Evaluation of the quality of sorghum malts

Abstract

Dorado, Malisor 84-7, Tortillero, ATx623*SC103-12E, and ATx630*R3338 sorghums were cleaned, steeped in tap water for 20h, immersed in a 2% sodium hypochlorite solution for 10 min, rinsed with distilled water 5 times, drained, germinated from 0 to 6 days at 28°C and 95% RH, and dried for 48 h at 50°C. Commercially available colorimetric assays and standard methods for determining barley malt quality characteristics were adapted to characterize these sorghum malts. The rapid visco-analyzer (RVA) was adapted to determine α-amylase activity by measuring changes in peak and final viscosity of a malt extract-corn starch slurry heated to 95°C for 3 min. Viscosity changes were significantly correlated with malt α-amylase activities determined by a specific colorimetric assay for α-amylase. The RVA method needs confirmation but appears to be simple, quick and reliable. The colorimetric assay to determine β-amylase showed significant differences in the levels of β-amylase activity among sorghum varieties. The colorimetric assay is a simple, direct, accurate, reproducible, fast assay which requires only a small sample (0.1 g, d.b.) and costs $0.40/sample for substrate and reagents. Dry matter loss (DML) increased and density decreased with germination time. The level of α- and β-amylase activity varied among sorghum varieties. The optimum germination time for malting sorghum under the conditions used in this study was 4 days. Significant correlations between malting quality parameters of 5-day germinated grain and physical measurements on raw and malted grains were found. Confirmation of these correlations with larger number of samples might indicate the usefulness of these measurements in selecting for malting potential. Dorado had malting properties most closely resembling those of barley malt. The proportion of β- to a-amylase activity (0.4) in Dorado malt was the same as that of barley malt although the extract was 70% compared to 80% or higher expected for barley malt. The Kolbach index and the free α-amino nitrogen of sorghum malt were similar to barley malt (30-40%, >150mg/L, respectively). ATx630*R3338, the waxy cultivar, had the lowest α-amylase activity; its diastatic power was only 33% of Dorado malt. Sorghums with soft or intermediate endosperm texture had the best malting properties in this study, which confirms observations made in South Africa and India.