Vacuum Freezing Properties of Blanched Apple Slices

WANG Haiou, FU Qingquan, CHEN Shoujiang, WANG Rongrong, ZHANG Wei, YANG Ping

(School of Food Science, Nanjing Xiaozhuang University, Nanjing 211171, China)

Abstract:The vacuum freezing properties of blanched apple slices were investigated by comparing with traditional refrigerator freezing. Results showed that after 40 min vacuum freezing, a mass loss of 27.5% and the lowest frozen temperature of -27.6 ℃ were achieved in apple slices pretreated by blanching, while a mass loss of 22.9% and the lowest frozen temperature of -26.5 ℃ were obtained in non-blanched apple slices. The total process of vacuum freezing was divided into low pressure flash evaporation stage, ice crystal formation stage and deep freezing stage. Vacuum freezing caused less microstructural changes and less signif cant cell disruption and contributed to smaller thawing loss and weaker relative electrical conductivity in frozen-thawed apple slices in contrast to refrigerator freezing. Blanching pretreatment before freezing caused more severe damage to cell microstructure in apple slices than non-blanching pretreatment, resulting in a signif cant increase in thawing loss and relative electrical conductivity in frozen-thawed apple slices.

Key words:vacuum freezing; blanching pretreatment; apple slices; micro-structure

Freezing has become one of the most important unit operations in food processing and preservation [1]. And it is the necessary process for food vacuum freeze-drying [2]. Usually, freezing consists of three stages [3-4]: precooling stage, phase transition stage and tempering stage, during which the sensible and latent heat from food product are removed by the traditional refrigerator cooling system. As a very effective and clean cooling technology, vacuum cooling is characterized by the rapid evaporation of the water in the product itself, and quick removal of the heat contained in the product [5-7]. It was widely used in the processing of fruits, vegetables, meat, fish, sauces, soups, bakery, and ready meals [8-9].

The vacuum cooling turns into vacuum freezing if the vapor pressure in the vacuum chamber drop below 0.6 kPa,i.e., the saturation pressure of water at 0 ℃. So vacuum freezing is known as a new special freezing technique that is attracting more and more researchers’ interests. And some studies on vacuum freezing of food have been reported recently. Cogné et al. [10]developed a numerical simulation to model heat and mass transfer during vacuum freezing of puree droplet. Chen et al. [11]studied the morphological changes of water during vacuum cooling and vacuum freezing, finding that the liquid water inside the vessel in a vacuum cooling system was frozen into two layers: irregular porous layer on the top and dense layer on the bottom. In order to simplify the food freeze-drying process and shorten the drying time, the author proposed a novel freeze-drying technology in which the traditional air-freezing or plate freezing process of food product was replaced by vacuum freezing conducted in the same vacuum freeze dryer [12]. And some laboratory experiments were performed on fruits and vegetables with this novel freeze-drying technology. Zhang Haifeng et al. [13]analyzed the process characteristics of water vaporization and solidization, the organizational structure and mass loss during the vacuum freezing of fresh mutton.

In the freezing or drying process of fruits and vegetables, blanching pretreatment is generally applied in order to undermine and suppress the enzyme activity, prevent nutrients loss, retain original colors and taste, reduce bacterial contamination, and so on [14-19]. There is few report on the vacuum freezing properties of fruits and vegetables with blanching pretreatment. The main objectives of the current study are to evaluate the vacuum freezing properties of blanching-treated apple slices in contrast with traditional refrigerator freezing method, including mass loss, temperature variation, thawing loss, relative electrical conductivity and micro-structure of the tissue in the apple materials. Due to the fact that there have been lots of reports about the influence of blanching pretreatment on the sensory and nutritional qualities of apples [14-19], so these quality indices were not involved in this experiments. This study aimed to provide basic knowledge for improving the effect of vacuum freezing in fruits and vegetables and promoting the practical application of this novel freezing technique.

1 Materials and Methodds

1.1 Materials

Fresh Fuji apples were obtained from a farm located in Yantai, Shangdong, China. The apples were selected according to apparent color and size. Then, the selected apples were washed, pitted and cut into 3 cm × 3 cm × 0.5 cm slices. The initial moisture content of these apple slices was (86.34 ± 0.52)% (m f). Some apple slices were blanched for 1 min in 95 ℃ distilled water heated by an induction cooking plate and then cooled quickly to room temperature using cold distilled water according to the method of Wang Yuchuan et al. [16]. The blanching parameters were chosen on the basis of the literatures [17-19]. And it was also verified by the pre-experiments that the chosen blanching parameters can suppress the enzyme activity and retain good colors for the fresh slices. The non-blanched apple slices were taken as the control group.

