Study of the growth parameters of the yeast Phaffia rhodozyma and its production of astaxanthin in peat hydrolysates

Acheampong, Edward Asafo-Adjei (1993) Study of the growth parameters of the yeast Phaffia rhodozyma and its production of astaxanthin in peat hydrolysates. Masters thesis, Memorial University of Newfoundland.

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Abstract

Salmonid aquaculture as a supplement to the world’s protein resources increased substantially in the 1980s resulting an increase in the market for astaxanthin, the principal carotenoid of salmon. Consumer acceptance of farmed salmonids depends on the color of the flesh. Since fish are unable to synthesize these pigments, they must be supplied to the diet. Currently, synthetic astaxanthin and canthaxanthin are used as pigmenters but there is considerable interest in using biological sources of astaxanthin in the aquaculture industry. – The yeast Phaffia rhodozyma has the potential as an industrial pigment source but its use is limited by the low quantities of astaxanthin in the yeast and also the high cost of growth media. Peat is one of the most abundant and inexpensive resources of the world. The liquid extract from peat contains fermentable carbohydrates which have been used in the production of various microorganisms. – A comprehensive study on the efficiency and yield of the production of yeast biomass and astaxanthin in peat hydrolysate has been carried out by a biotechnological process using Phaffia rhodozyma. The peat hydrolysates used in the present study were prepared by acid and non-acid hydrolyses process at various temperatures ranging from 185-225°C. The sugars present in the peat hydrolysate were determined. The total reducing sugars constitute about 20.66% of the total carbohydrate concentration (TCH). – The substrate concentration that supported the best growth of P. rhodozyma was 30gL⁻¹ for the non-acid hydrolysate and 15gL⁻¹ for the acid hydrolysate. The non-acid hydrolysate produced a dry biomass concentration of 4.04 ± 0.11 gL⁻¹, a yield coefficient of 34.82 ± 0.98% and an efficiency of 13.48 ± 0.1% whilst the acid hydrolysate produced a biomass concentration of 4.30 ± 0.04 gL⁻¹, a yield coefficient of 36.74 ± 1.13% and an efficiency of 14.33 ± 0.1%. These results were obtained at a pH of 7.0, an incubation temperature of 18°C, and a fermentation time of 120 hours. The best inoculum ratio was 5% (v·v⁻¹) and agitation speed was 200 r.p.m. – Of the six peat hydrolysates provided, the acid hydrolysate prepared at a temperature of 185°C (PH4-02-185) was found to support the best growth. The astaxanthin content of the yeast grown in the best acid and non-acid hydrolysates were determined. The acid hydrolysate produced an astaxanthin content of 1567 μg·g⁻¹ yeast. These values compare favorably with those reported by other researchers. – The nutritional requirements of P. rhodozyma grown in the best peat hydrolysate were studied in an effort to enhance the growth and astaxanthin production by this yeast. Organic source of nitrogen were found to promote the growth of and astaxanthin production by the yeast better than ammonium salts of inorganic acids. The addition of increasing concentrations of yeast extract resulted in increase in biomass concentration, yield coefficient, efficiency and astaxanthin content. On the other hand, addition of potassium phosphate to the medium increased the dry biomass concentration but decreased the astaxanthin concentration of the yeast. – Experiments were conducted in 2L fermenters to study the growth kinetics and astaxanthin production by the yeast in peat hydrolysate. The best growth produced a biomass concentration of 4.86 ± 0.12 g·L⁻¹ and an astaxanthin content of 1079 ± 17 μg·g⁻¹ yeast after 120 hours of fermentation. The optimum agitation speed was 250 r.p.m. The optimal aeration required for the production of astaxanthin was found to be 0.5 vvm. Higher agitation speeds reduced biomass production but had no effect on astaxanthin production. Similarly, aeration rates did not have any significant impact on astaxanthin production. The maximum specific growth rate, μmax of the yeast grown in peat hydrolysate was found to be 0.038 h⁻¹. – The chemical composition of the P. rhodozyma cell biomass was analyzed. The crude protein content was found to be 47%, with 5% ash, 19% total lipids, and reasonable amounts of tryptophan, arginine, leucine phenylalanine, threonine, glycine and other amino acids. The yeast was found to contain high quantities of unsaturated fatty acids.

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