Endogenous and exogenous factors involved in sorghum germination with reference to malting.
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Date
1997
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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.
Description
Thesis (Ph.D.)-University of Natal, Durban, 1997.
Keywords
Sorghum--Preharvest sprouting., Malt liquors., Sorghum products., Brewing., Sorghum., Theses--Biological and conservation sciences.