Enzymatic degradation of granular potato starch by Microbacterium aurum strain B8.A

Microbacterium aurum strain B8.A was isolated from the sludge of a potato starch-processing factory on the basis of its ability to use granular starch as carbon- and energy source. Extracellular enzymes hydrolyzing granular starch were detected in the growth medium of M. aurum B8.A, while the type strain M. aurum DSMZ 8600 produced very little amylase activity, and hence was unable to degrade granular starch. The strain B8.A extracellular enzyme fraction degraded wheat, tapioca and potato starch at 37 °C, well below the gelatinization temperature of these starches. Starch granules of potato were hydrolyzed more slowly than of wheat and tapioca, probably due to structural differences and/or surface area effects. Partial hydrolysis of starch granules by extracellular enzymes of strain B8.A resulted in large holes of irregular sizes in case of wheat and tapioca and many smaller pores of relatively homogeneous size in case of potato. The strain B8.A extracellular amylolytic system produced mainly maltotriose and maltose from both granular and soluble starch substrates; also, larger maltooligosaccharides were formed after growth of strain B8.A in rich medium. Zymogram analysis confirmed that a different set of amylolytic enzymes was present depending on the growth conditions of M. aurum B8.A. Some of these enzymes could be partly purified by binding to starch granules

Particle size of milled barley and sorghum and physico-chemical properties of grain following extrusion

Milled barley and sorghum grains were separated into three size fractions (fine, 1.0 mm) and extruded at two maximum temperatures (100 °C; 140 °C). Mechanical resistance and specific mechanical energy during extrusion was significantly higher for fine fractions, and extrusion at high temperature resulted in higher mechanical resistance. Pressure generated during extrusion was higher for the fine fraction in sorghum but lower in barley. Expansion index was highest for the fine fraction for barley, but did not differ significantly between sorghum fractions or with extrusion temperature. For all samples, extrusion at low temperature resulted in a higher final paste consistency and lower water absorption index, but there was no significant effect on water solubility index (WSI). Fraction size showed a significant effect on WSI in sorghum but not in barley. The results are rationalised in terms of differences in grain composition between sorghum and barley. © 2010 Elsevier Ltd. All rights reserved