Effects Of Pressure Reduction Regulation On Vacuum Cooling Of Dry/Rainy Season Pakchoi(Brassica Chinensis L.)

Vacuum cooling（VC）is a cooling technology for moist and porous foodstuffs with superiority in cooling rate,uniformity and hygiene,and low energy consumption.The principle of VC is based on the evaporative heat transfer mechanism.Therefore,it is an efficient cooling technology for leafy vegetables.However,there are still some disadvantages that weaken the competitiveness of industrial applications such as the inevitable mass loss,uneven cooling in different parts of leafy vegetables and microbial safety.Numerous studies have shown that pressure reduction rate is a crucial processing parameter that influences the cooling performance.The coupling of heat and mass transfer during VC is closely related to the pressure that causes the phase transition of water.Therefore,pressure reduction regulation can be used to optimize the cooling performances of dry season vegetable.In addition,the hot and rainy climate in southern China has a great impact on postharvest vegetables,but little is known about the effects of VC of rainy season vegetables.It is thus expected that the synchronous dewatering of rainy season vegetable cooling can be achieved by VC with pressure reduction regulation.Therefore,in the current study,dry and rainy season pakchoi（Brassica chinensis L.）were processed by VC with pressure reduction regulation.The effect of pressure reduction rates on VC of different edible parts of dry season pakchoi was discussed first.The Feasibility of simultaneous water removal by VC of rainy season pakchoi based on the preservation characteristics was verified then.In addition,the principal component analysis（PCA）and hierarchical cluster analysis（HCA）were used to employed for further evaluation of the effects of pressure reduction regulation on vacuum cooling of dry/rainy season pakchoi.It is hope that the current study can provide some guidance for upgrading of VC equipment and expanding the applicability of VC technology.The main research results are as follows:1.The results showed that the leaf was more sensitive to pressure stress than the petiole during VC process,and more serious cell damages were observed on the adaxial epidermis,where the leaf tip and edge were the most severe areas.For the petiole,the positions near the leaf lost water easier.Lowering the pressure reduction rates could minimize the cooling difference within the edible parts,but it could also cause stagnation in temperature reduction,thus increasing the mass loss.Furthermore,the rapid pressure reduction rate possessed the best cooling efficiency and minimal quality deterioration.The moderate pressure reduction rate provided the best temperature uniformity,the smallestΔE and the lowest MDA content,while the slow pressure reduction rate could weaken the temperature increase during the pressure recovery stage but might cause the forced opening of stomata due to long vacuum duration,which was further supported by microstructure observation.2.Rainy season pakchoi was simulated by immersion treatments.The increased water during immersion treatments mainly played critical roles in cooling（water loss rates）,water replenishment（water absorption rates）and humidification（residual water rates）during VC.Lowering the pressure reduction rates could significantly increase the water loss rates and decrease the water absorption rates and residual water rates（p<0.05）.3.The water removal of VC could alleviate the effect of water stains caused by immersion treatment in rainy season pakchoi.During the preservation,I₆₀VC_UP treatment showed lower mass losses,comprehensive color differences and MDA contents as well as higher free water,vitamin C,chlorophyll contents and sensory scores in the non-packaging group.The perforated packaging was generally conducive to quality improvement due to the micro-spontaneous modified atmosphere.For packing group,I_60Ptreatment showed the highest moisture contents and sensory scores and lower respiration rates,but it was not as good as the I₆₀VC_P treatment in terms of nutrient preservation ratse and reduction of cell damages.4.Based on the comprehensive evaluations of multivariate statistical analysis,HCA showed that the quality of pakchoi using a rapid pressure reduction rate of VC was the closest to that of fresh one,but the effect of pressure reduction rates on the quality of pakchoi was not significant.As long vacuum duration could produce stagnation in temperature reduction,causing higher mass loss and cell damage,and decreasing the pressure reduction rates could have a positive contribution to temperature uniformity,if the pressure reduction modes can be adjusted according to the temperature change to alleviate the occurrence of the temperature stagnation while lowering the pressure reduction rates and shortening the vacuum duration as much as possible,the VC performance could be significantly improved.In addition,the results of PCA and HCA showed that VC combined with perforated packaging showed the best quality at the end of storage and performed the best storage stability.Therefore,VC showed high application potential in the simultaneous cooling and water removal of rainy season vegetables.