Utilization of sweet sorghum juice for the production of astaxanthin as a biorefinery co-product by phaffia rhodozyma

Authors: Stoklosa, Ryan; Johnston, David; Nghiem, Nhuan

Submitted to: ACS Sustainable Chemistry & Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/17/2018
Publication Date: 3/5/2018
Citation: Stoklosa, R.J., Johnston, D., Nghiem, N.P. 2018. Utilization of sweet sorghum juice for the production of astaxanthin as a biorefinery co-product by phaffia rhodozyma. ACS Sustainable Chemistry & Engineering. 3(6):3124-3134.

Interpretive Summary: Sweet sorghum is one underutilized crop in the United States that has the potential for conversion in a biorefinery. Unlike other cereal crops, sweet sorghum is drought tolerant and has high water usage efficiency. Crushed stalks of sweet sorghum produce a juice rich in sucrose, glucose, and fructose. A red-pigmented yeast known as Phaffia rhodozyma has been shown to grow in the presence of these three sugars. P. rhodozyma produces a carotenoid (an organic pigment) known as astaxanthin. This chemical has applications in aquaculture feed because it provides the characteristic red color in salmon, and as a nutraceutical for its high anti-oxidant properties. Moreover, astaxanthin can be sold for $2,000 per kg making it a high value product. This work utilized sweet sorghum juice (SSJ) to grow P. rhodozyma for the production of natural astaxanthin. Full strength SSJ fermentations in a 2 L bioreactor produced up to 29 g/L of yeast cell mass and 90 mg/L of astaxanthin. The concentrations achieved with SSJ fermentation are slightly higher than what has been found previously with other feedstocks. The results obtained now allow for further process optimizations to be implemented.

Technical Abstract: Co-product generation in a biorefinery process is crucial to allow ethanol production from agricultural feedstocks to be economically viable. One feedstock that has underutilized potential in the U.S. is sweet sorghum. The stalks of sweet sorghum can be crushed to produce a juice rich in soluble sugars (e.g. sucrose, glucose, and fructose) that can be biochemically converted to co-products. This work looks to utilize the red-pigmented yeast Phaffia rhodozyma to produce the secondary metabolite astaxanthin by fermenting sweet sorghum juice (SSJ). Shake flask fermentations on defined media indicated that all three sugars (sucrose, glucose, and fructose) could be fermented with nitrogen supplementation. SSJ without nitrogen addition could only produce modest biomass growth and astaxanthin production; however, combining nitrogen supplementation with yeast extract in diluted SSJ could metabolize all sugars present in 168 hours. A 2 L fermentation with full strength SSJ produced up to 29 g/L of biomass growth, around 90 mg/L of astaxanthin, and an overall productivity of 3.68 mg astaxanthin/g dry cell mass after 240 hours. Further process optimization is needed since glucose metabolism was incomplete after 240 hours on full strength SSJ.