| Starch plays a major role in supplying metabolic energy for human body,and its digestibility is closely related to postprandial glycemic response.Cereal endosperm is the main source of dietary starch for Chinese residents.In cereal endosperm,starch granules are embedded in protein matrix,and cell walls provided effective encapsulation for starch and protein.The effects of complex food matrix on starch digestibility cannot be ignored.Therefore,in this paper,the cereal endosperm was disassembled to particles with various sizes,and the effect and mechanism of the cell wall structure and protein matrix on starch digestibility were studied.In addition,with the aim to reduce starch digestibility,starch granules were encapsulated in calcium alginate beads to form starch-entrapped microspheres for the imitation of natural cereal endosperm structure.The main research contents of this paper are as follows:Firstly,the effect of endosperm cell wall structure on starch digestibility was investigated using oat groats as raw materials.Oat endosperm particles with different particle sizes（<0.15,0.15～0.42,0.42～0.85and 0.85～1.70 mm）were prepared by using milling and sieving.Confocal laser scanning microscope showed that the cellular integrity disrupted gradually through reductions in particle size,and when the particle size was less than 0.42 mm,no intact cell wall structure remained.The digestion rate increased from 0.0115 min-1 to 0.0195 min-1with the reduction of particle sizes,indicating that the cell wall could act as a physical barrier to impede the diffusion of amylolytic enzymes to starch.After cooking,the digestion rate and the rapidly digestible starch content of cell-wall-encapsulated starch were lower than those of dispersive starch.With the disruption of cellular integrity,the swelling power increased from10.48 g/g to 14.56 g/g,and the gelatinization enthalpy increased from 3.06 J/g·to 4.94 J/g,which indicated that the cell wall limited digestibility by restricting starch gelatinization during cooking.In addition,the oat cell wall absorbedα-amylase in a non-catalytic manner and inhibitedα-amylase activity to some extent.The adsorption ability was positively correlated with the cell wall content,and the inhibition rate reached a maximum of 13%when the cell wall content was 10 mg/m L.The interactions between cell wall andα-amylase providing a third mechanism of cell wall structure hindering starch digestibility.Secondly,sorghum flour,corn flour and oat flour were selected as raw materials to study the effect of protein matrix on starch digestibility.Scanning electron microscope showed that proteins attached to the surface of starch granules,preventing the binding of enzymes to starch.After deproteinization,the digestion rate increased from 0.013 min-1,0.0214 min-1 and 0.0182min-1 to 0.0164 min-1 and 0.0256 min-1,0.0216 min-1,respectively.The relative crystallinity of starch decreased by 24%,5%and 25%,respectively.The ration of 1047 cm-1/1022 cm-1 also decreased,and the O-H absorption peak shifted to the high wavenumber direction,indicating that the protein improved the structural order of starch by hydrogen bond.The presence of proteins reduced the swelling power during cooking.Deproteinization treatment before cooking increased the starch digestion rate of all three grain flours,and pepsin hydrolysis treatment after cooking increased the content of rapidly digestible starch of corn flour and oat flour,but had no effect on sorghum starch digestion.Finally,indigestible starch-entrapped microspheres were constructed by embedding starch granules in calcium alginate gel.Scanning electron microscope showed that starch granules were tightly packed in the microspheres,and the calcium alginate gel was wound around starch granules to form a physical barrier.Starch hydrolysis occurred through a layer-by-layer progression to the core of the microspheres,and the digestion rate of starch decreased from0.0140 min-1to 0.0103 min-1.When the particle size of the microspheres increased,due to the extension of the enzyme diffusion route,the digestion rate further decreased to 0.0062 min-1.Compared with native starch,the swelling power and solubility of starch-entrapped microspheres were decreased,and the gelatinization enthalpy of starch-entrapped microspheres（2）was only 72%of that of native starch.The lower gelatinization enthalpy and the residual polarized cross represented that the gelatinization degree of starch in the microspheres was limited.After cooking,a compact and continuous starch-calcium alginate gel network structure was formed inside the microspheres,and the content of rapidly digestible starch decreased from84.54%to 47.06%,while the content of slowly digestible starch increased from 7.90%to34.90%. |
