Effects of operation processes and conditions on enhancing performances of vacuum cooling of foods: A review

Trends in Food Science & Technology 85 (2019) 67–77

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Trends in Food Science & Technology journal homepage: www.elsevier.com/locate/tifs

Review

Effects of operation processes and conditions on enhancing performances of vacuum cooling of foods: A review

T

Zhiwei Zhua,b,c, Yi Genga,b,c, Da-Wen Suna,b,c,d,∗ a

School of Food Science and Engineering, South China University of Technology, Guangzhou, 510641, China Academy of Contemporary Food Engineering, South China University of Technology, Guangzhou Higher Education Mega Center, Guangzhou, 510006, China c Engineering and Technological Research Centre of Guangdong Province on Intelligent Sensing and Process Control of Cold Chain Foods, Guangzhou Higher Education Mega Center, Guangzhou, 510006, China d Food Refrigeration and Computerized Food Technology (FRCFT), Agriculture and Food Science Centre, University College Dublin, National University of Ireland, Belfield, Dublin 4, Ireland b

A R T I C LE I N FO

A B S T R A C T

Keywords: Vacuum cooling Process optimization Mass loss Cooling rate Energy consumption

Background: Vacuum cooling (VC) is a cooling technology for moist and porous foodstuffs with superiority in cooling rate, uniformity and hygiene, and low energy consumption. However, there are still some disadvantages that weaken the competitiveness of industrial applications such as the inevitable mass loss and uneven cooling in different parts of leafy vegetables. In order to improve the performance of VC, many efforts have been performed. Scope and approach: In this review, the basic information of VC and the main applications for fruit, vegetables and meat products as well as limitations are described first. Improvements for enhancing performances of this technology in terms of operation processes including pre-treatment, technical optimization and integration, as well as equipment upgrade, and optimized operation conditions such as final pressure and pressure reduction rate are discussed. In addition, the limitation and further prospects of VC in the food industry are also mentioned. Key findings and conclusions: The findings presented in this review demonstrated that optimizing operation processes and controlling operation conditions can effectively improve the performance of VC including mass loss, cooling rate and energy consumption and temperature distribution. However, many enhancing strategies still have limitation for applied in the food industry. Future trends of VC are the study of moisture migration characteristics based on food microstructure and multi-functional optimization of VC process or system. It is hoped that the current review can provide some guidance for further developments of the VC technology.

1. Introduction

satisfactory time-temperature management can guarantee the quality of fresh produce (Hsiao & Huang, 2016). Precooling, as the first important step of cold chain, can effectively reduce postharvest respiration, inhibit microbial growth and reduce enzyme activity by timely removal of field heat from freshly-harvested products (Berry, Defraeye, Nicolaï, & Opara, 2016; Zhao, Liu, Tian, Yan, & Wang, 2018). However, traditional cooling methods, such as air blast cooling, hydrocooling or immersion cooling, are ineffective due to low cooling rates, increasing the risk of product spoilage. In addition, surface drying in air cooling (Lu, Chen, & Wang, 2016) and potential contamination in hydrocooling (De et al., 2017) also promote the development of a more efficient, hygienic and energy-saving cooling method. Therefore, vacuum cooling (VC), as a rapid cooling technology based on the principle of moisture

Many fresh and perishable products can generate unexpected waste due to lack of effective temperature control strategies (Badia-Melis, Mc Carthy, Ruiz-Garcia, Garcia-Hierro, & Robla Villalba, 2018; ÁlvarezHernández et al., 2018). Therefore, techniques such as drying (Liu, Pu, & Sun, 2017; Ma, Sun, Qu, & Pu, 2017; Ma, Qu, & Sun, 2017; Pu & Sun, 2015; Qu, Sun, Cheng, & Pu, 2017), cooling (Feng, Drummond, Zhang, & Sun, 2014; Kiani, Sun, & Zhang, 2012; Zhou et al., 2017; Zhu, Wu, et al., 2018) and freezing (Cheng, Sun, Pu, & Wei, 2018a, 2018b; Li, Zhu, & Sun, 2018; Luo, Sun, Zhu, & Wang, 2018; Xie, Sun, Xu, & Zhu, 2015; Xie, Sun, Zhu, & Pu, 2016) are often used to maintain their quality and safety, in particular, an integrated cold chain with

∗

Corresponding author. School of Food Science and Engineering, South China University of Technology, Guangzhou, 510641, China. E-mail address: [email protected] (D.-W. Sun). URLs: http://www.ucd.ie/refrig, http://www.ucd.ie/sun (D.-W. Sun).

https://doi.org/10.1016/j.tifs.2018.12.011 Received 20 September 2018; Received in revised form 29 December 2018; Accepted 29 December 2018 Available online 03 January 2019 0924-2244/ © 2019 Elsevier Ltd. All rights reserved.

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Fig. 1. Vacuum cooling innovations: (a) A typical vacuum cooler; (b) Novel vacuum cooler components for: (i) change of gas composition, (ii) immersion VC, (iii) bubbling VC.

