Understanding the impact of wheat bran on the dynamic behaviour of starch and gluten in breadmaking, using bran modification as a tool

Interest in the enrichment of dietary fibre in cereal-based foods has grown significantly due to increasing health awareness. As a complement of wheat flour in milling, wheat bran represents a self-evident source of dietary fibre. In addition, wheat bran is a good source of minerals, vitamins and bioactive compounds. However, consumption of wheat bran-enriched foods remains low since its incorporation in cereal-based foods poses important issues during processing and concerning final product quality. Some of these issues are addressed by using expensive food additives. The lack of insight into the root causes of these problems prevents developing strategies to solve them in another way. The incorporation of bran in wheat bread, a staple food all around the world, represents an interesting system to study the impact of bran on the various aspects of starch and gluten functionality since starch and gluten constitute the major structure-determining components in bread. Indeed, in breadmaking, proper gluten network development as a result of hydration and mixing is required; proper leavening relies on the stability of the gluten-starch matrix during fermentation and oven rise, and gelatinisation of starch upon baking determines the eventual crumb structure. Against this background, this doctoral dissertation aimed to contribute to a better understanding of the impact of wheat bran on gluten and starch functionality in breadmaking. To this end, the alteration of bran properties by different innovative modification techniques was investigated and used as a research tool. The possibility to induce physicochemical modifications in wheat bran was investigated using microfluidisation, wet milling or dry milling. Microfluidisation processing parameters (pressure, number of passes, bran concentration and initial particle size) and water content during milling strongly influenced the physicochemical properties of wheat bran. The three applied techniques were able to produce ultrafine wheat bran samples with a median particle size between 21 µm and 29 µm, but the physicochemical characteristics were not only determined by the particle size but also by the technique of particle size reduction. Wet milling and microfluidisation with subsequent freeze-drying resulted in a higher surface area and strong water-retention capacity than dry milling. The wet environment during particle size reduction with microfluidisation or wet milling results in penetration of water into the wheat bran structure and an increase in porosity after freeze-drying. It also promotes the activity of endogenous enzymes, which increases the water-extractable content of the wheat bran. With the different modification techniques, a sample set with diverse physicochemical properties was obtained. To study the effect of (modified) wheat bran on the starch gelatinisation temperature, the water distribution between starch and bran was investigated. Dynamic water vapour sorption and water-retention capacity experiments showed that fine dry-milled wheat bran bound up to three times more water than starch. However, examining starch gelatinisation in starch-bran-water mixtures with differential scanning calorimetry showed that the effect of substituting starch by bran differed from that of moving into a regime of limiting water. Modelling the effect of the mixture composition on starch gelatinisation behaviour indicated that the onset and peak gelatinisation temperatures were positively correlated with the bran concentration in water. Extractable wheat bran components, such as potassium and phosphorus, were mainly held responsible for this effect as they decrease the plasticisation capacity of the solvent. The conclusion temperature was negatively correlated with the water content. Our observations were mechanistically explained with the side-chain liquid-crystalline polymeric model for starch. The effect of (modified) wheat bran on gluten functionality was investigated in a gluten-starch mixture as a simplified model system for flour. A proper gluten network microstructure, as visualised with confocal laser scanning microscopy, could be achieved in the presence of (modified) wheat bran. However, significant effects of the type of wheat bran, water absorption and mixing time on uniaxial extensional dough rheology and loaf volume were observed. Wheat bran addition decreased the strain hardening of dough despite optimisation of water absorption and mixing time. The deleterious effect of wheat bran on dough rheology increased by adding modified wheat bran with high strong water-retention capacity and surface area. The strain hardening behaviour of dough proved to be a valuable predictor of bread loaf volume also in the presence of (modified) wheat bran. Finally, a wheat flour-based bread system was used to obtain more insight into the impact of (modified) wheat bran during the different phases of the breadmaking process. The incorporation of 10% bran affected the volume increase of dough mainly from the baking phase on, while in the presence of 20% bran, the fermentation phase was also affected. The decrease in gas retention during fermentation could be linked to a decrease in strain hardening, as was also observed for gluten-starch mixtures. However, the behaviour of native gluten in flour could not be entirely mimicked by isolated gluten. The major effect of (modified) wheat bran on dough rheology was already present immediately after mixing. Water redistribution during fermentation caused by the hydration properties of wheat bran was, therefore, suggested to be of minor importance. The changes in dough rheology were mainly attributed to the production of yeast metabolites and the expansion itself. The effect of wheat bran on the oven rise could not be attributed to the impact of wheat bran on starch gelatinisation or water/ethanol evaporation during baking. Dough rheology was, therefore, also during baking, identified as the main reason for the observed differences in the volume increase of the dough. The strong water-retention capacity of wheat bran was furthermore shown to affect evaporation during baking, which will affect the final bread quality. In conclusion, this dissertation studied the effect of modified wheat bran on starch and gluten functionality during breadmaking. It was shown that mainly the hydration properties of wheat bran influence gluten functionality and consequently dough rheology. This leads to a decrease in gas retention during fermentation and even more pronounced during baking. Finally, this results in a decreased loaf volume. Although starch gelatinisation is affected, mainly by water-extractable components of wheat bran, this is of minor importance during breadmaking. The explored wheat bran modification techniques could help to align wheat bran functionality with different food applications. The insights obtained in this doctoral dissertation can, therefore, lead to strategies to increase wheat bran incorporation in food systems and consequently increase the consumption of dietary fibre.

Jaar van publicatie:2021

Toegankelijkheid:Open