Fiber classification and characteristics
Fiber is classified as soluble or insoluble based on fiber solubility in water. The fiber characteristics relevant to swine nutrition include fermentability, viscosity, and hydration. Natural fibrous feed ingredients are usually composed of both soluble and insoluble fiber.
Soluble fibers are more rapidly fermented in the hindgut and produce more volatile fatty acids, such as acetate, propionate, and butyrate, which are used as sources of energy to promote gut development (Montagne et al., 2003). Soluble fibers also promote a prebiotic effect by enhancing beneficial bacteria fermentation and production of volatile fatty acids while reducing gut pH to eliminate pathogens. The presence of soluble fibers increases digesta viscosity, which delays digesta passage rate, interferes with nutrient digestion, and predisposes proliferation and colonization of pathogens (McDonald et al., 2001). However, soluble fibers have better solubility, swelling capacity, water-holding capacity, and water-binding capacity that are important for digestion.
Soluble fibers include pectins, gums, and β-glucans. Feed ingredients such as sugar beet pulp and citrus pulp have predominantly soluble fiber (Jha and Berrocoso, 2015).
Insoluble fibers are relatively resistant to fermentation in the hindgut and do not contribute much to production of volatile fatty acids (Montagne et al., 2003). The presence of insoluble fibers increases fecal bulkiness and accelerates digesta passage rate, which prevents proliferation and colonization of pathogens (Wellock et al., 2008).
Insoluble fibers include cellulose and hemicellulose. Feed ingredients such as wheat middlings, wheat bran, rice hulls, oat hulls, and distillers dried grains with solubles have predominantly insoluble fiber (Jha and Berrocoso, 2015).