Study Of Combined Hot Air And Microwave Vacuum Drying Colocasia Esculenta(L.)Schott Chips

Taro(Colocasia esculent a(L.)Schott),a kind of characteristics of economic crop,featured by short harvest period,high content of moisture and ployphenols,easy to be browning and degenerative,is hard for storage and preservation.Therefore,drying is one of the most effective ways to extend the supply period.This study,using fresh taro as the raw material,is aimed to obtain taro chips with high nutritional value,good taste and quality,through comparing the the quality of fresh and dried products,optimizing the pretreatment methods and drying technologies of taro chips.The study is of great significance to open up a new way for taro's deep processing and promote the taro industry development.The main results are as follows:1.The nutritional compositions and the qualities of eight varieties of dried taros were analyzed.Results showed that total contents of soluble protein,polyphenols,Vc content in Longxiang,Hongxiang,Xianghe and Wugu taro were significantly higher than those of the other four varieties.The highest soluble protein,polyphenols,Vc content were observed in the Hongxiang,Fenghua and Longxiang taro,which were 1455,72.99 and 4.48 mg/100g,respectively.Through comparing the color of taro before and after microwave drying,the result showed that Wugu,Xianghe,Hongxiang taro had low brightness value and high color change value.Polyphenols content and the oxidation degree of Fenghua and Xiangsha taro were higher in pretreatment which made the chips yellowish-brown.The structure was strongly influenced by taro starch and moisture content.For example,due to the low moisture content of Lipu taro and Pinang,starch can't absorb enough water when heated and starch granules expansion was limited,therefore,it was difficult to expand during microwave drying.While for Longxiang,Fenghua,Xiangsha and Wugu taro,whose water content were higher,starch could absorb adequate water and expand fully.Thus,the four varieties of taro chips had better puffing rate,texture,crispness and uniformity.2.This section,using Longxiang taro as the material,is aimed to study the effect of different pretreatments on the quality of taro chips.Results showed that,after being heated and blanched,the amount of amylose and amylopectin were reduced significantly,and the gumminess and chewiness also decreased,which was conducive to the expansion of taro slices during microwave vacuum drying.Maltodextrin molecular spread into the internal material after impregnating,which strengthened the structure dense,make the chip structure more uniform.Freezing-thawing process reduced the interaction between chemical components,decreased puffing resistance during microwave drying,and it also made the taro chips looser in structure,smaller in its hardness.B+M+F pretreatment could produce the highest brightness value,puffing rate,moderate hardness,uniform texture and loose structure,thus it was a suitable pretreatment method for taro slice with vacuum microwave drying.3.The effects of seven combination drying methods on drying time and quality were investigated.The coefficient of the moisture effective diffusion of taro slices dried by LMD+HMD,HMD+LMD was higher than the others significantly.Due to the high efficiency and uniformity of microwave,the center of products appeared brown.Products had lower brightness and high browning degree.The temperature during MD+AD process was relative lower,which inhibited material non enzymatic browning reaction and made taro chips appear ivory.But because of the microwave energy was mainly used to evaporate water at the early stage,the taro slice couldn't expand,and the volume also shrunk seriously during hot air drying,which made the crispness of chips worse.Compared with the MD+AD,AD+MD drying time shorted by 62.5%,Vc retention rate increased nearly by 10%.And the taro chips dried by this method had porous structure and obvious taro flavor.For AD+HMD+LMD,the chip hardness and crispness were moderate,the value and Vc retention were 83.8,35%,respectively,which were higher than other kinds of drying methods.4.The study,based on the above research,used single factor experiments,optimized the technology for combining hot air and vacuum microwave in the drying of taro chips through adopting the response surface central composite design,studying the influence of hot temperature,transition point expressed in moisture content,pulse ratio on the hardness,L value,Vc retention rate and the establishment of a quadratic polynomial model equation.The optimal processing parameters were hot air temperature of 63?,transition point expressed in moisture content of 63%,pulse ratio of 4,and on this condition the quality was better,hardness,the L*value,Vc retention rate of taro chips were 4155g,83.3 and 30.36%,respectively.