Effects Of Processing Methods On Characteristic Components Of Dried Chinese Yam Products

Chinese Yam contains unique nutrients,such as starch,protein,and a variety of characteristic components.And it also has polysaccharide,diosgenin,allantoin and polyphenol,which make yam have both edible and medicinal value at the same time.Currently,the relevant standards of Chinese Yam are usually based on water content,ash content and other indexes as the quality evaluation criteria and grading basis of yam,so the setting of nutritional quality indexes is still not perfect.As a traditional food and medicine substance,the active characteristic components of Chinese Yam are the decisive factors affecting the efficacy.The active characteristic components are included in the evaluation index,and the evaluation of Chinese Yam products combined with other quality indexes can further standardize the Chinese Yam products market.In this study,the traditional variety "Xishi Seed" Chinese Yam(National Geographic Indication product)produced in Heze Chenji,Shandong Province,was dried in different ways,including direct drying(natural drying,atmospheric drying,vacuum freeze-drying,microwave drying),curing(atmospheric pressure curing,high pressure curing,dry heat curing,microwavesteam curing)and then drying.The experimental factors were investigated,and the content change of characteristic components was taken as the index.The influence of different processing methods on the content changes of characteristic components of Chinese Yam was explored to provide certain theoretical basis for the optimization of Chinese Yam processing technology and achieve the purpose of "precise nutrition and moderate processing".Different drying methods had significant difference on the quality of Chinese Yam tablets.The results of hot air drying showed that the contents of resistant starch,protein and diosgenin in Chinese Yam tablets after hot air drying at 45 ℃ were higher than those under other conditions,as follows: 4.89 %,11.70 %,0.07 mg/g,etc.The content of polyphenols is the highest at 55 ℃(1.2 mg/g),while the content of flavonoids is the highest at 65 ℃(0.24 mg/g).From the drying characteristics,the microstructure after hot air drying at 45 ℃ has little damage to the structure of the mountain tablet.The microstructure is compact,the surface is flat,the corrugated folds of different degrees are present,there are cracks,and the debris particles are less.When the temperature was 65 ℃,the moisture ratio of Chinese Yam tablet was relatively small,the drying rate was faster,and the rehydration rate was higher at 45 ℃ and 55 ℃,which were 1.55 g/g and 1.56 g/g,respectively.Microwave drying results showed that the contents of resistant starch,polysaccharide and allantoin after microwave drying of P4(280 W)were 11.8mg/g and 0.34 mg/g,respectively.The contents of resistant starch and flavonoid were the highest after P5(140 W)microwave drying,which were 4.02 % and 0.11 mg/g,respectively.From the drying characteristics,the microstructure of microwave drying P5(140 W)was orderly,and the moisture ratio of Chinese Yam tablet was the smallest and the drying rate was the fastest when the power was P1(700 W).The rehydration of P1(700 W)and P2(560 W)was higher than that of P1(2.23 g/g)and P2(2.27 g/g).The resistant starch,polysaccharide,diosgenin,polyphenols and flavonoids in vacuum freeze-drying group were all higher than those in other groups,indicating that vacuum freeze-drying had better retention of characteristic components.From the drying characteristics,after vacuum freeze-drying,the microscopic structure was honeycomb pores,relatively flat,with minimal shrinkage compared with other drying methods,and the original structure of cells was relatively intact.The drying rate was fast and the rehydration rate was the highest(3.03 g/g).The retention degree of characteristic components of Chinese Yam was less than that of vacuum freeze-drying.From the drying characteristics,the cell wall of Chinese Yam tablet was not damaged greatly after natural drying,and there were some cracks and complete microstructure.The drying rate of natural drying was slower,and the rehydration rate was lower(1.51 g/g).The nutritional quality of Chinese Yam was evaluated by analytic hierarchy process(AHP).The higher comprehensive scores included vacuum freeze drying group > 45 ℃ hot air drying group > P4(280 W)microwave drying group >P5(140 W)microwave drying group > 65 ℃ hot air drying group > 50 ℃ hot air drying group >55 ℃ hot air drying group > natural drying group,etc.In conclusion,the optimal drying conditions for Chinese yam were 45℃ for hot air and P4(280 W)for microwave.The characteristic components of Chinese Yam were different among different drying methods after curing.The content of resistant starch after high pressure cooking and vacuum freeze-drying was the highest,which was 8.74 %.The polysaccharide content of Chinese Yam obtained by high pressure cooking and drying was the highest,which was 20.15 mg/g.The highest content of diosgenin was 0.40 mg/g after high pressure cooking and natural drying.The highest allantoin content was 0.59 mg/g after hot air drying.The content of polyphenols of microwave-dried yam was the highest,which was 1.05 mg/g.The highest flavonoid content was 0.11 g/g after high pressure boiling and vacuum freeze-drying.According to the comprehensive evaluation of Chinese Yam by AHP,the groups with higher comprehensive score were: high pressure cooking and curing group > atmospheric steam freeze-drying group >atmospheric steam natural drying group > atmospheric steam hot air drying group > baking and curing natural drying group > microwave steam freeze-drying group,etc.Due to the differences in processing methods,the nutritional components and active substances of Chinese Yam will be different,and the direction of product development will also be different.