Physical and chemical effects of bran on brown bread

Abstract

Brown bread was produced by adding three bran fractions of different sizes (Pollard < 0.75 mm diam., Select> 0.75 mm < 1.8 mm and Digestive > 1.8 mm), from the same grist, to a common base white flour. The addition of bran caused loaf volumes to be depressed and crumb structures to be coarse. Bran components appeared to affect gluten functionality by changing its physicochemical characteristics through a subtle interplay of chemical and physical effects. Pollard depressed loaf volumes the most, and Digestive bran the least. This could probably be attributed to differences in chemical composition. Pollard had the highest fat content, and therefore probably the highest lipoxygenase and glutathione concentrations, which adversely affected loaf volume. Subjecting the brans to a dry heat-treatment, which inactivated lipase and reduced the total reducing substances of which glutathione is part, increased loaf volume. This suggests that a chemical effect of the bran (probably the lipid metabolising enzymes lipase and lipoxygenase, as well as glutathione) is at least in part responsible for loaf volume depression. Heat treatment had the greatest effect on decreasing loaf volume depression in breads baked with Pollard and the least on breads Baked with Digestive bran, indicating a greater chemical effect in brans with smaller particle size. A similar baking study was conducted with bran from 10 widely differing wheat samples (all of the same nominal particle size range). The different brans caused different levels of loaf volume depression. In all cases, dry heat-treatment of the brans decreased total reducing substances, inactivated lipase and increased loaf volume and height. However, the loaves still differed somewhat in volume. It is possible that differences in chemical composition of the different bran sources also accounted for these differences in loaf volume depression. In addition to the difference in chemical compositions of the different brans, the higher loaf volumes of breads baked with Digestive bran, compared to those baked with Pollard, could also possibly be explained by the large, flaky Digestive bran particles trapping air bubbles in the dough. These air bubbles possibly provided extra nucleation sites for gas cells, as well as oxygen, which improved oxidation of the gluten and functioning of the oxidising agent ascorbic acid. The theory that Digestive bran particles trap air, could also probably explain why at low levels of addition, breads baked with Digestive bran had higher loaf volumes than the white bread controls. However, for large bran particles to be able to increase loaf volume, they have to be free of epicarp hairs. Heat treatment did not result in all the brans producing loaves of the same volume, suggesting that a physical effect of the bran is also responsible for loaf volume depression. CAT scanning of proofing dough showed a uniform crumb structure in white bread dough in contrast with bubbles in brown bread dough, which were large and irregular. Bubbles became more elongated as proofing progressed. Bran probably ruptured the gas cell walls of the foam structure, leading to coalescence of the bubbles. Coalescence caused larger and irregular bubbles with lower internal pressures, probably resulting in lower loaf volumes. It is concluded that there is a subtle interplay between the chemical and physical effects of bran on brown bread, which determines the extent of loaf volume depression. The addition of either large, epicarp hair free bran particles, or heat-treated smaller bran particles to white base flour are potential methods of optimising brown bread loaf size.