1.2 Instruments and equipment

50F vacuum freeze dryer Ningbo Scientz Biotechnology Co. Ltd.; BCD-182DTB freeze refrigerator Hefei Meiling Co. Ltd.; YNK/TH-50 constant temperature and humidity chamber Suzhou Unique Environmental Test Equipment Co.; FE38-Standard conductivity meter Mettler-Toledo Instruments Co. Ltd.; HNY-1102C shaker Nuoji Instrument Co. Ltd..

1.3 Methods

1.3.1 Vacuum freezing and refrigerator freezing

A laboratory-scale vacuum freeze dryer and a freeze refrigeratorwere used for the freezing operation of apple samples which were divided into 4 groups (group A, B, C, D). Apple slices of group A and group B were selected from the blanched slices, and those of group C and group D from the non-blanched slices. Group A and group C were performed vacuum freezing operation for 40 min in the vacuum freeze dryer. The refrigeration units of the freeze dryer were switched on half an hour earlier to reduce the cold trap temperature below -50 ℃, then apple slices of group A and group C were put into the freeze dryer at the same batch ensuring the two groups’ vacuum freezing were performed under the same operation parameters such as the environmental pressure and the cold trap temperature. Apple slices in group B and D were put into the freeze refrigerator to perform traditional freezing for 4 h at -30 ℃.

1.3.2 Mass loss analysis in vacuum freezing

Three slices of apple samples in each group were randomly selected and individually weighed before and after vacuum freezing to determine the mass loss (ML), taking the average value of 3 slices as the final result. ML was calculated as the percentage loss of initial weightmass using the formula (1).

where m 0/g and m 1/g were the weight of apple slice before and after vacuum freezing, respectively.

1.3.3 Sample temperature analysis in vacuum freezing

The temperature of the geometric center of apple slices in each group was measured using the thermocouple temperature sensors of the vacuum freeze dryer to determine the sample temperature variation during vacuum freezing.

1.3.4 Thawing loss analysis

After freezing, 3 frozen slices were taken out from each group, separately placed into a high-density polyethylene bag and thawed in a constant temperature and humidity chamber maintained at (20 ± 0.5) ℃ and (70 ± 5)% relative humidity until the temperature in the geometric center of the samples reached 4 ℃. Each thawing experiment was undertaken in triplicate. Then thawing loss (TL) was calculated according to the formula (2).

where m 2/g was the weight of samples before freezing; m 3/g was the weight of samples after thawing.

1.3.5 Relative electrical conductivity analysis

The method was based on Lebovka et al. [20]. The relative electrical conductivity (REC) of fresh slices (before blanching), blanched slices (after blanching), and freezethawed slices in group A, B, C, D (after freeze-thawing) were all measured with the following method. Ten small apple slices with 1 cm in diameter were obtained from each group using the 1 cm-diameter hole puncher, then were washed twice with deionized water, placed in the Erlenmeyer flask containing 100 mL deionized water, and measured the initial conductivity E 0with the conductivity meter. The second conductivity E 1was also measured after shaking the Erlenmeyer flask for 1 h at temperature of 15 ℃ in the shaker. Subsequently, the Erlenmeyer flask was boiled for 15 min on the electric furnace, then cooled down to the room temperature and added with deionized water until achieving the original weight before boiling. Finally, the third conductivity E 2was measured. All the measurements were undertaken in triplicate. REC was calculated as the formula (3).

1.3.6 Micro-structure analysis by light microscope

The method was modified from that of Ignat et al. [21]. Some of the fresh, blanched and freeze-thawed slices in group A, B, C, D were taken for micro-structure analyses by light microscope. All the prepared apple slices were fixed in Formalin acetic acid alcohol (FAA) solution (90% ethanol, 5% acetic acid, 5% formalin) for 3 d. After fixation, gradient elution with 30%, 50%, 70%, 90% and 100% ethanol was performed for 15 min in each ethanol concentration. Then the slices were processed by an automatic histoprocessor to embed the tissue in paraffin which was cut into 5 μm paraffin-tissue slices with a tissue slicer. The paraffin-tissue slices were baked to remove paraffin, stained with Safranin O/Fast Green, and finally were sealed in glass slide to be ready for microscope imaging.