(Feng, Drummond, Zhang, & Sun, 2013a, b), IVC with agitation (Feng et al., 2014) and IVC with bubbling (Song, Guo, Liu, & Jaganathan, 2018) were investigated. On the other hand, effects of operation conditions including final pressure (Ozturk, Ozturk, & Koçar, 2017) and pressure drop rate (Cheng & Hsueh, 2007; Drummond & Sun, 2012; Schmidt & Laurindo, 2014) to reduce mass loss, improve the uniformity of temperature distribution and reduce tissue injury were also studied. In previous reviews, Sun and Zheng (2006) summarized the development of VC for the agri-food industry including the advantage and disadvantages of this technology, while Feng, Drummond, Zhang, Sun, and Wang (2012b) mainly introduced the evolution of VC in meat products. Most recently, Zhu, Li, et al. (2018) and Zhu, Wu, et al. (2018) focused on the developments of mathematical simulations of VC for food products. The current review focuses on discussing the effects of operation processes and conditions on enhancing the performance of VC technology for rapid cooling of foods. In this review, vacuum cooling principles and its recent applications in fruit, vegetables and meat products are introduced, and the limitations and future trends of VC technology for the food industry are also highlighted.

evaporation under low pressure, has attracted much attention in the food industry (Ding et al., 2016; Ozturk, Ozturk, & Kocar, 2011). VC was first successfully practiced in cooling of lettuce (Ozturk & Ozturk, 2009) and mushroom (Singh, Langowski, Wani, & Saengerlaub, 2010), which was then extended to other fruit and vegetables (Ding et al., 2016) and later on rapid cooling of cooked meat products (Sun, 2014). VC was also applied to cool other moist products such as cut flowers (Brosnan & Sun, 2003), bakery products (Novotni et al., 2017), fishery products (Huber, Soares, Carciofi, Hense, & Laurindo, 2006) and steamed stuffed bun (Deng, Song, & Li, 2011). However, there are some limitations, such as the inherent disadvantage of mass loss, uneven temperature distribution and tissue changes, affecting the application effect of VC. In order to optimize VC performances, numerous approaches based on operation processes have been investigated. For fruit and vegetables, water spraying is a common method to reduce mass loss (Ding et al., 2016). Yesil, Kasler, Huang, and Yousef (2017) added gaseous ozone treatment during VC, while Zhu, Li, and Sun (2018) and Zhu, Wu, et al. (2018) invented the use of a gas mixture with high CO2 instead of air at pressure recovery stage, which was defined as modified atmosphere vacuum cooling (Sun, Wu, & Zhu, 2016a; 2016b), and this novel technique possesses great superiority in maintaining cooled product quality. Furthermore, many analytical techniques including simulated annealing (Tian et al., 2014), response surface methodology (Feng & Sun, 2014), genetic algorithm (Santana et al., 2018) and tabu search (Tian et al., 2018) have been used in optimizing VC operation conditions. For cooked meat products, effects of brine injection before cooking (Mcdonald & Sun, 2001b; Mcdonald et al., 2001) and after cooling, i.e., vacuum impregnation (Schmidt & Laurindo, 2014), immersion vacuum cooling (IVC) (Cheng & Sun, 2006a,b) and integration of cooking and cooling (Schmidt, Aragão, & Laurindo, 2010) during VC on mass loss and cooling efficiency were conducted. Considering the lengthening of the cooling time after introducing the cooling medium, some strategies such as changing sample size and porosity (Drummond & Sun, 2008a, b, 2012), IVC with different initial water temperature

2. Vacuum cooling process and applications 2.1. Thermodynamic principles The principle of vacuum cooling is based on the evaporative heat transfer mechanism (Wang & Sun, 2001, 2004; Sun & Zheng, 2006). The boiling point of water is reduced with the decrease of pressure. Therefore, for porous products with high free water content, the decrease of surrounding pressure can promote the evaporation of surface and internal moisture at the low temperature, which results in heat removal and rapid cooling. However, the pressure in the surrounding environment should be controlled to prevent chilling injury and frostbite of the cooled products (Feng et al., 2012b). The process of VC is mainly divided into three stages. The first stage is the constant temperature and depressurization stage, and no cooling 68

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inevitable evaporation in VC. Alibas and Koksal (2014, 2015) found that the mass loss of cauliflower by VC was 4.55%, which was significantly (p < 0.01) higher than forced air cooling (2.89%), and low flow hydrocooling and high flow hydrocooling generated mass gain of 2.81% and 3.65%, respectively. Also nonleafy vegetables especially root/stem ones with a small specific surface area usually do not yield a satisfactory temperature drop (Cheng, 2006). On the other hand, Li, Tajkarimi, and Osburn (2008) firstly reported that VC increased the risk of E. coli O157:H7 infiltration and survival in lettuce tissue. They indicated that the mechanism of infiltration might be the change of apertures of stomata, and the survival of bacteria was not because of greater nutrient inside the lettuce tissue. However, He et al. (2013) disagreed with this viewpoint based on their results from SEM observation of vacuum cooled cherry. They observed that the number of E.coli cells decreased with VC treatment and their morphology destroyed after one week of cold storage, indicating the contribution of VC to microbial safety, which was mainly due to the degradation of bacterial reproduction and functional destruction caused by DNA damage. However, the structural differences of biological materials should not be ignored and He et al. (2013) did not verify the problem of infiltration. Furthermore, Song, Liu, and Jaganathan (2016a, b) observed the temperature distribution of Brassica chinensis during VC by thermal infrared imaging (Fig. 2) and indicated that for the whole samples, the average temperature difference between leaf and petiole could reach 7 °C at the beginning of the VC process. The petiole needed more time to reduce the temperature because of thicker structure with higher water content and the wax coat outside skin of the petiole blocked the water evaporation. While for fresh cut samples, the results showed the opposite, i.e., the temperature at the edge of the cut dropped faster, which meant that the moisture was more likely to escape from the wound and cause local frostbite. They also found that the number of stomas and the capacity of water absorption from the petiole caused the final temperature of each point at the leaf part to increase with increase in the distance from the main vein. Besides, Garrido et al. (2015) found that VC treatment resulted in obvious mechanical injury to the cell, which was characterized by a visible gap between the plasma membrane and the cell wall as well as the cytoplasmic retraction, and the damage cell rate increased by 25.2% after cooling.

