Starch structure and degree of starch hydrolysis of small and large starch granules from barley varieties with varying amylose content

Starch structure is considered to have an impact on the rate of starch digestion in cereals, and is therefore an important factor in regard to optimise the starch quality in animal feed. The purpose of the study reported was to investigate the impact of amylose level and starch particle size on starch degradation in vitro. A two-step in vitro method was conducted to degrade starch enzymatically. The method consisted of a pre-incubation in HCl and pepsin, and thereafter an incubation with buffers, porcine pancreatic α-amylase and amyloglucosidase. Degree of starch hydrolysis was calculated based on free glucose content. Milled grains, isolated starch and A and B granules from barley cultivars with various level of amylose were hydrolysed. Particle size, thermal characteristics and chemical components were determined to characterise the material. Cultivars with low level of amylose had a higher degree of starch hydrolysis than cultivars with normal and high amylose content for all time intervals (P<0.05). This was observed both for incubated flour and purified starch. A higher degree of starch hydrolysis was found for normal amylose cultivars compared to high amylose cultivars when incubating flour (P<0.05). This difference was not significant for the purified starch fraction. Due to a higher surface area and lower crystallinity, small starch granules were degraded at a higher level than large granules (P<0.05), despite a slightly higher amylose content. The range in starch hydrolysis between amylose groups within granule size was similar to the results from incubations with total starch fractions. Low amylose cultivars contained, however, less small granules by volume than the two other amylose groups (P<0.05). This indicates that the amylose level and the amylose lipid complex are the limiting factors regarding starch degradation for these barley cultivars.