In conventional dry grind process, high glucose concentrations (>15% w/w) andliquefaction viscosities restrict slurry solids contents to 30 to 32% w/w. High slurry solidsfermentations (above 33%) are important in reducing energy costs; decreased water input resultsin less evaporation, dehydration and distillation. There are numerous challenges associated withhigh solids fermentations: high slurry viscosities, high glucose concentrations that exert osmoticstress on yeast and high ethanol concentrations that result in loss of cell viability. The objectivewas to determine an economical and process efficient enzyme combination to be employed duringsimultaneous saccharification and fermentation (SSF) to reduce slurry viscosities and glucoseconcentrations at high solids. We also evaluated effects of nitrogen source and dose on high solidsfermentation. The enzyme combination we considered for this study consisted of granular starchhydrolyzing enzymes (GSHE), alpha-amylase component of GSHE (GSHE AA) andglucoamylase (GA).In this study, we showed that using low temperatures (55 C) and a combination of phytaseand alpha-amylase during liquefaction reduced slurry viscosities at 35% solids by 81% comparedto the conventional process. We compared eighteen SSF enzyme treatments to optimize eachGSHE, 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 alltreatments except control, liquefaction (55 C for 90 min) was conducted at 35% solids using aformulation of alpha-amylase and phytase. SSF (32 C for 72 hr) was carried out using enzymetreatments, urea and yeast. The treatment containing 0.5 L glucoamylase and 1.25 L GSHE per gdry 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 glucoseiiconcentrations (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 modifiedprocess. Three nitrogen sources (urea, ammonium sulfate, glutamine) and protease werecompared for ethanol yields and other fermentation parameters. Urea and protease resulted insimilar ethanol yields. However, fermentation rates were higher for protease during initial 12 hrof 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 ethanolyields. However at 40% solids, 4.32 and 2.16 mg urea and 0.71 mg protease gave highestfermentation efficiencies and ethanol yields. At 35 and 40% solids, increasing protease levelsfrom 0.71 to 1.42 mg reduced fnal ethanol concentrations, ethanol yields and fermentationeffciencies. Increasing solids content from 35 to 40% decreased fermentation effciencies andsimultaneously reduced ethanol yields across all urea and protease levels.
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