Enzymatic and process technologies to increase corn dry grind slurry solids

In conventional dry grind process, high glucose concentrations (>15% w/w) and liquefaction viscosities restrict slurry solids contents to 30 to 32% w/w. High slurry solids fermentations (above 33%) are important in reducing energy costs; decreased water input results in less evaporation, dehydration and distillation. There are numerous challenges associated with high solids fermentations: high slurry viscosities, high glucose concentrations that exert osmotic stress on yeast and high ethanol concentrations that result in loss of cell viability. The objective was to determine an economical and process efficient enzyme combination to be employed during simultaneous saccharification and fermentation (SSF) to reduce slurry viscosities and glucose concentrations at high solids. We also evaluated effects of nitrogen source and dose on high solids fermentation. The enzyme combination we considered for this study consisted of granular starch hydrolyzing enzymes (GSHE), alpha-amylase component of GSHE (GSHE AA) and glucoamylase (GA). In this study, we showed that using low temperatures (55 C) and a combination of phytase and alpha-amylase during liquefaction reduced slurry viscosities at 35% solids by 81% compared to the conventional process. We compared eighteen SSF enzyme treatments to optimize each GSHE, GSHE AA and GA. These SSF enzyme treatments included two enzyme combinations: 1) GA and GSHE and 2) GA and GSHE AA, with different levels of enzyme concentrations. For all treatments except control, liquefaction (55 C for 90 min) was conducted at 35% solids using a formulation of alpha-amylase and phytase. SSF (32 C for 72 hr) was carried out using enzyme treatments, urea and yeast. The treatment containing 0.5 L glucoamylase and 1.25 L GSHE per g dry corn resulted in the highest fermentation efficiencies (92%) and ethanol yields (418 L/tonne). The control treatment resulted in the lowest fermentation efficiencies (84%) and ethanol yields (381 L/tonne). The above mentioned enzyme treatment also resulted in 34% lower peak glucose ii concentrations (9.87% w/v) compared to control treatment (13.49% w/v). Nitrogen source and dose effects were determined at 35 and 40% solids using modified process. Three nitrogen sources (urea, ammonium sulfate, glutamine) and protease were compared for ethanol yields and other fermentation parameters. Urea and protease resulted in similar ethanol yields. However, fermentation rates were higher for protease during initial 12 hr of fermentation. Effects of urea and protease levels were evaluated at 35 and 40% solids. At 35% solids, 2.16 mg urea and 0.71 mg protease resulted in highest fermentation efficiencies and ethanol yields. However at 40% solids, 4.32 and 2.16 mg urea and 0.71 mg protease gave highest fermentation efficiencies and ethanol yields. At 35 and 40% solids, increasing protease levels from 0.71 to 1.42 mg reduced fnal ethanol concentrations, ethanol yields and fermentation effciencies. Increasing solids content from 35 to 40% decreased fermentation effciencies and simultaneously reduced ethanol yields across all urea and protease levels