1.4 Statistics

Statistical analysis of variance (ANOVA) was performed using SPSS 20.0 software. Tests of significant differences between means were determined by Duncan’s multiple range tests at a significance level at 0.05 (P<0.05).

2 Results and Discussion

2.1 ML analysis in vacuum freezing

Fig. 1 ML of apple slices after different times of vacuum freezing

During vacuum freezing the water in the apple slices evaporated from liquid to vapor along with the performing time causing the result of continual ML. As shown in Fig. 1, ML in group A (27.5%) after vacuum freezing for 40 min was significantly higher than that of group C (22.9%, P<0.05). Due to the blanching pretreatment, the ML in group A was increased by 32.35%, 21.85%, 20.26%, 20.00% at 10, 20, 30, 40 min of the vacuum-freezing performing time compared with that in group C, respectively. Water evaporation rate (ML rate) varied with a slowing down trend. The first 10 min was the rapidest and main evaporation stage, contributing to more than half of the total ML in 40 min.

During the vacuum cooling and freezing process, water evaporation in apple slices was driven by the difference between the water vapor pressure on materials and thepressure in the vacuum chamber. ML and frozen temperature are the main indicators of vacuum freezing performance which is affected by some factors including the initial temperature of the material, moisture status and micro-structure of the inner tissue, vacuum pressure, vacuum performing time, the cold trap temperature and so on [10-11]. In our experiments, vacuum freezing operations of group A and C were performed at the same batch in the same equipment, ensuring that the material initial temperature and the vacuum processing conditions of the two groups were consistent. The significant difference of ML between group A and C may be mainly caused by the moisture status and micro-structure of material tissue. This might be due to the fact that blanching pretreatment resulted in some changes to the cell wall microstructure, contributing to the more ML in group A.

ML was inevitable phenomenon of vacuum freezing, which lead to different results for food different processing. ML was undesirable for the manufacturers if the vacuumfrozen materials were used as instant freezing products for the preserving purpose, and should be controlled as small as possible to reduce the economic losses. However, ML was desirable if the vacuum-freezing processing was used as the purpose of freezing stage in freeze-drying technology, and should be controlled as more as possible to obtain the maximum dehydration mass and the lowest freezing temperature in favor of enhancing freeze-drying performances.

2.2 Sample temperature analysis in vacuum freezing

Sample temperature variation was caused by the phase change of the moisture in the apple slices themselves, including evaporation from liquid to vapor and freezing from liquid to solid ice [10]. As shown in Fig. 2, the vacuum freezing process can be divided into 3 stages according the change curve of sample temperature which presented a similar trend in the studies of Zhang Haifeng et al. [13].

Fig. 2 Temperature changes of apple slices during vacuum freezing

The first stage: pressure-reducing and flashing stage. The pressure in the vacuum chamber dropped to saturation vapor pressure at the initial temperature of apple slices after 2 min of vacuum pumping, reaching to the flash-point of water evaporation. Then rapid evaporation and boiling status of water in apple slices happened, removing large quantity of heat from samples themselves and leading to a rapid reduction in sample temperature (dropped below -10 ℃). The temperature curves of group A and C almost overlapped, showing no obvious difference in slices temperature variation caused by the blanching pretreatment.

The second stage: ice formation stage. Apple slice samples turn into a super-cooling sate due to the happening of the flashing evaporation. And ice crystals in samples were gradually formed releasing a lot of latent heat, which slowed down the temperature reduction rate of samples caused by the removed heat of water evaporation. This stage lasted about 10 min, the temperature curve dropped with a slighter slope compared with the first stage. Moreover, the temperature curves of group A and C slightly separated showing that the blanched samples had a lower temperature.

The third stage: deep freezing stage. Water evaporation and freezing in slices get on simultaneously along with the continuation of vacuum freezing. And the temperature curves slowly leveled off at the limit frozen temperature at -27.6 ℃ in group A and -26.5 ℃ in group C, following by a slight rise. At this stage, most of water in the slices was frozen and water evaporation gradually weakened. Even at the later stage, sublimation drying happened due to the heat transferring from ambient environment to the slices, which contributed to the slight rise in slices temperature.

