Study of the growth parameters of the yeast Rhodotorula rubra and its carotenoid production

Abstract

A red yeast, Rhodotorula rubra, was grown in a batch fermenter with molasses and acid extract from peat as basic substrates in separate experiments. -- Different concentrations of water-diluted peat extract were tested in shake flask experiments in an attempt to determine the optimal TCH (total carbohydrate) concentration for the growth of the yeast. The best results were obtained with a TCH concentration of 15 g/L. The nutritional requirements of R. rubra grown in peat extract were studied to enhance the growth of the yeast biomass. The yeast required supplements of nitrogen, potassium, and magnesium for growth. Growth was also better when a combination of organic and inorganic sources of nitrogen were added than when the inorganic nitrogen source (ammonium salts) was used alone. The addition of 4 g/L ammonium sulphate, 1.5 g/L yeast extract, 0.174 g/L potassium sulphate, and 0.5 g/L magnesium sulphate to the peat extract medium produced 4.4 g/L dry biomass, with a biomass yield of 57%, and a biomass efficiency of 29%. -- Several operational parameters, namely, pH, agitation speed, temperature, and fermentation time, were studied to optimize the biomass production by R. rubra. The best results produced approximately 4.8 g/L of dry biomass with a yield of 70% and an efficiency of 32%. The cultivation time employed was 84 hours, with an inoculum ratio of 2% (v/v), an incubation temperature of 22 ± 1°C, an initial pH of 5.5, and an agitation speed of 200 rpm -- In separate experiments in a 2-Liter fermenter, the growth of R. rubra in the peat extract and molasses was studied to optimize the agitation and aeration rates. The optimal conditions produced 5.9 g/L of biomass with a yield of 71% in peat extract medium, and 4.5 g/L of biomass with a yield of 33% in molasses medium. These results were obtained with an agitation speed of 200 rpm, an aeration rate of 1.0 vvm, an inoculum ratio of 2% (v/v), an incubation temperature of 22 ± 1°C, a pH of 5.5 ± 0.1, and a cultivation time of 72 hours. The β-carotene contents of the yeast produced from the peat extract medium and the molasses medium were 1235 μg/g of dry biomass, and 1034 μg/g of dry biomass, respectively. -- The proximate compositions of the R. rubra biomass grown in the two different media were analyzed and compared. The yeast grown in peat extract contained approximately 48.5% crude protein, 4.1% ash, 27.7% total carbohydrate, and 5.7% total fat. The yeast grown in molasses contained 43.6% crude protein, 3.8% ash, 30.8% total carbohydrate and 10.2% total fat. The amino acids and fatty acids compositions of the yeast grown in peat extract were analyzed. The yeast protein contained 10 essential amino acids, and was high in leucine, arginine, lysine, and valine. It was, however, deficient in tryptophan when compared with the FAO standard. Eleven fatty acids were identified in the biomass.