The Utilization of Low-grade Carbohydrates

Abstract

This thesis is concerned with the application of fermentation technology to the production of useful materials from a class of carbohydrate resources presently under-utilized and even wasted. The use of both carbohydrate-containing crops and processing wastes as fermentation substrates was extensively reviewed. It was concluded that a simple cheap fermentation process for the production of "biomass" directly from low-grade starch containing materials could have widespread application. Experimental work was carried out using a Continuous Tower Fermenter System, cultures of Aspergillus niger M1 and three types of media. These were based on potato starch/salts (a semisynthetic medium), raw potatoes and a waste from a "dry caustic" potato peeling process. In all the fermentations, mould grew in the form of colonies, which were responsible in part, for the radical differences in behaviour which distinguished the tower fermenter from other continuous fermentation systems. The morphology of the mould both governed and was governed by fermenter conditions such as the oxygen transfer rate. The concentration of mould in the fermenter was determined largely by a balance of the fluidisation and sedimentation forces acting on the colony morphology. In contrast, the concentration in the effluent stream depended on the growth conditions and was usually significantly lower. Experiments were carried out to determine the responses to extremes of temperature and substrate limitation; in the letter, submerged sporulation occurred. When using alkaline media, steady states occurred in which the specific growth rate of the mould exceeded the culture dilution rate, attributed to a "froth flotation" effect. The disappointingly low productivity rates and yield coefficients observed were the direct result of oxygen transfer limitations imposed by the mould morphology. In the author's opinion, the system is best suited to the production of "biomass" from effluents containing low carbohydrate concentrations.