Studies On Technology And Mechanisms Of Varying Temperature And Pressure Puffing Drying For Citrus Peel Processing

China is the largest citrus grower and producer in the world. Cans and juices are the main current processed product forms of citrus. The pomace, as the by-product of orange juice, is about40%to50%of the fruit in weight, which contains abundant nutrients including soluble sugar, acid, pectin, crude fat, crude fiber, vitamins, amino acids and minerals, etc. Since in the pomace water content exceeds80%, and pectin content is more than20%(on dry matter basis), these factors increase the difficulty in the pomace drying. Furthermore, challenges remain on how to utmost utilize the citrus pomace within the short span of harvest. Varying temperature and pressure puffing drying technology is one of the flow puffing drying technologies. It can be used to process pomace into puffed citrus chips or other food nutrition additives. The manufacturing procedure is simple and ecological, and the products are crispy in taste, easy in preserving, which promise a good market.In order to discover the processing procedures and puffing drying mechanisms of explosion puffing frying technology, the peels of Satsuma orange was focused on to optimize the pre-treatment process and technological conditions of citrus peels using explosion puffing drying, respectively. In the meantime, the effects of drying methods on physical properties, compositions of its essential oil, the main antioxidant compounds and their antioxidant activities of dehydrated orange peels had also been investigated. After a series of experiments, the drying characteristics, kinetics model and isothermal hygroscopic properties had been finally obtained for explosion puffing drying of citrus peels. The experimental results were as follows:(1) The optimal pre-treatment technological conditions of explosion puffing drying were freezing and thawing twice,40%of sugar-impregnated concentration,30min of sugar impregnating at40℃.(2) The optimal processing technological conditions of explosion puffing drying were puffing temperature of95℃, the initial moisture content of60%and vacuum drying temperature of75℃.(3) Based on the analysis of the parameters such as apparent density, shrinkage rate, porosity, rehydration rate and hardness, studied the effects of freeze drying, explosion puffing drying and hot-air drying on quality characteristics of dehydrated citrus peel. The results showed that under freeze drying the color of citrus peel was satisfactory and its rehydration rate was the highest. The texture, color, rehydration rate and brittleness of the citrus peels treated by explosion puffing drying were similar to that of peels vacuum freeze drying, and obviously better than hot-air dried ones.(4) The essential oil content of citrus peels treated by explosion puffing drying, which was up to99.83%, were higher than that of the other two samples treated by freeze drying and hot-air drying. And the types of terpene (41), the main volatile components in citrus essential oil, were higher than that of sample treated by freeze drying (33) and slightly less than that of sample treated by hot-air frying (45). Moreover, the relative amount of citrus terpene (96.51%) of sample treated using explosion puffing drying was the highest among three kinds of samples. The results indicated that in present study explosion puffing drying was the best way to preserve the citrus essential oils. It was also found that the relative amounts of D-limonene and lR-a-pinene of the sample treated using explosion puffing drying were higher than the other two samples, and the relative amounts of (+)-4-carene was higher than that of the sample treated by freeze drying, close to that of the sample treated by hot-air drying. D-limonene,1R-α-pinene and (+)-4-carene are main characteristic flavor compounds of citrus, which demonstrated that explosion puffing drying can contribute to stress the particular flavor of citrus peels compared to other two drying methods.(5) The vacuum freeze-drying had the lowest effects on antioxidant active ingredients of citrus peel and their antioxidant activities. But the explosion puffing drying had an advantage over hot-air drying in preserving the amount of polyphenols of citrus peel. HPLC results showed that four kinds of flavonoids, including hesperidin, orange flavonoids, nobiletin, and tangeretin, were detected in citrus peels obtained from three kinds of drying methods, while naringin and neohesperidin were not detected. The contents of the four kinds of flavonoid of citrus peels treated by explosion puffing drying were higher than those of the sample treated by hot-air drying. It was also found that sugar-impregnating process was the main cause of loss of hesperidin in citrus peels.(6) During explosion puffing drying, there were definite processes of acceleration, constant speed and deceleration with the latter two as the main evaporation steps. The higher the initial moisture content of citrus peel, the longer its constant drying stage. The higher the puffing temperature and the vacuum drying temperature, the shorter the drying process. The optimal temperature for puffing was95℃, and the optimal vacuum drying temperature was (73±2)℃. The kinetic model of explosion puffing drying fitted to the Page equation. It could be predicted that the relationship between water content and drying time, drying temperature and puffing temperature during the process of explosion puffing drying. The model equation was as follows:(7) The adsorption isotherm shape of dried citrus slices from explosion puffing drying was similar to that of apple chips, jujube chips, showing the shape of cis-"S". The equilibrium moisture rate rose with relative humidity increase, and lower with temperature increase. MHE, GAB model could used to predict adsorption isotherms of the three kinds of fruit and vegetable chips. MBET model had no correlation with them. GAB model had widest applicability. Citrus peel crispy slices were best sealed stored at13%to30%of relative humidity, below10℃or higher than30℃.By these researches, the production process of explosion puffing drying was achieved. The changes of quality and antioxidant activity during the process were revealed. By the current researches the characteristics of citrus peel were obtained from explosion puffing drying, a drying model was formulated and the optimal parameters of storage of citrus peel crispy chips were obtained. It provides the theoretical basis for industrialization of citrus peel puffing chips, and opens a new way of deep processing of citrus peel.