Study On The Microwave Vacuum Drying Characteristics And Quality Of Pineapple Slices

Pineapple is one of the world's four famous fruits, and has special nutrition, cosmetology as well as medicinal value. Microwave vacuum drying (MVD) technology was applied to pineapple slices drying in this paper. The MVD characteristic of pineapple slices was studied firstly and BP neural network models of MVD were established correspondingly. Meanwhile, effects of different drying parameters on main quality of pineapple slices were studied; the operating conditions were optimized, also the pineapple slices of combined airflow and microwave vacuum drying (A MVD) were studied. Finally comparison with the traditional airflow drying (AD), analysis of three kinds of dried pineapple slices drying energy consumption mode and the main quality changes were studied to confirm the feasibility of MVD and AMVD process.1. MVD drying characteristics of pineapple slices were systematically studied. The MVD process to pineapple slices was divided into three stages:speed-up, constant-speed and speed-down. The more microwave power were, the quicker dehydration rate was and the shorter drying time was. The more loadage were, the shorter drying time was , but the drying rate increased first and then decreased. Vacuum degree had great influence on the dehydration rate of constant-speed and speed-down period. It could obviously increase dehydration rate and shorten drying time by increasing vacuum degree. However, the higher vacuum degree was, the more energy required. The high fiber content of dried pineapple slices presented the coking phenomenon which affected the quality and energy consumption of product. Therefore, it was very important to choose suitable vacuum degree to dry pineapple slices. The less slices thickness was, the quicker dehydration rate was and the shorter drying time was.2. According to the relation of pineapple slices of drying time, drying parameters and moisture ratio (MR), MVD BP neural network model were established, and it could be used to predict the moisture change,and it showed that the model has good application value which can be applied to other fruit and vegetable drying characteristics of the forecast area.3. Effects of MVD parameters on main quality of pineapple slices were studied. Influence of polysaccharide ,vitamin C contents and sensory quality of pineapple slices affected by microwave intensity,vacuum degree and slice thickness was got. The polysaccharide content of pineapple slices increased when microwave intensity and slice thickness decreased,and vacuum degree increased . The vitamin C content of pineapple slices increased when microwave intensity and slice thickness decreased,and vacuum degree increased . The sensory quality of pineapple slices decreased when microwave intensity and slice thickness increased,and vacuum degree decreased. To make the nutritional content of pineapple pieces are large reserved, and the sensory quality of pineapple slices ensured, so microwave vacuum drying parameters :the higher vacuum degree, medium microwave intensity, the smaller slice thickness should be chosen suitably.4. By microwave intensity, vacuum degree for test factors, quadratic regression general rotary unitized design was adopted, with pineapple slices drying time, unit energy consumption, polysaccharide contents and sensory quality as evaluation indexes, the process optimization of MVD of pineapple slices was studied. Based on the experimental data, the quadratic regression model of four evaluation indexes were established, the effects of independent and combined action of microwave intensity and vacuum degree on each indexes were analyzed, then variables were analyzed with response surface methodology, and Evaluation Function Method was applied to optimize drying technology. It was indicated that microwave intensity and vacuum degree had significant influences on each index. The optimal MVD technology for pineapple slices was determined as follows: microwave intensity was 6.5 W/g , vacuum degree was -90kPa and slice thickness was 6mm.5. Optimization of combined AD and MVD technology for pineapple slices was studied. Polysaccharide contents, unit energy consumption and sensory quality of dried pineapple slices were chosen as evaluation indexes, effects of airflow temperature, conversed moisture content and unit mass microwave power on the quality of dried pineapple slices and drying energy consumption were studied. By regression analysis, the optimal combined AD and MVD technology of pineapple slices was determined as follows: airflow temperature was 60℃, conversed moisture content was 25%, slice thickness was 6mm, and unit mass microwave power was 5 W/g. The better technology of combined AD and MVD for pineapple slices was compared with single AD and single MVD. It showed that the quality of dried pineapple slices could be slightly reduced by combined AD and MVD and the energy consumption could be slightly larger than MVD. It showed that the quality of dried pineapple slices could be greatly improved by combined AD and MVD and the energy consumption could be obviously reduced than single AD. Therefore, the technology of combined AD and MVD is a kind of popularization of drying technology.