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In terms of a number of operational metrics, the ruminant animal—particularly the grazing cow—represents the pinnacle of a cellulosic biomass processing system. The active conversion of cellulosic biomass is assisted by an effective physical pretreatment of the biomass by the cow. This pretreated biomass is fermented by a complex anaerobic microbial community, primarily in a specialized digestive organ, the rumen. Fermentation of the non-lignin components of the feed (carbohydrates, protein, nucleic acids, and some lipids) yields methane, carbon dioxide, and a mixture of volatile fatty acids (VFAs, C2–C6 monocarboxylic acids), the latter of which retain about two-thirds of the energy of the feed and are used by the host for energy and biosynthetic reactions. The rumen itself displays many of the properties of a bioreactor in a cellulose biorefinery, including high solids loading, temperature and pH control, adequate mixing, biphasic continuous flow, and absorptive removal of energetic products. Because ruminal fermentation can be readily conducted in bioreactors under non-aseptic conditions, it may represent a practical route to fuel production if the energy-rich VFA products can be chemically converted to more practical liquid or gaseous fuels. Several useful co-products of these fermentations have also been identified.

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