Sorghum hot water extract : influence of grain physico-chemical characteristics

Abstract

Traditionally, barley is the preferred cereal for traditional lager brewing. The use of sorghum for the production of European-style lager beers has recently been recognised. The use of sorghum as brewing adjunct could be a major socio-economic advantage in the developing countries in Africa. Limitations for its use, however, include its low amylolytic potential, high gelatinization temperatures, and the presence of tannins. Adjuncts are often combined with cereal malt during the brewing process to provide extra sources of fermentable carbohydrates. As with all cereals, the functional properties of sorghum grains are influenced by their physico-chemical characteristics. It is therefore critical to understand the structure, chemistry and functionality of the sorghum cultivar(s) considered for use as brewing adjuncts. Hot water extract describes the quality of the wort of an adjunct and depicts the amount of starch that was solubilised during mashing. The determination of hot water extract is expensive, laborious and time-consuming. The provision of a possible predictive marker(s) for sorghum hot water extract that is less complicated to determine, could be of great economical value to the brewer. Hot water extract was determined for 43 sorghum cultivars and then compared to various physico-chemical characteristics. Sorghum endosperm texture was visually determined. Suspensions of whole sorghum flour were pasted using a Rapid Visco Analyser (RVA) with an extended heating cycle. Significant negative correlations were obtained between extract content and pasting temperature and time in corneous endosperm samples. There was also a significant positive relationship between tannin-free sorghums and peak viscosity in pasted samples. Protein contents of 10 different sorghums were compared to their hot water extracts, where there was a significant negative relationship between these characteristics. Protein content could be used successfully as a predictive marker for extract. No significant relationship could be established between sorghum hot water extract and starch content. Tannin-containing sorghum cultivars gave significantly lower extracts and had higher malt diastatic power (DP) than non-tannin cultivars. There was no significant relationship between the DP and extract content of non-tannin sorghums. When only non-tannin sorghum cultivars that pasted were subjected to principal component analysis, it seemed that a positive relationship existed between peak viscosity and extract content. Low protein sorghum cultivars with no tannins and corneous endosperm would be suitable for use as brewing adjuncts. Protein content, the presence of tannins, endosperm texture and peak viscosity could be used as predictive markers for sorghum hot water extract