It is well known that vacuum freezing has an amazing freezing rate in contrast to traditional refrigerator freezing, which was verified by the above analyses of sample temperature in vacuum freezing. And the temperature variation and ML variation of apple slices showed a close relation in Fig. 1 and Fig. 2. The more and faster ML, the more and faster temperature reduction, which reflected the energy conservation principle during vacuum freezing process in some sense.

2.3 TL analysis

TL is an important indicator for the integrity in the cell tissue of frozen products. Thawing experiments of samples in this study were performed under the same temperature and humidity. TL of the 4 groups was presented in Fig. 3. TL in group B was highest (27.67%), followed by group D (24.94%), group A (21.87%) and the lowest group C (10.71%). The difference between any two groups in TLwas significant except group B and D (P<0.05). It can be concluded that blanching pretreatment caused more TL in terms of the same freezing method. Many available studies revealed that a series of changes in tissues of fruits and vegetables will happened during blanching pretreatment, including degeneration of inner cell protoplasm, increase in cell membrane permeability, extracellular and intracellular water exchange, increase in tissue elasticity and toughness and so on [22]. In this study, blanching pretreatment caused about 104% and 11% increase in TL for the vacuum freezing and the refrigerator freezing, respectively. This was probably due to micro-structure damage on cell tissue of apple slices caused by the blanching pretreatment. And TL in the refrigerator freezing method were significantly higher than that in the vacuum freezing method. This was probably due to the formation of larger ice crystals in the extracellular or intracellular space because of extremely slow freezing rate in refrigerator freezing, leading to more disruption of cells [23-24].

Fig. 3 TL of frozen-thawed apple slices from 4 groups

2.4 REC analysis

Fig. 4 Relative electrical conductivity of apple slices after different treatments

REC is an important indicator to measure permeability of cell membrane. Higher REC value represents greater damage of membrane intactness. The REC results were shown in Fig. 4. After blanching pretreatment and/or the freeze-thawing treatment in all groups, the RECs in the slices were increased with different degrees compared with the raw fresh slices. A similar difference trend of REC in group A, B, C and D was observed in Fig. 4 as TL in Fig. 3. The highest REC value of 63.45% was found in the thawing slices of group B, following down by 58.62% in thawing slices of group D, 55.77% in thawing slices of group A, 40.66% in thawing slices of group C, 39.33% in the blanched slices (blanching) and 31.22% in the raw fresh slices (control group). A significant difference in REC was observed between any two groups except the pair of blanching and thawing in group C, thawing in group A and thawing in group D. It can be concluded from the REC results that refrigerator freezing conducted more damage to cell tissue than vacuum freezing, and combined treatments of blanching and freezing conducted a further more damage on the micro-structure of apple tissue than the single one.

2.5 Micro-structure analysis by light microscope

Fig. 5 Photomicrographs of apple tissues after different treatments

Fig. 5 shows the photomicrographs of apple tissue under different treatment conditions, in which thin layers lined cells surface profile. It was observed from Fig. 5a that cells in untreated fresh apple were regular in shape, arranged in order and appeared plump with an apparent consistent cell wall structure. However, blanching treatment caused the loss of turgidity of the cells, appearing irregular in cell shape, distortion in tissue and disorderly in arrangement (Fig. 5b), which wasverif ed by the apparent phenomenon that blanched apple slices presented relatively more soft in tissue and smaller in volumesize compared with untreated fresh ones.

Fig. 5c shows the cell tissue of vacuum-freeze-thawed apple slices with blanching pretreatment, in which a similar tissue morphology as Fig. 5b was observed except that a small quantity of cell walls were disrupted. The cell tissue of vacuum-freeze-thawed apple samples without blanching pretreatment was shown in Fig. 5e with fewer structural changes in contrast to Fig. 5c, even retaining many regular and plump cells similar to those in Fig. 5a.

Cells in refrigerator-freeze-thawed apple slices with (Fig. 5d) and without (Fig. 5f) blanching pretreatment appeared more distortion and disruption in cell walls compared to Fig. 5c and Fig. 5e. It is well known that ice crystal size is closely related to the freezing rate. Vacuum freezing were performed with a much faster freezing rate than refrigerator freezing, resulting in smaller and more uniform ice crystals in the extracellular and intracellular region. In contrast, larger ice crystals in apple tissue were formed during refrigerator freezing contributing to more cell disruption and morphological changes.

