Effect Of Ripening On Physicochemical Properties,Starch Digestibility And Starch Fine Structure Of Banana Flour

China has become a major country of banana production.However,banana is perishable and low-commercialized,which is not conductive to the industry development.As a kind of good deep-processing products,banana flour can not only keep the nutrients in the banana,but also be convenient for transportation and storage,contributing to wide application in the food industry as ingredients and enhancing its additional value.With the development of social economy and the improvement of people's health consciousness,natural and functional foods are becoming more and more popular.Banana is rich in carbohydrate and native banana starch is resistant to digestion.It was reported that resistant starch could control type II diabetes and obesity,and prevent colon cancer.In this paper,banana flour at seven ripening stages was used as the materials.The chemical composition,physicochemical properties and physiologic functions of banana flour were explored.Additionally,the digestibility and multi-scale structural characteristics of banana starch were investigated,for the purpose of understanding their relationship.The main contents are as follows:Firstly,the results of chemical composition showed that the main components of banana flour were starch and soluble sugar(more than 80 g/100 g).The starch content of banana flour at ripening stage 1 was up to 63.30 g/100 g.As the banana ripens,the starch content decreased significant,while soluble sugar content increased.Meantime,starch was degraded rapidly and soluble sugar was accumulated rapidly during the process from ripening stage 2 to 3.With increasing maturity,the particle size gradually increased due to the adhesion among starch and other components in banana flour.Besides,gelatinization temperature and solubility increased while viscosity,gelatinization enthalpy and swelling power showed a downward trend.The rapidly available glucose quantity gradually increased from the 1st stage to the 7th stage(16.2 g/100 g to 26.3 g/100 g).Secondly,the banana flour at ripening stage 1,3,5 and 7,of which physicochemical properties showed significant differences,was used to investigate its effect on the type II diabetic mice during an 8-week intervention.The results showed that banana flour could improve the weight loss,insulin resistance,and dyslipidemia obviously,especially the banana flour at ripening stage 1 due to the highest content starch content.Moreover,the glucagon like peptide-1 in serum was increased significantly after intervention(p<0.05).It was found that the gene relative expression levels of glucose-6-phosphatase,related to gluconeogenesis,were down-regulated,while those of glucokinase,insulin receptor and glucose transporter 2 were up-regulated.Therefore,daily intake of banana flour could promote the liver glycogen synthesis,inhibit liver glucose output and gluconeogenesis,thus reducing blood glucose level,increasing insulin sensitivity and improving insulin resistance.The influence of banana flour at different ripening stages on each gene mRNA expression levels was different,leading to the differences of intervention effects.Finally,the effect of starch fine structures on digestive performances was studied.The banana starch at ripening stage 1 showed the strongest resistance to digestion both in vitro and in vivo,the resistant starch content of which was more than 80 g/100 g.The resistant starch content decreased with ripening.The rapidly and slowly digestible starch contents at ripening stage 5 and 7 were over 35 g/100 g,making the healthy rats keep relatively high blood sugar levels after 120-min gavage.The surfaces of starch granules became rough,showing the striations caused by enzymatic degradation,and the crystalline type changed from C_B to C_A.The relative crystallinity and the short-range order gradually decreased.On the other hand,the granules became loose and the amorphous background region and the amorphous region in the semi-crystalline lamella were damaged seriously through the analysis of semi-crystalline lamella.The proportions of degree of polymerization(DP)6-24 chains and branching degree both increased,but the proportions of DP?25 chains decreased.And the apparent amylose content decreased.These factors led to the changes of digestibility of banana starch at different ripening stages collectively,further affecting the physiologic functions of banana flour.