takes place in this stage. When pressure drops to the saturation pressure corresponding to the initial temperature of the product (flash point), the water begins to evaporate, when the second stage begins, the cooling occurs. With the continuous evaporation of moisture and discharge of vapor, the temperature drops rapidly to the required level. The third stage is pressure recovery stage, when the vent valve is opened and air flows into the chamber to restore the surrounding pressure. For cooling with liquid, the cooling process can also be divided into before boiling, during boiling and after boiling according to the flash point (Song, Liu, Jaganathan, & Chen, 2015). 2.2. Vacuum cooling system The vacuum cooler has a specific structure according to different handing capacity and product characteristics. The main systems and corresponding components of a typical vacuum cooler are shown in Fig. 1a: The vacuum system includes the vacuum chamber and vacuum pumps. The vacuum chamber is a closed container where products are put in during cooling process. Vacuum pumps are used to reduce the pressure of the vacuum chamber to required level and remove the vapor evaporated from the products. However, in order to avoid the generation of a large amount of water vapor that significantly increases the load, a vapor condenser acts as auxiliary vacuum pumps to maintain stable pressures by condensing partial vapor then draining it through a valve. The refrigeration system used in vacuum cooler is normally a vapor compression refrigeration system. 2.3. Applications and limitations 2.3.1. Fruit and vegetables Vacuum cooling technology should be prioritized for moist and porous products based on the principle of evaporative cooling. Therefore, it is an efficient cooling technology for leafy vegetables such as lettuce (He & Li, 2008; Ozturk & Ozturk, 2009), cabbage (Song, Liu, & Jaganathan, 2016b; Zhu et al., 2018), spinach (Garrido, Tudela, & Gil, 2015) and purslane (Ozturk et al., 2011). They can usually be cooled within 30 min, which contributes to the keeping of good quality during storage. There are also some studies showed the superiority of cooling effects for nonleafy vegetables. Alibas and Koksal (2014) found that cooling cauliflower by VC from the initial temperature of 23 ± 0.5 °C–1 °C showed an obvious advantage in cooling time (36 min) and energy consumption (0.51 kWh) compared to forced-air cooling (188 min, 1.13 kWh), high flow hydrocooling (64 min, 0.67 kWh) and low flow hydrocooling (84 min, 0.78 kWh). Meanwhile, the best brightness color value (L*, 80.15) and lowest hardness decrease rate (0.026%) were found after VC treatment. Ding et al. (2016) compared the effects of VC, ice-water cooling and cold room cooling on broccoli, and showed that after 30 min, VC resulted in a broccoli temperature of 5.5 °C, whereas the other two conventional cooling methods could only achieve a high final temperature of 10.0 °C and 11.8 °C, respectively, showing high cooling rate of VC for nonleafy vegetables. Other roots of plant as well as most of fruits are usually not suitable for VC due to the presence of dense peel, which makes it difficult to evaporate water. However, there are still some positive impacts on antioxidant enzymes for the certain fruits. He, Zhang, Yu, Li, and Yang (2013) indicated that VC treatment showed higher catalase (CAT) and peroxidase (POD) activity of cherry than that without VC during storage time (p < 0.05), and the lower malondialdehyde content in the vacuum cooled cherry also confirmed indirectly the inhibitory effect of oxidative injury. Similar results were also found in vacuum cooled blackberry. According to PCA analysis, the blackberry treated with 1methylcyclopropene and VC could be discriminated due to the higher antioxidant activities of the treatment after storage at 0 °C for 38 d (Li et al., 2018). Despite the superiority in high cooling rate, as mentioned previously, the inherent disadvantage is the high mass loss due to the

2.3.2. Meat products There exists a danger zone from 55 to 10 °C during cooling process for microorganism proliferation in cooked meat products (Feng & Sun, 2014). However, it is a big challenge for conventional cooling to pass this zone quickly due to the inherent low thermal conductivity of meats, which is about 0.5 W/m K (Sun & Zheng, 2006). For example, cooked pork can be cooled from 70 to 4 °C within 120 min, which was significantly faster than air blast cooling (565 min), slow air cooling (855 min), and water immersion cooling (855 min) (Sun & Wang, 2000). Therefore, VC is widely applied to rapidly cool meat products such as cooked beef (Mcdonald & Sun, 2001b), beef pieces (Zhang, Drummond, & Sun, 2013), sausages (Feng, Drummond, Zhang, & Sun, 2012a), meatball (Ozturk et al., 2017) and chicken breast (Schmidt & Laurindo, 2014). Even for cooked meats arranged in stacks with layers in the vacuum chamber, VC could still achieve more homogeneous cooling than air blast cooling, as shown in Fig. 3 (Schmidt, Silva, Zanoelo, & Laurindo, 2018). However, there are also some limitations. For cooked meat products, a large mass loss of about 10% occurs during VC treatment, which is around 2 times higher than conventional cooling methods (Sun & Zheng, 2006), seriously reducing the economic value of meat products, as meats are sold based on weights. In addition, the meat tissue usually shows slightly higher Warner–Bratzler shear force (WBS), hardness and slightly lower springiness after VC treatment due to the water loss, which causes negative impact on quality (Cheng & Sun, 2006b). 69

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Fig. 2. Thermal infrared images of Brassica chinensis at different stage during vacuum cooling (Song et al., 2016b).

