Endogenous and exogenous factors involved in sorghum germination with reference to malting.

Abstract

In Africa, the grain sorghum (Sorghum bicolor (L.) Moench), is malted to provide the most important ingredient in brewing, malt, which is used primarily for the production of traditional (opaque) sorghum beer. Malting is the germination of cereal grain in moist air under controlled conditions, the primary objective being to promote the development of hydrolytic enzymes which are not present in the ungerminated grain. The malting process can be physically split into three distinct unit operations (viz. steeping, germination and drying). To date, little attention has been given to optimising the conditions of steeping for sorghum. The effects of different steeping variables (time, temperature and aeration) on the quality (in terms of diastatic power (amylase activity), free amino nitrogen and hot water extract) of sorghum malt for brewing were investigated. Malt quality was found to increase with steeping time, over the range 16-40 hours and the optimum steeping temperature was found to be in the range 25 to 30°C. Aeration during steeping appeared to be necessary to maximise the malt quality, particularly when steeping was conducted for long periods at high temperatures. Of particular significance was the observation that final sorghum malt quality was highly significantly correlated (p<0.01) with grain moisture content at steep-out (the end of the imbibition period). When steeping conditions based on these findings were used, a germination temperature of 25-30°C was found to be optimal for sorghum malt quality. As with steep-out moisture, green malt (grain after the specified germination time) moisture content was correlated Significantly (p<0.01) with final sorghum malt quality. The finding that sorghum malt quality is related to steepout moisture content was given further substance when it was shown that the stimulatory effect on sorghum malt quality of steeping sorghum in a dilute solution of alkali, actually increases the amount of water taken up during steeping probably because the alkali disrupted the pericarp cell wall structure of the grain. Barley malting practices have taken advantage of the knowledge that the exogenous application of gibberellic acid can enhance the synthesis of the critically important malt hydrolytic enzyme, a-amylase. To date, literature on the effect of exogenous application of gibberellic acid on sorghum malt quality has been inconclusive; with reports both of no effects, and of positive effects, on amylase activity. To elucidate the possible control mechanisms involved in sorghum germination, a combined HPLC-radioimmunoassay technique was used to determine the levels of selected plant growth regulators from the groups auxin, cytokinins, gibberellins and abscisic acid in sorghum at various stages of germination. Levels of gibberellic acid were low throughout germination. During germination the levels of the other plant growth regulators declined, but a peak in cytokinins followed the first visible signs of root protrusion. The high level of the germination inhibitor and gibberellic acid antagonist, abscisic acid, in the germ (embryo inclusive of scutellum) portion of the mature non-germinated grains was noteworthy. Based on these findings, it was determined that sorghum malt quality could in fact be improved significantly by the application of exogenous gibberellic acid. However, this was effective only if it was administered during the end of steeping or at the beginning of the germination step. By optimising the conditions of steeping and germination and by steeping in dilute NaOH or in gibberellic acid not only should it be possible to enhance the quality of sorghum malt, it should be possible to reduce the time required to obtain the specific quality, thereby offering a saving to the sorghum maltster in terms of operation costs and enhancing the total throughput possible from the malting plant.