Journal of Cereal Science 56 (2012) 396-403

Particle size heterogeneity in milled barley and sorghum grains: Effects

on physico-chemical properties and starch digestibility

G.J. Al-Rabadi a, P.J. Torley b,1, B.A. Williams a, W.L. Bryden c, M.J. Gidley a,*

a Centre for Nutrition and Food Sciences, ARC Centre of Excellence in Plant Cell Walls, School of Agriculture and Food Sciences, The University of Queensland, St Lucia, Brisbane, Qld

4072, Australia

b Division of Chemical Engineering, School of Engineering, The University of Queensland, Brisbane, Qld 4072, Australia

c Centre for Nutrition and Food Sciences, School of Agriculture and Food Sciences, The University of Queensland, Gatton, Qld 4343, Australia

The average particle size of ground grains is known to influence properties related to processing (e.g.water absorption and solubility) and nutritional value (e.g. starch digestion rate) of human foods and animal feeds. The purpose of this study was to identify the contributions made by individual size fractions of hammer-milled barley and sorghum grains to average bulk compositional, hydration, rheological, and enzyme susceptibility properties. Barley and sorghum grains were each hammer-milled through

a 4 mm screen and subsequently fractionated on a set of eight sieves ranging from 0.125 mm to 2.8 mm. Individual fractions were characterised for (1) starch, aNDF, and water content, (2) water absorption index (WAI) and water solubility index (WSI), (3) viscosity profile during cooking and cooling in excess water, and (4) in vitro starch digestibility. Weighted average values based on fraction yields and property values for WAI, WSI, and starch digestibility were not significantly different from values obtained for non-fractionated ground grains of both barley and sorghum. Glucose yields from starch digestion varied about ten-fold between the smallest and largest particle fractions, and WAI and WSI had value ranges of 1.9e2.8 g/g (sorghum), 2.1e4.0 g/g (barley) and 1.3e4.5% (sorghum), 0.7e10.3% (barley), respectively. Viscosity profiles for milled sorghum grain fractions were dominated by starch swelling which became increasingly restricted as particle sizes increased. Viscosity profiles for milled barley grain fractions did not exhibit typical starch-based behaviour and were most likely dominated by soluble fibres. Taken together, the results show that there is considerable potential for designing combinations of hydration, rheological and digestibility properties of ground grains through informed selection of appropriate grains and particle size distributions.