Engineering Slowly Digestible Starch via Physical and Enzymatic Modification: Structural Mechanisms of Formation

Dietary starch varies in digestion rate: Rapidly digestible fractions cause sharp postprandial glucose spikes linked to chronic disease, whereas slowly digestible starch (SDS) breaks down gradually, producing a moderate glycemic response. Increasing SDS content is therefore a key objective in developing carbohydrate-rich foods with improved metabolic functionality. Physical, enzymatic, and combined modification strategies have been reported to enhance SDS, but outcomes are often inconsistent, with the same treatment increasing SDS under some conditions while favoring rapidly digestible or resistant fractions under others. The structural basis of this variability, and how conditions can be adjusted to reliably favor SDS formation, remains insufficiently synthesized. This review compiles studies reporting quantified SDS increases relative to unmodified controls following physical, enzymatic, and combined strategies, linking these changes to structural and functional transformations, and addressing conditions under which SDS declines. SDS formation is positioned as crystallization arrested at nucleation, distinct from the chain maturation yielding resistant starch, with chain length, branching density, and granule or matrix architecture governing the balance among SDS, resistant, and rapidly digestible fractions. These insights, together with strategies for recovering SDS when treatments underperform, aim to inform rational design of starch-based foods with tailored digestibility for glycemic management and to help close the gap between laboratory-scale research and commercial translation.