**2. Biohydrogenation in the rumen**

Forages and concentrate contain abundant PUFA such as α-linolenic acid (18:3n-3), linoleic acid (18:2n-6), and oleic acid (cis-9 18:1) [15] in the form of dietary lipid. In the rumen, dietary lipids are hydrolyzed and released nonesterified FA. Then, 18:3n-3, 18:2n-6, and cis-9 18:1 are converted to saturated FA via cis-trans isomerization to trans-FA intermediates, followed by hydrogenation of the double bonds [12]. This process is called biohydrogenation. However, a small percentage of PUFA passes into milk and meat. PUFA is toxic to bacteria, so it is believed that the bacteria perform the biohydrogenation process to reduce the toxicity of PUFA [16]. Bacteria prefer saturated FA for synthesizing their cell membranes because the double bonds in unsaturated FA distort their molecular shape and disrupt the structure of the lipid bilayer [17].

In general, the bacteria responsible for biohydrogenation are classified into groups A and B [18]. Group A bacteria hydrogenate 18:2n-6 and 18:3n-3 to trans-11 18:1 and related isomers, while group B bacteria convert those 18:1 isomers to 18:0. The *Butyrivibrio* group contains the most active bacterial species involved in the biohydrogenation of C18 unsaturated FA [19, 20], including the genera *Butyrivibrio* and *Pseudobutyrivibrio*, and *Butyrivibrio proteoclasticus* [21]. Within the complex ecosystem of the rumen, some bacteria could not be exclusively classified into one of the two groups. *B. proteoclasticus*, for instance, converts 18:1 isomers not only to 18:0 but also hydrogenates 18:2n-6 to 18:0 [22].
