bottom of the immersion cooling bath) and frozen using the ultrasoundassisted freezer. Samples were exposed to the ultrasound waves with the following conditions: 20 or 30 kHz frequency, 60 s on/60 s off duty cycle, and 0, 125, 150, 175 or 190 W power
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 4 1 ( 2 0 1 4 ) 8 2 e9 1
levels during these freezing tests. In order to study the effect of different ultrasound parameters on the freezing of broccoli, ultrasound was applied at different moments of the freezing process, for different exposure times and different pulse mode during the phase change and sub-cooling stages (Table 1). The freezing process ended as soon as the temperature of the sample reached 18 C. The temperature of sample during freezing was monitored by using T-type thermocouple at the geometric center of the sample. Temperatures of the samples and the coolant were measured and recorded at 5 s interval. These temperature data were used to generate the freezing curves and to calculate the freezing time. After freezing, each sample was placed into a double high-density polyethylene bag and stored at 18 1 C. All experiments were performed in triplicate and average values are reported.
2.3.
Quality analysis
2.3.1.
Observation of the microstructure
After two weeks of frozen storage, the frozen broccoli was freeze-dried in a freeze-dryer (Model 77530-30, Labconco Co., USA) to prepare sample for scanning electron microscopic (SEM) analysis (Model S-4800, Hitachi Co., Japan). The surface of the young stem of broccoli was sampled, and its bottom was fixed onto an aluminum disc with conductive, double-sided adhesive tape. Gold preprocessing was performed under vacuum conditions prior to observing under the SEM. The SEM images of the samples were digitally captured at different locations at 150, 500 and 1000 magnification.
2.3.2.
85
(ANOVA) was performed to determine the effects of different freezing treatments on freezing time, drip loss and the firmness of broccoli. The comparison among the mean values was performed using Duncan’s test to examine differences between different freezing processes. Comparisons that yielded P < 0.05 were considered significant.
3.
Results and discussion
3.1. The effect of ultrasound frequency and power level on immersion freezing of broccoli The freezing curves and total freezing times for normal immersion freezing and UAF are presented in Figs. 2 and 3. It can be seen from figures that the different treatments of UAF had different effect on the total freezing time. The total time here is defined as the time required for the broccoli sample to reach 18 C from an initial temperature of 8.5 C. The total freezing times for UAF at ultrasound power levels of 125 (30 kHz), 150 (20 and 30 kHz) and 175 W (20 kHz) were shorter than that of normal immersion freezing (P < 0.05), and the results showed clearly that broccoli-freezing was improved by ultrasound irradiation. Compared with the normal immersion freezing,
Measurement of drip loss
After two weeks of frozen storage, samples were thawed at 4 1 C in a refrigerator for 10 h and drip loss (DL) was calculated using the method suggested by Gonc¸alves et al. (2011) using Eq. (1) given below: DLð%Þ ¼
Mt Mo 100 Mi
(1)
where Mi (g) is the mass of the sample before thawing, M0 (g) is the mass of dry blotting paper, Mt (g) is the weight of wet blotting paper with exuded liquid. Four replications were carried out for each test.
2.3.3.
Measurement of firmness
Firmness was measured using a method suggested by Ferna´ndez-Leo´n et al. (2013) with some modification. The TAXT2i Texture Analyzer (Stable Micro Systems Ltd., Vienna 153 Court, Surrey, UK) was used for this purpose in compression mode. The whole broccoli heads were used as sample in these tests. The force was applied to produce a 5% deformation by a 100 mm flat aluminum probe. Force/deformation curves were recorded using the computer program associated with the texture analyzer and the maximum force (N) was calculated and used as the measure of the firmness. Ten replications were carried out for each test.
2.4.
Statistical analysis
The data were analyzed with SPSS software version 15.0 for Windows (SPSS Inc., Chicago, IL, US). Analysis of variance
Fig. 2 e The freezing curves of broccoli during different ultrasound-assisted freezing (UAF) processes and normal immersion freezing process.
86
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 4 1 ( 2 0 1 4 ) 8 2 e9 1
Fig. 3 e The total freezing times for broccoli under ultrasound-assisted freezing at different frequencies and power levels.
the UAF at ultrasound power levels of 125 (20 kHz), 175 (30 kHz) and 190 W (20 and 30 kHz) could significantly increase the total freezing time of broccoli (P < 0.05). The shorter freezing times were observed at relatively lower ultrasound power levels (except 125 W at 20 kHz). In contrast, longer freezing times were required when UAF is used at relatively high power levels. This observation indicates that relatively higher freezing rates can be achieved at relatively low ultrasound power levels. In addition, the ultrasound treatment also generated heat during the freezing process. The heat generated by ultrasound at lower power levels did not negatively impacted the freezing rate. However, when at relatively high power levels were used, the heating effect was found to dominate the freezing process, leading to significant increase in the freezing time. Regarding the effect of ultrasound frequency, at relatively low power levels (especially at 125 W), the freezing time for UAF was found to be significantly (P < 0.05) shorter at 30 kHz than at 20 kHz. This might be due to the fact that the ultrasound generates greater acoustic streaming at higher frequencies at the same power intensity (Delgado and Sun, 2011). The acoustic streaming increases the heat transfer rate which results into a shorter freezing time (Zheng and Sun, 2006). However, at relatively high power levels (especially at 175 W), the freezing time at 30 kHz was significantly (P < 0.05) longer than at 20 kHz. The shorter freezing time at 20 kHz can be attributed to the fact that it is much easier to generate greater cavitation at lower frequencies. At low frequencies, the bubbles produced by the ultrasound are bigger in size. When these bigger bubbles collapse, higher amount of energy is released and increase the effect of acoustic cavitation (Delgado and Sun, 2011). In fact, total freezing time does not accurately reflect the effect of ultrasound irradiation on freezing process of broccoli. As can be seen from Fig. 2, each freezing curve was divided into three stages: pre-cooling stage (8.5e0 C), phase change stage (0 to 5 C) and sub-cooling stage (5 to 18 C). It can be observed from Fig. 2 that the dependence of the freezing time on ultrasound frequency and power level is different for each stage. In order to better evaluate the impact of ultrasound frequency and power level on the immersion freezing of
broccoli, the freezing time of each stage of the freezing process is presented in Fig. 4. In the pre-cooling stage, UAF has a significant effect in shortening the freezing time at 150 (20 and 30 kHz) and 175 W (20 kHz) power levels (P < 0.05). However, at 125 W (20 kHz) and 190 W (30 kHz) power levels, the freezing efficiency was found to decrease. It is known that a rapid temperature decrease occurs at the pre-cooling stage due to the removal of sensible heat. When power ultrasound is employed, the removal of sensible heat is accelerated due to the presence of acoustic cavitation and acoustic streaming. This is because both the cavitation and acoustic streaming are known to cause macroscopic turbulent flow and high speed collisions of microscopic particles which reduce the heat transfer resistance by decreasing the thickness of boundary layer at solidewater interface (Kiani et al., 2013a,b; Legay et al., 2011). However, the thermal effect of ultrasound cannot be neglected, because it hinders the removal of heat from the broccoli sample. Broccoli is compositionally heterogeneous material and contains water and many solid compounds. The generation of heat in the broccoli was caused by the absorption and dissipation of ultrasound energy due to heterogeneity of the sample. And the more heterogeneous the material is the greater heat would be generated (Hu et al., 2012; Lee et al., 2004; McClements and Gunasekaran, 1997). In this research, at 150 (20 and 30 kHz) and 175 W (20 kHz) power level, the effect of increased rate of heat transfer was greater than the effect of heat generation. Because of this reason, the freezing
Fig. 4 e The time used up at each freezing stage at different ultrasound frequencies and power levels.
