Crystalline Properties of Starch
Abstract: The crystalline properties of starch constitutes a puzzling area of research. Our understanding of the crystalline structure of amylopectin, that is the crystalline component in starch, is based on the observations from investigations on modified amylose. The first part of this thesis includes a discussion of the unit cells (the smallest crystalline units) suggested in the literature. The different unit cells are discussed in relation to each other, and also to what could be reasonable to find in nature. The second part of the thesis is focused on how the crystalline properties of starch are influenced by factors in the native state, and during processing, i.e., heating and cooling. Phosphate groups located at the C-6 position of potato starch were observed to influence the degree of crystallinity. The effect of these bulky phosphate groups can be altered by annealing, i.e., careful heat treatment in excessive amounts of water. Potato starches with a high content of phosphate groups were observed to have the largest shift in gelatinization enthalpies over the examined annealing period. This is not an effect of hydrolysis but rather their orientation towards positions causing less dislocations in the clusters. The crystallinity will be lost in a two-stage process during gelatinization at intermediate levels of water. The X-ray and DSC results in that investigation show that the gelatinization of starch must be discussed in relation a glass transition of a partially crystalline polymer system, as previously suggested in the literature. The gelatinization parameters of starch can be changed by addition of the anionic surfactant sodium dodecyl sulphate (SDS). A brightfield microscopy study confirmed that SDS has the capability to penetrate granules and cause an extensive swelling of the starch granules. Ageing of starch based foods is a problem for the consumers´ acceptability of many products. The early retrogradation of mainly the amylose fraction was investigated by microcalorimetric investigations at 25°C. This method was also used to study the complex formation between different starches and glycerol-monolaurin or SDS. The complex formation between amylopectin from potato and SDS was quantified by surface tension measurements. The degree of binding between amylopectin and SDS was 6 to 7 surfactant molecules per 1000 glucose units. The inclusion complexes cannot form crystalline junction zones, but are believed to be the true anti-staling mechanism of many starch based foods.
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