| Highland barley(HB)is a naturally slow-digesting grain,and the consumption of HB results in a lower glycemic index than other grains.Compared with starches extracted from other grains,highland barley starch(HBS)shows no significant difference.Protein,β-glucan and lipid are the main endogenous non-starch constituents in HB,which are the main factors affecting the digestibility of starch in HB.The interactions between different endogenous nonstarch components and HBS show significant effects on starch digestion.In addition,there were significant differences in protein types among different cereals,so it can be speculated that the types of protein would also affect the digestion of starch.Based on these,this research aims to explore the effects and mechanisms of different endogenous non-starch components on the in vitro digestibility of starch in HB.Furthermore,different soluble proteins extracted from HB and β-glucan are as impact factors,and combined with the spray drying(SD)technology,preliminarily exploring the feasibility of the construction of simulated grain structure,to providing important reference values and models for the construction of slowdigesting starch.The main contents in this research are as follows:1.The effects and mechanisms of different endogenous non-starch components on in vitro digestibility of HBS were investigated.The changes of structural,physicochemical and digestive properties of starch in highland barley flour(HBF)were discussed after removing endogenous protein,β-glucan and lipids respectively.The results showed that the removals of different non-starch components did not change the crystal form of HBS,while reduced the degree of starch order and thermal stability,and enhanced the gelatinization and digestion of HBS.The co-removal of protein and β-glucan showed the most significant effect on HBS.It can be observed by Scanning electron microscope(SEM)and Confocal laser scanning microscopy(CLSM)that the surfaces of starch granules in HBF were covered by the continuous matrix composed of protein and β-glucan,which may limit the HBS granules absorb water,swell and gelatinize,and further hinder the contact between digestive enzymes and starch,thus slowing down the digestion of starch.2.The synergistic effects of endogenous proteins with different solubilities and β-glucan on digestibility of starch in HBF were explored.The water-soluble,salt-soluble,alcoholsoluble and alkali-soluble proteins in the defatted HBF(mainly containing protein,β-glucan and starch,DG-HBF)and the both defatted and deβ-glucan HBF(mainly containing protein and starch,DG-Dβ-HBF)were removed one by one.The results showed that the contents of resistant starch(RS)in DG-HBF and DG-Dβ-HBF were significantly increased after the removals of water-soluble and salt-soluble protein.However,the RS content in DG-HBF decreased significantly after the removal of alcohol-soluble protein,while the most significant decrease of RS content in DG-Dβ-HBF was caused by the removal of alkali-soluble protein.In conclusion,the inhibitory effect of alcohol-soluble,alkali-soluble protein and β-glucan on starch digestion was slightly higher than that of alcohol-soluble and alkali-soluble protein.The inhibitory effect of alkali-soluble protein and β-glucan on starch digestion was significantly lower than that of alkali-soluble protein.Combined with SEM and CLSM,it can be conducted that the removal of water-soluble and salt-soluble protein increased the aggregation degree of alcohol-soluble,alkali-soluble protein and β-glucan on the surfaces of starch granules,thus inhibiting the swelling and gelatinization of starch and slowing down the in vitro digestibility of starch.3.The supplementation of β-glucan on the effect of HBS digestion were studied,and the potential of SD treatment to improve the effect of β-glucan was discussed.β-glucan was added into HBS at proportions of 0%,2%,4% and 6%,and dispersed in deionized water,then SD treatment was conducted at inlet temperature of 165±5℃ and outlet temperature of82±2℃,respectively.It was found that the gelatinization,in vitro digestibility and glucose diffusion rate of HBS decreased with the increase of β-glucan addition,and it shows concentration-dependent on β-glucan.SD treatment significantly accelerated the digestion and glucose diffusion rate of starch,while the addition of β-glucan can offset the effects of SD treatment on the amorphous region,water distribution,gelatinization characteristics and digestibility of HBS gel,especially SD-6% β+S showed the slowest hydrolysis and glucose diffusion rate.CLSM showed that SD treatment could change β-glucan from aggregation state to uniform dispersion in HBS,and the uniform dispersion state was more conducive to β-glucan inhibiting starch digestion.4.The addition of highland barley protein isolated(HBPI),promin(Pro),glutelin(Glu)and SD treatment on the effects of HBS digestion were studied.HBPI,Pro,Glu and Pro+Glu(Pro and Glu were simultaneously added at a relative ratio of 1:1)were added into HBS at a ratio of 10%,and dispersed in 0.02% Na OH solution.The SD treatment was carried out under the same SD conditions as described above,and then the p H of spray-dried powders was adjusted to neutral before vacuum freeze-drying.It was found that the inhibitory effects of different soluble proteins on the digestibility of HBS was in the order of Pro > Glu > Pro+Glu >HBPI,especially the additions of Pro and Glu resulted in similar content of RDS and C∞ of HBS.Additionally,SD treatment can significantly enhance the inhibitory effect of different soluble proteins on the digestibility of HBS.After SD treatment,the hydrolysis rate curve of HBS added with Glu was the lowest overall.CLSM showed that SD treatment resulted in the transformation of different proteins from large aggregates and scattered around starch granules to wrapped on the surface of starch granules,and different soluble proteins showed different wrapping states,among which Pro and Glu show more significant differences.After SD treatment,Pro coated a few starch granules to form a large aggregate,while Glu coated evenly on the surface of starch granules,indicating that uniform and continuous physical barrier is more conducive to block enzyme attack on starch.5.The effects of different soluble proteins(Pro,Glu and Pro+Glu),β-glucan and SD treatment on digestibility of HBS were studied.Different soluble proteins and β-glucan were added into HBS at 10% and 6% proportions,respectively,and dispersed in 0.02% Na OH solution.The SD treatment and later steps were carried out under the same SD conditions as described above.The results showed that the synergistic inhibitory effects of different proteins and β-glucan on HBS digestibility were in the order of Pro+β-glucan > Pro+Glu+β-glucan >Glu+β-glucan.The co-addition of Pro and β-glucan resulted in the slowest digestibility of HBS with RS content of 25.67% and C∞ of 74.73%.After SD treatment,the digestibility of HBS added with Glu and β-glucan was significantly decreased,while the digestibility of the HBS added with Pro and β-glucan was significantly accelerated.CLSM showed that small starch granules(<5.5 μm)were aggregated in the HBS co-supplemented with Pro and β-glucan after SD treatment,while there was no protein-formed barrier on the surface of small starch granules.The starch supplemented with Glu and β-glucan after SD treatment was still covered with continuous matrix,and small starch granules did not aggregate.Combined with the above studies,it can be concluded that Pro is more likely to compound with β-glucan than HBS.However,β-glucan has little effect on the matrix formed by Glu on the surface of HBS granules. |
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