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

Promovendus/a: Yamina De Bondt

Promotor(en): Prof. dr. ir. Christophe Courtin

A dietary fibre intake of more than 25 g/day is recommended for a healthy and balanced diet. However, in many countries, the daily intake is much lower. Therefore, interest in the enrichment of foods with dietary fibre has grown significantly. Wheat bran is a side stream of the production of refined flour and a concentrated source of dietary fibre. Furthermore, it contains vitamins, minerals, essential amino acids and antioxidants and its consumption is associated with several health benefits. Epidemiological studies indicate that wheat bran consumption may reduce the risk of cardiovascular diseases, various types of cancer and type 2 diabetes. As bread is a staple food worldwide, the incorporation of wheat bran in bread is interesting from a health perspective. However, the incorporation of wheat bran in food products poses important issues during processing and concerning final product quality. In wheat-based products, starch and gluten are the major structure-determining components, and their functionality is a major determinant of product quality.

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.

The approach of this PhD study was twofold. First, insight into the effect of different modification techniques on the physicochemical properties of wheat bran was obtained. Second, these wheat bran samples with different properties were incorporated in dough systems with increasing complexity. In this way, fundamental insights into the effect of the different wheat bran physicochemical properties on the different components in the dough system during breadmaking could be obtained.

First, the possibility of changing the properties of wheat bran with microfluidisation and wet and dry milling was explored. 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 median particle size of wheat bran was reduced from ca. 1.5 mm to ultrafine particle sizes of ca. 25 µm. Wet milling and microfluidisation with subsequent freeze-drying resulted in a higher surface area and strong water-retention capacity than dry milling. A sample set with diverse physicochemical properties was obtained.

When starch is heated in water, starch granules swell and lose their molecular order, which is called starch gelatinisation. This process has a major impact on the textural properties of products. Therefore, the effect of (modified) wheat bran on the starch gelatinisation temperature was determined in a mixture of starch, water and wheat bran with differential scanning calorimetry (DSC). It was shown that wheat bran increases the onset and peak gelatinisation temperature. The extractable components of wheat bran, such as potassium and phosphorus, were responsible for this effect as they decrease the plasticisation capacity of the solvent. The conclusion gelatinisation temperature was independent of bran concentration but negatively correlated with the water content. These observations were mechanistically explained with the side-chain liquid-crystalline polymeric model for starch.

The gluten proteins are responsible for the unique viscoelastic properties of wheat flour dough that allow expansion of gas cells in the dough. 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 (CLSM), could be achieved in the presence of (modified) wheat bran. However, dough rheology and loaf volume were affected by the type of wheat bran, water absorption and mixing time. Uniaxial extensional rheology was determined with an extensional viscosity fixture (EVF) mounted on a rheometer. 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. 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. To overcome the detrimental impact of wheat bran on breadmaking, it will be necessary to improve gluten functionality and dough rheology. Possible strategies that could further be investigated to counteract the impact of wheat bran are: decreasing the water-retention capacity of wheat bran, adapting the breadmaking recipe (water content or additives) or adapting the mixing procedure. Furthermore, 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.