Study On Combined Drying Technique Of Nelumbo Nucifera Gaertn Chips

Nelumbo nucifera Gaertn is belonging to the genus of nelumbo of nymphaeaceae. It is a kind of large perennial herbaceous aquatic vegetation, with high edible value, unique nutritional value and various healthcare effects, loved by consumers. Browning reaction of lotus root happened easily, which caused, the fresh lotus root is not suitable for long-term storage or transportation, while drying is one of the most effective ways to extend the supply period. At present, dried products of lotus root included lotus root starch, dehydrated lotus root and lotus root chips. The first two products were dried by sunshine or hot air, their additional value were low but nutritive loss was serious. Currently the drying method of lotus root chips on the market was frying, on this occasion, the oil content of products was high and controllability of quality was poor. Therefore, it is significant to study new drying technology of lotus root, in order to improve the quality and additional value of dried products. In this thesis, effect of different drying methods on quality of lotus root chips, optimal processing parameters of combined vacuum microwave-explosion puffing drying of lotus root chips and dynamics models of hot air drying and vacuum microwave drying of lotus root were studied. The main results were as follows:The effects of 6 drying methods including hot air drying, vacuum microwave drying, hot air-vacuum microwave combination drying, hot air-explosion puffing combination drying, vacuum microwave-explosion puffing combination drying and hot air-vacuum microwave-explosion puffing combination drying on physico-chemical properties (color, shrinkage, hardness, crispness, vitamin C, polyphenol and micro-structure) of dried Nelumbo nucifera Gaertn were investigated. The results showed that compared with single dried-products, the four kinds of combination-dried lotus root chips had acquired much better quality. And shrinkage and browning degrees of hot air-vacuum microwave-explosion puffing combination-dried lotus root chips were significantly lower than vacuum microwave-dried products; vitamin C and polyphenols retention of this drying method had an obvious advantage over hot air drying; hardness and crispness of chips were moderate and organizational structures were porous. Overall, hot air-vacuum microwave-explosion puffing combination drying method was the most suitable way for processing Nelumbo nucifera Gaertn chips among all the six drying methods.In order to optimize the technology for combined vacuum microwave-explosion puffing drying of Nelumbo nucifera Gaertn chips, based on the single factor experiments, a three-factor quadratic regression combination design was adopted. The influence of microwave power per unit mass, water content at conversion point and puffing temperature on the L value, hardness, crispness and water content of Nelumbo nucifera Gaertn chips producted by combined vacuum microwave-explosion puffing drying was studied, and variables were analyzed with response surface methodology. The optimal processing parameters were microwave power per unit mass of 12.48W/g, water content at conversion point of 48.73% and puffing temperature of 86.56℃.Hot air and vacuum microwave drying techniques were used to dry lotus root slices to research its drying properties respectively. The results showed that both drying procedures of lotus root slices could accurately be described by the Page model. The suitable kinetics models of both drying techiques and hot air-vacuum microwave combined drying method which could be used to describe the relationship of moisture ratio and drying time were established base on experimental data and the proof tests were performed. The effective diffusivity coefficients were also computed. The results showed that the predicted values of the models were nearly consistent with the observed values; the effective diffusivity coefficients of vacuum microwave drying were fairly higher than hot air drying.