Interactions between surfactants and starch : from starch granules to amylose solutions

Starch is a mixture of two polysaccharides, amylose (AM) and amylopectin, which occurs naturally in the form of microscopic granules that are abundantly found in tubers, roots, cereal grains and fruits. In order to bring out their functional properties as thickeners and texture enhancers, starch granules are often disrupted by heating in excess water. This process, which is referred to as gelatinisation, causes the granules to swell and exude a fraction of the starch polysaccharides, resulting in a dramatic increase in the viscosity of the starch suspension. Surfactants are known to affect the different aspects of the gelatinisation process and, in particular, the swelling properties of starch. Surfactants are also known to form helical inclusion complexes with AM, the formation of which plays an important role in many of the instances in which starch and surfactants interact. This work was carried out in order to gain insight into how the surfactant structure (head group and chain length) influences the swelling properties of starch and the molecular mechanisms behind these effects. The investigations involved the study of the temperature-induced gelatinisation of starch in the presence of surfactants as well as studies on the association of surfactants to AM in solution and the solubility of the resulting AM-surfactant complexes. Information on the extent of granule swelling upon heating was indirectly obtained by means of viscometry while insight on the molecular events taking place during gelatinisation was sought by means of differential scanning calorimetry (DSC) and confocal laser scanning microscopy (CLSM). Viscometric studies revealed that, with the exception of the cationic surfactants (alkyl trimethyl ammonium bromides), short-chain (C10, C12) surfactants induce an early swelling (swelling at lower temperatures than the control sample) in normal wheat starch granules, whereas their longer chain counterparts (C14, C16) have the opposite effect. Contrary to this finding, the effect of surfactants on the swelling of waxy wheat starch granules, an AM-free starch variety, is not influenced by the surfactant chain length but by the head group charge of the surfactant. The enhancing/restricting effect of surfactants on the swelling of normal wheat starch is not correlated to their effect on the early aspects of gelatinisation (onset of the gelatinisation transition) but is, in most cases, associated with the dissociation temperature of AM-surfactant complexes formed simultaneously as the granules gelatinise. CLSM studies revealed that, compared to a longer-chain surfactant (C16), a short-chain (C12) surfactant has the ability to penetrate further into the granule matrix during gelatinisation, which may favour its availability for interacting with different starch granule components during gelatinisation. Studies on the interactions between AM and surfactants with different chain length (C12 vs. C16) and head group (sodium sulphates vs. maltosides) revealed that the presence of a charged head group favours the water solubility of the resulting AM-surfactant complexes. However, this effect can be counteracted by the effect of the surfactant chain length: an increase in the chain length (C12 vs. C16) decreases the solubility of the complex.QC 2010091