87
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 4 1 ( 2 0 1 4 ) 8 2 e9 1
time was shorter than that of the normal immersion freezing. In contrast, the effect of increase in the rate of heat transfer less than the effect of heat generation at 125 (20 kHz) and 190 W (30 kHz) power level. As a result, the freezing time of pre-cooling stage was longer than that of normal immersion freezing. A similar effect of ultrasound on the freezing time during the pre-cooling stage of dough was reported by Hu et al. (2012). These authors reported that at 288 and 360 W (25 kHz) power level the freezing time of the pre-cooling stage decreased compared to the corresponding freezing time for conventional freezing. This decrease in the cooling time at the pre-cooling stage was attributed to the dominance of heat transfer enhancement inside the dough. But at 175, 224, and 418 W (25 kHz) power level, the freezing efficiency was depressed due to the domination of heat generation (Hu et al., 2012). In the phase change period, a large amount of latent heat generated from water transformation into ice should be removed. Most of the freezable water crystallizes in this stage. Achieving appropriately short freezing time in this stage is crucial to obtain frozen food products of high quality (Delgado and Rubiolo, 2005). As can be seen from Fig. 4, UAF has significantly shortened the time required in this phase change stage at 175 (20 kHz) and 150 W (30 kHz) power level (P < 0.05). The reduction of freezing time in this stage is achieved primarily due to inducing better nucleation by ultrasound. During this phase change stage, the ultrasound cavitation bubble provides a favorable condition for heterogeneous primary nucleation of ice. The shock waves and micro-jets also promote the secondary nucleation by breaking up the preexisting dendritic ice crystals into smaller nuclei (Chow et al., 2003, 2005; Saclier et al., 2010). However, when the frozen broccoli was exposed to low (125 W at 20 kHz and 30 kHz) or high power ultrasound (175, 190 W at 30 kHz and 190 W at 20 kHz), the ultrasound was not effective enough to enhance the nucleation or unacceptably high amount of heat was generated inside of the broccoli. Because of this reason, the freezing time of phase change stage was significantly (P < 0.05) increased. After the phase change stage, since there is no need to remove the latent heat, the apparent specific heat decreases and the thermal conductivity increases all of these facilitate faster removal of the sensible heat. Because of these reasons, temperature of broccoli sample starts to decrease rapidly once again. Although the UAF at 175 (20 kHz) and 150 W (30 kHz) power levels were particularly suited in decreasing the freezing time during pre-cooling and phase change stages, the ultrasound treatment required more freezing time at subcooling stage of freezing of broccoli. The longer freezing time at sub-cooling stage can be explained from the fact that
most of the moisture within the sample was already frozen and the acoustic characteristics of this sample at sub-cooling stage were different compared to its acoustic characteristics at pre-cooling and phase change stages. The ice crystals within the broccoli or the grain boundaries among crystals are capable of scattering the ultrasound energy, which decreases the rate of heat transfer (Gu¨lseren and Coupland, 2008). As explained above, less heat transfer enhancement and heat generation increased by the ultrasound at this sub-cooling stage which increased the freezing time.
3.2. Effect of ultrasound parameters on the freezing time during phase change and sub-cooling stages The phase change stage is very important in the freezing process. The time taken for the temperature of a food item to pass through this stage determines both the number and the size of ice crystals. Minimizing the elapsed time of the phase change period contributes to optimizing the quality of frozen food products (Delgado and Rubiolo, 2005; Sun and Li, 2003). It has been reported that the UAF could improve the food-freezing process by shortening the time required in the phase change stage. The effectiveness of ultrasound-assisted freezing process depends on the power of the ultrasound, exposure time, the initial temperature for irradiation and pulse mode (Comandini et al., 2013; Delgado et al., 2009; Hu et al., 2012; Li and Sun, 2002; Zhang et al., 2001, 2003). We have shown in preceding sections that UAF at 175 (20 kHz) and 150 W (30 kHz) power levels decreased the time spent in the pre-cooling and phase change stages; however, it increased the freezing time at sub-cooling stage. In order to optimize ultrasound-assisted broccoli-freezing process, different ultrasound parameters were studied during the phase change and sub-cooling stages (Tables 2and 3). During the phase change stage, the initial temperature at which the ultrasound power was applied was selected to be close to the nucleation temperature. This temperature was experimentally determined from the freezing curve and was found to be around 1.1 C for broccoli. During the sub-cooling stage it was found that the closer the temperature of broccoli to the cooling medium the greater the (negative) heating effect due to the application of the ultrasound. This resulted into significant decrease in the freezing rate (Fig. 2). Therefore, the initial temperature at which the ultrasound power was applied was fixed at 5 C. Table 2 shows the average freezing times when broccoli samples were frozen using normal immersion (control) freezing and when ultrasound was applied during the phase change stage. The freezing time at the phase change stage was significantly decreased (P < 0.05) when treatments P20-1, P202 and P30-4 were applied (the detailed conditions of P20-1,
Table 2 e Freezing times at the phase change stage of ultrasound-assisted freezing process.a Treatment Freezing time (s) Treatment Freezing time (s) a
Control
P20-1
P20-2
P20-3
P20-4
295 10.0d e e
205 18.0a P30-1 290 18.0d
240 10.0b P30-2 255 13.2bc
255 15.0bc P30-3 275 8.7cd
275 13.2cd P30-4 235 18.0b
Values of different treatments having different letters in superscript are significantly different (P < 0.05). Results are mean standard deviation (n ¼ 3).
88
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 4 1 ( 2 0 1 4 ) 8 2 e9 1
Table 3 e Freezing times for sub-cooling stage of ultrasound-assisted freezing process.a Treatment Freezing time (s)
Control
S20-1
S20-2
S20-3
S30-1
S30-2
S30-3
325 8.7c
315 18.0c
225 13.2a
275 13.2bc
300 18.0c
310 10.0c
250 13.2b
a
Values having different letters in superscript in the same line are significantly different (P < 0.05). Results are mean standard deviation (n ¼ 3).
P20-2 and P30-4 are shown in Table 1). Treatments P20-1, P20-2 and P30-4 with freezing times of 205, 240 and 235 s, respectively, which resulted in an increase in the freezing efficiency of about 18.6e30.5%. The results were similar to those earlier reported by Comandini et al. (2013) and Delgado et al. (2009). These authors also observed that ultrasound could change the nucleation temperature during freezing of potato (Comandini et al., 2013) and apple (Delgado et al., 2009). We did not observe the effect of ultrasound on the nucleation temperature most probably due to the fast freezing rate. Hence, further investigations are required to verify the effect of ultrasound on the nucleation temperature. During sub-cooling stage, the ultrasonic attenuation was increased due to the freezing of most of the available water (Gu¨lseren and Coupland, 2008). The loss of ultrasonic energy occurs during the sub-cooling stage can be due to the increase in the concentration of scattering objects, i.e., the ice crystals. The energy of ultrasound could be scattered by the crystals or the crystal grain boundaries. When the freezing effect (due to enhanced heat transfer) does not overcome the negative effect due to the ultrasound attenuation, the freezing time at subcooling stage increases. It can be seen from Table 3 that the minimum freezing time required for the sub-cooling stage was 225 and 250 s when treatments S20-2 and S30-3 were applied (the detailed conditions of S20-2 and S30-3 are shown in Table 1). These results show that during the ultrasound-assisted broccoli-freezing process, freezing time depends on the applied ultrasound parameters. Taking all factors into consideration and the optimal design of parameters for the UAF of broccoli is presented in Table 4. As can be seen from this table the total freezing time when these optimized protocols are used is shorter than the freezing time when normal immersion freezing (730 s) is used. When the protocols UAF-I (20 kHz/175 W) and UAF-II (30 kHz/150 W) were used, the total processing time decreased by 29.5% and 22.6% (P < 0.05), respectively, compared to the time required in the normal immersion freezing.
3.3.
Microstructure of frozen-thawing broccoli
Fig. 5 shows the microstructure of broccoli stem under SEM. It has been reported that the extent of cryo-damage to plant tissue depends on the distribution and the size of ice crystals both of which are governed by the freezing time or freezing rate (Li and Sun, 2002; Pre´stamo et al., 2004). Fast freezing rates produce small crystals evenly distributed throughout the tissue and maintain the cell turgor, while slow freezing rates generally produce large ice crystals exclusively in extracellular areas and damage the structure of cell (Delgado et al., 2009). The SEM micrographs show that the tissue of
fresh broccoli stem visualized in the transverse section is composed of uniformly distributed oval cells. The cell walls of these cells are intact and there is no sign of damage (Fig. 5A). A comparison of SEM micrograph of the fresh tissue with the SEM micrograph of sample frozen under normal immersion freezing shows that the latter had suffered a distinct damage of the structure (Fig. 5B). The cells of the sample subjected to the normal immersion freezing appear to be collapsed and the shrinkage of the tissue occurred as a result of long freezing time. This damage to the cellular structure can be attributed to the formation of extracellular crystal matrix and large ice crystals (Go´ral and Kluza, 2009). Fig. 5 panels C and D show SEM micrographs of broccoli frozen using UAF (UAF-I and UAF-II). The cells of these samples appear slightly fractured and irregular. This may be due to the fact that UAF produces a large number of fine crystals inside and outside the cells. When the extent of intracellular and extracellular ice crystals is very high, cells can become irregular and get fractured as shown in Fig. 5 panels C and D. In spite of this, the microstructure of broccoli frozen using UAF is better preserved than the microstructure of broccoli frozen using normal immersion freezing. As can be seen from Fig. 5 panels B, C and D, the microstructure of broccoli tissues subjected to both UAF-I and UAF-II has less distortion of cells than the microstructure of samples subjected to normal immersion freezing. These observations suggest that the UAF reduces the freezing time and favors the formation of smaller ice crystals inside and outside the cells compared to normal immersion freezing. As a result, the structural integrity is better preserved.
