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University of Pretoria (1999)

Enzymic decortation of sorghum grain

Siwela, Muthulisi

Titre : Enzymic decortation of sorghum grain

Auteur : Siwela, Muthulisi

Université de soutenance : University of Pretoria

Grade : MSc (Food Science) 1999

Sorghum is a drought resistant indigenous African cereal and traditional food crop. Unfortunately. milling sorghum. as is normally done using a Prairie Research Laboratory (PRL)-type abrasive dehuller is not effective. Because in sorghum the germ is integral in the grain. the process of removing the outer bran layers by abrasion leads to high losses, up to as much as 40% if a flour of less than I% fat is to be produced. The pericarp of sorghum is rich in cellulose and hemicelluloses, and the germ, lipids. Specific hydrolytic enzymes should degrade these chemical components and thus remove the bran and germ with minimal loss of grain material. In this work. four industrial enzymes. endo-β-glucanase. xylanase. pectinase and lipase were applied either singly or in combination to two different commercial batches of sorghum to determine whether hydrolytic enzymes could be used to decorticate sorghum grain. Sorghum grain was either de-waxed with hexane or scraped to remove the waxy material on the pericarp and seed coats. The grain was then soaked in a 10% (w/w) solution of either single or combined enzymes in flasks which were then incubated at 50°C in a water bath and left overnight with shaking. SEM showed that endo-β-glucanase caused decortication of the sorghum grain at the seed coat/aleurone layer interface with both the hexane de-waxed grain and the scraped one. The germ was not removed. The results suggested that endo-β-glucanase hydrolysed the exposedpericarp cell wall material and thus loosened the pericarp, and hence its removal. The germ could not be removed perhaps due to the fact that, in sorghum grain the germ/endosperm interface is at right angles to the surface of the grain, thus enzymes have only a small area to act on and all the underlying interface is not accessible. Different endo-β-glucanase concentrations were applied to partially decorticated sorghum grain to establish the optimum concentration to be used to remove the pericarp material to an acceptable level. The lowest level when there was a good decortication effect, as indicated by grain ash content and colour, was at 0.01% endo-β-glucanase concentration (ml/l00 g grain). The effectiveness of enzymic and mechanical decortication was compared in terms of the relationships between decortication yield and grain ash content, and between decortication yield and grain colour. To reduce the grain ash to an acceptable level (about 1.1%), enzyme decortication resulted in about 10% saving in grain material relative to the mechanical process. However. at the same decortication yield. the mechanical process gave a lighter product than the enzymIc process. It appears that endo-β-glucanase can remove the pencarp from sorghum grain at an economically feasible concentration (0.01%). and that there is significant reduction of grain material loss relative to the mechanical process. However, more work needs to be done to improve the colour of the product. and to find a way to remove the germ.

Mots Clés : sorghum milling ; sorghum commercial processing


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