Structural modifications of granular starch upon acylation with short-chain fatty acids

Low (LAMS) and high (HAMS) amylose maize starches acylated with acetic, propionic and butyric acids at three different degrees of substitution (DS) have been structurally characterized. SEM, XRD, SAXS and FT-IR were used to ascertain the effects of the molecular size of the esterified acid, the composition of the base maize starch and the DS on their structural properties. LAMS and HAMS did not differ significantly at the microscopic level. However, substantial differences were found at the nanostructural levels as a function of the esterified fatty acid, highlighting the importance of these length scales for the understanding of the macroscopic functional characteristics. Although the crystal unit cell remained practically unchanged, as deduced from the similar X-ray diffraction patterns, an increase in long-range order was observed with increasing fatty acid chain length. Conversely, shorter acyl chain lengths were related to greater decreases in the electron density difference between the amorphous and crystalline lamellae, suggesting the introduction of defects in the lamellar structure. From the SAXS results, it appears that the longer esterified chains are able to be accommodated within the crystalline lamellar regions so that the nanostructure is affected less by butyrylation than by the addition of acetyl or propionyl groups.