In vitro amylase digestion of extruded maize and high amylose maize starches and evolution of starch structure

摘要

Background/Aims: Starch digestibility influences the calorie intake aswell as gastro-intestinal health. High amylose maize starches (HAMS) areknown to have greater enzyme resistance than those with lower amyloselevels. So far, structural features responsible for the slow digestion ofextrudate are not fully understood. The current work is aimed to compare the molecular, mesoscopic and microscopic structures of partially digestedextruded maize starches varying in amylose content and digestion rate.Methods: Three maize starches with 27, 50 and 80% amylose levels wereextruded at different moisture feed rates. Extrudates with varying moisturecontents were digested (in vitro) at various times and undigestedresidues were freeze-dried. Raw, extrudate and partially digested starcheswere analysed for %starch digestibility, resistant starch, amylopectinbranch length profile, helices, crystallinity, lamellar periodicity etc.Results: Digestibility data showed the normal starch was close to 100%digested, whereas HAMS were ~30% digestible as granules and ~75% inextrudates. There was a significant decrease in long amylopectin chainsdue to shear during extrusion, resulting in shorter chains influencingstarch digestibility. NMR and crystallinity data showed that extendeddigestion of extrudate results in a significant increase in molecular order orhelical content. Mechanism of enzyme resistance of granular high-amylosestarches is qualitatively different to that for processed starches.Conclusions: Incorporation of HAMS to enhance resistant starch in foodscan be achieved from either cooked or uncooked forms, but that theconsequences may not be the same based on the different mechanismsinvolved in amylase digestion.