Impact of heating rate on starch granule structure and functionality.

摘要

The quality and shelf life of baked products is affected by the time and temperature of the baking process. In this study, dough was baked at 219°C by using different ovens (conventional, impingement or hybrid) and with different dough weight (520g or 80g) for varying times. During baking the temperature profile at the dough center was recorded. The texture, thermal properties and pasting characteristics of baked products with reference to baking conditions were investigated. Small breads baked in the hybrid oven had the highest heating rate (25.1°C/min) while large breads baked in conventional oven had the lowest heating rate (6.0°C/min). When data were evaluated as a function of heating rate in this study, the enthalpy of amylopectin recrystallization, rate of bread firmness development and the amount of soluble amylose were lower at the slower heating rate. The differences observed in product firmness following storage are potentially a consequence of the extent of starch granule hydration, dispersion and the extent of amylose reassociation; all of which are affected by the heating rate during baking.;Structural attributes of the baked goods are mainly determined by the gelatinized starch granules, which form the basis for the second study. The changes in starch granule hydration and dispersion were followed visually by using a hot stage microscopy and at a molecular level by using the differential scanning calorimetry (DSC). Hot stage microscopy revealed changes in starch granule morphology and size during heating at either 5 or 25°C/min when monitored continuously. The results showed that granule morphology was different as a function of the heating rate and granule swelling was governed kinetically. These observations visually demonstrated the significant influence of processing conditions, i.e., heating rate, on starch granule size and morphology; and therefore potentially have implications in determining structure and texture of baked products.;The changes observed visually in starch granules indicate the potential difference in the gelatinization process of the starch granules. The visual changes observed by microscopy were further investigated using the DSC at molecular level. Differential Scanning Calorimeter tracked the processes of starch gelatinization and retrogradation at molecular levels in terms of peak temperature and enthalpy of endothermic events associated with gelatinization as well as retrogradation. The effect of heating rate on baked product structure, texture and firmness development following storage has been reported. However, the differences in the mechanistic processes occurring during starch gelatinization, as a function of heating rate, are unclear. The objective of this study was to investigate the effect of heating rate at different moisture contents on starch retrogradation and gelatinization processes. Starch suspensions at a water:starch ratio of 0.7 or 2.0 were heated by using a DSC at heating rates between 5°C/min and 40°C/min for the retrogradation study. The thermal profiles were evaluated, as a function of heating rate, and the retrogradation behavior following storage were also determined. The effect of heating rate was not evident on the retrogradation behavior of starch suspensions at either moisture content. To determine the effects of heating rate on starch-water interactions, starch suspensions at a water:starch ratio ranging from 0.7 to 3.0 were heated at 5, 15 or 25°C/min by using a DSC to different final temperatures and rescanned. The deconvoluted G and M1 endotherms and the corresponding additional unfrozen water (AUW) were determined. The results showed that the G and M1 endotherms merged at higher heating rates and at higher moisture contents. A significant interaction was observed between moisture content and heating rate demonstrating that the gelatinization process was governed by moisture content at the lower heating rates and by heating rate at the higher heating rates. Results from the AUW data suggest that the M1 component of gelatinization dominated at lower moisture content and lower heating rates. However, at higher moisture contents, an interaction was observed between moisture content and heating rate. The data suggest that at higher moisture content and at higher heating rate, there is still a kinetic limitation to the complete melting of the M1 endotherm. All these results support the premise that baking process can be a potential tool in developing the baked product quality and shelf life.