Pasting properties and (chemical) fine structure of acetylated yellow pea starch is affected by acetylation reagent type and granule size

Yellow pea starch was fractionated into small and large size granule fractions and then modified with acetic anhydride and vinyl acetate (acetylation after sieving) or first acetylated in the same way and then fractionated into small and large size fractions (acetylation before sieving). Acetylation with different procedures (acetylation before or after sieving) resulted in different degrees of substitution for small size granule fractions when acetic anhydride was used as reagent. Modification with vinyl acetate gave products with much higher peak viscosities than modification with acetic anhydride for the same granule size and same degree of substitution (DS). The location and distribution of acetyl groups was investigated by analyzing the α-amylase hydrolysates of isolated amylose and amylopectin with chromatography and mass spectrometry. Mass spectra showed that the substituent distribution mainly depended on the type of reagent and was not affected by the granule size. Both amylose and amylopectin were modified in an heterogeneous way and the reactions took place in different regions of the granule. It is postulated that acetylation occurs more homogeneously throughout the granule when vinyl acetate is used as reagent, while the reaction with acetic anhydride to a large extent takes place in the outer lamellae of granule.