Influence Of High-amylose Maize Kernel Characteristics On Starch Extraction And Optimization Of Soaking Process

Starch is a polysaccharide widely found in nature and consists mainly of amylose and amylopectin.High-amylose maize starch is usually defined as maize starch with more than 50%amylose content.As a starch with unique physical and chemical properties,high-amylose maize starch has high application value in food,materials,textiles,healthcare,paper and packaging,but its high selling price hinders its large-scale application.The extraction of starch from domestic high-amylose maize with independent intellectual property rights can effectively reduce its cost and provide a new strategy for the deep processing of high-amylose maize.However,at present,high-amylose maize starch has the problem of low extraction rate and purity.This paper investigates the differences in kernel characteristics between high-amylose maize and normal maize,analyses the reasons why high-amylose maize starch is difficult to extract and optimizes the high-amylose maize soaking process by adding lactic acid.The main contents of the paper are as follows.Firstly,two different high-amylose maize（G50 and G70）and one normal maize were selected as raw materials to investigate the differences between high-amylose maize and normal maize in terms of basic components,physical properties and internal microstructure,and the effects of their starch extraction.The results showed that as the amylose content of the maize increased,the total starch content decreased significantly and the protein content and density increased significantly.The horny percentage of normal,G50 and G70 maize was 42.43%,49.00%and 64.49%respectively.In floury endosperm,starch and protein are loosely bound,whereas in horny endosperm,protein and starch are very tightly bound and the density and protein content are greater than in floury endosperm,resulting in starch not being easily separated from protein.The strong hydrogen bonding in horny endosperm and the presence and strength of g-g-g and g-g-t disulphide bonds make it difficult to break the protein matrix network.Secondly,the effect of adding lactic acid on the soaking effect was investigated,and the optimal soaking process was determined by orthogonal experiments using the extraction rate and purity as the investigated indexes.The results showed that the water absorption of G50 and G70 increased by 2.56%and 6.11%,respectively;the soluble protein content increased by 0.61mg/m L and 0.69 mg/m L,respectively;the free sulfhydryl content increased by 72.45μmo L/g and 76.48μmo L/g,respectively,and good soaking results were achieved.Finally,the optimal process parameters for G50 were 50 h soaking time,52°C soaking temperature,0.15%（w/w）SO₂ addition and 0.6%（w/w）lactic acid addition,and the extraction rate and purity were 86.15%and 93.74%,respectively;the optimal process parameters for G70 were 50 h soaking time,56°C soaking temperature,0.15%（w/w）SO₂ addition and 0.6%（w/w）lactic acid addition.The extraction rate and purity under these conditions were 73.14%and 92.68%,respectively.Finally,the physicochemical properties of the extracted high-amylose maize starch and the remaining maize starch with different amylose contents were studied to investigate the feasibility of the lactic acid addition process and the effect of the amylose content on the physicochemical properties of the maize starch.The results showed that both extracted high-amylose maize starches were able to meet the requirements of the national first grade,and the particle morphology did not change.This shows that lactic acid does not affect the structural properties of the starch and that the process is feasible.The high-amylose maize starch granules were smaller and had a smoother surface,with special shapes such as long and oval.As the amylose content increases,the particle size,relative molecular weight,crystallinity,short range orderliness,enthalpy of gelatinization and light transmission of the starch all tend to decrease,while the average chain length,pasting temperature and resistant starch content tend to increase.