Table 4 e The optimal combination of parameters for ultrasound-assisted freezing of broccoli. Ultrasound application stage Pre-cooling
Phase change
Sub-cooling Total freezing time (s)
Ultrasonic parameters Ultrasound application/ interval (s) Exposure time (s) Initial temperature for application ( C) Ultrasound pulse mode (s) Exposure time (s) Initial temperature for application ( C) Ultrasound pulse mode (s) Exposure time (s)
UAF-I (20 kHz/175 W)
UAF-II (30 kHz/ 150 W)
60/60
60/60
60 0 60/60 120
60 0.5 30/60 120
5 60/60
5 30/60
180
180
515 22.9
565 13.2
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 4 1 ( 2 0 1 4 ) 8 2 e9 1
89
Fig. 5 e SEM microphotographs (1503, 5003 and 10003 magnification) of the fracture surface of the broccoli stems.
3.4.
Drip loss of frozen-thawing broccoli
Fig. 6 shows the drip losses of broccoli samples subjected to normal immersion freezing and UAF. As can be seen from this figure, the broccoli sample frozen using normal immersion freezing had the highest drip losses. The water holding capacity of the sample frozen using normal immersion freezing was poor due to the damage to the microstructure of the broccoli (Fig. 5B). The microstructure of the samples frozen using UAF was much better preserved (Fig. 5C and D) as a result the drip loss was also considerably low. Slightly higher drip loss was observed in samples subjected to UAF-II than in samples in samples subjected to UAF-I, which can be attributed to the greater distortion of cells (Fig. 5D) and longer freezing time (Table 4) in the UAF-II samples.
3.5.
firmness of samples frozen using UAF was significantly higher (P < 0.05) compared to that of the samples frozen using normal immersion freezing. Moreover, as seen in Fig. 6, the drip loss and firmness results, the judges rated the quality of UAF samples was better than normal immersion freezing.
Textural firmness of frozen-thawing broccoli
Fig. 6 also presents the firmness of the broccoli. As can be seen from this figure, the samples treaded with normal immersion freezing have the lowest firmness. The higher extent of damage of microstructure of broccoli treated with normal immersion freezing (Fig. 5B) decreased the cell turgor and the firmness. The samples frozen using UAF showed softer texture compared to that of fresh broccoli due to the formation of ice in the tissues. It is worth noting that the
Fig. 6 e The drip loss and firmness of thawed broccoli samples that had been frozen with normal immersion freezing, UAF-I and UAF-II.
90
4.
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 4 1 ( 2 0 1 4 ) 8 2 e9 1
Conclusions
Similar to conventional food-freezing processes, the UAF process could be divided into three stages named as precooling, phase change and sub-cooling stage. With the enhancement of heat transfer by ultrasonic cavitation the total freezing time and times required at pre-cooling, phase change and sub-cooling stages of broccoli were significantly reduced by the application of ultrasound-assisted freezing (UAF) at 150 (30 kHz) or 175 W (20 kHz) power level and with judicious combination of process parameters (exposure time, ultrasound irradiation temperature and pulse mode). The results showed that select the appropriate conditions during the ultrasound-assisted freezing broccoli are very important. Compared to normal immersion freezing, the microstructure and the textural firmness of the broccoli tissue were better preserved and the drip loss was minimized by the optimal application of ultrasound-assisted freezing of broccoli. Although this work only shows a qualitative vision of the influence of ultrasound application in freezing process, the results provided useful information for the application of UAF in food processing in the near future.
Acknowledgments The authors gratefully acknowledge the financial support provided by China National Natural Science Foundation to carry out this project (Contract No. 21176104).
references
Ando, H., Kajiwara, K., Oshita, S., Suzuki, T., 2012. The effect of osmotic dehydrofreezing on the role of the cell membrane in carrot texture softening after freeze-thawing. J. Food Eng. 108, 473e479. Chow, R., Blindt, R., Chivers, R., Povey, M., 2003. The sonocrystallisation of ice in sucrose solutions: primary and secondary nucleation. Ultrasonics 41, 595e604. Chow, R., Blindt, R., Chivers, R., Povey, M., 2005. A study on the primary and secondary nucleation of ice by power ultrasound. Ultrasonics 43, 227e230. Comandini, P., Blanda, G., Soto-Caballero, M., Sala, V., Tylewicz, U., Mujica-Paz, H., Fragoso, A.V., Toschi, T.G., 2013. Effects of power ultrasound on immersion freezing parameters of potatoes. Innov. Food Sci. Emerg. Technol. 18, 120e125. Delgado, A.E., Rubiolo, A.C., 2005. Microstructural changes in strawberry after freezing and thawing processes. Food Sci. Technol. 38, 135e142. Delgado, A.E., Sun, D.W., 2011. Acoustic cavitation. In: Feng, H., Barbosa-Ca´novas, G.V., Weiss, J. (Eds.), In Ultrasound Technologies for Food and Bioprocessing. Dordrecht Heidelberg London, New York, pp. 2e3. Delgado, A.E., Zheng, L., Sun, D.W., 2009. Influence of ultrasound on freezing rate of immersion-frozen apples. Food Bioprocess Technol. 2, 263e270. Ferna´ndez, P.P., Otero, L., Guignon, B., Sanz, P.D., 2006. Highpressure shift freezing versus high-pressure assisted freezing:
effects on the microstructure of a food model. Food Hydrocolloid. 20, 510e522. Ferna´ndez-Leo´n, M.F., Ferna´ndez-Leo´n, A., Lozano, M., Ayuso, M., Gonza´lez-Go´mez, D., 2013. Altered commercial controlled atmosphere storage conditions for ‘Parhenon’broccoli plants (Brassica oleracea L. var. italica). Influence on the outer quality parameters and on the health-promoting compounds. LWT e Food Sci. Technol 50, 665e672. Gonc¸alves, E.M., Abreu, M., Branda˜o, T.R., Silva, C.L., 2011. Degradation kinetics of colour, vitamin C and drip loss in frozen broccoli (Brassica oleracea L. ssp. Italica) during storage at isothermal and non-isothermal conditions. Int. J. Refrigeration 34, 2136e2144. Go´ral, D., Kluza, F., 2009. Cutting test application to general assessment of vegetable texture changes caused by freezing. J. Food Eng. 95, 346e351. _ Coupland, J.N., 2008. Ultrasonic properties of Gu¨lseren, I., partially frozen sucrose solutions. J. Food Eng. 89, 330e335. Hu, S.Q., Liu, G., Li, L., Li, Z.X., Hou, Y., 2012. An improvement in the immersion freezing process for frozen dough via ultrasound irradiation. J. Food Eng. 114, 22e28. Inada, T., Zhang, X., Yabe, A., Kozawa, Y., 2001a. Active control of phase change from supercooled water to ice by ultrasonic vibration 1. Control of freezing temperature. Int. J. Heat. Mass Transf. 44, 4523e4531. Inada, A., Zhang, X., Yabe, A., Kozawa, Y., 2001b. Active control of phase change from supercooled water to ice by ultrasonic vibration 1. Control of freezing temperature. Int. J. Heat. Mass Transf. 44, 4523e4531. Kiani, H., Zhang, Z., Delgado, A., Sun, D.W., 2011. Ultrasound assisted nucleation of some liquid and solid model foods during freezing. Food Res. Int. 44, 2915e2921. Kiani, H., Sun, D.W., Delgado, A., Zhang, Z., 2012. Investigation of the effect of power ultrasound on the nucleation of water during freezing of agar gel samples in tubing vials. Ultrason. Sonochem. 19, 576e581. Kiani, H., Sun, D.W., Zhang, Z., 2013a. The effect of ultrasound irradiation on the convective heat transfer rate during immersion cooling of a stationary sphere. Ultrason. Sonochem. 19, 1238e1245. Kiani, H., Sun, D.W., Zhang, Z., 2013b. Effects of processing parameters on the convective heat transfer rate during ultrasound assisted low temperature immersion treatment of a stationary sphere. J. Food Eng. 115, 384e390. Lee, S., Pyrak-Nolte, L.J., Cornillon, P., Campanella, O., 2004. Characterisation of frozen orange juice by ultrasound and wavelet analysis. J. Sci. Food Agricul. 