3. Optimization strategies of operation processes

3.1. Water spraying

As discussed previously, the inherent disadvantage of mass loss and other limitations weaken the cooling effects. In order to further enhance the performances of VC, some optimization strategies for operation processes were developed, and their effects are listed in Table 1 and Table 2.

Water spraying, as a pre-treatment, is usually used for vegetables to reduce the mass loss. Ding et al. (2016) added water sprayer in the vacuum cooler and found that there was no significant difference of mass loss in broccoli between VC (1.47%), ice-water cooling (0.72%) and cold room cooling (1.27%) after storage for 30 days, which meant positive effects of water spraying (p > 0.05). However, Tian et al. (2014) indicated that a large amount of water would impact final

Fig. 3. Effect of vacuum cooling and air cooling on the cooling rate of stack-arranged chicken breasts (3 kg) (Schmidt et al., 2018). 70

Broccoli, spinach

Lettuce

Spinach

Cabbage

Broccoli

Broccoli

Water spraying

Moisture control

VC + GOT

MAVC

SA

RSM with GA or TS

E. coli populations decreased by 2.1, and 2.8 log CFU/g compared with initial populations of 107, and 105 CFU/g (p < 0.05) Higher color, sensory score, ascorbic acid, chlorophyll and CAT than VC; reducing the rate of respiration and the activity of POD Best conditions: weight: 274 g, pressure: 200 Pa, WV: 6%, time: 40 min; Results: mass loss: 0.35%, Tf: 1.48 °C Best conditions: weight: 273.5–278.0 g, WV: 3.0%, time: 40 min Pressure: 200 Pa; Results: mass loss: 0.34 ± 0.01%, Tf: 2.0 ± 0.0 °C

Reducing the association and infiltration of bacteria (e.g. E. coli O157:H7)

Decreasing mass loss

Results

Unclear

Unclear

Leaf morphology characteristics did not affect gas retention

Tian et al. (2018), Santana et al. (2018)

Zhu, Li, et al. (2018) and Zhu, Wu, et al. (2018) Tian et al. (2014)

Yesil et al. (2017)

Ding et al. (2016), Garrido et al. (2015) Vonasek and Nitin (2016)

Water-soak on the leaves, high final temperature VC with low moisture level increased the risk of infiltration of E. coli O157:H7 Treatment efficacy decreased with inoculum level increased

References

Disadvantages

71 A mass loss (25.4%) lower than VC (28.8%); moisture content and mechanical properties were similar with CC Decreasing mass loss; higher water content and lower WBS than VC Energy saving and sanitation Low IWT decreased cooling time; better effects on large cooked meat Decreasing cooling time

Chicken breast fillet

Both small and large cooked meat

Chicken breast cuts

Pork ham, sausage

Pork ham, sausage

Cooked pork

VC + VI

IVC

Integration of cooking and cooling IVC with IWT

IVC with agitation

BVC

Cooling rate was lower than IVC

Only significant effect at later stage (from 10 to 4 °C)

Cooking solution was a common choice (hot water)

Increasing cooling time

Weak effect on mass loss; increasing cooling time and saltness Cooling time was 2 times longer than VC; the impact of impregnation solution viscosity was unknown Lower cooling rate than VC; increasing energy consumption; excessive boiling phenomenon

Disadvantages

Feng Feng Feng Feng Song

et et et et et

al. al. al. al. al.

(2013a), (2013b) (2013a), (2014) (2018)

Cheng and Sun (2006b, 2007), Drummond, Sun, Vila, and Scannell (2009) and Drummond et al. (2015) Schmidt et al. (2010)

Schmidt and Laurindo (2014)

Mcdonald et al. (2001), Desmond et al. (2002)

References

Note: VC = vacuum cooling, CC = cold-chamber cooling, IVC = immersion vacuum cooling, IWT = initial water temperature, VI = vacuum impregnation, BVC = bubbling vacuum cooling, WBS = Warner–Bratzler shear force.

Higher cooling rate and lower mass loss than VC

Increasing total yield

Cooked beef, ham

Brine solution injection

Results

Products

Methods

Table 2 Operation processes for improving the performance of VC for cooked meat products.

Note: HC = hydrocooling, VC = vacuum cooling, GOT = gaseous ozone treatment, MAVC = modified atmosphere vacuum cooling, SA = simulated annealing technique, RSM = response surface methodology, GA = genetic algorithm technique, TS = tabu search technique, WV = water volume, Tf = final temperature, CAT = catalase, POD = guaiacol peroxidase.

Products

Methods

Table 1 Operation processes for improving the performance of VC for vegetables.

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3.3. Introduction of brine solution

temperature and Garrido et al. (2015) found that water-soaking might appear on leafy vegetables such as baby spinach after water spraying due to rapid increase in pressure at pressure recovery stage. Therefore, in order to determine the best cooling conditions with low mass loss and the expected final temperature, Tian et al. (2014), Santana et al. (2018) and Tian et al. (2018) used meta-heuristics techniques including simulated annealing technique, response surface methodology with genetic algorithm technique and response surface methodology with tabu search, respectively, to optimize VC process of broccoli. Under optimal conditions with water spraying (Table 1), the mass loss could be decreased to about 0.34% and the final temperature could reach about 2 °C. However, the moisture level in VC process can affect the microbial safety of the cooled products. Vonasek and Nitin (2016) revealed the relationships between moisture level and microbial infiltration using a multiphoton-imaging approach. It was suggested that the mechanism of such bacterial infiltration could be due to the structure change of stomata promoted by vacuum (Li et al., 2008). However, further research of Vonasek and Nitin (2016) indicated that VC treatment could not significantly influence stomatal opening and association of E. coli O157:H7 with stomata (p > 0.05), whereas moisture content showed stronger effects. In this experiment, the low moisture level was achieved under a laminar airflow, and this act might force bacteria to move into guard cells of the stomata and the crevices around the epithelial cells, leading to the increase of nearby distribution of bacteria. Furthermore, they also confirmed that the interaction of VC and moisture significantly contributed to stoma infiltration rather than VC itself, which was dependent on inoculation. Therefore, the risk of microbial association with stomata and its infiltration in fresh produce may disappear under the natural level with non-artificial inoculation and avoiding low moisture condition during VC should be a reasonable choice for microbial safety.

