Protease Use In Ethanol Production From Dry Fractionated Corn

corn ethanol;protease application;corn dry fractionation;yeast nitrogen source;starch fermentation

Vidal, Bernardo-Jr C.

关键词: corn ethanol;    protease application;    corn dry fractionation;    yeast nitrogen source;    starch fermentation;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/18269/Vidal_Bernardo-Jr.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Fractionating the corn kernel to separate endosperm from germ and pericarp improves corn ethanol processing by increasing fermentation throughput and generating salable coproducts.One such technology, dry fractionation, suffers from loss of germ derived nutrients and amino acids, resulting in poor fermentation kinetics.In the fuel ethanol industry, such deficiencies may be addressed by increasing inorganic nitrogen and other nutritional supplements.We investigated the addition of a commercial protease as an alternative to exogenous nitrogen supplementation.Our goal was to understand how protease can be used more effectively in corn dry fractionation for ethanol production.We compared and evaluated the relative impacts of protease use among wet fractionation (E-Mill) and dry fractionation processes (using both conventional and granular starch hydrolyzing enzyme processes, referred to as dry conv and dry RS, respectively).With protease treatment, residual starch in the endosperm fiber was reduced by 1.9% w/w (22% relative reduction) in dry conv and 1.7% w/w (8% relative reduction) in dry RS, while no reduction was observed in E-Mill.Protease treatment increased ethanol production rates early in fermentation (0 to 36 hr), especially in dry conv and dry RS (0.3 and 0.6 g/L/hr higher than no protease controls, respectively).We therefore observed a greater benefit from using protease in dry fractionation process than in wet fractionation process.To understand protease efficacy in dry fractionation ethanol process, we studied protease’s effects on each unit operation step.We found no evidence to suggest that protease pretreatment of dry fractionated endosperm increased glucose production rates during conventional liquefaction and saccharification.Instead, protease affected fermentation performance via free amino nitrogen (FAN) generated for yeast consumption.Protease generated FAN resulted in fermentation being 99% complete in 48 hr, compared to 93% with a urea supplemented control.Yeast growth and FAN consumption rates were not different between fermentations that were supplemented with both urea and FAN and with FAN alone, indicative of FAN being utilized preferentially.Only when urea was limiting (<2.5 mg N/g glucose) did FAN supplementation increased ethanol yields.With high protease loading (generating 1.6 mg FAN/g glucose), ethanol yields were 2 g/L lower than a urea control.This reduced ethanol yield was attributed to poorer utilization of maltose, evident from the increase in maltose concentrations after fermentations with increasing initial FAN.Using glucose and maltose solutions, we observed high residual maltose during fermentations with high FAN supplementation.However, in contrast to conventional process employing separate high temperature liquefaction, a granular starch hydrolyzing enzyme (GSHE) process did not result in unutilized maltose or reduced ethanol yields, including at high initial FAN concentrations (435 mg/L in the mash supernatant).At intermediate concentrations of protease generated FAN (244 mg/L), ethanol yields from GSHE process were higher by 3 g/L compared to a urea control. Finally, we studied protease use to generate FAN from germ as a supplement for endosperm fermentation.Ethanol yields were dependent on mash FAN concentrations, increasing to a maximum when FAN level was 80 to 90 mg FAN/100 g ds.At half the optimal FAN level (40 mg FAN/100 g ds), nitrogen limitation occurred.As was observed with endosperm derived FAN, maltose concentrations at the end of fermentation increased with increasing initial germ derived FAN.The magnitude of the residual maltose concentrations resulting from these two FAN sources differed; germ derived FAN resulted in residual maltose concentrations <50% of those resulting from endosperm derived FAN (for the same FAN levels).Ethanol production rates at 0 to 24 hr fermentation period were higher with germ FAN supplementation than with a urea control.Protease use to generate optimal FAN levels (80 to 100 mg FAN/100 g ds) in mash could improve economics of dry fractionated corn ethanol production by increasing fermentation rates and, consequently, reducing fermentation time.