Studies On Technologies Of Hot-air Combined Microwave Vacuum Drying Of Litchi Pulp

Litchi (Litchi chinensis Sonn.) is a kind of special and famous fruit in South China. The fruit deteriorate and decay rapidly after harvest since it mature in hot season, so litchi is often processed to dried litchi. The long drying time and high temperature associated with conventional drying of litchi often causes browning and nutritional value loss of products. Microwave vacuum-drying is a rapid and efficient dehydration method with low temperature that the color, flavor, taste and nutrient of food can be retained better after microwave vacuum drying. Combined hot air and microwave vacuum drying can reduce drying time and improve energy efficiency of the dehydration and the quality of the final product. The objective of this thesis was to produce dried litchi pulp using combined hot air and microwave vacuum drying technology, study the blanching and color protection technology of litchi pulp, analyze the hot air drying and microwave vacuum drying characteristics and kinetics model, optimize the combined drying parameters and investigate the storage stability of dried litchi pulp.In order to reduce the browning reaction, the blanching and color protection technology of litchi pulp was studied. The litchi pulp was blanched in hot water (80℃、90℃and100℃) for1min,2min and3min respectively to inactivate the polyphenol oxidase (PPO). After immersed in vitamin C, citric acid, phytic acid and CaCl2solutions in different concentrations for1h, litchi pulp was then dried with hot air drying. The water content, rehydration ratio, color and sensory evaluation of dried litchi pulp was compared to ascertain the best complex color protectors.The hot air drying characteristics of litchi pulp was studied. Effects of size, maturity, hot-air temperature and air velocity on hot air drying characteristics of litchi pulp were analyzed. The microwave vacuum drying characteristics of litchi pulp was studied. Effects of microwave power, vacuum pressure and loading capacity on drying rate of litchi pulp were investigated. Hot air drying and microwave vacuum drying kinetic models of litchi pulp were also investigated.Orthogonal test was carried out to investigate the effects of hot air temperature, conversed moisture content, microwave intensity and pulse ratio on the L value of the dried litchi pulp, specific energy consumption and sensory score respectively. The optimum conditions for drying of litchi pulp by hot air and vacuum microwave combination were found.In order to investigate the storage stability of dehydrated litchi pulp, the adsorption isotherms of dehydrated litchi pulp were determined and simulated. The quality changes of dried litchi pulp at different packing manner (0.05mm polythene film bag packing, vacuum packing and nitrogen packing) and storage temperatures (0℃,30℃) were studied, including water content, water activity, color, browning index, and5-HMF content.