Influence of mixed culture conditions on yeast-wall hydrolytic activity of Bacillus circulans WL-12 and on extractability of astaxanthin from the yeast Phaffia rhodozyma

O kagbue , R.N. & L ewis , M.J. 1985. Influence of mixed culture conditions on yeast-wall hydrolytic activity of Bacillus circulans WL-12 and on extractability of astaxanthin from the yeast Phaffia rhodozyma. Journal of Applied Bacteriology 59 , 243–255.
In mixed culture Bacillus circulans WL-12 hydrolysed cell walls of Phaffia rhodozyma and rendered astaxanthin extractable from the yeast. pH control was critical to survival and lytic activity of the bacillus; the optimum range was 6.2–6.8. The optimum range of temperature was 20–24C. Glucose (1–2%) was efficient in minimizing catabolite repression of the lytic enzyme complex of the bacillus. Slow-feeding of glucose improved ultimate yields of lytic enzyme but did not acclerate yeast cell wall modification. A relatively high inoculum level of B. circulans accelerated modification of P. rhodozyma in the mixed culture: when the bacterial inoculum was four times that of the yeast, over 80% of total astaxanthin was extractable in 48 h. High bacterial inoculum size also stimulated yeast autolysis and necessitated early harvest of the mixed culture. Results obtained in shake flasks were duplicated in 5-litre fermentors and suggest that the mixed culture has potential industrial value for producing a biomass containing biologically-available astaxanthin. Extractability of astaxanthin was also achieved when mixed culture filtrate was incubated with pure cultured Phaffia cells. When suitably fortified with nutrients, the filtrate also supported simultaneous yeast growth and modification of the yeast cell walls. A scheme incorporating mixed culture with B. circulans WL-12 and re-use of culture filtrate has been proposed for enzymatic processing of Phaffia rhodozyma for inclusion in animal diets.