84, 405e410. Legay, M., Gondrexon, N., Le Person, S., Boldo, P., Bontemps, A., 2011. Enhancement of heat transfer by ultrasound: review and recent advances. Int. J. Chem. Eng. 2011, 1e17. Li, B., Sun, D.W., 2002. Effect of power ultrasound on freezing rate during immersion freezing of potatoes. J. Food Eng. 55, 277e282. McClements, D.J., Gunasekaran, S., 1997. Ultrasonic characterization of foods and drinks: principles, methods, and applications. Crit. Rev. Food Sci. 37, 1e46. Petzold, G., Aguilera, J.M., 2009. Ice morphology: fundamentals and technological applications in foods. Food Biophys. 4, 378e396. Pre´stamo, G., Palomares, L., Sanz, P., 2004. Broccoli (Brasica oleracea) treated under pressure-shift freezing process. Eur. Food Res. Technol. 219, 598e604. Saclier, M., Peczalski, R., Andrieu, J., 2010. A theoretical model for ice primary nucleation induced by acoustic cavitation. Ultrason. Sonochem. 17, 98e105. Sun, D.W., Li, B., 2003. Microstructural change of potato tissues frozen by ultrasound-assisted immersion freezing. J. Food Eng. 57, 337e345. Volkert, M., Puaud, M., Wille, H.-J., Knorr, D., 2012. Effects of high pressure-low temperature treatment on freezing behavior,
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 4 1 ( 2 0 1 4 ) 8 2 e9 1
sensorial properties and air cell distribution in sugar rich dairy based frozen food foam and emulsions. Innov. Food Sci. Emerg. Technol. 13, 75e85. Wang, L.L., Chen, Y.M., Li, Z.G., 2013. The effects of freezing on soybean microstructure and qualities of soymilk. J. Food Eng. 116, 1e6. Zhang, X., Inada, T., Yabe, A., Lu, S., Kozawa, Y., 2001. Active control of phase change from supercooled water to ice by ultrasonic vibration 2. Generation of ice slurries and effect of bubble nuclei. Int. J. Heat. Mass Transf. 44, 4533e4539.
91
Zhang, X., Inada, T., Tezuka, A., 2003. Ultrasonic-induced nucleation of ice in water containing air bubbles. Ultrason. Sonochem. 10, 71e76. Zhang, S.Z., Wang, H.Y., Chen, G.G., 2010. Addition of icenucleation active bacteria: Pseudomonas syringae pv. panici on freezing of solid model food. LWT e Food Sci. Technol. 43, 1414e1418. Zheng, L., Sun, D.W., 2006. Innovative applications of power ultrasound during food freezing processes-A review. Trends Food Sci. Technol. 17, 16e23.
Available online at www.sciencedirect.com
ScienceDirect w w w . i i fi i r . o r g
journal homepage: www.elsevier.com/locate/ijrefrig
The effects of ultrasound-assisted freezing on the freezing time and quality of broccoli (Brassica oleracea L. var. botrytis L.) during immersion freezing Ying Xin a, Min Zhang a,b,*, Benu Adhikari c a
State Key Laboratory of Food Science and Technology, Jiangnan University, 214122 Wuxi, Jiangsu, China School of Food Science and Technology, Jiangnan University, 214122 Wuxi, Jiangsu, China c School of Applied Sciences, RMIT University, City Campus, Melbourne, VIC 3001, Australia b
article info
abstract
Article history:
The application of ultrasound during immersion freezing of broccoli was studied and
Received 3 August 2013
particular attention was given to the effects on freezing time, microstructure, firmness and
Received in revised form
drip loss of broccoli. Broccoli florets were immersion frozen in an ultrasound-assisted freezer
26 December 2013
at two frequencies and four different power levels. The results showed that the total freezing
Accepted 28 December 2013
time and times required for pre-cooling, phase change and sub-cooling stages of broccoli
Available online 8 January 2014
were significantly reduced by the application of ultrasound-assisted freezing (UAF) at 150 (30 kHz) or 175 W (20 kHz) power level and with judicious combination of process parameters
Keywords:
(exposure time, ultrasound irradiation temperature and pulse mode). The microstructure
Ultrasound-assisted freezing
and the firmness of broccoli tissue were better preserved and the drip loss was significantly
Freezing time
reduced by the application of optimized UAF compared to the normal immersion freezing.
Microstructure
These findings indicate that there is a great potential of UAF in immersion freezing of food.
Drip loss
ª 2014 Elsevier Ltd and IIR. All rights reserved.
Firmness Broccoli
Effets de la conge´lation assiste´e par ultrasons sur la vitesse de conge´lation et la qualite´ de broccoli (Brassica oleracea L. var. botrytis L.) pendant la conge´lation par immersion Mots cle´s : Conge´lation assiste´e par ultrasons ; Temps de conge´lation ; Microstructure ; Pertes par e´coulements ; Fermete´ ; Brocoli
* Corresponding author. School of Food Science and Technology, Jiangnan University, 214122 Wuxi, China. Tel./fax: þ86 510 85877225. E-mail address: [email protected] (M. Zhang). 0140-7007/$ e see front matter ª 2014 Elsevier Ltd and IIR. All rights reserved. http://dx.doi.org/10.1016/j.ijrefrig.2013.12.016
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 4 1 ( 2 0 1 4 ) 8 2 e9 1
1.
Introduction
Freezing is one of the most successful methods for long term preservation of perishable food products. This method is also capable of preserving initial sensory characteristics of food products and ensures that the products remain suited for further processing. However, fresh food products having higher water content are more susceptible to the formation of large ice crystals during freezing. The large ice crystals present in the tissues of frozen foods cause mechanical damage and drip loss which results into poor product quality (Ferna´ndez et al., 2006; Wang et al., 2013). It has been shown that the quality of frozen food is closely related to the size and location of ice crystals, which in turn, largely depends on the freezing time or the rate of freezing (Petzold and Aguilera, 2009). Therefore, conscious efforts are made to shorten the freezing time, to increase the freezing rate and to reduce the size of the ice crystals in the frozen products. All of these can be achieved by applying the advanced methods of food freezing, such as high-pressure food freezing, application of ice nucleation active bacteria, dehydrofreezing and ultrasound-assisted freezing (UAF) (Comandini et al., 2013; Ando et al., 2012; Volkert et al., 2012; Zhang et al., 2010). Ultrasound has attracted great interest in recent years and its application in food freezing has shown promising
83
advantages. Power ultrasound has been proven to be effective in the initial nuclei formation process and also in the subsequent crystallization process during freezing (Li and Sun, 2002). The cavitation bubbles produced by ultrasound can serve as nuclei for ice crystal formation once they reach the critical nucleus size. In addition, the strong forces originating from the collapse of cavitation bubbles fragment the dendritic ice crystals into small size. These fragmented crystals then act as new nuclei (Chow et al., 2003, 2005; Saclier et al., 2010). The vigorous collapse of cavitation bubble creates local zone of high pressure (50 MPa or more) for very short time (nano-seconds), which results in high degrees of supercooling. The supercooling can act as a driving force for instantaneous nucleation (Inada et al., 2001a,b; Zhang et al., 2001). Furthermore, power ultrasound has been recognized to enhance convective heat transfer by acoustic cavitation or acoustic streaming (Legay et al., 2011). These two phenomena arise from the propagation of power ultrasonic waves into a fluid. The passage of power ultrasound waves causes macroscopic turbulence, increases the frequency of high speed collision of microscopic particles, reduces the thickness of solideliquid boundary layer and decreases the heat transfer resistance (Kiani et al., 2013a,b; Legay et al., 2011). Therefore, ultrasound-assisted freezing is able to decrease the freezing time, reduce the size of the
Fig. 1 e Schematic diagram and photograph of the ultrasound-assisted freezer.
