Study On The Microwave Drying Characteristics And Drying Technology Optimization Of Lotus-seed

Lotus-seed (Nelumbo nucifera Gaertn) is the ripe seed of lotus, which has rich nutrition and specific curative effects. For the traditional product, dry lotus-seed is favoured by customers at home and abroad. Microwave technology was applied to lotus-seed drying in this paper. The microwave drying characteristics and the rule of quality change for lotus-seed were studied, and kinetic models of microwave drying and colour were established correspondingly. Meanwhile, effects on quality characteristics of different drying technologies were studied to search optimal drying technology for lotus-seed. Therefore an important recommendation for the practical industrialization of lotus-seed by microwave drying was offered.1．The rule of moisture and temperature change during microwave drying for lotus-seed were known. The microwave drying process for lotus-seed was divided into three stages that were speed-up, constant-speed and speed-down. During the drying, the bigger the microwave power per gram, the more frequent the dehydrating rate and the shorter the required time. The changement of central temperature, superficial temperature and environmental temperature were also divided into three stages that were preheating, steady-going and warming-up which was correspongding to dehydrating stages. The bigger the microwave power per gram, the higher the central and superficial temperature of lotus-seed, and furthermore the central temperature was always higher than the superficial temperature and environmental temperature.2．Microwave drying kinetic model for lotus-seed was established. The suitable model was MR=e^(-rt)N, of which r=e^{(-49．038+44．767P-10．077P)2} and N = 1．051 + 0．456P-0．202P²ï¼ŽIt was proved that predicted date and experimental date were nearly accordant and the model could be used for describing the change of moisture in lotus-seed during microwave drying.3．Changes of poly phenol oxidase (PPO) , resistant starch (RS) and colour of lotus-seed during microwave drying were studied. Microwave was used to inactivate PPO activity. It was verfied that the higher the microwave power or the longer the heating time, the better the result. But materials would carbonize if the power was too high or the time was too long. It was good for RS when the lower power and proper time were employed. On the contrary, the yield of RS would be affected. When heating time was the same, lightness (L) and yellowness (b) of lotus-seed would drop down if the power enhanced, but changes of redness (a) , hue angle (H^o) and chroma (C) weren't well-regulated. Moreover, when the heating tme was the same, L and H^o would drop down, a would go up, and that b and C would change according with a parabolic curve if the power increased gradually.4．The kinetic model of colour change for lotus-seed was established by linear and nonlinear regression. Chroma was the best index for describing colour during microwave drying, for it responsed the change of colour more truly and precisely. Its kinetic model was C =-0．019W + 2．308t-0．154t² +0．003t³ + 29．285 , of which R²=0．964, P (=0．000) <0．01．5．Microwave and hot-air combine drying was applied to lotus-seed processing. The optimal technology which microwave power, loadage, transform moisture and hot-air temperature were considered, was obtained by employing uniform design of quasi-level of rehydration rate, RS content and hue angle. It was indicated that effects of four factors on the index were different, but the impact of microwave power was the most significantly. Meanwhile, regressive equation of each index were significantly which was proved by statistical test. The best result was obtained when microwave power was 539W, loadage was 200g, transform moisture was 30% and hot-air temperature was 70℃.6．Effects of different drying technologies on shrinkage, rehydration ratio, content of RS, colour and organization structure of lotus-seed were studied. Six drying technologies were considered that were sun drying, hot-air drying, vacuum drying, freezing drying, microwave drying and microwave and hot-air combine drying. It was indicated that the different drying technologies had different effects on the shrinkage, and the order of shrinking degree was that freezing drying < microwave drying