Effect of polyhydroxy compounds on the thermal and mechanical properties of ageing starch gels
Prokopowich, Dale John
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Starch based products, such as cakes and cookies, are formulated with relatively high amounts of sugar. Relatively little research has been done on the effects of sugars upon starch retrogradation and the anti-retrogradation mechanism of sugars is not yet understood. This study was conducted to provide further information on the effect of polyols (polyhydroxy compounds) on the thermal and mechanical properties of concentrated starch gels and provide further insight into the anti-retrogradation mechanism(s) of polyols. The development of ordered structures in ageing starch, in the presence of polyols, was probed by small strain dynamic rheometry and differential scanning calorimetry (DSC). Polyols were added at a ratio of 1:0.5:1.5 (w/w) for starch:polyol:water mixtures. The effect of polyols on the formation of "ordered" structures within ageing amylopectin networks was studied by DSC. The addition of glucose oligosaccharides of DP 1 to 3 retarded retrogradation with increasing DP, oligomers with a DP 4 to 7 exerted little effect, while maltooctaose promoted this process. The effect of glucose-based disaccharides with different glucosidic linkages on the reorganization of amylopectin short DP chains was also examined. Disaccharides with more extended rigid structures (i.e. cellobiose, B (1->4) glucosidic linkage) retarded retrogradation of amylopectin to a greater extent than disaccharides with more flexible glucosidic linkages (i.e. maltose, a (1->4) linkage); however, differences among the disaccharides were rather minor over longer storage periods. Minor differences were found when the anti-retrogradation behaviour of sugars and their respective sugar alcohols was compared. However, the addition of glyceraldehyde completely inhibited the formation of "ordered" structures in the ageing waxy maize starch network over the storage period examined. The effect of pentoses and hexoses on the retrogradation of waxy maize starch gels was studied. Pentoses (i.e. ribose and xylose) were found to retard the reorganization of amylopectin chains more effectively than hexoses (i.e. fructose and glucose). In fact, fructose was shown to accelerate the rate of retrogradation as compared to the control (starch-water). Finally, it was shown that retrogradation of amylopectin was strongly influenced by the concentration of polyol added. Ribose continuously retarded the formation of "ordered" structures within ageing waxy maize starch gels with increasing concentration (0-37.5% w/w), whereas fructose promoted this process at concentrations greater than 7.5% (w/w). A comparative study was undertaken to examine the effect of polyols on the thermal and mechanical properties of waxy maize, wheat, potato and pea starch gels. The addition of polyols, at a ratio of 1:0.5:1.5 (w/w) for starch:polyol:water mixtures inhibited chain reorganization of starch gels, as followed by DSC and dynamic rheology, in the following order: ribose > sucrose > maltotriose > water alone, glucose > fructose. The effects of polyols on the development of the retrogradation endotherm (AH) and gel rigidity (increase in G') were found to be less pronounced for potato and pea starches than waxy maize or wheat starches. Significant correlations (p <_ 0.05) were observed between the effect of monosaccharides on the retrogradation endotherm (AH) and their hydration properties (n H and n DHN). An anti-retrogradation mechanism of polyols was proposed on the basis of their effect on the three-dimensional hydrogen-bonded structure of water.