Multi-stage Vacuum Cooling Pressure Reservation And Storage Characteristics Of Chinese Kale

The Chinese kale is rich in a variety of nutrients,and can effectively prevent cancer,has long been popular with consumers.The leaves have large surface area and high water content,after harvest,they are easily affected by field heat and their own transpiration and respire action,which eventually leads to the decline of nutritional quality and commodity value.Vacuum cooling has outstanding advantages in the field of vegetable pre-cooling because of its remarkable cooling efficiency and energy saving and health safety characteristics,but some limitations such as uneven temperature distribution and large temperature difference weaken its wide application.Therefore,the fresh Chinese kale was took as the research object in this paper to start researches focus on the vacuum cooling effect and storage characteristics.Firstly,the effects of different pressure reduction modes on the cooling effect and quality of Chinese kale were studied,and got the most suitable pressure reduction mode by analyzing and compared synthetically;then the best storage conditions were selected by setting different cooling final temperatures and different storage temperatures.Finally,the prediction model of Chinese kale shelf life was established by using quality dynamics law and Arrhenius equation.The aim is to provide theoretical basis for technical optimization in cold chain process of leafy vegetables.The main findings are as follows:(1)Evaluation of the cooling effect of Chinese kale by the pressure reduction modeDifferent vacuum cooling pressure reduction modes were used to give Chinese kale the cooling treatment,the cooling effect of the pressure reduction mode on Chinese kale was analyzed by cooling curve,temperature distribution field,temperature difference and quality index(weight loss,color,chlorophyll,respiratory intensity)synthetically.The results showed that different pressure reduction modes could significantly affect the temperature distribution of Chinese kale leavers and rhizomes(p<0.05)during vacuum cooling.At the end of cooling,the final pressure of 0.6-0.8 kPa and the leaf temperature reached-1.17?,which led to frostbite,so the minimum pressure should be kept above 0.8 kPa.Compared with other pressure reduction modes,the MSVPR cooling effect was the best,and the average temperature difference between leaf and rhizome was 0.53?at the end of cooling,which effectively solved the problem of large temperature difference during cooling of Chinese kale.After cooling,the temperature of the central vein of the Chinese kale rhizome was the lowest,the temperature of the edge of the leaf was the highest,and the temperature from the central vein to the edge of the leaf increased gradually.Moreover different pressure reduction modes could affect the respiratory intensity,color L*?b*and ?E value of Chinese kale(p<0.05),the frostbite because of too low vacuum cooling pressure could cause metabolic disorder in vegetables and enhance respiratory effect.(2)Study on the quality of Chinese kale shelf based on cooling final temperature in MSVPR technology.Using fresh Chinese kale as raw material with MSVPR technology for cooling treatment,comparing with the traditional vacuum cooling continuous pressure reduction mode by setting different cooling final temperature(0?,4?,8?)explored effect of the cooling final temperature on the weight loss,chlorophyll,color,respiration intensity,Vc and MDA content of Chinese kale during storage.The results showed that MSVPR could effectively and uniformly reduce the temperature difference between the leaves and rhizomes of Chinese kale(the average temperature difference was controlled within 1?),and the storage quality of Chinese kale cooled to 4? was the best(p<0.05);when storage to 20 d,compared with other cooling final temperatures,the Vc loss rate of kale at a final temperature of 4? was 41.41%,the weight loss was 2.73%,and the chlorophyll decreased by 17.28%,it could effectively reduce the respiratory intensity and inhibit the increase of MDA content,extend shelf life.(3)Study on quality characteristics of Chinese kale based on storage temperature under MSVPR technologyThe Chinese kale was cooled based on MSVPR technology.By setting different storage temperatures(0?,4?,8?),the influence of temperature on the quality characteristics of Chinese kale during the storage was studied.The results showed that the higher the storage temperature,the greater the effect on the nutritional quality and physiological activity of Chinese kale(p<0.05).On the 20th day of storage,the quality of Chinese kale stored at 0? was the best,and the weight loss of Chinese kale was 2.23%,the content of chlorophyll and Vc decreased by 8.54%and 36.13%respectively,and effectively inhibited the respiratory intensity of Chinese kale,maintained a better color,and at the same time increased APX activity during storage,prevented excessive accumulation of MDA content,and reduced cell membranes damage.Finally,the commercial shelf days of kale can be extended to 20 days.(4)Construction of model for the shelf life prediction of Chinese kale based on MSVPR technologyBased on the MSVPR technology,the kinetics variation law of the quality index of Chinese kale with storage time under different storage temperatures(0?,4?,8?)was studied,the comprehensive shelf life prediction model was established based on temperature-time quality changes combined with the Arrhenius equation,and the model was validated based on the end value of sensory assessment.The results showed that the lower the storage temperature,the slower the rate of quality change,the weight loss,chlorophyll,and b*value conformed to the zero-order reaction kinetics,and the L*value and VC content conformed to the first-order reaction kinetics.Validation showed that the model fit well(R2>0.9),and the relative error of the kale L*value,b*value and Vc content was small(<10%),which can be used to predict the shelf life based on quality index of kale under different storage temperatures.