Isolation and chemical analyses of nonfermented fiber fractions of oat hulls and cottonseed hulls.

The purpose of this study was to isolate, using both in situ and in vivo methodology, the nonfermented fiber fraction of oat hulls (OH) and cottonseed hulls (CSH) and to compare the concentrations of alkali-labile phenolic monomers, nitrobenzene oxidizable phenolic monomers, and neutral monosaccharides, as well as the cross polarization/magic angle spinning (CP/MAS) carbon-13 (13C) nuclear magnetic resonance (NMR) spectra, of the nonfermented fraction with the original OH or CSH. The in situ isolation procedure involved a 30-h ruminal pretreatment and an 8-h acid:pepsin pretreatment followed by 1 to 7 additional days of incubation in the rumen. Fractions not fermented in vivo were isolated from duodena, ileal, and fecal material obtained from a site and extent of digestion trial in which these byproducts were fed to sheep at 80% of the diet (as-fed basis) and they represented the sole source of dietary fiber. Based on nonfermented fraction composition, both in situ and in vivo, all components analyzed were degraded to some extent. Also, all components present in original byproduct material were present in both the in situ and in vivo nonfermented fractions. Based on NMR analysis, cellulose crystallinity did not change during either long-term in situ or in vivo fermentation. However, CSH cellulose was more crystalline than that of OH. The ADL content of OH and CSH was 6.1% and 19.4%, respectively, and very little (15%) of the ADL disappeared during either in situ or in vivo fermentation. Much of the p-coumaric and ferulic acid of OH, associated with the cell wall matrix as lignin-carbohydrate and phenolic-carbohydrate complexes, was recovered in the fermented fractions. Data are interpreted to indicate that lignin encrustation and cellulose crystallinity are factors affecting CSH fermentation. Lignin encrustation and the presence of lignin-carbohydrate/phenolic-carbohydrate complexes are factors that inhibit OH fermentation.