84
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 4 1 ( 2 0 1 4 ) 8 2 e9 1
Table 1 e Ultrasound treatments at phase change and sub-cooling stages of broccoli-freezing process. Treatment Control P20-1 P20-2 P20-3 P20-4 P30-1 P30-2 P30-3 P30-4 Control S20-1 S20-2 S20-3 S30-1 S30-2 S30-3
Ultrasound application stage
Frequency (kHz)/ power (W)
Initial temperature at which the ultrasound was applied ( C)
Ultrasound pulse mode (s)
Exposure time (s)
Phase change
e 20/175
e 0 0 0.5 0.5 0 0 0.5 0.5 e 5 5 5 5 5 5
e 60/60 30/60 60/60 30/60 60/60 30/60 60/60 30/60 e 60/60 60/60 30/60 60/60 60/60 30/60
e 120 120 120 120 120 120 120 120 e 120 180 180 120 180 180
30/150
Sub-cooling
e 20/175
30/150
ice crystals, better maintain the cell structure and finally improve the quality of frozen food. The efficacy of ultrasound-assisted freezing depends on the frequency, power level, time of exposure to ultrasound and pulse mode (Comandini et al., 2013; Delgado et al., 2009; Hu et al., 2012; Li and Sun, 2002). It is commonly accepted that the higher the power of the ultrasound, the longer the exposure time, the stronger is the effect of sonication. However, the amount of heat generated due to the application of ultrasound can increase the temperature of the food and refrigerating medium and which reduces the freezing rate (Delgado et al., 2009; Hu et al., 2012; Li and Sun, 2002). Furthermore, the inherent characteristics of food also affect the efficacy of ultrasound-assisted freezing (Hu et al., 2012; Lee et al., 2004; McClements and Gunasekaran, 1997). Although research have been undertaken to study the effect of ultrasoundassisted freezing on a number of food materials such as potato, apple, dough, sucrose solution and agar gel (Comandini et al., 2013; Delgado et al., 2009; Hu et al., 2012; Kiani et al., 2011, 2012; Li and Sun, 2002), the application of ultrasoundassisted freezing on broccoli has not been reported. In this study, we have applied the UAF for freezing of broccoli. The main objective of this work was to quantify the effect of ultrasound frequency and power level on immersion freezing of broccoli, with particular attention to their effect on the freezing time. In order to obtain an appropriate protocol for UAF, ultrasound was applied at different frequencies, different power levels, different exposure times and different pulse modes both at phase change and sub-cooling stages. The microstructure, firmness and drip loss of broccoli when frozen using normal immersion freezing and optimized UAF were also measured in order to assess the effects of UAF on the quality of broccoli.
2.
Materials and methods
2.1.
Materials
Fresh broccoli (Brassica oleracea L. var. botrytis L.) was obtained directly from cultivated area in Wuxi (Jiangsu, China). Broccoli
inflorescences were divided into small florets of about 5 0.5 cm in diameter, cut 2 0.5 cm below the lowest ramification. Then, broccoli florets were blanched in a thermostated water bath (HH-1, Aohua Instrument Inc., Changzhou, China) at 95 1 C for 3 min. After blanching, samples were kept in a refrigerator at a temperature of 8.5 0.5 C for 12 h to achieve uniform initial temperature.
2.2.
Experimental set up
2.2.1.
The ultrasound-assisted freezing system
An ultrasound-assisted freezer (Hechuang Co., China), which operates at 20 and 30 kHz frequency and dissipated powers from 125 to 190 W was used for broccoli immersion freezing (Fig. 1). The coolant (40% calcium chloride solution, 7500 cm3) was cooled at 25 C and was circulated through the immersion cooling bath (25 20 20 cm) (Fig. 1-D) by a refrigeration cycle system (Fig. 1-C). The flow rate of the coolant used for freezing was set to1 L min1. The samples (Fig. 1-H) were suspended into the circulating coolant and the temperature of the samples was recorded by T-type thermocouples (Fig. 1-I) connected to a data acquisition system (Fig. 1-J). The temperature variation at the geometric center of sample was monitored and recorded. The power output from the generator (Fig. 1-F) to the transducers (Fig. 1-E) could be adjusted by the control system of ultrasound (Fig. 1-G) when the UAF was used. And two refrigeration systems (Fig. 1-A, B) were used for cooling.
2.2.2.
Immersion freezing process
Broccoli samples (90.5% w/w moisture content) were removed from the refrigerator and immediately immersed into the coolant (25.0 0.5 C). Although the ultrasound transducers are evenly located at the bottom of the immersion cooling bath, the intensity of the ultrasound waves delivered to the medium is different at different positions in the tank (Kiani et al., 2011). Therefore, for each experimental trial, samples were kept at the same position (10 cm from the bottom of the immersion cooling bath) and frozen using the ultrasoundassisted freezer. Samples were exposed to the ultrasound waves with the following conditions: 20 or 30 kHz frequency, 60 s on/60 s off duty cycle, and 0, 125, 150, 175 or 190 W power
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 4 1 ( 2 0 1 4 ) 8 2 e9 1
levels during these freezing tests. In order to study the effect of different ultrasound parameters on the freezing of broccoli, ultrasound was applied at different moments of the freezing process, for different exposure times and different pulse mode during the phase change and sub-cooling stages (Table 1). The freezing process ended as soon as the temperature of the sample reached 18 C. The temperature of sample during freezing was monitored by using T-type thermocouple at the geometric center of the sample. Temperatures of the samples and the coolant were measured and recorded at 5 s interval. These temperature data were used to generate the freezing curves and to calculate the freezing time. After freezing, each sample was placed into a double high-density polyethylene bag and stored at 18 1 C. All experiments were performed in triplicate and average values are reported.
2.3.
Quality analysis
2.3.1.
Observation of the microstructure
After two weeks of frozen storage, the frozen broccoli was freeze-dried in a freeze-dryer (Model 77530-30, Labconco Co., USA) to prepare sample for scanning electron microscopic (SEM) analysis (Model S-4800, Hitachi Co., Japan). The surface of the young stem of broccoli was sampled, and its bottom was fixed onto an aluminum disc with conductive, double-sided adhesive tape. Gold preprocessing was performed under vacuum conditions prior to observing under the SEM. The SEM images of the samples were digitally captured at different locations at 150, 500 and 1000 magnification.
2.3.2.
85
(ANOVA) was performed to determine the effects of different freezing treatments on freezing time, drip loss and the firmness of broccoli. The comparison among the mean values was performed using Duncan’s test to examine differences between different freezing processes. Comparisons that yielded P < 0.05 were considered significant.
3.
Results and discussion
3.1. The effect of ultrasound frequency and power level on immersion freezing of broccoli The freezing curves and total freezing times for normal immersion freezing and UAF are presented in Figs. 2 and 3. It can be seen from figures that the different treatments of UAF had different effect on the total freezing time. The total time here is defined as the time required for the broccoli sample to reach 18 C from an initial temperature of 8.5 C. The total freezing times for UAF at ultrasound power levels of 125 (30 kHz), 150 (20 and 30 kHz) and 175 W (20 kHz) were shorter than that of normal immersion freezing (P < 0.05), and the results showed clearly that broccoli-freezing was improved by ultrasound irradiation. Compared with the normal immersion freezing,
Measurement of drip loss
After two weeks of frozen storage, samples were thawed at 4 1 C in a refrigerator for 10 h and drip loss (DL) was calculated using the method suggested by Gonc¸alves et al. (2011) using Eq. (1) given below: DLð%Þ ¼
Mt Mo 100 Mi
(1)
where Mi (g) is the mass of the sample before thawing, M0 (g) is the mass of dry blotting paper, Mt (g) is the weight of wet blotting paper with exuded liquid. Four replications were carried out for each test.
2.3.3.
Measurement of firmness
Firmness was measured using a method suggested by Ferna´ndez-Leo´n et al. (2013) with some modification. The TAXT2i Texture Analyzer (Stable Micro Systems Ltd., Vienna 153 Court, Surrey, UK) was used for this purpose in compression mode. The whole broccoli heads were used as sample in these tests. The force was applied to produce a 5% deformation by a 100 mm flat aluminum probe. Force/deformation curves were recorded using the computer program associated with the texture analyzer and the maximum force (N) was calculated and used as the measure of the firmness. Ten replications were carried out for each test.