Introducing brine solution before cooking or after cooling for cooked meat products can reduce the mass loss. Injecting brine solution before cooking was researched by Mcdonald et al. (2001) for cooked beef and Desmond, Kenny, and Ward (2002) for cooked ham. The meats were injected to levels from 120% to 145%, and 120%–130% of their green weight with the brine solution, respectively. Although there was no significant effect between injection level and mass loss (p > 0.05), the total yield of cooked beef and ham was increased from 85.8% to 115.97% (Mcdonald et al., 2001) and from 102.7% to 110% (Desmond et al., 2002), respectively. However, it was noted that the consequence arising from increasing injection levels was longer cooling time and decrease in sensory attributes. Introducing brine solution after cooling is based on the principle of vacuum impregnation (VI), i.e., introducing a liquid phase, in which the substances are dissolved or dispersed, into the food with porous structures by macroscopic pressure (Rydzak et al., 2017). Schmidt and Laurindo (2014) submersed the cooked chicken breast fillet in a saline solution with 2% NaCl solution at 5 °C under vacuum for 15 min after VC. The results showed that about 5% of the mass loss could be compensated. And the increase in porosity of the meat tissue subjected to VC also contributed to the dispersion of liquid. In addition, after VI treatment, products with moisture contents and mechanical properties similar to those of cold-chamber cooling were obtained except for the hardness, which might be related to the tenderization caused by the repetitive pressure variations. 3.4. Immersion vacuum cooling (IVC) IVC was specifically developed by Cheng and Sun (2006b) to target the high mass loss during VC of cooked meats. In IVC process, the meat is immersed in the liquid for cooling together and the surrounding liquid can enter the meat pores due to hydrodynamic mechanisms when the vacuum is broken (Fig. 1bii). Therefore, the mass loss can be compensated (Drummond et al., 2009, 2015). However, the heat transfer of IVC includes heat conduction and convection in addition to water evaporation, so the low cooling rate of IVC becomes a main obstacle (Cheng & Sun, 2007). However, because of the water compensation, inferiority of meat quality during VC including high WBS and hardness could be effectively prevented (p < 0.05) (Cheng & Sun, 2006b). Later, Schmidt et al. (2010) integrated cooking and cooling process into the same operating container and applied it on chicken breast cuts, both VC and IVC cooling methods were available depending on if draining the cooking water or not. Such design did not need to transfer the cooked products to other vessel so that it was an energyefficient method to reduce product handing time and avoid contamination during transfer. The results showed that, after the water immersion cooking, IVC treatment could reduce about 5% of mass loss compared with VC, and IVC took about 2800 s to cool from 80 °C to 14 °C while VC took about 800 s. Similar device was also applied to cooked vegetables for ready-to-eat meals such as carrots (Rodrigues, Cavalheiro, Schmidt, & Laurindo, 2012) and potatoes (Rodrigues, Cavalheiro, Schmidt, & Laurindo, 2013). These researches all concluded that combined immersion cooking with IVC showed the lowest mass loss with high meat quality. However, it did not contribute to improving cooling rates. In order to enhance the cooling rate of IVC, many measures were investigated. Both experiment and mathematical simulation indicated that the cooling time of IVC was significantly affected by meat size (p < 0.01) (Drummond & Sun, 2008a, b). When the size was increased from 1.0 to 4.3 kg, the cooling time was extended from 2.8 to 5.5 h. Although small-size meat can be cooled fast, it is more economical from commercial point of view to cool large meat joints. Compared with conventional cooling methods, VC with water could cool cooked pork ham (2.2 ± 0.2 kg) from 72 °C to 4 °C within 165.3 min, which

3.2. Change of gas composition at pressure recovery stage New attempts to replace air with other gases during pressure recovery stage provide the novel ideas for the sterilization and preservation of the leafy vegetables (Fig. 1bi). Yesil et al. (2017) added gaseous ozone during VC to observe the inactivation effect of population size of inoculated pathogen on spinach leaves inoculated with E. coli O157:H7. During VC, when the spinach was cooled to 4 °C, i.e., vacuum pressure reached 96.51 kPa, ozone (1.5 g of per kg of gas mixture) was introduced into the vacuum chamber until the pressure reached 170.27 kPa and the pressure was held for 30 min. Their results showed that the E. coli populations inoculated at three levels of 8, 7, 5 log CFU/g were decreased by 0.2, 2.1 and 2.8 log CFU/g, respectively. Although the gaseous ozone efficacy weakened as the inoculum levels increased. It was possible that E. coli cells formed biofilms to provide protection against sterilization at a high inoculum level. However, the pathogen levels in natural contamination were usually lower than that in the experiment. Therefore, in practical applications, gaseous ozone treatment during VC should exert more effective sterilization (Yesil et al., 2017). In addition, Zhu, Li, et al. (2018) and Zhu, Wu, et al. (2018) experimented the invented modified atmosphere vacuum cooling (MAVC) technology on cooling of three leafy cabbages with different morphological structures. In MAVC, a gas mixture with 7% CO2 (v/v) was injected into the vacuum chamber at pressure recovery stage instead of air. Unlike modified atmosphere preservation, MAVC aims to use some special structures of plants such as stomata for gas exchange or different leaf morphologies and tightness to retain the mixed gas. Their results indicated that the internal porous structure played a positive role in gas retention although the effect of leaf morphology characteristics was weak. Furthermore, MAVC treatment could reduce the respiration rate and delay the decline of chlorophyll content, confirming that MAVC was an effective strategy to extend the shelf life of cabbages and keep high quality during storage. 72

