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Abstracts: Several pre-cooling methods for bok choi were used, such as natural convection pre-cooling (NCPC), strong wind pre-cooling (SWPC), vacuum pre-cooling (VPC), cold water pre-cooling (CWPC), electrolyzed water pre-cooling (EWPC), and fluid ice pre-cooling (FIPC), in order to determine the most suitable precooling method. It was found that VPC reduced the respiration rate, inhibited the increase of malondialdehyde (MDA) and relative electrolyte leakage, and significantly decreased the total bacterial count. This may be due to the rapid decompression process during vacuum pre-cooling, which disrupts the microbial structure and has a certain sterilizing effect. Bok choi pre-cooled by VPC had the best color, hardness value, chlorophyll, titratable acid (TA) content, vitamin C (VC) content, total phenolic (TP) content, soluble sugar content, superoxide dismutase (SOD) activity, ascorbate peroxidase (APX) activity, and catalase (CAT) activity. Therefore, the most suitable pre-cooling method for bok choi among the above pre-cooling methods was the VPC method.

To optimize the treatment conditions for vacuum pre-cooling of baby cabbage and maintain the fresh quality of postharvest baby cabbage, a central composite response surface experiment with two factors and three levels was conducted on the basis of analyzing the effects of different vacuum pre-cooling final temperature (0, 2 and 4 ℃) and different vacuum pre-cooling final pressure (400, 600, 800 and 1000 Pa) on the pre-cooling of postharvest baby cabbage. The results showed that compared with the control without pre-cooling and the pre-cooling group with final temperature of 0 ℃, the pre-cooling with final temperature of 2~4 ℃ at the final pressure of 600 Pa could better maintain the appearance quality of postharvest baby cabbage, inhibit the increase of respiratory rate and malondialdehyde (MDA) content. Under the condition of final temperature of 2 ℃, compared with the control without pre-cooling and the pre-cooling group with a final pressure of 1000 Pa, pre-cooling with final pressures of 400~800 Pa could also better maintain the appearance quality of postharvest baby cabbage, inhibit the increase of respiratory rate and MDA content. Further central composite response surface experiments showed that the optimal conditions for vacuum pre-cooling of postharvest baby vegetables were final temperature of 4.0 ℃ and final pressure of 600 Pa. The vacuum pre-cooling condition not only maintained good quality of postharvest baby vegetable, but also increased the content of soluble protein, total phenols, and ascorbic acid by 18.78%, 18.47% and 33.23%, respectively. Therefore, appropriate vacuum pre-cooling treatment is a highly potential commercialization technology that could effectively maintain the good quality of postharvest baby vegetable during circulation and shelf life.

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This work aimed to study the effect of atomized water spray during vacuum pre-cooling on the quality of pakchoi during low temperature circulation and the shelf life period. Quality and nutritional parameters of pakchoi subjected to vacuum pre-cooling combined with atomized water spray were analyzed during simulated circulation for one day at (4 ± 1) ℃ and the shelf life period of five days at (20 ± 1) ℃. The results showed that the co-treatment not only decreased the pre-cooling time by 68%, but also reduced the water loss rate of pakchoi by 73.29% and the mass loss rate after the shelf life by 15.6% in comparison with vacuum precooling alone. The co-treatment also delayed the yellowing process, and maintained higher contents of titratable acids, soluble sugars, and soluble proteins in pakchoi. In addition, on the last day of shelf life, the contents of nitrite and malondialdehyde in the co-treatment group were decreased by 13.70% and 14.82%, respectively compared with the vacuum precooling group. To sum up, vacuum pre-cooling combined with atomized water spray could not only solve the problem of water loss caused by vacuum pre-cooling and shorten the pre-cooling time, but also delay the yellowing of leaves and maintain better nutritional quality of pakchoi.

