Retrogradation Properties of Starch
Sammanfattning: The main purpose of this thesis was to examine various factors influencing starch retrogradation. The reassociation events of starch, which are closely linked to the staling of bakery products, were mainly studied by thermal analysis. Some aspects of starch gelatinization are also included. The gelatinization and retrogradation were described as non-equilibrium processes where the phase transitions of the partially-crystalline food polymer are controlled by the plasticizing effect water has on the amorphous regions. Isothermal microcalorimetry (IMC) was introduced as a convenient method to study the first 24 h of retrogradation. It was possible to monitor in detail the initial phase separation and crystallization of amylose, as well as the recrystallization of amylopectin during the early stages of retrogradation. The retrogradation properties of starch are much dependent on the botanical source and the level of amylopectin recrystallization, as studied by differential scanning calorimetry (DSC), followed in general the X-ray diffraction patterns of the starches, A-pattern low, B-pattern high, and C-pattern intermediate. DSC-measurements showed that amylopectin recrystallization was significantly reduced in the presence of oils and emulsifiers, while dietary fiber increased the extent of retrogradation. The IMC study also showed that the effect of added lipids varied and influenced different periods in the first 24 h of starch retrogradation, depending mainly on the amylose content, the botanical source of starch, and the type of lipid used. Indications of the interaction between amylopectin and added lipids/surfactants were obtained in the IMC investigation of the retrogradation of waxy maize starch. The temperature dependence of the amylopectin recrystallization was extensively studied in this thesis. In order to affect crystallinity, starch was exposed to different temperature cycles during storage in order to favor the nucleation and propagation of the crystallites. The overall quality and quantity of recrystallized amylopectin was greatly affected when gelatinized starch was treated with the time-temperature cycles. Principal component analysis emphasized several correlations between the chemical data, and the gelatinization and retrogradation properties of ten starches from various botanical origins. Two main trends were identified: the correlation between the amylopectin unit-chain length distribution and the thermo-analytical variables describing the melting temperatures of recrystallized amylopectin (except the onset temperature) as well as the gelatinization and retrogradation related enthalpies. The other trend was the negative correlation between the amylose content and the gelatinization temperatures of the starches. The two main trends were observed in almost orthogonal directions in the loading plot indicating that they described two independent features of the starch samples. The correlation coefficients between the melting enthalpy of recrystallized amylopectin (deltaHc) of ten starches from different botanical sources after exposure to various time-temperature treatments and the relative amount of amylopectin unit-chains between DP 6 and 56 were calculated. From the correlations the amylopectin unit-chains could be sorted into distinct groups with either positive or negative correlation to deltaHc. The most stable amylopectin crystallites melt per definition at the offset temperature of the retrogradation related endotherm (Tf). A development of crystallites melting at higher temperatures, noticed as an increase in Tf, was observed for cereal starches after exposure to various time-temperature treatments. The changes found in Tf correlated to the same distinct amylopectin populations as the previous ones for the relation to deltaHc. However, the signs of the correlation coefficients were changed.
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