73

PIVC

Bamboo shoots, cabbage, water spinach Cooked chicken breast MSVPR

Note: VC = vacuum cooling, IVC = immersion vacuum cooling, PRR = pressure reduction rate, MSVPR = multi-stage vacuum pressure reserving, PIVC = pulse immersion vacuum cooling.

Limitation of cycle numbers

High device cost

Cooked ham, Brassica chinensis PRR control by volumetric displacement of the pump

Reducing cooling time, energy saving; satisfactory of temperature drop Reducing mass loss

Unclear

Iceberg lettuce, sausage, pork ham, cooked beef PRR control by valve

VC: low PRR increased total yield, and reduced mass and tissue damage loss IVC: reducing cooling time (first stage) Uniform temperature distribution; avoiding excessive boiling

Meat size and casing type (sausage) were constrains

Ozturk and Ozturk (2009), Ozturk et al. (2011), Pichaya et al. (2012), Ozturk et al. (2017) McDonald and Sun (2001a), He et al. (2004) Increasing final temperature and risk of bacterial contamination under a high final pressure Reducing mass loss and energy consumption Iceberg lettuce, purslane, organic chayote, meatball

Ozturk and Hepbasli (2017) assessed energy consumptions of vacuum cooling systems by energy and exergy analysis based on mushroom experiments and presented that high coefficient of performance of 12 and exergy efficiency of 80% could be reached at the lowest pressure during cooling process. However, the choice of the final pressure should consider not only avoiding chilling injury and frostbite of the products but also its impact on the cooling time and mass loss (Ozturk et al., 2011; Ozturk & Ozturk, 2009), energy consumption (Pichaya, Danai, & Kasem, 2012) and microbial growth rate (Ozturk et al., 2017) (Table 3). Ozturk and Ozturk (2009) researched the effect of cooling iceberg lettuce with different final pressures. They found from the comparison of 0.7 kPa and 1 kPa, the cooling time of former was 220 s faster than latter but the mass loss was higher of 0.28%. Similar results were obtained in VC of purslane (Ozturk et al., 2011) and meatball (Ozturk et al., 2017). Therefore, the increase in final pressure can reduce the

Increasing final pressure

4.1. Final pressure

Products

4. Effects of operation conditions

Methods

Table 3 Operation conditions for improving the performance of vacuum cooling.

Results

Disadvantages

References

shortened about half of cooling time of air blast cooling (320.7 min), and also significantly reduced mass loss from 13.71% for VC to 6.99% for IVC (p < 0.05) (Cheng & Sun, 2006b). In addition, according to the mathematical simulation results of Drummond and Sun (2012), an increase of porosity from 2% to 5% could reduce the cooling time by more than half, but with higher mass loss. Therefore, it might be possible to compensate long cooling time of large product by changing product porosity appropriately. Generally speaking, cooking solution with high temperature was used in early studies of IVC, However, it would produce a large amount of vapor that increase the load of vacuum pump and the time of heat transfer at later stage of IVC. Considering that low initial temperature of cooling medium can promote the heat exchange between meat products and water cooking is not a common choice in the industry for large meat products (Feng et al., 2013a, b), hygienic tap water (as low as 7 °C in winter) can be considered as an alternative. Cooling cooked sausage by IVC with initial water temperature of 20 °C could shorten 10 min of cooling time as compared to that of 80 °C (Feng, Sun, García, Juan, & Zhang, 2013b), such time advantage is more obvious in large-size meats such as pork hams when the initial water temperature is lower (Feng et al., 2013a). It is also necessary to minimize the amount of water to keep just covering the meat to reduce cooling time and pump load. In addition, Drummond and Sun (2012) suggested that adding some turbulence by agitating in surrounding liquid could significantly improve cooling rate. After employing agitation, 47.39% cooling time reduction was found in the experiment of cooling pork ham from 72 °C to core temperatures of 4.6 °C (Feng et al., 2013a). Similar results were found in Feng et al. (2014), who cooled sausages from 72 to 4 °C. However, the effect of stirring on reducing cooling time was only significant (p < 0.05) at the later stage (from 10 °C to 4 °C) and it was influenced by pressure drop rate. Recently, Song et al. (2018) designed a novel bubbling vacuum cooler for cooling small-size cooked pork by adding a bubble generation system and expectedly a high cooling rate was achieved due to high coefficient of boiling heat transfer (Fig. 1biii). The artificial bubbles were only generated at “before boiling” and “after boiling” stage by pressure difference to avoid excessive boiling phenomenon at “during boiling” stage. However, the cooling rate from 60 °C to 4 °C after introducing the artificial bubbles (0.1 ± 0.01 °C/s) was only slightly higher than IVC (0.07 ± 0.02 °C/s) with no significant difference (p > 0.05) and still lower than VC (0.17 ± 0.01 °C/s). This was due to the change of flash point of the surrounding water after dissolution of soluble components and to excessive vapor produced at “during boiling” stage. Although the improvement of cooling rate was not obvious, the continuous bubbling promoted sample porosity and increased water penetration, thus showing a weight gain of about 2.3%.

