Starch Modification, Destructuration and Hydrolysis during O-Formylation

摘要

The mixing of dry starch with 40 or 99% (v/v) formic acid (FA) produces an O-formyla-tion reaction which causes a combination of acid hydrolysis and starch destructura-tion, 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-controlledconditions 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 Theological 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,