Techno-economic implications of improved high gravity corn mash fermentation

Abstract

The performance of Saccharomyces cerevisiae MBG3964, a strain able to tolerate >18% v/v ethanol, was compared to leading industrial ethanol strain, Fermentis Ethanol Red, under high gravity corn mash fermentation conditions. Compared to the industrial ethanol strain, MBG3964 gave increased alcohol yield (140 g L⁻¹ vs. 126 g L⁻¹), lower residual sugar (4 g L⁻¹ vs. 32 g L⁻¹), and lower glycerol (11 g L⁻¹ vs. 12 g L⁻¹). After 72 h fermentation, MBG3964 showed about 40% viability, whereas the control yeast was only about 3% viable. Based on modelling, the higher ethanol tolerant yeast could increase the profitability of a corn–ethanol plant and help it remain viable through higher production, lower unit heating requirements and extra throughput. A typical 50 M gal y⁻¹ dry mill ethanol plant that sells dried distiller’s grain could potentially increase its profit by nearly $US3.4 M y⁻¹ due solely to the extra yield, and potentially another $US4.1 M y⁻¹ if extra throughput is possible.

Introduction

Recent reviews have highlighted continued improvements in efficiency of ethanol production from corn (Johnson, 2006, Wu, 2008, Hettinga et al., 2009). These improvements include an increase in corn yields from 2 t ha⁻¹ in 1950 to 10 t ha⁻¹ in 2004, a 60% reduction in production cost between 1975 and 2005 and a 45% reduction in corn processing costs (excluding corn and fossil fuels) from 1983 to 2005 (Hettinga et al., 2009). The total cost including capital and corn feedstock declined 60% from the 1980s to 2005 (Hettinga et al., 2009). Improvements in corn to ethanol technology continue to be made with dry mill ethanol yields increasing from 2.64 to 2.81 denatured gallons of ethanol per bushel between 2001 and 2007, average water use decreasing from 4.7 to 3.5 L of water per litre of ethanol, and energy use for production decreasing by 22% (Wu, 2008). These gains have resulted from a number of improvements in plant design and operation as well as the widespread adoption of simultaneous saccharification and fermentation. A significant number of these changes were made to ethanol plants in the late 1980s and early 1990s as the industry shifted away from traditional beverage alcohol technology (Hettinga et al., 2009).

A number of plant design changes have reduced both the capital and operating costs of corn to ethanol plants. The introduction of dry milling increased the yield from 0.37 to 0.40 m³ t⁻¹ and no new wet mills have been built since 1990 (Hettinga et al., 2009). During the late 1980s and early 1990s molecular sieves replaced azeotropic distillation and heat integration became more widespread, both innovations leading to reductions in steam requirements (Hettinga et al., 2009). More recent reductions in energy consumption have resulted from selling wet instead of dried distiller’s grain. In 2008, 37% of mills sold distiller’s grain in wet form (Wu, 2008). One of the most significant changes in ethanol production from corn was the uptake of simultaneous saccharification and fermentation (Casey and Ingledew, 1986) resulting in increased ethanol titres from 80 to 125 g L⁻¹. Increased ethanol titres achieved using simultaneous saccharification and fermentation procedures is not necessarily due to an increase in ethanol tolerance of the yeast per se, but rather a decrease in the osmotic stress that the yeast is placed under during fermentation (Kotrba, 2006). The introduction of new more ethanol-tolerant yeast strains has further increased the alcohol concentration achieved, typically in excess of 125 g L⁻¹ (Kotrba, 2006). At the same time there has been improvement in the activities of the amylase enzymes and a drop in their price with a 70% price reduction between 1990 and 2005 (Hettinga et al., 2009). In this paper we report on the performance of a recently developed yeast strain with improved ethanol tolerance and model the potential effects of introducing the new yeast strain into modern dry mill ethanol plants.