Preparation And Characterization Of Yam Powder And Cross-linked Starch

Yam is rich in nutrients and medicinal ingredients,and is a high-value medicinal and food homologous food.The concentration of yam harvest season and the difficulty of storage and transportation directly affect the application of yam,so it is possible to apply yam powder or extract the starch to the food industry.At present,yam powder is usually peeled,which produces a large amount of waste residue and loses nutrients and bioactive components in the yam peel.In addition,the processing suitability of the raw starch extracted from yam has some defects,which need to be modified to meet the processing needs and broaden its application range.Therefore,in this paper,yam was used as raw material to prepare non-peeled yam powder,and the extracted raw starch was cross-linked to prepare cross-linked starch,and the yam powder and its cross-linked starch were systematically characterized.The research results are as follows:1.Taking yam as raw material,the effects of peeling process on the preparation,physicochemical properties and biological activity of yam powder were investigated.Firstly,the peeled yam powder and the non-peeled yam powder were prepared.The physicochemical properties and structural characteristics of the yam powder were investigated;the total flavonoid content,total soluble polyphenol content,reducing power and DPPH free radical scavenging ability were determined by spectrophotometry.The results showed that the yield of non-peeled yam powder was 21.37%;the non-peeling treatment improved the water binding capacity（194.53 g/100g）,swelling power and solubility of yam powder;and increased the total flavonoids in the yam powder（0.22 mg RE/g）and total soluble polyphenols（2.8 mg GA/g）and DPPH free radical scavenging ability(EC₅₀ value 43.86 mg/mL)and reducing power（0.56 V_C/g）;the morphological characteristics and crystal structure of the two kinds of yam powders are not much different.2.Starch was extracted from yam by water extraction,alkali extraction and alkali-alcohol extraction,and the quality of the extracted starch was evaluated.The yield and purity of the starch extracted by the three methods were analyzed;and the product index of the starch was determined according to the national standard of the food starch.The results showed that the starch yield of the alkali method was 15.9%higher;the purity of the water extraction method was 98.73%;the moisture content extracted by the alkali method,the lead content of the pollutants,the total number of colonies,and the number of molds and yeasts were respectively 7.6%,0.03 mg/kg,2.3 lg CFU/g,2 lg CFU/g,and no coliform bacteria were detected.Considering the comprehensiveness of product indicators,the alkali method is a superior method for extracting yam starch.3.Using yam starch as raw material,cross-linked starch was prepared by phosphorylation of starch and its system was characterized.Using the resistant starch content as an indicator,the response surface method was used to optimize the preparation process of phosphate starch.The processing characteristics,stability,in vitro digestibility,gelatinization properties and structural properties of the prepared cross-linked starch were measured.The results showed that the optimum conditions for the preparation of cross-linked starch were as follows:40%starch milk was adjusted to pH 10,10%sodium trimetaphosphate/sodium tripolyphosphate（99:1）was added,and the reaction was carried out at 48℃for 3.3 h.From the characterization of cross-linked starch,the solubility,swelling power and light transmittance of cross-linked starch were lower than that of the raw starch;its stability was improved;the viscosity value decreased sharply,the gelatinization temperature increased to 86.43℃,and the gelatinization value was△H also increased to 15.1;compared with the raw starch,the morphology of cross-linked starch granules changed,and there was a phenomenon of encapsulation between the granules.In this study,a systematic comparison was made between the peeled yam powder and the non-peeled yam powder,so that suitable yam powder could be selected for different foods according to the need.Yam starch was extracted and cross-linked starch was prepared,and the two were systematically compared,which could improve its application value in the field of food processing.These results are helpful to open up new avenues for the utilization of yam resources and increase the added value of yam processing.