2.4.
Statistical analysis
The data were analyzed with SPSS software version 15.0 for Windows (SPSS Inc., Chicago, IL, US). Analysis of variance
Fig. 2 e The freezing curves of broccoli during different ultrasound-assisted freezing (UAF) processes and normal immersion freezing process.
86
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 4 1 ( 2 0 1 4 ) 8 2 e9 1
Fig. 3 e The total freezing times for broccoli under ultrasound-assisted freezing at different frequencies and power levels.
the UAF at ultrasound power levels of 125 (20 kHz), 175 (30 kHz) and 190 W (20 and 30 kHz) could significantly increase the total freezing time of broccoli (P < 0.05). The shorter freezing times were observed at relatively lower ultrasound power levels (except 125 W at 20 kHz). In contrast, longer freezing times were required when UAF is used at relatively high power levels. This observation indicates that relatively higher freezing rates can be achieved at relatively low ultrasound power levels. In addition, the ultrasound treatment also generated heat during the freezing process. The heat generated by ultrasound at lower power levels did not negatively impacted the freezing rate. However, when at relatively high power levels were used, the heating effect was found to dominate the freezing process, leading to significant increase in the freezing time. Regarding the effect of ultrasound frequency, at relatively low power levels (especially at 125 W), the freezing time for UAF was found to be significantly (P < 0.05) shorter at 30 kHz than at 20 kHz. This might be due to the fact that the ultrasound generates greater acoustic streaming at higher frequencies at the same power intensity (Delgado and Sun, 2011). The acoustic streaming increases the heat transfer rate which results into a shorter freezing time (Zheng and Sun, 2006). However, at relatively high power levels (especially at 175 W), the freezing time at 30 kHz was significantly (P < 0.05) longer than at 20 kHz. The shorter freezing time at 20 kHz can be attributed to the fact that it is much easier to generate greater cavitation at lower frequencies. At low frequencies, the bubbles produced by the ultrasound are bigger in size. When these bigger bubbles collapse, higher amount of energy is released and increase the effect of acoustic cavitation (Delgado and Sun, 2011). In fact, total freezing time does not accurately reflect the effect of ultrasound irradiation on freezing process of broccoli. As can be seen from Fig. 2, each freezing curve was divided into three stages: pre-cooling stage (8.5e0 C), phase change stage (0 to 5 C) and sub-cooling stage (5 to 18 C). It can be observed from Fig. 2 that the dependence of the freezing time on ultrasound frequency and power level is different for each stage. In order to better evaluate the impact of ultrasound frequency and power level on the immersion freezing of
broccoli, the freezing time of each stage of the freezing process is presented in Fig. 4. In the pre-cooling stage, UAF has a significant effect in shortening the freezing time at 150 (20 and 30 kHz) and 175 W (20 kHz) power levels (P < 0.05). However, at 125 W (20 kHz) and 190 W (30 kHz) power levels, the freezing efficiency was found to decrease. It is known that a rapid temperature decrease occurs at the pre-cooling stage due to the removal of sensible heat. When power ultrasound is employed, the removal of sensible heat is accelerated due to the presence of acoustic cavitation and acoustic streaming. This is because both the cavitation and acoustic streaming are known to cause macroscopic turbulent flow and high speed collisions of microscopic particles which reduce the heat transfer resistance by decreasing the thickness of boundary layer at solidewater interface (Kiani et al., 2013a,b; Legay et al., 2011). However, the thermal effect of ultrasound cannot be neglected, because it hinders the removal of heat from the broccoli sample. Broccoli is compositionally heterogeneous material and contains water and many solid compounds. The generation of heat in the broccoli was caused by the absorption and dissipation of ultrasound energy due to heterogeneity of the sample. And the more heterogeneous the material is the greater heat would be generated (Hu et al., 2012; Lee et al., 2004; McClements and Gunasekaran, 1997). In this research, at 150 (20 and 30 kHz) and 175 W (20 kHz) power level, the effect of increased rate of heat transfer was greater than the effect of heat generation. Because of this reason, the freezing
Fig. 4 e The time used up at each freezing stage at different ultrasound frequencies and power levels.
87
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 4 1 ( 2 0 1 4 ) 8 2 e9 1
time was shorter than that of the normal immersion freezing. In contrast, the effect of increase in the rate of heat transfer less than the effect of heat generation at 125 (20 kHz) and 190 W (30 kHz) power level. As a result, the freezing time of pre-cooling stage was longer than that of normal immersion freezing. A similar effect of ultrasound on the freezing time during the pre-cooling stage of dough was reported by Hu et al. (2012). These authors reported that at 288 and 360 W (25 kHz) power level the freezing time of the pre-cooling stage decreased compared to the corresponding freezing time for conventional freezing. This decrease in the cooling time at the pre-cooling stage was attributed to the dominance of heat transfer enhancement inside the dough. But at 175, 224, and 418 W (25 kHz) power level, the freezing efficiency was depressed due to the domination of heat generation (Hu et al., 2012). In the phase change period, a large amount of latent heat generated from water transformation into ice should be removed. Most of the freezable water crystallizes in this stage. Achieving appropriately short freezing time in this stage is crucial to obtain frozen food products of high quality (Delgado and Rubiolo, 2005). As can be seen from Fig. 4, UAF has significantly shortened the time required in this phase change stage at 175 (20 kHz) and 150 W (30 kHz) power level (P < 0.05). The reduction of freezing time in this stage is achieved primarily due to inducing better nucleation by ultrasound. During this phase change stage, the ultrasound cavitation bubble provides a favorable condition for heterogeneous primary nucleation of ice. The shock waves and micro-jets also promote the secondary nucleation by breaking up the preexisting dendritic ice crystals into smaller nuclei (Chow et al., 2003, 2005; Saclier et al., 2010). However, when the frozen broccoli was exposed to low (125 W at 20 kHz and 30 kHz) or high power ultrasound (175, 190 W at 30 kHz and 190 W at 20 kHz), the ultrasound was not effective enough to enhance the nucleation or unacceptably high amount of heat was generated inside of the broccoli. Because of this reason, the freezing time of phase change stage was significantly (P < 0.05) increased. After the phase change stage, since there is no need to remove the latent heat, the apparent specific heat decreases and the thermal conductivity increases all of these facilitate faster removal of the sensible heat. Because of these reasons, temperature of broccoli sample starts to decrease rapidly once again. Although the UAF at 175 (20 kHz) and 150 W (30 kHz) power levels were particularly suited in decreasing the freezing time during pre-cooling and phase change stages, the ultrasound treatment required more freezing time at subcooling stage of freezing of broccoli. The longer freezing time at sub-cooling stage can be explained from the fact that
most of the moisture within the sample was already frozen and the acoustic characteristics of this sample at sub-cooling stage were different compared to its acoustic characteristics at pre-cooling and phase change stages. The ice crystals within the broccoli or the grain boundaries among crystals are capable of scattering the ultrasound energy, which decreases the rate of heat transfer (Gu¨lseren and Coupland, 2008). As explained above, less heat transfer enhancement and heat generation increased by the ultrasound at this sub-cooling stage which increased the freezing time.
