干燥方式对脱水桑叶品质的影响

Effects of Drying Methods on Dried Quality of Mulberry Leaf

以新鲜桑叶为原料,研究冷冻干燥、热泵干燥、微波真空干燥和自然晒干4种干燥方式对桑叶营养成分、复水比、色差、组织结构及风味等品质指标的影响。结果表明:干制后的桑叶营养成分流失严重,色泽由鲜绿转为深绿,酯类和烃类物质明显增多;其中冷冻干燥的桑叶品质最佳,对桑叶的叶绿素、酚类和蛋白保留率较高,色泽较为鲜亮,组织结构没有发现明显皱缩,复水率可达93%;微波真空干燥和热泵干燥的桑叶品质次之,自然晒干的桑叶品质最差;通过分析干燥方式与风味物质相对含量的关联程度发现,酯类和烃类在干制桑叶风味评价中具有重要作用。热泵干燥和冷冻干燥的风味物质相近,且主要体现在酯类含量上的差异;微波真空干燥和自然晒干的风味物质比较相近。综合来看,桑叶脱水干燥方式首选冷冻干燥,其次为微波真空干燥和热泵干燥,不宜选择自然晒干,而在脱水桑叶的工业化推广上应首选热泵干燥。

The quality indexes of fresh mulberry leaf, including nutritional component, rehydration rate, color difference, organization structure and flavor, could be affected by drying methods, such as freeze-drying, heat pump drying,microwave drying and sun drying. The results showed that nutritional components of dried mulberry leaf lost seriously,color changed from bright green to dark green, esters and hydrocarbon materials increased obviously. The quality of freeze dried mulberry leaf had higher rehydration potential, higher chlorophyll, phenols and protein content, better color and lower shrinkage in organization structure than those prepared by other drying methods. As a result the final product of freeze-drying was the best. The quality of microwave dried and heat pump dried mulberry leaf followed by freeze dried products, however, the quality of mulberry leaf using sun drying was the poorest. The results also showed that esters and hydrocarbon materials played important roles in the evaluation of dried mulberry flavor through analysis about the correlation degree between drying methods and relative content of flavor compounds. Freeze drying yielded a product similar in volatile compounds to that obtained in heat pump drying and the difference between them was mainly in esters content.in addition, the flavor compounds of microwave dried product and sun dried material were nearly the same. Comprehensively, the first choice of drying method on mulberry leaf is vacuum freeze-drying, followed by microwave vacuum drying and heat pump drying, but sun drying was not recommended. But considering energy consumption and economic benefits, heat pump drying is the best choice in the industrial extension process of dehydrated mulberry leaf.