Ethanol is used as a fuel additive has resulted in rapid of growth of ethanol production. Thus the bio-based ethanol production has been one of the fastest growing industries in the U.S. The dry grinding corn ethanol process is more predominant than other ethanol production process in the U.S. In the dry grinding process, the corn is fermented to produce ethanol and distillers dried grains with solubles (DDGS). The generated DDGS are primarily used by farmers to feed livestock. However, drying distillers grains consume energy and costs money. As a result, DDGS are more expensive than other distiller grains (Gorden, 2008). To reduce operational costs through drying process, some other distiller grains with relatively high moisture content, including whole stillage, thin stillage, and syrup could be considered as an alternative animal feed ingredient.

The physical and biological properties tests provided the information background information about operational processes, and valuable components change over time. The thin stillage and whole stillage had high initial average moisture contents of 92% (w.b.) and 87% (w.b.) respectively, and initial water activity of 0.99; the high water content marked samples easily susceptible to rapid spoilage. Time had a significant effect (P < 0.05) on properties of co-products. Both thin stillage and whole stillage samples got mold growth after 5 days incubation at 32oC. Thin stillage had the greatest separation rate in settling experiment. However, syrup had relative low initial average moisture content of 62% and initial water activity of 0.92. No mold growth and settling separation happened in syrup samples. There were no evidence showing a linear relationship exists between Hunters L*, a* and b*, and mold growth. The Solvitaà ® test showed that high-temperature treatment caused high CO2 production in all samples. The exponential models described the relationship between storage time (from 0 to 5 days at 25oC and 35oC) and CO2 concentration for three co-products.

Evaporation is the typical method used to concentrate solids in these co-products, but it requires a large amount of water and energy consumption. In order to overcome the problems that associated with the evaporator, membrane filtration could be applied that may provide a cheaper and efficient way to improve value for whole stillage, thin stillage, and syrup. Fractionation of these wet co-products by using ultrafiltration was conducted to evaluate membranes as an alternative to evaporators in ethanol production. A study has been showed that ultrafiltration required less energy than evaporation (Rausch and Belyea 2006). This study indicates that ultrafiltration could be a better choice that can be applied in biotechnology industries to concentrate valuable components. However, an important problem associated with membrane technologies is flux decline and membrane fouling. An understanding of causes of flux decline is necessary to minimize or avoid fouling and to make membrane application economic.

The membrane size, stirring speed and volume capacity had significant effects (P < 0.05) on flux during the ultrafiltration for whole stillage and thin stillage. The flux increased by 30% maximum as siring speed increased from 160 to 320 rpm for YM 10 membrane (10KDa) in these two samples. The effect of membrane size on solid recovery was significant (P < 0.05). The solid recovery for YM 100 membrane in whole stillage ranged from 75% to 83%, and 74% to 84% for thin stillage, however, the YM 10 kDa was ranging from 80% to 90% in whole stillage, and 84% to 90%. Retentate products from ultrafiltration could be further used as an ingredient to feed animals, and the permeate stream could be recycled in dry grind plants to help in reducing process water requirement.