Effect of extrusion temperature and pre-extrusion particle size on starch digestion kinetics in barley and sorghum grain extrudates

Barley and sorghum milled grains were separated into three size fractions (fine, <0.5 mm;medium, 0.5'1.0 mm; coarse, >1.0 mm) and extruded at two temperature levels (maximumof 100 'C or 140 'C), to determine the effect of pre-extrusion fraction size on starchdigestibility. Following extrusion, in vitro enzymatic starch digestibility was markedlyenhanced with a first order rate, typically ten times greater than before extrusion, but followingthe same trend i.e. faster digestion for finer pre-extrusion grain sizes. For sorghum,the projected maximum digestibility was also reduced with pre-extrusion grain size. Therewere only minor differences in digestibility following extrusion at 100 'C compared with140 'C. Grains were not pre-conditioned, so moisture uptake during extrusion may havebeen limited, resulting in extrudates containing residual ungelatinized starch granules. It isproposed that the main effect of extrusion processing was to open up the grain endospermstructure, thereby reducing the diffusion path lengths for amylase to digest starch. Theresults indicate that an efficient use of extrusion processing could be to target separatedlarger particles from milled grains for re-combination with finer ground grains for improvedenergy utilization in monogastric feeds.