Dilution rate and pH effects on the conversion of oleic acid to trans C18:1 positional isomers in continuous culture

In a previous in vitro study, mixed ruminal microorganisms converted oleic acid to a variety of trans monenes when grown in batch cultures under constant environmental conditions. To determine whether a similar conversion occurs under environmental conditions more typical of the rumen, conversion of C-13-labeled oleic acid to biohydrogenation intermediates was determined in ruminal microorganisms grown in continuous culture at two pH (5.5 and 6.5) and liquid dilution rates (0.05 and 0.10/h) arranged factorially. After each morning feeding of the dual-flow continuous cultures, 250 mg of oleic acid in 5 mL of ethanol were injected into each culture. On d 10, 250 mg of oleic-1-C-13 replaced the unlabelled oleic acid in ethanol. Trans fatty acids were isolated from culture samples by solid phase extraction, and C-13 enrichment and identity of double bond position was determined by gas chromatography-mass spectroscopy. At pH 6.5 and 0.10/h dilution rate, C-13 enrichment was detected in all trans-C-18:1 isomers having double bond positions from C-6 through C-16 in the acyl chain. However, when pH or dilution rate in fermentors was lowered, no C-13 enrichment was detected in any trans isomer with a double bond position beyond C-10. Enrichment in stearic acid increased by reducing culture pH from 6.5 to 5.5, but decreased when dilution rate dropped from 0.10 to 0.05/h. The stearic acid carbons that originated from oleic acid biohydrogenation increased from 30 to 72% when pH dropped from 6.5 to 5.5. The C-13 enrichment of trans-10 was reduced under low pH and dilution rate conditions. The results of this study confirm that ruminal microorganisms are capable of converting oleic acid to a wide variety of trans-C-18:1 positional isomers when ruminal conditions are favorable (such as the pH 6.5 and 0.10/h dilution rate treatment). However, at low pH and dilution rate, the conversion of oleic acid to trans-C-18:1 still occurs, but positional isomers produced are restricted to double bond positions from C-6 to C-10. Low pH conditions also increased the conversion of oleic acid to stearic acid.