3.2. Effect of ultrasound parameters on the freezing time during phase change and sub-cooling stages The phase change stage is very important in the freezing process. The time taken for the temperature of a food item to pass through this stage determines both the number and the size of ice crystals. Minimizing the elapsed time of the phase change period contributes to optimizing the quality of frozen food products (Delgado and Rubiolo, 2005; Sun and Li, 2003). It has been reported that the UAF could improve the food-freezing process by shortening the time required in the phase change stage. The effectiveness of ultrasound-assisted freezing process depends on the power of the ultrasound, exposure time, the initial temperature for irradiation and pulse mode (Comandini et al., 2013; Delgado et al., 2009; Hu et al., 2012; Li and Sun, 2002; Zhang et al., 2001, 2003). We have shown in preceding sections that UAF at 175 (20 kHz) and 150 W (30 kHz) power levels decreased the time spent in the pre-cooling and phase change stages; however, it increased the freezing time at sub-cooling stage. In order to optimize ultrasound-assisted broccoli-freezing process, different ultrasound parameters were studied during the phase change and sub-cooling stages (Tables 2and 3). During the phase change stage, the initial temperature at which the ultrasound power was applied was selected to be close to the nucleation temperature. This temperature was experimentally determined from the freezing curve and was found to be around 1.1 C for broccoli. During the sub-cooling stage it was found that the closer the temperature of broccoli to the cooling medium the greater the (negative) heating effect due to the application of the ultrasound. This resulted into significant decrease in the freezing rate (Fig. 2). Therefore, the initial temperature at which the ultrasound power was applied was fixed at 5 C. Table 2 shows the average freezing times when broccoli samples were frozen using normal immersion (control) freezing and when ultrasound was applied during the phase change stage. The freezing time at the phase change stage was significantly decreased (P < 0.05) when treatments P20-1, P202 and P30-4 were applied (the detailed conditions of P20-1,
Table 2 e Freezing times at the phase change stage of ultrasound-assisted freezing process.a Treatment Freezing time (s) Treatment Freezing time (s) a
Control
P20-1
P20-2
P20-3
P20-4
295 10.0d e e
205 18.0a P30-1 290 18.0d
240 10.0b P30-2 255 13.2bc
255 15.0bc P30-3 275 8.7cd
275 13.2cd P30-4 235 18.0b
Values of different treatments having different letters in superscript are significantly different (P < 0.05). Results are mean standard deviation (n ¼ 3).
88
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 4 1 ( 2 0 1 4 ) 8 2 e9 1
Table 3 e Freezing times for sub-cooling stage of ultrasound-assisted freezing process.a Treatment Freezing time (s)
Control
S20-1
S20-2
S20-3
S30-1
S30-2
S30-3
325 8.7c
315 18.0c
225 13.2a
275 13.2bc
300 18.0c
310 10.0c
250 13.2b
a
Values having different letters in superscript in the same line are significantly different (P < 0.05). Results are mean standard deviation (n ¼ 3).
P20-2 and P30-4 are shown in Table 1). Treatments P20-1, P20-2 and P30-4 with freezing times of 205, 240 and 235 s, respectively, which resulted in an increase in the freezing efficiency of about 18.6e30.5%. The results were similar to those earlier reported by Comandini et al. (2013) and Delgado et al. (2009). These authors also observed that ultrasound could change the nucleation temperature during freezing of potato (Comandini et al., 2013) and apple (Delgado et al., 2009). We did not observe the effect of ultrasound on the nucleation temperature most probably due to the fast freezing rate. Hence, further investigations are required to verify the effect of ultrasound on the nucleation temperature. During sub-cooling stage, the ultrasonic attenuation was increased due to the freezing of most of the available water (Gu¨lseren and Coupland, 2008). The loss of ultrasonic energy occurs during the sub-cooling stage can be due to the increase in the concentration of scattering objects, i.e., the ice crystals. The energy of ultrasound could be scattered by the crystals or the crystal grain boundaries. When the freezing effect (due to enhanced heat transfer) does not overcome the negative effect due to the ultrasound attenuation, the freezing time at subcooling stage increases. It can be seen from Table 3 that the minimum freezing time required for the sub-cooling stage was 225 and 250 s when treatments S20-2 and S30-3 were applied (the detailed conditions of S20-2 and S30-3 are shown in Table 1). These results show that during the ultrasound-assisted broccoli-freezing process, freezing time depends on the applied ultrasound parameters. Taking all factors into consideration and the optimal design of parameters for the UAF of broccoli is presented in Table 4. As can be seen from this table the total freezing time when these optimized protocols are used is shorter than the freezing time when normal immersion freezing (730 s) is used. When the protocols UAF-I (20 kHz/175 W) and UAF-II (30 kHz/150 W) were used, the total processing time decreased by 29.5% and 22.6% (P < 0.05), respectively, compared to the time required in the normal immersion freezing.
3.3.
Microstructure of frozen-thawing broccoli
Fig. 5 shows the microstructure of broccoli stem under SEM. It has been reported that the extent of cryo-damage to plant tissue depends on the distribution and the size of ice crystals both of which are governed by the freezing time or freezing rate (Li and Sun, 2002; Pre´stamo et al., 2004). Fast freezing rates produce small crystals evenly distributed throughout the tissue and maintain the cell turgor, while slow freezing rates generally produce large ice crystals exclusively in extracellular areas and damage the structure of cell (Delgado et al., 2009). The SEM micrographs show that the tissue of
fresh broccoli stem visualized in the transverse section is composed of uniformly distributed oval cells. The cell walls of these cells are intact and there is no sign of damage (Fig. 5A). A comparison of SEM micrograph of the fresh tissue with the SEM micrograph of sample frozen under normal immersion freezing shows that the latter had suffered a distinct damage of the structure (Fig. 5B). The cells of the sample subjected to the normal immersion freezing appear to be collapsed and the shrinkage of the tissue occurred as a result of long freezing time. This damage to the cellular structure can be attributed to the formation of extracellular crystal matrix and large ice crystals (Go´ral and Kluza, 2009). Fig. 5 panels C and D show SEM micrographs of broccoli frozen using UAF (UAF-I and UAF-II). The cells of these samples appear slightly fractured and irregular. This may be due to the fact that UAF produces a large number of fine crystals inside and outside the cells. When the extent of intracellular and extracellular ice crystals is very high, cells can become irregular and get fractured as shown in Fig. 5 panels C and D. In spite of this, the microstructure of broccoli frozen using UAF is better preserved than the microstructure of broccoli frozen using normal immersion freezing. As can be seen from Fig. 5 panels B, C and D, the microstructure of broccoli tissues subjected to both UAF-I and UAF-II has less distortion of cells than the microstructure of samples subjected to normal immersion freezing. These observations suggest that the UAF reduces the freezing time and favors the formation of smaller ice crystals inside and outside the cells compared to normal immersion freezing. As a result, the structural integrity is better preserved.
Table 4 e The optimal combination of parameters for ultrasound-assisted freezing of broccoli. Ultrasound application stage Pre-cooling
Phase change
Sub-cooling Total freezing time (s)
Ultrasonic parameters Ultrasound application/ interval (s) Exposure time (s) Initial temperature for application ( C) Ultrasound pulse mode (s) Exposure time (s) Initial temperature for application ( C) Ultrasound pulse mode (s) Exposure time (s)
UAF-I (20 kHz/175 W)
UAF-II (30 kHz/ 150 W)
60/60
60/60
60 0 60/60 120
60 0.5 30/60 120
5 60/60
5 30/60
180
180
515 22.9
565 13.2
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 4 1 ( 2 0 1 4 ) 8 2 e9 1
89
Fig. 5 e SEM microphotographs (1503, 5003 and 10003 magnification) of the fracture surface of the broccoli stems.
3.4.
Drip loss of frozen-thawing broccoli
Fig. 6 shows the drip losses of broccoli samples subjected to normal immersion freezing and UAF. As can be seen from this figure, the broccoli sample frozen using normal immersion freezing had the highest drip losses. The water holding capacity of the sample frozen using normal immersion freezing was poor due to the damage to the microstructure of the broccoli (Fig. 5B). The microstructure of the samples frozen using UAF was much better preserved (Fig. 5C and D) as a result the drip loss was also considerably low. Slightly higher drip loss was observed in samples subjected to UAF-II than in samples in samples subjected to UAF-I, which can be attributed to the greater distortion of cells (Fig. 5D) and longer freezing time (Table 4) in the UAF-II samples.
3.5.
firmness of samples frozen using UAF was significantly higher (P < 0.05) compared to that of the samples frozen using normal immersion freezing. Moreover, as seen in Fig. 6, the drip loss and firmness results, the judges rated the quality of UAF samples was better than normal immersion freezing.
Textural firmness of frozen-thawing broccoli
Fig. 6 also presents the firmness of the broccoli. As can be seen from this figure, the samples treaded with normal immersion freezing have the lowest firmness. The higher extent of damage of microstructure of broccoli treated with normal immersion freezing (Fig. 5B) decreased the cell turgor and the firmness. The samples frozen using UAF showed softer texture compared to that of fresh broccoli due to the formation of ice in the tissues. It is worth noting that the
Fig. 6 e The drip loss and firmness of thawed broccoli samples that had been frozen with normal immersion freezing, UAF-I and UAF-II.
90
4.
