Evaluation of the potential malting, brewing and antiyeast characteristics of selected Zambian sorghum cultivars

[摘要] ENGLISH ABSTRACT: The potential malting, brewing and antiyeast characteristics of twelve selected sorghum cultivars from Zambia (MMSH 375, 625, 707, 1038, 1077, 1194, 1257, 1356, 1363, 1365, 1376 and 1401) was evaluated. This was done through the determination of tannin content, proximate composition, %germinative energy (%GE), micro-structure, diastatic power (DP) and free amino nitrogen (FAN); as well as the antiyeast activity of N-containing compounds for each cultivar.All cultivars were tannin-free and thus suitable for brewing potential exploration. Seven cultivars (MMSH-375, 625, 1038, 1194, 1257, 1401 and 1376) had a %GE considered appropriate for malting and brewing. A maximum of 25% of the DP level recommended for commercial malting was achieved in this study. In spite of the low DP observed in all cultivars after malting, high extract levels ranging between 18.7 and 25.5˚P were achieved. In unmalted grains a significantly FAN level (p≤0.05) was observed in three cultivars (MMSH-375, 1194 and 1077). These FAN values, however, represented less than 10% of the recommended FAN level necessary to support proper yeast functioning during wort fermentation. The 150 mg/L threshold which is necessary to support proper yeast function and is employed in modern day brewing was achieved in malts of sorghum cultivars MMSH-375 (intermediate cultivar) and 1401 (intermediate cultivar). A FAN level between 100 and 140 mg/L, which is considered enough to support optimal fermentation, was achieved in cultivars MMSH-625 (intermediate cultivar), 1077 (hard cultivar), 1194 (intermediate cultivar), 1356 (soft cultivar) and 1376 (intermediate cultivar) after malting. Mashing did not significantly increase FAN levels in all sorghum cultivars. The addition of potassium metabisulphite (KMS) in the mash did, however, lead to a further increase of FAN levels in all cultivars. Scanning electron micrographs revealed that the corneous endosperm was inaccessible to degradation by enzymes and only the soft endosperm was modified in all cultivars during malting.The N-containing compounds from selected malts of sorghum cultivars with good malting and brewing potential (MMSH-375, 625, 1401 and 1194) were isolated and their antiyeast activity was determined. It was found that the N-containing compounds were playing an essential role during fermentation by contributing positively towards yeast growth in all four cultivars. However, a 20-30% reduced growth at the lower concentrations of the putative %yeast growth promoting N-containing compounds was observed in all cultivars. Irrespective of the reduced %yeast growth observed in all four cultivars at low concentrations of the putative yeast growth N-containing compounds, it could not be concluded that the sorghum cultivars utilised in this study possess antimicrobial peptides and proteins because only small compounds (<1 kDa) were detected with UPLC-MS. The dose-response data showed that extracts of MMSH-375 and MMSH-625 resulted in generally a higher %yeast growth when compared to other cultivars. The UPLC-MS also confirmed the dose response data that MMSH-375 has a better qualitative yeast-feeding value. MMSH-375 is thus rated as the best cultivar out of the four that were screened and is recommended to be use in the commercial production of sorghum lager beer.

[发布机构] Stellenbosch University