engleski

PHYSICO-CHEMICAL PROPERTIES OF GLUTEN-FREE BREAD WITH MILLET BRAN DEPENDING ON THE PARTICLE SIZE

Millet bran is an edible by-product, rich in dietary fibre and other bioactive compounds often deficient in a gluten-free diet. This work aimed to investigate the influence of adding proso millet bran of different particle sizes, with or without xylanase, on bioactive compounds and physical properties of gluten-free bread. The millet bran contained 10.8 g/100 g moisture, 10.1 g/100 g proteins, 6.7 g/100 g fat, and 5.4 g/100 g ash. It was obtained by industrial decortication and sifting through a 500 µm screen. This 'coarse' bran with an average particle size (d(50)) of 223 µm was further grinded into ‘medium’ (d(50) = 157 µm) and ‘ultra-fine’ size (d(50) = 8 µm). Prior to bread-making, the bran (40 g) was soaked in water (100 mL) without or with xylanase (10 XU/g bran) for 16 h at 55 °C and pH 5.6. A control recipe consisted of rice flour (400 g), water (356 g, 89% on flour basis), sugar (16.7 g), instant yeast (5.7 g), salt (5.3 g), and butter (4.2 g). Otherwise, 10% of rice flour was replaced with the bran, and the amount of added water (on flour/bran basis) was optimized for each formulation: 102% for bread with added coarse or medium size bran, and 97% for bread with ultra-fine bran. Bread volume was determined according to the method AACC 10-05.01 and specific volume was calculated as volume to mass ratio. Crumb colour was determined colourimetrically in the CIELab system and texture profile was determined with a texture analyser. Content of insoluble dietary fibre (IDF), fibre soluble in water but not in 78 % ethanol (SDFP), and fibre soluble in water and 78 % ethanol (SDFS) was determined according to the AOAC 2011.25 method. Total free phenolics content (TPC) was determined with Folin-Ciocalteu method. Rice bread was low in fibre (1.8 g/100 g), whilst the bread with added bran was a source of fibre ranging from 3.2 to 3.9 g/100 g (of which 18-25% was soluble) in dependence on the bran particle size and xylanase addition. TPC of rice bread was 121 µg/100 g bread, whereas in breads with added bran it ranged from 200 to 249 µg/100 g. TPC was significantly (p < 0.001) lower upon adding xylanase. Compared to the control, breads with added bran had specific volume unchanged or even improved up to 11%. After adding bran, as well as xylanase, crumb colour was darker while yellow and red colour were more pronounced. All properties of enriched breads were influenced by the interaction of bran particle size and xylanase addition, except crumb springiness and SDFP content. In the case of using the medium particle size bran, xylanase addition had the most positive effect in terms of higher SDFS content (45%), improved bread volume (6%), crumb softness (16%), resilience (7%), cohesiveness (2%), and lower chewiness (15%). In conclusion, proso millet bran can be successfully used for enrichment of gluten-free bread with dietary fibre and phenolic compounds, but its particle size should be considered. Yet, the influence of xylanase on quality of enriched gluten-free bread is limited. Further studies should investigate multiple enzyme treatments of bran used for enhancement of gluten-free bread nutritional and physical properties.

gluten-free bread ; proso millet bran ; dietary fibre ; particle size ; xylanase

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