Loss of crystalline structure and swelling kinetics of maize starch and flour granules in glycerol excess: The role of the envelope structure

• Gelatinization is more difficult for flours with increased amylose content.
• Interactions between glycerol and starch are influenced by heating rate.
• Granule swelling depends on amylose content.
• Strength of granule envelope due to a network including proteins, lipids, amylose.
• Viscosity evolution during gelatinization is due to granule swelling.

Temperatures of loss of crystallinity and kinetics of swelling in glycerol excess were investigated on various maize flours and starch. The increase of amylose content in flour leads to a more persistent “crystalline” structure. This appears more clearly in differential scanning calorimetry (DSC) than in loss of birefringence, because a significant portion of the starch structure does not come only from amylopectin crystallites but also from helical arrangements. The effect of heating rate on the loss of order and on the glycerol/starch interactions was highlighted. Granule swelling tests in glycerol excess confirmed that the melting of crystallites is not a sufficient condition for complete dissolution of the granule. The granule swelling takes place in two steps for flours containing amylose, and occurs largely after gelatinization due to the importance of the granule envelope. The strength of this envelope, due to the presence of networks including proteins, lipids and amylose, is different between standard maize flour and starch. Tests in presence of lauric acid highlight the role of lipids on the granule envelope strength. The change in the viscosity of the granule/glycerol suspension during gelatinization is explained by granule swelling and can be described by a Krieger–Dougherty equation.