Impact of granule hydration on maize and wheat starch chemical reactivity at the granular and molecular levels

Granular and molecular reaction patterns of maize and wheat starches (normal and waxy genotypes) were investigated in relation to extent of granule hydration (equilibrated at 25, 55, 75, 86 or 100% relative humidity [RH]; moisture range ≈ 10–40 g/100 g starch) within a model reaction system utilizing a fluorescent reagent (5-(4,6-dichlorotriazinyl)aminofluorescein). The greatest incremental increase in granule hydration (49–80%) and overall extent of reaction (30-76-fold) occurred between 86 and 100% RH, likely accentuating the starch glass transition temperature (Tg), which was depressed to room temperature via gradual granule hydration. Under limited hydration conditions (25–86% RH; presumably below Tg), reaction was confined to granule surfaces, whereas under conditions of sufficient hydration/plasticization (100% RH; presumably above Tg), reaction occurred throughout the granule matrix, dramatically increasing reaction of amylose and amylopectin branch chains. For normal starches, amylose was more inert to reaction in low moisture conditions (25–87% RH), indicating either a lesser prevalence at granular surfaces or insufficient hydration to react. Waxy starches exhibited more homogenous granular reaction patterns and greater overall extents of reaction than their respective normal starch counterparts, due to reactivity differences amongst amylose and amylopectin medium and long chains. Findings also provide insights for manipulating granular/molecular reaction locale in “dry” starch modifications. [Display omitted] •A threshold level of hydration appeared to trigger the starch glass transition (Tg).•Below threshold hydration levels, reaction was confined to starch granule surfaces.•Reaction migrated into the granule matrix as threshold hydration levels were met.•Total starch chain reactivity increased 30–76 fold above threshold hydration levels.•Starch hydration level impacted granular and molecular reaction extent and locale.