Inactivation of tannins in milled sorghum grain through steeping in dilute NaOH solution

Abstract

Steeping milled sorghum in up to 0.4% NaOH was investigated as a method of tannin inactivation. NaOH steeping substantially reduced assayable total phenols and tannins in both Type III and Type II sorghums and with Type III sorghum caused a 60–80% reduction in a-amylase inhibition compared to a 20% reduction by water steeping. NaOH treatment also reduced starch liquefaction time and increased free amino nitrogen. Type II tannin sorghum did not inhibit a-amylase and consequently the NaOH treatment had no effect. HPLC and LC–MS of the tannin extracts indicated a general trend of increasing proanthocyanidin/ procyanidin size with increasing NaOH concentration and steeping time, coupled with a reduction in total area of peaks resolved. These show that the NaOH treatment forms highly polymerised tannin compounds, too large to assay and to interact with the a-amylase. NaOH pre-treatment of Type III sorghums could enable their utilisation in bioethanol production.