Song and Liu (2014), Song et al. (2015), Song et al. (2016b) Cheng (2006), Cheng and Hsueh (2007) Schmidt and Laurindo (2014)

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mass loss. However, if the final pressure is higher than 1.5 kPa, the final temperature of the products will be higher than 10 °C, which cannot achieve the required cooling temperature (Ozturk & Ozturk, 2009). Ozturk et al. (2017) also found that the highest microbial growth of meatball occurred at pressure of 1.5 kPa because it did not fall to the desired final temperature, which might increase the risk of bacterial contamination. Additionally, Pichaya et al. (2012) added the pressure holding operation to VC of organic chayote shoot under final pressure. When the holding time for the final pressure at 1.0 kPa was increased to 2 min, the mass loss rate was increased by 0.4%. Furthermore, Pichaya et al. (2012) found that cooling organic chayote shoot from 1923 °C–8 °C using a final pressure of 1.1 kPa with holding time of 5 min produced the mass loss of 0.5% and energy consumption of 0.1098 kWh, while the final pressure of 1.0 kPa with holding time of 3 min led to the higher mass loss of 0.6% and energy consumption of 0.1159 kWh (Pichaya et al., 2012). Under the former optimal conditions with storage temperature of 8 °C, the cooled organic chayote shoot showed better appearance and longer shelf life of 3 more days.

also suitable for vegetables to achieve a more even temperature distribution by using a constant moderate volumetric displacement of 0.033 m2/s (Song et al., 2016b). Furthermore, it is worth noting that the phenomenon of excessive boiling and spillage of solution especially at boiling stage during IVC can pollute the chamber and bring in risks of the microbial growth. Song et al. (2015) researched this phenomenon by observing cooling of water with a high-speed camera and they found more than 46% of water loss during boiling stage in the cooling process. They also reported that a low volumetric displacement of 0.0012 m3/s from 10 to 2 kPa showed a positive cooling effect that weakened the degree of boiling and spillage due to the slow pressure drop and prolonged the time for heat transfer from the surface to inside of the sample (Cheng & Liu, 2007; Song et al., 2015). Multi-stage vacuum pressure reservation (MSVPR) operated by intermittent start-up of the vacuum pump breaks the conventional continuous depressurization mode and performs some improvements of cooling performance. Cheng (2006) combined VC and hydrocooling with MSVPR to solve the problem that nonleafy vegetables such as bamboo shoots were difficult to drop temperature due to the low surface to volume ratio. When the desired reserved pressure of 13.3 kPa was reached, the pumping pipe was closed to isolate the vacuum chamber and keep the pressure, which allowed thorough heat transfer between the bamboo shoots and the surrounding water. After the temperature of water was closed to that of bamboo shoots, the vacuum pump worked again to continue reducing pressure until it was lower than the cooled products, which achieved a satisfactory cooling with final temperature of 4.75 °C as well as a reduction in running cost and energy consumption. Similar multi-stage processes were also applied to cabbage and water spinach, in order to avoid temperature increase during pressure recovery stage, the external high-temperature air was cooled before injecting into the vacuum chamber (Cheng & Hsueh, 2007). It is also worthy of attention to compensate for mass loss in VC by changing traditional continuous depressurization way. Pulsed immersion vacuum cooling (PIVC), namely the pressure returns to atmospheric pressure within a few seconds then continues to decline, was researched by Schmidt and Laurindo (2014) and applied in cooked chicken breast during IVC. During the pressure drop, the first cycle began when the surrounding liquid temperature was below 15 °C and the interval of three cycles was 10 min. After accomplishing cooling, a significant reduction in mass loss of 2.8% (p < 0.05) was obtained by PIVC as compared to 4.8% for IVC and 11.6% for VC. The repeated mutations during PIVC caused gas expansion of pores in the meat tissue, which resulted in more cooking liquid to enter the porous structure by multiple pressure recovery stages and the higher moisture contents contributed to the lower hardness and WBS than that of VC (p < 0.05) (Schmidt & Laurindo, 2014). However, it was noted that more cycle numbers did not yield better results, since the limitation of expansion determined that the mass loss cannot be decreased indefinitely (Cheng & Sun, 2006a).

