Corn steep liquor as a cost-effective nutrition adjunct in high-performance Zymomonas ethanol fermentations

The ethanologenic bacterium Zymomonas mobilis has been demonstrated to possess several fermentation performance characteristics that are superior to yeast. In a recent survey conducted by the National Renewable Energy Laboratory (NREL), Zymomonas was selected as the most promising host for improvement by genetic engineering directed to pentose metabolism for the production of ethanol from lignocellulosic biomass and wastes. Minimization of costs associated with nutritional supplements and seed production is essential for economic large-scale production of fuel ethanol. Corn steep liquor (CSL) is a byproduct of corn wet-milling and has been used as a fermentation nutrient supplement in several different fermentations. This study employed pH-controlled batch fermenters to compare the growth and fermentation performance of Z. mobilis in glucose media with whole and clarified corn steep liquor as sole nutrient source, and to determine minimal amounts of CSL required to sustain high-performance fermentation. It was concluded that CSL can be used as a cost-effective single-source nutrition adjunct for Zymomonas fermentations. Supplementation with inorganic nitrogen significantly reduced the requirement for CSL. Depending on the type of process and mode of operation, there can be a significant contribution of nutrients from the seed culture, and this would also reduce the requirement for CSL. Removal of the insolubles (40% of the total solids) from CSL did not detract significantly from its nutritional effectiveness. On an equal-volume basis, clarified CSL was 1.33 times more "effective" (in terms of cell mass yield and fermentation time) than whole CSL. For fermentations at sugar loading of >5% (w/v), the recommended level of supplementation with clarified CSL is 1.0% (v/v). Based on CSL at US $50/t, the cost associated with using clarified CSL at 1.0% (v/v) is 88 cents/1000 L of medium and 5.3 cents/gal of undenatured ethanol for fermentation of 10% (w/v) glucose. This cost compares favorably to estimates for using inorganic nutrients. The cost impact is reduced to 3.1 cents/gal if there is a byproduct credit for selling the insolubles as animal feed at a price of about US $100/t. Therefore, the disposition of the CSL insolubles can significantly impact the calculations of cost associated with the use of CSL as a nutritional adjunct in large-scale fermentations.