Effect of chemical conditioning on the milling of high-tannin sorghum

Suitable methods are required for milling tannin-containing sorghums. Sorghum varieties SV2 (tannin-free), Chirimaugute (medium-tannin) and DC-75 (high-tannin) were milled using a simple roller mill and a multi-sample tangential abrasive decortication device. Grain was conditioned up to 20% moisture prior to milling using HCl (0.9%, v/v), formaldehyde (HCHO; 0.05%, v/v), NaOH (0.3%, w/v) and water as control. Abrasive decortication and roller milling reduced levels of the polyphenols. Polyphenol reduction was affected significantly by variety, chemical treatment and conditioning moisture (P<0.001). NaOH and HCHO treatments gave lower polyphenol content in the meal and offal after roller milling. Enzyme inhibition (EI) by polyphenols was reduced by 52% after decortication or roller milling. Chemical treatment did not significantly affect EI levels after decortication. NaOH and HCHO treatments gave the lowest EI when Chirimaugute was roller milled at 20% moisture. The yield of product obtained after decortication was high at 12% moisture for SV2 and Chirimaugute and at 16% conditioning moisture for DC-75. Conditioning to 20% moisture prior to milling did not improve product yield. Roller milling and decortication resulted in products that were lighter in colour than the grain. Both abrasive decortication and roller milling plus NaOH and HCHO reduced tannin content of offal. It is concluded that conditioning using NaOH could be advantageous in roller milling but not in abrasive decortication of tannin-containing sorghums. (C) 2000 Society of Chemical Industry.