It can be concluded from Fig. 5 that vacuum freezing without blanching pretreatment (Fig. 5e) caused the smallest morphological changes and the lowest breakage to apple cell tissue, f owing increasingly by Fig. 5c, f and d. And the aforementioned difference of ML, thawing loss and relative electrical conductivity in different treatments groups could be attributed to the micro-structural changes at cellular level in the sample tissue.

In Fig.1, ML in group A was significantly higher than that in group C. It might be due to that blanching pretreatment on apple slices softened cells tissue, enhanced cells permeability and promoted more water evaporation or ML during vacuum freezing. And TL in Fig. 2 and relative electrical conductivity in Fig. 3 indirectly ref ect the degree of structural changes in apple cells shown in Fig. 5. For example, vacuum-freeze-thawed apple samples without blanching pretreatment had the smallest TL value and REC value which was confirmed by the minimum structural changes in Fig. 5e. Refrigerator-freeze-thawed apple samples with blanching pretreatment had the highest TL value and REC value conforming to the highest degree of disruption in cells tissue.

And for the purpose of freezing preservation, the least micro-structure disruption in Fig. 5e (vacuum freezing without blanching pretreatment) was desired and accepted in priority in order to achieve a good quality of frozen products. However, the micro-structure changes on apples tissue due to blanching treatment and freezing methods would also cause different drying properties if the frozen products were used for vacuum freeze drying [25-26], which needed further studies in the near future.

3 Conclusions

The vacuum freezing properties of blanching-treated apple slices were investigated in this study by comparing with the traditional refrigerator freezing. Results from this work demonstrated that blanching pretreatment provided significant increase in ML of apple slices during vacuum freezing. In addition, more than half of the ML and most of temperature reduction happened within the first 10 min of the vacuum freezing period. The total process of vacuum freezing can be divided into 3 stages: pressure-reducing and flashing stage, ice formation stage and deep freezing stage. Based on the analyses of thawing loss, relative electrical conductivity and photomicrographs, it has been concluded that vacuum freezing causes less micro-structural changes and disruption to apple cells tissue and contributes to smaller TL and REC value in contrast to refrigerator freezing, and that blanching treatment before freezing conducts further damage to cell micro-structure than non-blanching pretreatment.

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热烫处理苹果片真空冻结特性

王海鸥,扶庆权,陈守江,王蓉蓉,张 伟,杨 平
(南京晓庄学院食品科学学院,江苏 南京 211171)

摘 要:本研究与传统冰箱冷冻相比较,研究了热烫处理苹果片的真空冻结特性。真空冻结40 min后,经热烫处理的苹果片冻结最低温度达-27.6 ℃,质量损失为27.5%,而未经热烫处理的苹果片的冻结最低温度和质量损失分别为-26.5 ℃、22.9%。依据温度变化,苹果片真空冻结过程可分为:减压闪发段、冰晶形成段、深层冻结段。相对于冰箱冷冻而言,真空冻结苹果片对组织微观结构改变小,冻融后汁液流失率低,电导率低,另外,冷冻前热烫处理会引起更大的组织结构损坏,显著增加冻融后的汁液流失率和电导率。

关键词:真空冻结;热烫处理;苹果片;微观结构

中图分类号:TS255.3

文献标志码:A

文章编号:1002-6630(2016)23-0057-07

引文格式:

收稿日期:2016-07-24

基金项目:国家自然科学基金青年科学基金项目(31301592);常州市科技支撑计划项目(CE20152017);江苏省教育厅自然科学基金项目(15KJB550008);南京晓庄学院人才引进项目(2013xzrc04)

作者简介:王海鸥(1978—),男,副教授,博士,主要从事食品冷冻与干燥加工技术研究。E-mail:who1978@163.com

DOI:10.7506/spkx1002-6630-201623010

WANG Haiou, FU Qingquan, CHEN Shoujiang, et al. Vacuum freezing properties of blanched apple slices[J]. 食品科学, 2016, 37(23): 57-63.

DOI:10.7506/spkx1002-6630-201623010. http://www.spkx.net.cn

WANG Haiou, FU Qingquan, CHEN Shoujiang, et al. Vacuum freezing properties of blanched apple slices[J]. Food Science, 2016, 37(23): 57-63. (in English with Chinese abstract) DOI:10.7506/spkx1002-6630-201623010. http://www.spkx.net.cn