Influence of Particle Size and Xylanase Pretreatment of Proso Millet Bran on Physical, Sensory and Nutritive Features of Gluten-Free Bread
1University of Zagreb, Faculty of Food Technology and Biotechnology, Pierottijeva 6, 10000 Zagreb, Croatia
2University of Applied Sciences ‘Marko Marulić’, Petra Krešimira IV 30, 22300 Knin, Croatia
3Division of Food Sciences, Nutrition and Dietetics, School of Biosciences, University of Nottingham, Sutton Bonington Campus, Loughborough, LE12 5RD, UK
Received: 26 April 2022
Accepted: 2 November 2022
cereal by-product; cryogenic milling; dietary fibre; mineral bioaccessibility; phenolic acids
Research background. Millet bran is a by-product rich in dietary fibre, micronutrients and bioactive compounds which are often deficient in a gluten-free diet. Previously, cryogenic grinding has been shown to improve the functionality of bran to some extent, although offered limited benefits for bread making. This study aimed to investigate the effects of adding proso millet bran depending on its particle size and xylanase pretreatment on the physicochemical, sensory, and nutritional properties of gluten-free pan bread.
Experimental approach. Coarse bran (d50=223 μm) was ground to medium size (d50=157 μm) using an ultracentrifugal mill or to superfine particles (d50=8 μm) using a cryomill. Millet bran presoaked in water (for 16 h at 55 °C) without or with the addition of fungal xylanase (10 XU/g) replaced 10 % of the rice flour in the control bread. Bread specific volume, crumb texture, colour and viscosity were measured instrumentally. Along with proximate composition, the content of insoluble and soluble fibre, total phenolic compounds (TPC) and phenolic acids as well as total and bioaccessible minerals of bread were assessed. Sensory analysis of breads included a descriptive, hedonic, and ranking test.
Results and conclusions. Dietary fibre content (7.3-8.6 g/100 g dm) and TPC (42-57 mg/100 g dm) of the breads depended on bran particle size and xylanase pretreatment. The effect of xylanase pretreatment was most evident in breads with medium bran size in terms of higher content of fibre soluble in ethanol (45 %) and free ferulic acid content (5 %); improved bread volume (6 %), crumb softness (16 %), and elasticity (7 %), while lower chewiness (15 %) and viscosity (20-32 %). Bread bitterness and dark colour were increased after adding medium-sized bran but its bitter aftertaste, crust crookedness, crumb hardness and graininess were reduced with xylanase pretreatment. Although bran addition impaired protein digestibility, it enriched the bread with iron (341 %), magnesium (74 %), copper (56 %) and zinc (7.5 %). Xylanase pretreatment of the bran resulted in improved bioaccessibility of zinc and copper of the enriched bread compared to the control and bread without xylanase.
Novelty and scientific contribution. Application of xylanase to medium bran obtained by ultracentrifugal grinding was more successful than its application to superfine bran obtained by the multistage cryogrinding as it resulted in more soluble fibre in gluten-free bread. Moreover, xylanase was proven beneficial in maintaining desirable bread sensory properties and mineral bioaccessibility.