Bioprocessing to improve oat bread quality: Dissertation

The health-promoting properties of whole grain oat have made it a desirable ingredient for use in breads. However, the absence of gluten-forming proteins and high fibre content pose technological challenges with respect to product texture. Fundamental understanding about the role of oat components on the structure formation of dough and bread is needed to facilitate the development of new healthy variants of oat breads with consumer appealing properties. A concept was created for using whole grain oat flour as a base in an oat-wheat bread with high ?-glucan content and good textural and sensory quality. Ingredient and process parameters for optimised texture and taste of the oat-wheat bread were established without extensive degradation of ?-glucan. The potential of bioprocessing methods, such as the use of sourdough and enzymes, to modify the chemical and rheological properties of oat doughs, and to improve the texture and flavour of oat breads were also investigated. The maximal specific volume 3.6 (cm3/g), minimal instrumental hardness (0.1 kg after 2 h, and 0.3 kg after 72 h storage), and optimised sensory properties were attained for an oat-wheat bread by adding 15.2 g gluten and 91.5 g water/100 g flour to the dough, which was proofed at 40 °C for 75 min and baked at 210 °C. The optimized recipe and processing parameters provided the baking conditions for preparing an oat-wheat bread containing 51% oat by weight of flour mixture with good taste and structure as well as long shelf life. The use of an optimized wheat sourdough process in the production of oat-wheat bread provided a feasible method of producing a new type of bread with high ?-glucan content. The optimal sourdough conditions for enhanced crumb texture and flavour of the bread were achieved by a small addition of wheat sourdough (10 g/100 g dough) which had been fermented at 40 °C for 20 hours. The use of optimized sourdough resulted in a bread with similar specific volume and staling rate as the corresponding straight dough bread. The use of wheat sourdough did not affect the content or molecular weight of ?-glucan as compared to straight dough bread. The amount of ?-glucan in both breads was 1.5±0.1 g/100 g fresh weight. This means that a portion (2 slices, á 34 g) of the bread contained 1.0 g ?-glucan, which is the amount required for a cholesterol-lowering health claim in the EU. The molecular weight of the ?-glucan was reduced from 1.0 million in the oat flour to 0.55 million in both breads, showing that a slight degradation of ?-glucan occurred during the bread-making phase. This was most likely due to the endogenous ?-glucanase activity present in the wheat flour. The potential of cross-linking enzymes, such as laccase (LAC) and tyrosinase (TYR), to modify oat macropolymers during oat bread making was studied either alone or together with xylanase (XYL). TYR was more effective than LAC in improving the specific volume and reducing the firmnesss of the gluten-free oat bread, especially in combination with XYL. The degradation of arabinoxylan by XYL together with slight degradation of ?-glucan by ?-glucanase side activity in LAC preparation was suggested to improve the specific volume of whole grain oat, gluten-free oat, and oat-wheat breads, and the softness of fresh oat-wheat bread. The polymerization of oat globulins by TYR together with degradation of arabinoxylan by XYL was suggested to be the main contributors to improved volume and softness of gluten-free oat bread. In conclusion, the flavour and texture of oat bread was found to be effectively modified with the sourdough and enzyme bioprocessing treatments employed. Sourdough improved the intensity of flavour without excess acidity or pungency. Wheat sourdough did not reduce the content or molecular weight of ?-glucan in oat-wheat bread. Use of LAC and XYL was most effective in improving the texture of oat-wheat bread. It was shown in this work that TYR efficiently cross-linked oat globulins, which was suggested to improve the textural properties of gluten-free oat bread.