4.1

Cost Effective Growth Media

In order for ethanol production to be commercially feasible, the growth media cost should be kept at a minimum. In addition to engineering strains to require less nutritional supplementation, the design of simpler, and therefore cheaper, growth media is important for the expansion ofbioethanol production.

AM1 [76] and NBS mineral salts media [77] are two simple mineral salts media developed in our laboratory. Both have been shown to support high levels of cell growth and ethanol production. AM1 is a derivative of NBS, with a 65% reduction in salts. With low total alkali (4.5 mM) and total salts (4.2 gL-1), AM1 was able to support production of ethanol from xylose and lactate from glucose with average productivities of 18 -19 mmol L-1 h-1.

OUM1 medium contains corn steep liquor, mineral salts, and urea as sole nitrogen source; K. oxytoca BW21 produced over 40 gL-1 ethanol (0.47 g ethanol per gram glucose) in this medium within 48 h [57]. The use of urea as sole nitrogen source has the benefit of cost reduction while also reducing media acidification [78].

On-site preparation of crude yeast autolysate from spent yeast offers poten­tial synergy between grain-based and lignocellulosic processes. Preparation of this autolysate, optimization of the resulting media, and ethanol production by KO11 were demonstrated by [31], with ethanol yields comparable to LB.

K. oxytoca is able to utilize urea as sole nitrogen source, where urea has roughly half the cost of ammonium on an equivalent nitrogen basis. Addition­ally, because urea metabolism does not contribute to media acidification [79], the use of urea reduces the cost of pH control. With the goal of reducing the nutrient cost of K. oxytoca-based ethanol production, optimized urea medium (OUM1) was developed. In addition to containing urea as the sole nitrogen source, OUM1 contains corn steep liquor, mineral salts, and glucose [57].

4.2