In vitro fermentation of polydextrose by bovine ruminal microorganisms

Polydextrose (PD) is a synthetic additive to human foods that has many physical properties of fat, but is poorly metabolized by nonruminant animals. PD was fermented in vitro by mixed ruminal microorganisms, but the kinetics of the fermentation were complex and varied with different ruminal inocula. Gas production during digestion of PD was described by a two-pool digestion model. One pool, which contained 20–47% of the substrate, was fermented without a lag period and at rates similar to that of purified potato starch or corn starch. The second pool was fermented considerably more slowly than was cellulose. The acetate/propionate ratio in products of PD fermentation ranged from 2.7 to 3.5. Growth rate of Selenomonas ruminantium D was higher on PD than on starches, while the reverse was true for Streptococcus bovis JB1, Prevotella ruminicola B14, and Butyrivibrio fibrisolvens H17c. For all four strains, the low final culture densities obtained suggest that only a fraction of PD was utilized. The fermentation properties of PD suggest that this material, even if available at competitive prices, is a not an effective substitute for starch in ruminant feeds.