UBC Theses and Dissertations
Thermal degradation of thiamine in bread Nadeau, Louise
Thiamine is an important nutrient found in significant amounts in wheat flours. This vitamin is heat labile thus destruction occurs during bread baking. Using a kinetic approach, the effect of heat and pH on thiamine degradation in a model system were studied. In order to compare the stability of thiamine from natural (whole wheat) and synthetic (enriched white) sources, thermal destruction of thiamine in the two breads was investigated. Destruction rates of thiamine hydrochloride in phosphate buffer at pH 6.0 and temperatures between 80 and 120°C were measured. The breakdown reaction could be described by first order kinetics. An energy of activation of 34.2. kcal/mole was obtained. Destruction rates of thiamine hydrochloride in phosphate buffer at 120°C were measured for pH values between 4.0 and 7.0. The reaction rate increased as the system was made more alkaline, with greater destruction at pH 6.0 and above. Thiamine losses in an enriched white flour system baked at a nominal temperature of 246°C (475°F) for 60, 75 and 90 min were found to be 2.4, 27.9 and 29.2%, respectively. Two experiments were carried out with 450 g (1 lb) enriched white loaves baked at 221°C (430°F). Baking times were 30 min for the first experiment, and 15, 37 and 60 min for the second experiment. No appreciable thiamine destruction were found in either experiment. The main investigation was with a semi-model system of 12g bread loaves made from enriched white and whole wheat flours. Four different nominal oven temperatures of 177, 221, 246 and 288°C (350, 425, 475 and 550°F) were used with four different baking times for each run. The pH of the dough and baked bread were determined. Oven, crust and loaf center temperatures were monitored. The mass average temperature data of the bread during baking showed a changing rate of temperature rise, and because of this, it was not possible to obtain kinetic data. However, a linear relationship was obtained when the logarithm of the percent thiamine retention was plotted against time. This experiment showed a lower thiamine stability with higher oven temperature. Thiamine was less stable in whole wheat bread than in enriched white bread. This might be explained by higher pH and ash content in whole wheat bread. Thiamine losses during normal baking of whole wheat and enriched white bread were found to be in the range of 28.3 to 47.8%.
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