4.2. Pressure reduction rate Numerous studies have shown that pressure reduction rate is a crucial processing parameter that influences the cooling performance (Table 3). Different pressure reduction rates are usually achieved by adjusting air bleed or electronic valve to control the time that reaches the final pressure under a constant pumping speed. McDonald and Sun (2001a) found that the fastest pressure reduction rate (9.35 mbar/min) shortened 50 min of cooling time than the slowest one (1.56 mbar/ min). However, the lowest pressure reduction rate could increase total yield of about 1.74% and decrease mass loss from 12.40% to 10.66%, especially an increase in the ratio of average temperature reduction caused by percentage mass loss was obtained. Furthermore, the pressure reduction rates also influence ultrastructure and storage quality of vegetables. He, Feng, Yang, Wu, and Li (2004) indicated the iceberg lettuce treated by a moderate pressure reduction rate of 3.13 mbar/min could relieve the cell injury such as plasmolysis and irregular membrane structure as well as achieving the maximum values of tissue firmness, ascorbic acid and catalase after two weeks of storage. The performance of IVC commonly used in the cooked meat products is more susceptible to the rate of depressurization. However, unlike previous expectance, the increase in pressure reduction rates can only significantly (p < 0.05) shorten the time of the first stage and has little effect on the remaining stage, thus slightly increasing the whole cooling time (p > 0.05). During the cooling process, the fast pressure reduction rate increases the rate of water evaporation. If the vapor cannot be removed timely, it will condense on the surface of the chamber wall and evaporate again, resulting in the decline of pressure reduction rate. In addition, a high pressure drop rate can achieve rapid temperature reduction of surrounding liquid, but the reduction in meat temperature is not as fast as the liquid because of the low thermal conductivity of meat. Consequently, the big temperature difference between the meat and liquid will increase the time in the third stage, which is dominated by heat convection and conduction (Feng et al., 2013a). On the other hand, due to that mass loss mainly occurs in early evaporative stage, the effect of different pressure reduction rates on cooling mass loss is appreciable during the first 60 min of cooling, but the effect is weakened gradually in the subsequent process (Drummond & Sun, 2012). Therefore, the restriction of different stages as well as product size should be considered when choosing pressure reduction rate in IVC. Song and Liu (2014) designed a vacuum pump with a frequency converter, which could change the pressure reduction rate by controlling the volumetric displacement of the pump, and found that a varying volumetric displacement with pressure could not only uniformize the temperature distribution of ham section but could also reduce about 3% of mass loss and 2% of energy consumption. Similar optimization was

5. Challenges and future trends Although many advantages about VC are reported, it cannot completely replace the traditional cooling technologies yet. Firstly, the applicability of this technology is limited because of the principle of water evaporation. Secondly, the inherent mass loss remains a technical bottleneck to the optimization of application performance. Thirdly, the high initial investment of VC equipment is also one of the reasons. However, VC, as a novel cooling technology, is worth popularizing due to its remarkable cooling efficiency and energy-saving feature. Therefore, deeper research in the future to ensure further developments of VC for the food industry is necessary.

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porosity, agitation, controlling the initial water temperature or adding agitation, as well as introducing bubbles in the surrounding liquid. From the perspective of operation conditions, high final pressure can reduce mass loss and energy consumption, but it will cause undesired high final temperatures. A proper pressure reduction rate can avoid excessive boiling during IVC and reach a balance between cooling time and mass loss, as well as ensuring an even temperature distribution of the cooled products. Moreover, intermittent operation of vacuum pump in MSVPR and PIVC saves energy, which can also compensate the mass loss, promote the temperature drop of non-leafy vegetables and prevent quality deterioration caused by dehydration. Future trends of VC are the study of moisture migration characteristics to enhance further understanding of mass transfer at cellular level, leading to the improvement of the cooling performance, the comprehensive performance, and multi-functional optimization of VC process and system. It is believed that with further technological progress, VC can achieve a wider range of commercial applications for the food industry.

5.1. Moisture migration characteristics Although most researches on VC have focused on the mass loss of the products, it is only a global mass difference. So far, the main commercial application of VC is vegetables, however, the temperature distribution and final temperature of the leafy vegetables is not absolutely uniform and consistent during VC process especially for those leaves that are inevitably damaged during harvest and transport, as mentioned previously, which causes the difference in cooling rate of different parts and has the risk of local chilling injury. Although this can be improved by adjusting the pressure reduction rate, it is not a onceand-for-all solution due to the wide variety, complicated structure and different moisture contents of each part in vegetables. As the cooling principle of VC is based on the water evaporation, and moisture content affects microbial infiltration, microstructure and product quality, therefore, it is necessary to study the migration characteristics of water during vacuum cooling process at the cellular level including the migration path, water distribution and state of water loss to further understand the moisture transfer mechanism, leading to the improvement of the cooling performance. Additionally, new observation and measurement methods to assist in the study of the moisture migration of VC should also be explored.

Acknowledgements The authors are grateful to the Key R&D Program of Ningxia Hui Autonomous Region (2018BCF01001) for its support. This research was also supported by the National Key R&D Program of China (2017YFD0400404), the Agricultural Development and Rural Work of Guangdong Province (2018LM2170, 2018LM2171, 2017LM4173), the International and Hong Kong – Macau - Taiwan Collaborative Innovation Platform of Guangdong Province on Intelligent Food Quality Control and Process Technology & Equipment (2015KGJHZ001), the Guangdong Provincial R & D Centre for the Modern Agricultural Industry on Non-destructive Detection and Intensive Processing of Agricultural Products, the Common Technical Innovation Team of Guangdong Province on Preservation and Logistics of Agricultural Products (2016LM2154).

5.2. Multi-functional optimization Despite numerous studies have been conducted to minimize mass loss and enhance cooling rate, it is still a challenge to find a feasible solution to translate laboratory results into industrial scale applications. Considering many factors that affect the mass loss and cooling rate such as size, porosity, pressure, cooling medium characteristics, etc., multifunctional optimization of operation processes and conditions with low operational complexity should be explored. In addition, optimization with more efficient algorithms can become a good tool to determine optimal cooling conditions for improving cooling performance. Furthermore, VC technology has great potential for application in cooling some cooked wheaten food such as steamed stuffed bun and bakery products, however for industrial application, it is necessary to optimize the ingredients and production process to increase the initial water content to balance the problems of high mass loss and texture change due to water loss. Last but not least, multi-functional optimization of equipment by combing with other process or technique, such as combining cooking with VC, and combining VC with modified atmosphere, should have a good market prospect.

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