| Fresh fruits and vegetables are rich in carbohydrates,vitamins and various micronutrients,which are of great significance on strengthening the immune system and improving physiological function of human.World Health Organization(WHO)had taken the daily intake of them as a radical index to appraise human health.China is a large agricultural country with annual yields of fruits and vegetables always occupying the first place in the world recent years.The obvious regionalism,seasonality and lack in complete and independent cold chain,however,lead to the loss of heavy postharvest fruits and vegetablesbe significantly greater than that of developed countries.Therefore,further study on preservation method of fruits and vegetables is essential to improve the agricultural production in our country.On the other hand,with the increasing concern on food safety,chemical preservation methods have been forbidded gradually in developed countries.The study of physical preservation methods has become an important field in preservation technology of fruits and vegetables.In this paper,based on multi-subject approaches,cold shock treatment system and cold shock treatment technology for postharvest fruits and vegetables were theoretically and experimentally researched.The main contents of the dissertation are as following:(1)A heat transfer model was proposed based on the structural features of fruits and vegetables in present study,which was solved by numeric method.The cold shock processes of cucumbers with chilled air and cold water are simulated by the validated model,and the effects of flow velocity of medium,size and thermophysical properties difference of fruits and vegetables on the heat transfer process are also analyzed.(2)Cucumbers were cold shocked at 0oC,3oC and 6oC with different duration,then stored above or below critical chilling injury temperature.The differences of life characteristics among groups with cold shock treatments were analyzed based on physiology and bio-chemistry indexes changes during storage.Combination with the heat transfer characteristics,the influence mechanism of cold shock parameters to physiology and bio-chemistry indexes was researched to find the heat transfer and biology mechanisms that cold shock treatments improve the storage quality and prevent the degradation of fruits and vegetables.(3)A new fuzzy evaluation model based on entropy weight method was built to assess the preservation quality of cucumber quantitatively,in which the variance contribution method was used to determine the weight of evaluation index every day,and the maximum index was chosen as the standard matter-element.The preservation quality of cold shocked cucumbers was comprehensively evaluated by using weight loss,color,firmness,antioxidant enzyme activities(POD,CAT,SOD),malondialdehyde(MDA)and respiration intensity as evaluation indexes.(4)A method based on thermodynamics entropy was promoted to describe the state change of fruits and vegetables,based on which the cold shock intensity was quantitatively described,and a model was established to predict the preservation quality of fruits and vegetables with cold shock treatment.(5)A cold shock treatment system was designed and built based on thermodynamic inverse cycle,which was auto-controlled by Siemens S7-200 programmable logic controller.The temperature change,energy consumption,coefficient of performance,suction and discharge pressures of compressor during cooling and stable period were comparatively studied in different cold-shock modes,and operation characteristics of different modes were also analyzed.Furthermore,the advantages and disadvantages of different deicing methods were compared with each other. |
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