Optimisation of kafirin extraction parameter for commercial applications

Abstract

Kafirin is a prolamin protein unique to sorghum grain. To make kafirin viable for commercial development there needs to be an optimised extraction process. For this purpose insight is needed into how the process is affected by the extraction temperature, grain/solvent ratio, grain/solvent contact time, and various amounts and types of reducing agents and extraction aids. Such insights are currently unavailable. Significant insights into the extraction of kafirin were gained by performing extractions under various conditions to determine how extraction parameters affected the extraction process. Extraction temperature, grain/solvent ratio, grain/solvent contact time and various amounts and types of reducing agents/extraction aids were tested to determine the optimum extraction conditions. The extraction aids tested were acids (glacial acetic acid, citric acid and phosphoric acid) and metal bisulphite complexes (sodium metabisulphite and potassium metabisulphite) were tested as reducing agents. Using a bench-scale extraction setup, the following important results were obtained. ? With regard to the commercialization of the process aqueous ethanol is the most promising solvent and was used throughout the study. ? The maximum amount of protein after extraction was roughly 45 g of protein for 1 000 g of grain out of a total kafirin content of roughly 62 g. This is an extraction efficiency of 70 - 75 % based on the total amount of kafirin available in the grain. Acids were found to have a limited role in the extraction process and can be excluded. ? The quantity of metalbisulphite complex used played a large role in the extraction process, reducing the yield by about 50 % when absent. The type of metalbisulphite used, however, did not play a role. ? The grain/solvent contact time was deemed to be the most important factor, with the highest yield occurring somewhere between 10 and 20 min. ? After 20 min the yield decreased due to increased solvent retention. The solvent retention and increased filtering time associated with the drop in yield were investigated. The weakened hydrogen bonding in the starchy endosperm made it possible for the starchy material to bond to other starchy particles or less likely the solvent, and was most likely the mechanism of solvent retention and thickening of the slurry. This increased solvent retention resulted in approximately 50 % of the total solvent being retained; which was discarded with spent grain. At 20 min the solvent retained was roughly 12 % of the total original solvent used. Further study is recommended with regard to: ? The mechanism of thickening to determine whether there are chemical or mechanical methods which would overcome the thickening and increased solvent retention ? The recovery, purification and subsequent reuse of ethanol ? Investigation into the mechanism that is responsible for the increased filtration time and whether this is related to the tannin-protein complex formation as the literature suggests ? The viability of the spent grain in agricultural applications such as cattle feed or fertiliser ? The effect of changes mentioned in this thesis on the end-use of the protein, e.g. film making quality