Submitted to: Process Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/9/2019
Publication Date: 4/20/2019
Citation: Stoklosa, R.J., Johnston, D., Nghiem, N.P. 2019. Phaffia rhodozyma cultivation on structural and non-structural sugars from sweet sorghum for astaxanthin generation. Process Biochemistry. 83:9-17. https://doi.org/10.1016/j.procbio.2019.04.005.
Interpretive Summary: Sweet sorghum is an agricultural feedstock that can be utilized in biorefineries to produce value-added co-products using fermentation processes. This crop can be harvested to supply two separate sugar streams: one stream containing soluble sugars in the form of juice, and the second containing sugars obtained by breaking down polysaccharides such as cellulose or hemicellulose. Sweet sorghum juice (SSJ) can be extracted from the crop using mechanical processes. The remaining solids after extraction, known as sweet sorghum bagasse (SSB), can be chemically pretreated followed by enzymatic hydrolysis to convert the polysaccharides into additional soluble sugars. Through fermentation processes the sugars can be a carbon source for yeast strains that produce high value co-products. One such yeast strain, Phaffia rhodozyma, can be grown on sugar subtstrates to produce the carotenoid astaxanthin. This product can be applied to aquaculture feed as both a colorant for farm raised salmon and an antioxidant for physiological development. P. rhodozyma was found to grow well in supplemented SSJ to produce up to 65.8 mg/L of astaxanthin. This corresponds to an astaxanthin content in the yeast cell of 1.82 mg astaxanthin per g of yeast dry cell mass. Sugars from SSB supported the growth of P. rhodozyma, however, the cultivation of the yeast strain was inhibited by the presence of other compounds from the bagasse. An improvement in P. rhodozyma cultivation only occurred after the hydrolysate was detoxified to remove inhibitory compounds. Detoxification with activated carbon (AC) produced a cell astaxanthin content of 2.07 mg astaxanthin per g of yeast dry cell mass. While both sugar streams could cultivate Phaffia rhodozyma and generate astaxanthin, SSJ appears to be more well suited as a substrate for cultivation since it requires less chemical processing.
Technical Abstract: Because it contains both structural and non-structural sugars, sweet sorghum has potential applications in biorefineries seeking to produce co-products. Differences between sugar composition in sweet sorghum juice (SSJ) and sweet sorghum bagasse (SSB) requires a robust organism that can be cultivated in both sources. The red pigmented yeast strain Phaffia rhodozyma can be grown indiscriminately on sugars originating from agricultural feedstocks and produces the carotenoid astaxanthin which has applications in high value markets. In this study we determined cultivating conditions for Phaffia rhodozyma in both SSJ and SSB hydrolysate for biomass growth and astaxanthin generation. Batch bioreactor studies on SSJ with supplementation produced 36.6 g/L biomass with 65.8 mg/L astaxanthin at a productivity rate of 0.352 mg/L/h after 168 hours of cultivation. Phaffia cultivated in SSB hydrolysate showed improved growth after activated carbon (AC) detoxification and only marginal improvement after laccase detoxification. AC detoxification produced 23.6 g/L biomass, 48.89 mg/L astaxanthin, and a productivity of 0.291 mg/L/h. Undiluted SSB hydrolysate produced inferior results due to the assumed presence of phenolic inhibitors. While both structural and non-structural sugars from sweet sorghum can be utilized by Phaffia rhodozyma, it is apparent that SSJ provides a better substrate to produce naturally occurring astaxanthin.