Starch Modification, Destructuration and Hydrolysis during <b><i>O‐</i></b>Formylation

Abstract

Abstract The mixing of dry starch with 40 or 99% (v/v) formic acid (FA) produces an O ‐formylation reaction which causes a combination of acid hydrolysis and starch destructuration. Moreover, this esterification reaction is highly exothermic in the presence of pure FA. When O‐ formylation is performed at temperatures higher than 20°C, starch formate content is high (degree of substitution, DS, of 2.15 after 60 min at 105°C) but then molecular weight decreases (η red ≶ 10 mL/g). Under thermally‐controlled conditions at 20°C in formic acid, degrees of substitution reach 1.5–1.6 after 6 h reaction times and polymer degradation seems to be limited (η red = 110 to 140 mL/g). The degrees of substitution obtained in water/formic acid mixtures are below those in formic acid alone. The level of destructuration of starch in formic acid and water/formic acid mixtures was also evidenced by dynamic rheological measurements and optical microscopy. Plots of storage modulus ( G ’) versus frequency (ω) was used to characterize both the gelatinization and the gel destruction processes as a function of reaction temperature ( T r ) and FA concentration.