Interaction between culture pH and corn oil concentration on NDF digestibility and biohydrogenation of unsaturated fatty acids in batch culture
Effects of culture pH and corn oil (CO) concentration on NDF digestibility (NDFD) and biohydrogenation (BH) of unsaturated fatty acids (FA) in batch culture were evaluated in a 2x3 factorial design. Cultures (4/treatment/time point) included alfalfa hay plus increasing concentrations of CO (0, 1, 2% DM) incubated at culture pH 5.8 or 6.2 for 0, 6, 12, 18, and 24 h. Effects of culture pH, CO, time, and their interactions were determined. Increasing CO in cultures increased total FA concentration which averaged 2.19, 3.36, and 4.54% DM for 0, 1, 2% CO, respectively (P<0.001). Culture pH did not affect total FA concentration (P=0.29). Main effects of treatments (culture pH and CO concentration) across time were significant for the response variables of interest despite significant (P<0.10) interactions (both 2-way and 3-way interactions) for these variables. Lower culture pH reduced NDFD across CO treatments and time (11.6 vs. 21.6%; P<0.001) whereas increasing CO increased NDFD across pH treatments and time (14.2, 16.9, and 18.8%; P<0.001); NDFD increased over time for all treatments (P<0.001). Addition of CO increased the concentration of cis-9, cis-12 18:2 across pH treatments and time (7.88, 17.4, 23.0 g/100 g FA; P<0.001) and higher culture pH reduced its concentration across CO treatments and time (13.8 vs. 18.4 g/100 g FA; P<0.001). Lower culture pH reduced BH extent for cis-9, cis-12 18:2 (34.4 vs. 53.0%; P<0.001), which increased with time (P<0.001) and was affected to a lesser extent by increasing CO (43.0, 45.3, and 42.7%; P<0.001 quadratic). Lower culture pH and increasing CO reduced the concentration of 18:0 (20.1 vs. 24.9 g/100 g FA and 28.7, 21.7, and 17.1 g/100 g FA, respectively; P<0.001). Lower culture pH increased the concentration of trans-10 18:1 (1.04 vs. 0.73 g/100 g FA; P<0.001) and reduced the concentration of trans-11 18:1 (4.73 vs. 6.11 g/100 g FA; P<0.001) across CO treatments and time. Increasing CO increased the concentration of trans-10 18:1 (0.72, 0.92, and 1.03 g/100 g FA; P<0.001) and trans-11 18:1 (3.46, 5.85, and 6.95 g/100 g FA; P<0.001) across pH treatments and time. In conclusion, higher culture pH increased NDFD, BH of cis-9, cis-12 18:2, and formation of trans-11 18:1. Increasing the inclusion of CO increased the formation of BH intermediates as well as NDFD (unexpectedly). Treatments interacted with each other and with time for all variables of interest, particularly formation of trans-10 and trans-11 18:1.
Keywords: batch culture, biohydrogenation, digestibility