Mixed Culture Fermentation in the Hydrolysis of Cereal Flour for the Production of Industrial Biochemicals
A Thesis Submitted to the School of Postgraduate Studies, University of Lagos
Hydrolysis of a complex medium (whole-wheat flour) using a mixed culture of Aspergillus. awamori and Aspergillus. oryzae was carried out for the purpose of producing a fermentation medium that can be used to produce industrial chemicals.. The experimentation was carried out in shake flasks as well as in a 10-litre fermenter and the results obtained in all cases indicated that the two microorganisms could cooperate together to hydrolyse both the starch and the protein in wheat flour concurrently. The mixed culture hydrolysis produced a medium with an optimum total reducing sugar of about 50 g/l and free amino nitrogen concentration of about 120 mg/l. The amount of reducing sugar obtained for the pure cultures of the two organisms when compared with that of the mixture showed that A. awamori gave a slightly higher rate of fermentation. A. awamori produced 50 g/l of reducing sugar in 30 h when compared with A. oryzae and the mixed culture producing the same amount in 42 h and 39 h respectively. The glucoamylase activity of the mixture was higher in the mixed culture than in either of the pure cultures. The highest yield of free amino acid (120 mg/l) was obtained in the fermentation with A. oryzae and the least value of (56 mg/l) was with that of A. awamori. The mixed culture gave a value of (115 mg/l) which is comparable to that obtained with A. oryzae. The protease activity followed the same trend as was observed for the production of free amino acid with the highest activity being observed with A. oryzae and the least with A. awamori. A mathematical model was developed for the hydrolysis of starch, based on proposed mechanism assuming a synergistic relationship in the action of the amylases (amylase and glucoamylase) produced by the two organisms. The model incorporates the syntheses of the two enzymes by the organisms and assumes a Michaelis-Menten model without substrate or product inhibition for the hydrolysis. Futhermore the mechanism assumes a homogeneous structure of the substrate predicts only one final product and lacks information on all possible intermediate products. The solution of the multiparameter model as it is usual with this type of model did not permit a wide variation of most of the parameters. Some parameters such as the ratio of Michaelis-Menten equilibrium constant and the turnover number for the two enzymes and represented by and could be varied between 10-110 and 1000-3800 respectively. Others like specific growth rates ( ), enzyme constant (a) and the number of cleavable bonds in the oligosaccharides per molecule of substrate (n) had a narrow range of values. The value of ( ) was between 0.04-0.25 h-1 and (n) had a value of 2 and the enzyme constant (a) had a value of 1-5. . The experimental data were very well predicted by the developed model. The model can therefore be used to predict the extent of hydrolysis of complex media by a mixed culture of A. awamori and A. oryzae.