Due to the different effects of trans fatty acids on human health and cows’ milk production, it is necessary to better understand the reactions of ruminal isomerisation of polyunsaturated fatty acids, which lead to different isomers. Linoleic acid can be isomerised by ruminal bacteria according to two isomerisation pathways (delta12 and delta9), leading to trans-11 and trans-10 isomers, respectively. This experiment was conducted in vitro at pH 5.5 or 7.0 with two enzymatic solutions originating from rumen fluids collected from two cows, one receiving a high-starch high-fat diet (33% starch, 7.3% fat, DM basis) inducing a high ruminal trans-10 isomers production and the other receiving a low-starch low-fat diet (22% starch, 3% fat, DM basis) inducing a high trans-11 isomers production. Before incubation, bacterial growth was inactivated in ruminal fluids with chloramphenicol. Then, 1 ml of each enzymatic solution was incubated with 1 ml of a buffer solution (pH 5.5 or 7.0) in vials containing 0.5 mg of pure linoleic acid (Sigma®) for 1 hour in four replicates. Fatty acids of incubated vials were analysed by gas chromatography. As expected, trans-11 isomers production was lower at pH 5.5 than at pH 7.0 (P < 0.01), but interestingly the low pH also resulted in a lower trans-10 isomers production (P < 0.01). The delta9-isomerisation was clearly expressed only in incubations prepared with rumen fluid from the high starch and high fat diet fed cow (P < 0.01). By contrast, the delta12-isomerisation was only slightly affected by the donor cow diet (P = 0.09). Our results make a contribution to the understanding of the two isomerisation pathways, showing that the enzymatic capacity of the ruminal fluid to produce trans-10 isomers depends on the cow’s diet but that a low pH does not directly promote the activity of the delta9 isomerase
