Rice Starch Molecular Fine Structure Determines Its Thermal And Digestive Properties

Starch is an important part of rice grains,which determines the cooking and eating quality of rice to some extent.The fine structure of starch is one of the characteristics of starch,affecting many aspects of starch properties.Understanding the molecular structure of starch helps understand the digestive and thermodynamic properties of starch.The fine structure of starch is the number distribution or weight distribution of starch chains with different degrees of polymerization.In this study,size exclusion chromatography and fluorophore-assisted carbohydrate electrophoresis were used to measure the fine structure of starch.Digestive enzymes were used to hydrolyze the gelatinized starch and retrograded starch to investigate their digestive properties.A differential scanning calorimeter was used to measure the thermodynamic properties of starch.Finally,through correlation analysis,the effect of the fine structure of starch on the digestibility and thermodynamic properties of starch is obtained.The main results are as follows:1.Although amylose CLDs didn’t affect starch gelatinization temperatures in this research,the relative length of amylose medium chains was negatively correlated with the gelatinization temperature range.Furthermore,gelatinization onset and peak temperature as well as maximum gelatinization rate were negatively correlated with the relative length of amylopectin short chains,while the conclusion temperature were related to the relative length of amylopectin medium chains.Based on these results,a model for the arrays of amylopectin and amylose molecules within semi-crystalline lamellas of rice starch granules was proposed.2.By fitting the in vitro digestion curves of fully gelatinized rice starches to the parallel first-order kinetics model,it was shown that two simultaneous digestion fractions,fast versus slow,are existed in the system.More importantly,the rate constants for the slow digestion fraction were shown to be significantly and positively correlated with the starch molecular size.That is,starches with larger amylopectin and amylose molecules would have a faster digestion rate.Additionally,it was shown that larger amylopectin molecules are also associated with relatively shorter amylopectin chains.The faster in vitro digestion rate of the slowly digestible fraction was thus rationalized by that larger amylopectin molecules would restrict the α-amylase hydrolysis,and they also have more non-reducing ends for the amyloglucosidase as substrates.3.Amy lose fine molecular structure is a major factor,in addition to the commonly considered amylose content,controlling the digestibility of retrograded rice starch during retrogradation.A slower digestion rate arises from restricted physical access of digestion enzymes to the interior of the gel network formed by tightly-packed small and thin cells.The higher density is mainly driven by amylose short-medium chains.However,the amylopectin-amylopectin and amylose-amylopectin retrogradation seems only play the minor role on digestibility in the system studied here.