Effects of substrate, passage rate, and pH in continuous culture on flows of conjugated linoleic acid and trans C18:1

A dual-flow continuous culture system consisting of 4 fermenters was used in a 4 x 4 Latin square design. The objective of the research was to evaluate the effects of solid dilution rate (SDR), pH, and concentration of linoleic acid ( LA) in the feed mixture on the production of conjugated linoleic acid (CLA) and trans-C-18:1. The 4 treatments were 1) control = pH 6.5, 1% LA, 4%/ h SDR; 2) high solid dilution rate (HSDR) = pH 6.5, 1% LA, 8%/ h SDR; 3) high linoleic acid (HLA) = pH 6.5, 3% LA, 4%/ h SDR; and 4) low pH (LPH) = pH 5.8, 1% LA, 4%/ h SDR. Inoculum was collected 6 h after feeding from a cow fed 40% alfalfa hay and 60% grain. Liquid dilution rate was held at 0.12/h. All treatments except HLA contained 2% tallow. The LA was dissolved in buffer and continuously infused into the fermenters. The CLA flows were 16.5, 20.4, 23.2, and 25.2 mg/d for control, HSDR, HLA, and LPH, respectively. Compared with control, LPH increased flows of CLA, cis-C-18:1, and C-18:2, and decreased flow of C-18:0. The neutral detergent fiber (NDF) and acid detergent fiber (ADF) digestibilities were not affected by pH. The HSDR tended to increase CLA flow compared to control, possibly because a shorter solid retention time led to incomplete biohydrogenation (BH). The NDF and ADF digestibilities and bacterial numbers were reduced by HSDR. With more LA available as a substrate for CLA, HLA resulted in a higher flow of CLA than control. The HLA resulted in the highest acid detergent fiber and fatty acid digestibilities, bacterial numbers, and BH. Increasing solids passage rate, reducing pH, and increasing dietary LA appears to increase in vitro CLA production.