The production of microbial protein from waste carbohydrates by filamentous fungi

A process for the production of microbial biomass from agricultural by-products was proposed. The use of define, aerobic, submerged culture to utilise waste carbohydrates and produce microbial biomass was assumed. Three potential substrates, whey, orange pulp, and potato wastes, were selected on the basis of local availability and published research data. These three waste substrates are produced from factories, over a six month season, in a scale of approximately five tonne per day. Current strategies for disposal include landfill, biological oxidation, and fodder supplements. Current information from the literature was used to select potential organisms for the proposed process. Reports of saprophytic filamentous fungi oxidising plant material were found to be prevalent. Ten fungi, isolated from local agricultural crops, were used in this study. The behaviour of the different organisms, growing in batch culture, on each substrate was observed. The problem of quantifying organism behaviour in media containing suspended substrate, was addressed. Changes were observed in the macromolecular composition of fungi over the batch culture cycle, making biomass determinations by some reported methods, approximate. Chemical methods for analysis of the substrates were also investigated. Other analytical chemical methods associated with the study were examined thoroughly. The determination of protein by the Biuret method in the present of interfering substances was investigated. Methods for determining the relative amounts of carbon, hydrogen, oxygen, and nitrogen in heterogeneous materials were developed and tested. Problems associated with the submerged culture of filamentous fungi are discussed. Members of the genus Aspergillus were seen to utilise high percentages of all substrates at relatively fast growth rates. Aspergillus niger was observed to grow at an optimum temperature of 35 degress C, and at pH values as low as 2.0. The Monod growth parameters of Aspergillus niger were determined. Continuous culture experiments using glucose as substrate, identified the true growth yield and maintenance energy of this fungus. The stoichiometry of the fermentation reactions between A.niger and glucose and A.niger and whey were determined. Oxygen utilisation rates predicated by the stoichiometric reaction between A.niger and glucose, were compared with those found by direct measurement with oxygen electrodes. A close agreement was observed between the measured and predicted rates of oxygen utilisation. A mathematical description of the diauxic growth pattern, often observed when the substrate consists of two or more chemically distinct fractions, was developed. A cybernetic computer model of microbial growth was written and tested using published data of a diauxic growth pattern. The program was then used to match Monod parameters with the behaviour exhibited by A.niger when growing on whey. Current rheological theory and observations of A.niger were used to calculate the power requirements necessary for mass and energy transport in bath culture. The observed stoichiometry was used to predict the maximum demand, during the batch culture cycle, for oxygen and heat transfer. These factors were in turn used to determine the maximum possible loading of a stirred tank fermenter. The feasibility and economic viability of microbial protein production processes operating at the scale of ca. 5 tonne day-1 were discussed