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 4 1 ( 2 0 1 4 ) 8 2 e9 1
Conclusions
Similar to conventional food-freezing processes, the UAF process could be divided into three stages named as precooling, phase change and sub-cooling stage. With the enhancement of heat transfer by ultrasonic cavitation the total freezing time and times required at pre-cooling, phase change and sub-cooling stages of broccoli were significantly reduced by the application of ultrasound-assisted freezing (UAF) at 150 (30 kHz) or 175 W (20 kHz) power level and with judicious combination of process parameters (exposure time, ultrasound irradiation temperature and pulse mode). The results showed that select the appropriate conditions during the ultrasound-assisted freezing broccoli are very important. Compared to normal immersion freezing, the microstructure and the textural firmness of the broccoli tissue were better preserved and the drip loss was minimized by the optimal application of ultrasound-assisted freezing of broccoli. Although this work only shows a qualitative vision of the influence of ultrasound application in freezing process, the results provided useful information for the application of UAF in food processing in the near future.
Acknowledgments The authors gratefully acknowledge the financial support provided by China National Natural Science Foundation to carry out this project (Contract No. 21176104).
references
Ando, H., Kajiwara, K., Oshita, S., Suzuki, T., 2012. The effect of osmotic dehydrofreezing on the role of the cell membrane in carrot texture softening after freeze-thawing. J. Food Eng. 108, 473e479. Chow, R., Blindt, R., Chivers, R., Povey, M., 2003. The sonocrystallisation of ice in sucrose solutions: primary and secondary nucleation. Ultrasonics 41, 595e604. Chow, R., Blindt, R., Chivers, R., Povey, M., 2005. A study on the primary and secondary nucleation of ice by power ultrasound. Ultrasonics 43, 227e230. Comandini, P., Blanda, G., Soto-Caballero, M., Sala, V., Tylewicz, U., Mujica-Paz, H., Fragoso, A.V., Toschi, T.G., 2013. Effects of power ultrasound on immersion freezing parameters of potatoes. Innov. Food Sci. Emerg. Technol. 18, 120e125. Delgado, A.E., Rubiolo, A.C., 2005. Microstructural changes in strawberry after freezing and thawing processes. Food Sci. Technol. 38, 135e142. Delgado, A.E., Sun, D.W., 2011. Acoustic cavitation. In: Feng, H., Barbosa-Ca´novas, G.V., Weiss, J. (Eds.), In Ultrasound Technologies for Food and Bioprocessing. Dordrecht Heidelberg London, New York, pp. 2e3. Delgado, A.E., Zheng, L., Sun, D.W., 2009. Influence of ultrasound on freezing rate of immersion-frozen apples. Food Bioprocess Technol. 2, 263e270. Ferna´ndez, P.P., Otero, L., Guignon, B., Sanz, P.D., 2006. Highpressure shift freezing versus high-pressure assisted freezing:
effects on the microstructure of a food model. Food Hydrocolloid. 20, 510e522. Ferna´ndez-Leo´n, M.F., Ferna´ndez-Leo´n, A., Lozano, M., Ayuso, M., Gonza´lez-Go´mez, D., 2013. Altered commercial controlled atmosphere storage conditions for ‘Parhenon’broccoli plants (Brassica oleracea L. var. italica). Influence on the outer quality parameters and on the health-promoting compounds. LWT e Food Sci. Technol 50, 665e672. Gonc¸alves, E.M., Abreu, M., Branda˜o, T.R., Silva, C.L., 2011. Degradation kinetics of colour, vitamin C and drip loss in frozen broccoli (Brassica oleracea L. ssp. Italica) during storage at isothermal and non-isothermal conditions. Int. J. Refrigeration 34, 2136e2144. Go´ral, D., Kluza, F., 2009. Cutting test application to general assessment of vegetable texture changes caused by freezing. J. Food Eng. 95, 346e351. _ Coupland, J.N., 2008. Ultrasonic properties of Gu¨lseren, I., partially frozen sucrose solutions. J. Food Eng. 89, 330e335. Hu, S.Q., Liu, G., Li, L., Li, Z.X., Hou, Y., 2012. An improvement in the immersion freezing process for frozen dough via ultrasound irradiation. J. Food Eng. 114, 22e28. Inada, T., Zhang, X., Yabe, A., Kozawa, Y., 2001a. Active control of phase change from supercooled water to ice by ultrasonic vibration 1. Control of freezing temperature. Int. J. Heat. Mass Transf. 44, 4523e4531. Inada, A., Zhang, X., Yabe, A., Kozawa, Y., 2001b. Active control of phase change from supercooled water to ice by ultrasonic vibration 1. Control of freezing temperature. Int. J. Heat. Mass Transf. 44, 4523e4531. Kiani, H., Zhang, Z., Delgado, A., Sun, D.W., 2011. Ultrasound assisted nucleation of some liquid and solid model foods during freezing. Food Res. Int. 44, 2915e2921. Kiani, H., Sun, D.W., Delgado, A., Zhang, Z., 2012. Investigation of the effect of power ultrasound on the nucleation of water during freezing of agar gel samples in tubing vials. Ultrason. Sonochem. 19, 576e581. Kiani, H., Sun, D.W., Zhang, Z., 2013a. The effect of ultrasound irradiation on the convective heat transfer rate during immersion cooling of a stationary sphere. Ultrason. Sonochem. 19, 1238e1245. Kiani, H., Sun, D.W., Zhang, Z., 2013b. Effects of processing parameters on the convective heat transfer rate during ultrasound assisted low temperature immersion treatment of a stationary sphere. J. Food Eng. 115, 384e390. Lee, S., Pyrak-Nolte, L.J., Cornillon, P., Campanella, O., 2004. Characterisation of frozen orange juice by ultrasound and wavelet analysis. J. Sci. Food Agricul. 84, 405e410. Legay, M., Gondrexon, N., Le Person, S., Boldo, P., Bontemps, A., 2011. Enhancement of heat transfer by ultrasound: review and recent advances. Int. J. Chem. Eng. 2011, 1e17. Li, B., Sun, D.W., 2002. Effect of power ultrasound on freezing rate during immersion freezing of potatoes. J. Food Eng. 55, 277e282. McClements, D.J., Gunasekaran, S., 1997. Ultrasonic characterization of foods and drinks: principles, methods, and applications. Crit. Rev. Food Sci. 37, 1e46. Petzold, G., Aguilera, J.M., 2009. Ice morphology: fundamentals and technological applications in foods. Food Biophys. 4, 378e396. Pre´stamo, G., Palomares, L., Sanz, P., 2004. Broccoli (Brasica oleracea) treated under pressure-shift freezing process. Eur. Food Res. Technol. 219, 598e604. Saclier, M., Peczalski, R., Andrieu, J., 2010. A theoretical model for ice primary nucleation induced by acoustic cavitation. Ultrason. Sonochem. 17, 98e105. Sun, D.W., Li, B., 2003. Microstructural change of potato tissues frozen by ultrasound-assisted immersion freezing. J. Food Eng. 57, 337e345. Volkert, M., Puaud, M., Wille, H.-J., Knorr, D., 2012. Effects of high pressure-low temperature treatment on freezing behavior,
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 4 1 ( 2 0 1 4 ) 8 2 e9 1
sensorial properties and air cell distribution in sugar rich dairy based frozen food foam and emulsions. Innov. Food Sci. Emerg. Technol. 13, 75e85. Wang, L.L., Chen, Y.M., Li, Z.G., 2013. The effects of freezing on soybean microstructure and qualities of soymilk. J. Food Eng. 116, 1e6. Zhang, X., Inada, T., Yabe, A., Lu, S., Kozawa, Y., 2001. Active control of phase change from supercooled water to ice by ultrasonic vibration 2. Generation of ice slurries and effect of bubble nuclei. Int. J. Heat. Mass Transf. 44, 4533e4539.
91
Zhang, X., Inada, T., Tezuka, A., 2003. Ultrasonic-induced nucleation of ice in water containing air bubbles. Ultrason. Sonochem. 10, 71e76. Zhang, S.Z., Wang, H.Y., Chen, G.G., 2010. Addition of icenucleation active bacteria: Pseudomonas syringae pv. panici on freezing of solid model food. LWT e Food Sci. Technol. 43, 1414e1418. Zheng, L., Sun, D.W., 2006. Innovative applications of power ultrasound during food freezing processes-A review. Trends Food Sci. Technol. 17, 16e23.