Composition of fatty acids (FA) of ruminant products has a potential impact on human health. Among them, trans FA, which are intermediates of the ruminal biohydrogenation of dietary unsaturated FA, deserve interest, in particular trans-10 and trans-11 isomers, which would have negative and positive effects on human health, respectively (Tricon et al., 2004). A large variability in the ratio of trans-10 to trans-11 isomers (t10/t11) has been observed (Shingfield et al., 2006). Some dietary factors shifting from t11 to t10 have been identified, like the proportion of concentrate (Griinari et al., 1998), or the addition of oil (Roy et al., 2006). The use of in vitro systems to simulate rumen fermentation presents technical, economical and ethical advantages compared to in vivo experiments, and allows screening studies. The aim of the present study was to investigate if the ruminal fluid of a cow having the t11 to t10 deviation results in the same deviation during in vitro batch incubation and if the pathway of biohydrogenation in vitro depends mainly on the donor cow or on the fermentative substrate. This study showed that the t10/t11 ratio of the ruminal fluid after 5 hours in vitro incubation reflects the in vivo values. This ratio in vitro did not depend on cultures substrates, but related to the diet of the donor cows, suggesting a major importance of the ruminal inoculum on the biohydrogenation pathway. This might be due to the short incubation time preventing the bacterial communities to evolve according to the in vitro substrate. As a consequence, short duration batch in vitro cultures cannot be used to study the dietary conditions of the t11 to t10 shift. Nevertheless, they could possibly be used for the study of the effects of feed additives on the t10/t11 ratio