Copyright of Shipin Kexue/ Food Science is the property of Food Science Editorial Department and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Vacuum pre-cooling is a multi-physics process coupling heat transfer, water transport, and shrinkage, the stresses caused by water evaporation and temperature variation in food interact with its deformation. However, the developed models usually assume that the shrinkage of food is insignificant so that it is ignored, which is inconsistent with the actual vacuum pre-cooling process and makes the simulation results inaccurate. Taking strawberry as an example, a multiphase porous media model with heat-water and mechanics (HWM) bi-directional coupling was developed for food vacuum pre-cooling. The accuracy of the HWM model was verified by comparing the heat-water (HW) model without considering shrinkage and the experiments. The results show that the development of the evaporation front predicted by the HWM model is more sensitive to the vacuum chamber pressure change compared with the HW model. The maximum average relative errors corresponding to the temperature and shrinkage of the HWM model are 0.27% and 6.95%, respectively, which are better than the HW model. In addition, the edge gradient effect that occurs in the middle stage of vacuum pre-cooling was observed. When the vacuum chamber pressure approached the lowest point, the evaporation region rapidly developed toward the core. The increase of evaporation driving force and the decrease of boundary moisture caused drastic temperature and shrinkage changes on the product surface. The model developed in this paper considering shrinkage provides technical support to optimize the vacuum pre-cooling process and improve product quality. • Shrinkage of strawberry during vacuum pre-cooling was modeled. • Heat and mass transfer model coupled with shrinkage was developed. • An improved method of predicting vacuum chamber pressure was proposed. • The edge gradient effect due to pressure drop was predicted. [ABSTRACT FROM AUTHOR]

Vacuum pre-cooling of food products after harvesting is a multiphysics problem coupling the fluid flow, heat and mass transfer, on which a series of numerical simulations have been performed. However, Multiphase flow with cross-regional nature is not coupled with heat and mass transfer processes, which is inconsistent with the actual vacuum pre-cooling process and makes the simulation results inaccurate. Taking shiitake mushroom (Lentinula edodes) as an example, a multiphase porous media model modified by the cross-regional phase transition equation for the thermo-hydro coupling of food vacuum pre-cooling (MTH) was established in this paper. The accuracy of the proposed model is validated by experimental results. The results show that the average errors corresponding to pressure, temperature, and weight loss were 23.47%, 6.29%, and 17.6% respectively. Compared with the traditional thermo-hydro coupling model (TH), the MTH model is more sensitive to pressure change, the average errors of temperature and weight loss are reduced by 4.19%, 9.89% respectively. Both models result in the lowest weight loss when the humidification ratio is close to 2%. In further, the simulation results predict the development of the evaporation front from the edge of the evaporation zone towards the inner core. The model developed in this paper considering cross-regional phase transition contributes to an in-depth understanding of the mechanism of vacuum cooling. • A cross-regional phase transition model based on saturation temperature and latent heat of evaporation was developed. • The model considered multiphase flow, permeation, and diffusion. • The effect of different humidification proportions on the weight loss rate was studied. • The model predicted the development of the evaporation front. [ABSTRACT FROM AUTHOR]

Heat and mass transfer of fruit and vegetables is significantly affected by vacuum pre-cooling end temperature (VPET). The simulations and experiments on the heat and mass transfer of sugarcane stem were performed to investigate the trends of total system pressure, temperature distribution, the water evaporation rate, weight loss, and thermal conductivity versus time during vacuum pre-cooling. A cylindrical model of heat and mass transfer of sugarcane stem was established, taking the transient heat transfer and mass conservation into consideration. Based on mass and energy conservation, the governing equations for water involved were coupled. What is more, the respiratory heat was regarded as the internal heat source. The finite volume method (FVM) was employed to precisely derive differential equations. The results of the numerical simulation and experiments showed that the maximum deviation of mean temperature between simulation and experiment was less than 2.6 °C. The maximum weight error between the simulation results and the experimental data was less than 2%, while the error of weight loss was 16.35%. The thermal conductivity in the radial direction of the sugarcane stem was lower than that of the axial direction. Compared with the simulation process, the two peaks of water evaporation rate in the experiment were caused by the change in relative humidity. The maximum errors in simulation and experiment operation for water evaporation rate and relative humidity were below 25% and 8.4%, respectively. The results implied that simulation results were consonant with the data obtained in the experiment. The reliably cylindrical model can be taken as an ideal reference for simulation research of other processes in the vacuum pre-cooling. • Cylindrical model of sugarcane stem during vacuum pre-cooling was established. • VPET was set to 2 °C, 4 °C, 6 °C, 8 °C, 10 °C, and 12 °C, respectively. • Heat and mass transfer processes of vacuum pre-cooling were studied. • Axial thermal conductivity of sugarcane stem was higher than radial direction. • Errors between simulation results and experimental data were analyzed. [ABSTRACT FROM AUTHOR]