title: The production of single cell protein from piggery slurry
Creators: name: Davey, Gwendoline Mary
Publisher: name: University of Glasgow
Year: 1980
Subjects: subject: Chemical engineering, agricultural engineering.

description: The aim of this project is to produce from an agricultural waste, namely pig slurry, a source of protein which is suitable for feeding to animals. It is considered important that the end-product is nontoxic, free from pathogens, of a high nutritional quality and can be produced in such a way that its composition does not vary. Analysis showed that the major carbohydrate components of pig slurry are cellulose and hemicellulose, which constitute approximately 17.5% and 23.5% of the dry weight of the slurry respectively. In this form the carbohydrate is not readily degraded by microorganisms, and in order to overcome this problem it was decided to hydrolyse the cellulosic material with dilute sulphuric acid (H?SO?). Temperatures of 80°C, 90°C and 100°C and acid concentrations of between 1% and 10% w/v were investigated. Thin-layer and paper chromatography of the liquid produced by mild H2SO4 hydrolysis of pig slurry indicated that xylose, arabinose, glucose, galactose and the uronic acids of glucose and galactose are released in detectable amounts. By using quantitative High Pressure Liquid Chromatographic (HPLC) analysis of the monosaccharides released by acid treatment, combined with a quantitative assessment of the residual cellulosic fractions, it was possible to optimise the hydrolysis conditions for maximum monomer release. Mild acid hydrolysis causes degradation of the hemicel 1ulosic fraction of slurry, and produces liquid hydrolysates which contain mainly pentose sugar and large amounts of ammonium nitrogen. The α-cellulose fraction remains almost intact in the solid residue. Three hydrolysis systems were adopted for further growth work with yeast species. These were 1% H2SO4 at 90°C for 56 h, 1% acid at 100°C for 24 h and 5% acid at 90°C for 3 h. A number of yeast species were selected for their ability to grow on vitamin-free media, with ammonia as a nitrogen source, and xylose and arabinose as carbon sources. Initial experiments indicated that the hydrolysate had an inhibitory effect on yeast growth. Extensive investigation into the source and nature of this inhibition showed that an aromatic phenolic breakdown product of carbohydrates was responsible for this phenomenon. It was further shown that the inhibitory effect could be overcome by dilution of the hydrolysate or treatment with activated charcoal. Of all the yeasts tested, the growth of a species, later identified as Candida tropicalis, was least affected by the presence of this inhibitory compound. This yeast species was therefore used in all subsequent growth trials with diluted or charcoal-treated hydrolysate. The highest yield of yeast biomass was obtained on a diluted hydrolysate produced with 5% acid at 90°C for 3 h. The yield was such that 9 g of dry biomass were produced from each litre of raw slurry hydrolysed. The slurry used throughout this work had a solids concentration of 110 g/l-1. The liquid remaining after yeast growth contains a large amount of residual ammonia. It is therefore possible to add this liquid back to the solid cellulosic residue of hydrolysis as a source of nitrogen for the growth of the cellulolytic, white-rot fungus, Sporotrichum pulverulentum. The cellulosic constituents of the solid residues produced by the hydrolysis systems employing 1% acid were demonstrated to be only sparingly available to Sporotrichum pulverulentum. Treatment of pig slurry with 5% acid at 90°C for 3 h, however, disrupts the structure of the α-cellulose sufficiently to allow extensive fungal attack. Growth trials indicated that for every litre of raw slurry hydrolysed 4.6 g of fungus were produced. Finally, a process was tested in which the yeast growth stage was omitted and the fungus grown on the unseparated liquid and solid residues of hydrolysis. In fermenter growth trials yields of fungal biomass of almost 20 g.l-1 of raw slurry were possible using this system. Amino acid analyses of the products of microbial growth on the liquid and solid residues from acid hydrolysis of pig slurry indicated that they are potentially useful as feed supplements for pigs. It was also found that the ultimate liquid residue from microbial growth still contained significant quantities of ammonia. It is therefore considered that the liquid residue may be usefully sprayed on agricultural land.