Aeration and rheology of high fibre bread doughs

Although there is continuous advice to increase consumption of dietary fibre, the intake of dietary fibre by individuals remains inadequate. Bread is potentially a major source of dietary fibre in the population; however, a factor mitigating against adequate consumption of high fibre bread is the damage caused by the fibre to the aerated structures of bread, which is key to its palatability and appeal. Wheat bran is a rich source of dietary fibre but its presence in wholemeal bread damages the aerated structures and lessens its appeal.

Addition of wheat bran and other sources of dietary fibre to bread tends to give decreased dough strength and loaf volume, impaired crumb structure and reduced crumb softness. Potential mechanisms reported in the literature by which bran exerts its deleterious effects include dilution of the gluten protein, mechanical disruption of gluten films, and starch gelatinisation at a lower temperature during baking (as a consequence of the increased water availability) giving less oven spring and lower loaf volumes. The particle size of the wheat bran mediates its detrimental effect, with smaller particles generally giving finer crumb textures, although not necessarily producing larger loaf volumes or the most appealing or healthy bread.

However, the full complexity of the effects of bran within the range of dough formulations and breadmaking processes is not yet understood, making it hard to find ways to produce acceptable high-fibre breads. Meanwhile, another potential new class of fibre-based bakery ingredient are Arabinoxylans (AX) which can be extracted from biorefinery by-products such as wheat bran, oat bran and sugarcane bagasse. AX can have either beneficial or detrimental effects on the dough depending on the type or level of AX used. This thesis therefore presents work to understand effects of bran and AX on dough aeration and rheology in order to better understand their effects in bread.

The effects of ethanol and retardation time (over 18 hours at 4°C) were investigated with the use of the Dynamic Dough Density (DDD) system, to investigate the hypothesis that retardation affected dough expansion through the production of ethanol by the yeast during the retardation period, and to demonstrate the sensitivity and usefulness of the DDD system prior to its use to investigate fibre effects in doughs. The addition of ethanol even at small levels decreased the maximum expansion of dough, while retardation showed the reverse, giving an increase in maximum dough expansion over time. It was therefore concluded that the effects of retardation did not arise as a result of ethanol. The DDD system proved a sensitive discriminator of these effects.

The Solvent Retention Capacity (SRC) test was used to determine the effect of fibre addition on water absorption. The SRC test uses four solvents to distinguish effects related to protein, starch damage and pentosans. The test was sensitive to addition of bran and AX, but its interpretation was ambiguous as it is conventionally used for characterising white flours.

Rheological studies were carried out using creep-recovery measurements and the expansion capabilities of dough formulations were investigated using the DDD system. Bread doughs were found to be less compliant with an increase in the level of fibre added; AX also inhibited the expansion capacity of bread doughs.

Bread aeration and dough rheology were investigated simultaneously by varying processing and ingredient factors during mixing. Dough aeration was quantified using dough density measurements, while dough rheology was characterised under dynamic oscillatory deformations using a Kinexus rheometer. Doughs were prepared using a bench top Minorpin mixer and a high-speed laboratory scale Tweedy 1 mixer. The high-speed Tweedy 1 mixer developed the gluten network better, leading to greater DDD expansion than the doughs produced from the Minorpin mixer. Dough formulations containing wheat bran gave less expansion in the DDD system. Dough formulations containing AX from wheat bran and from sugarcane bagasse also decreased DDD expansion, more so for AX from bagasse that wheat bran. Bread loaf volume decreased in all formulations with added fibre.

The current work has expanded understanding of the effects of fibre on aspects of dough and bread quality: aeration and rheology of doughs, water absorption, expansion of doughs, and baked loaf volume.