The solids remaining at the end of the fermentation (distillers dried grain with solids, or DDGS: see figure 1.21) are a high-protein animal feed — a saleable by-product that has been suggested to be so commercially desirable that reduced ethanol yields could be tolerated to support its increased production, although, in practice, high — sugar residues pose severe practical difficulties to DDGS drying and processing.122 The rapid rise of ethanol production from cornstarch has, however, demanded some remarketing of this coproduct:269

• The product is less dark because sugars are more efficiently fermented and less available to react chemically and caramelize in the dried product.

• The essential amino acid contents are higher (figure 4.13).

• Although ruminant animals can certainly benefit from feeding with DDGS, pigs are geographically much closer to ethanol plants in the midwestern United States.

Reducing phosphate content would widen the use of DDGS by addressing animal waste disposal issues, and the development of more efficient methods for removing water in the preparation of the DDGS could greatly reduce processing costs.270 Adding on a second fermentation (or enzymic biotransformation), a dry-grind processing to generate plant oils and a higher-value animal feed from the DDGS, and separating more useful and saleable fine chemicals from the primary fermentation would increase the total mass of recovered bioproducts to the maximum achievable (figure 4.14).271 Pric­ing is crucial because the increased supply of DDGS is likely to significantly reduce its market price, and its alternate use as the feedstock for further ethanol production itself has been worthy of investigation: steam and acid pretreatments can convert the residual starch and fiber into a substrate for yeast-based ethanol production with a yield 73% of the theoretical maximum from the glucans in the initial solids.272

A much simpler option is to realize the potential in the fermented solids to provide nutrients and substrates for a new round of yeast (or other ethanologen) growth and eth­anol production: such spent media (“spent wash,” stillage, or vinasse) can be recycled in the process known as “backsetting,” found to be beneficial for yeast growth and a practical means of reducing water usage in a fuel alcohol facility.273 Backsetting is not without its accepted potential drawbacks, including the accumulation of toxic nonvola­tiles in the fermentor, increased mash viscosity, and dead cells causing problems with

High Value Animal
Feed

Carbon Dioxide Plant Oils Inositol Succinic Acid Glycerol Ethanol

FIGURE 4.14 Projected recovery of product and coproducts from the ethanol fermentation of corn starch. (Data from Dawson.271)

viability measurements, but as a crude means of adapting the fermentation to a semi­continuous basis, it has its advocates on both environmental and economic grounds. Furthermore, a study by Novozymes demonstrated that mixtures of fungal enzymes could decrease vinasse viscosity and liberate pentose sugars from soluble and insoluble arabinoxylans that would be suitable for fermentation by a suitable pentose-utilizing ethanologen.274-277 Portuguese work has also shown that such a pentose-rich product stream can be the starting point for the fermentative production of xylitol (a widely used noncalorific sweetener) and arabinitol by the yeast Debaryomyces hansenii.278,279 As a support for the immobilization of yeast cells, brewer’s spent grains were a very effective means of supplying “solid-phase” biocatalysts for ethanol production from molasses, and the solids from bioethanol plants could serve a parallel function.280

Brazil has, by far, the longest continuous history of devising methods for eco­nomically viable disposal for vinasse and the solid waste product (bagasse), espe­cially because neither had at times been considered to be saleable and both could even represent negative value as incurred disposal costs:281 [43]

• Important for minimizing fertilizer use, material dissolved in the digestion wastewater represents 70% of the nutrient demand of sugarcane fields.