Stabilization of semi-solid-state starch by branching enzyme-assisted chain-transfer catalysis at extreme substrate concentration

In this study a branching enzyme (BE, alpha-1,4 --> alpha-1,6 glycosyltransferase) was used to modify starch granules of different structures at high temperature and at extreme starch dry matter content (30-40%, w/v) to restrict temperature-induced swelling. As opposed to diluted systems, such conditions led to stabilization of the granular structure for low-phosphate starch types at the highest BE activity. Bright field/polarized light and scanning electron microscopy confirmed maintenance of granular structure. The product compared to the control had significantly increased degree of branching as identified by shorter branch-length of the alpha-1,4 chains assessed by chromatography and larger proportion of alpha-1,6 links to alpha-1,4 links as assessed by H-1 nuclear magnetic resonance spectroscopy. Size exclusion chromatography demonstrated the presence of uniform molecules with smaller size. Further the polysaccharide product was 40% more soluble at 25 degrees C than the corresponding heat treated control. Both of these observations were supported by C-13 solid-state MAS NMR. Hence, significant chain transfer took place in the semi-solid state starch permitting conservation of the main granular organization in the final product. A hypothetic model is presented to account for the observed phenomenon. (C) 2012 Elsevier B.V. All rights reserved.