Technologies for improving the quality of bread doughs made with barley spent grain and sorghum

Abstract

Expenditure on wheat importation in sub-Saharan African countries is increasing greatly arising from the region’s rapidly expanding human population, urbanisation, and unfavourable conditions for wheat cultivation. Adoption of composite flours is encouraged to reduce wheat importation and promote local agriculture. Barley brewer’s spent grain (BSG), a high-fibre by-product of the brewing industry, is relatively inexpensive and available at large quantities. Sorghum, which is well-adapted to cultivation in sub-Saharan Africa, is an underutilized grain-crop. BSG and sorghum are potential vehicles for producing less expensive bread of improved nutritional properties. However, both lack functional gluten, which is responsible for good viscoelastic dough and high bread volume. BSG particle size reduction in combination with a sourdough fermentation were investigated as BSG pre-treatment technologies to improve wheat-BSG composite dough and bread quality. Fractionation of dried BSG through roller milling enriched the protein of BSG flour, but gave poor loaf volume and denser crumb. Additionally, the much lower flour extraction yields compared to hammer milling which gives a 100 % extraction rate flour would impact negatively on the product economic viability. Mixolab dough rheology showed that a 15 % BSG substitution significantly increased dough development time and flour water absorption. However, application of a short (3 h) ‘sponge and dough’ sourdough process improved the gas-holding properties of composite, increased loaf volume and crumb softness compared to a straight dough method. At 20 % BSG substitution, the composite wheat bread had 71.4 % more dietary fibre as well as higher protein and mineral contents than a commercial wheat brown bread. The effects of chemical (using glacial acetic acid) and physical treatment (through sheeting) on the functionality of sorghum doughs from normal and transgenic high protein digestibility (TG-HD) lines with supressed ?-kafirin expression were investigated. Normal sorghum flour doughs were subjected to sheeting in combination with sourdough addition. Partial flour pre- gelatinization, by cooking, was a pre-requisite for formation of a cohesive dough and was hence applied throughout this study. Upon baking, the combination of sheeting (15 passes) and sourdough addition (50% w/w of total flour) produced bread with a more aerated crumb and greater volume compared to the untreated control. Tensile tests of TG-HD doughs showed 38 and 42 % higher extensibility, compared to their null control doughs. These effects were attributed to the greater accessibility of ?-kafirins in the invaginated protein bodies of these high protein digestibility lines. Shear forces applied by manual sheeting and glacial acetic acid treatment were used in attempt to free the protein body-encapsulated kafirins and hence functionalise them in sorghum dough. Transmission electron microscopy (TEM) of these doughs revealed successful disruption of protein bodies by the respective treatments. Starch granules observed by scanning electron microscopy (SEM) seemed to remain intact, indicating the effects to be protein-related. However, the elevated temperature (>50oC), glacial acetic acid treatment and combination thereof, reduced dough extensibility. This was possibly due to the presence of other components in the dough system apart from the kafirins, mainly the starch granules, as well as insufficient plasticisation. The study shows that a combination of physico-chemical treatments, with emphasis on functionalising inert components such as fibre and protein, can substantially improve the dough functionality and consequent bread quality of gluten-void cereal grain materials.