Study On The In Vitro Starch Digestibility And Its Influencing Factors Of Semi-dried Highland Barley Noodles

Worldwide,the risk of residents suffering from chronic diseases（such as diabetes,obesity,and cardiovascular diseases,etc.）is increasing,and noodles of low glycemic index（GI）can control these diseases effectively.Highland barley（HB）has a good functional feature due to its nutritional properties and composition.Applying HB to noodles is conducive to dietary intervention for chronic diseases and low GI staple foods developing.Currently,high content highland barley noodles are mostly made as the form of dried noodles because of storage and circulation.The taste and cooking properties of semi-dried noodles are better than that of dried noodles.Therefore,semi-dried highland barley noodles（SHBN）will meet the sensory needs of consumers better.There are many controlling factors when processing SHBN.While current research mainly focused on preservation and quality improvement,with less study on the impact of processing on the starch digestibility of SHBN.In this paper,the effects of different varieties of highland barley,sorbitol,hydrocolloids,and drying conditions on the in vitro starch digestibility of SHBN and the intrinsic mechanisms would be investigated.Firstly,the differences in the in vitro starch digestibility of SHBN with different color varieties and the reasons were studied.In vitro starch digestion showed that the estimated glycemic index（e GI）of semi-dried white highland barley noodles（SWHBN）,semi-dried yellow highland barley noodles（SYHBN）,semi-dried blue highland barley noodles（SB₁HBN）,and semi-dried black highland barley noodles（SB₂HBN）were 82.43,73.82,77.67,and 70.01,respectively.The starch hydrolysis extent of SB₂HBN was significantly（p<0.05）lower than that of the others,which was because the relative crystallinity（RC）,the degree of short range ordered structure,and gelatinization enthalpy（ΔH,）of starch in SB₂HBN were the largest.The protein cross-linking and protein-starch composite structure were the strongest in SB₂HBN.In addition,Pearson correlation analysis observed that the content of protein,free total phenol,andβ-glucan were significantly（p<0.05）negatively correlated with the e GI,while significantly（p<0.05）positively correlated with the resistant starch（RS）content of SHBN.Secondly,the effect of sorbitol on the in vitro starch digestibility of SB₂HBN and the reasons were studied.In vitro starch digestion showed that adding sorbitol（0.0%-2.5%,w/w）could inhibit starch digestibility.However,when the addition was exceeded 2.0%,the inhibitory effect of sorbitol on starch digestion decreased.The e GI of noodles was reduced from 72.71 to63.44 significantly（p<0.05）by adding 2.0%of sorbitol.Through scanning electron microscopy,it was found that the microstructure of starch in cooked noodles with sorbitol was more compact.RC,V-type crystal structure,short-range ordered structure,hydrogen bond strength andΔH of starch were also enhanced by adding sorbitol.In addition,the swelling power and amylose leaching during gelatinization of SB₂HBN with sorbitol were reduced.Pearson correlation analysis found that the degree of short-range ordered structure andΔH of starch had a significantly（p<0.05）positive correlation with RS content.Then,the effects of three hydrocolloids on the in vitro starch digestibility of SB₂HBN and the underlying reasons were studied.Within the adding range（0.0%-1.50%,w/w）,the starch digestion inhibition of konjac glucomannan（KGM）increased gradually,the starch digestion inhibition of curdlan gum（CG）increased first and then decreased,and the starch digestion inhibition of guar gum（GG）decreased gradually.KGM,CG and GG showed the most inhibition on starch digestion at the adding level of 1.5%,1.0%,and 0.5%,respectively,making the RS content increasing significantly（p<0.05）from 38.66%to 49.26%,44.93%,and 43.78%,respectively.The reasons were listed as follows.All three hydrocolloids could promote free water to bound water in cooked noodles.For the pasting properties of SB₂HBN,KGM and GG increased the viscosity significantly（p<0.05）,while CG had little effect on it.Adding three hydrocolloids reduced the amorphous structure and increased theΔH of starch in noodles.Besides,proper amount of three hydrocolloids reduced the swelling power of raw SB₂HBN and strengthened the protein-starch composite structure in cooked SB₂HBN.Next,the effect of heat-humidity drying treatment（HHDT）on the in vitro starch digestibility of SB₂HBN and the related reasons were studied.The appearing of plateau for starch hydrolysis curves of noodles dried by HHDT was delayed,and the hydrolysis extent and rate were lower than control.After HHDT,the slowly digested starch（SDS）content in noodles was significantly（p<0.05）decreased,while the RS content was significantly（p<0.05）increased.At the humidity of 50%or 70%,the reduction extent in e GI of SB₂HBN increased first and then decreased as the temperature increased（105-135℃）.At the humidity of 90%,three drying temperatures had almost the same reduced effect in e GI of SB₂HBN.The e GI of noodles could be significantly（p<0.05）reduced from 70.08 to 56.56 when HHDT was set as 120℃and 50%（recorded as 120-50）.The reasons were that HHDT induced more V-type crystal structure,and raised the RC,hydrogen bond strength and short-range ordered structure of starch in cooked noodles.Meanwhile,HHDT promoted the temperature of starch gelatinization,iodine binding,blue value,and amylose content of noodles to increase.Observing the microstructure of cooked noodles,it was found that proper drying conditions hindered the starch swelling,densified the protein networks,and strengthened the protein-starch composite structure.Finally,black highland barley was used as the raw material,with 2.0%of sorbitol and 1.5%of KGM added,and SB₂HBN were prepared under 120-50 of HHDT conditions.The comprehensive treatments reduced the e GI of noodles from 70.07 to 47.60,and increased the RS content from 36.17%to 47.60%.Meanwhile,the texture and sensory quality of cooked SB₂HBN were also improved.