Anteiso-fatty acids (aFA) with odd carbon number are a class of branched-chain fatty acids (BCFA) mainly produced by bacteria. Bacterial sources are also made responsible for their occurrence in the low percent-range in lipids of ruminants (meat and milk) and fish. aFAs are chiral molecules and typically occur predominantly in form of (S)-enantiomers, and their primary precursor has been noted to be isoleucine. Yet, low proportions of (R)-aFAs were also detected in fish and cheese samples. Here we investigated the potential formation of (R)-aFAs by means of incubation experiments with rumen fluid from fistulated cows. Supplementation of rumen fluid with both L- and DL-isoleucine, resulted in a significant (α <0.05) increase of the aFA concentrations but in both cases enantiopure (S)-aFAs were observed. By contrast, incubations without addition of any isoleucine lead to a significant (α <0.05) formation of small proportions of (R)-aFAs similarly to those previously observed in fish and cheese. These results were consistently reproduced in three different years with rumen fluid from different cows fed different diets. All findings point to the existence of a further biosynthesis pathway of aFAs with different stereospecificity than the classic one using isoleucine as primer.

Bovine milk fat contains a large variety of structurally different fatty acids. In this study, we describe the presence of aromatic fatty acids in a butter fat sample. Fatty acids were released from butter fat and converted into the corresponding methyl esters (FAME). Urea complexation was used to separate the main saturated fatty acids. GC/MS screening of the FAME in the filtrate of the urea complexation indicated the presence of aromatic fatty acids. By (1) conversion of two representatives into picolinyl esters which were analyzed by GC/MS, (2) linear log t R over carbon number plots (R 2 = 0.95) and by the use of two reference standards we were able to show that the phenyl unit was located on the terminal carbon of the straight acyl chain of the FAME. In a fraction gathered by countercurrent chromatography we were able to identify 3-phenylpropionic acid (Ph-3:0), 4-phenylbutyric acid (Ph-4:0), 5-phenylpentanoic acid (Ph-5:0), 6-phenylhexanoic acid (Ph-6:0), 7-phenylheptanoic acid (Ph-7:0), 8-phenyloctanoic acid (Ph-8:0), 9-phenylnonanoic acid (Ph-9:0), 10-phenyldecanoic acid (Ph-10:0), 11-phenylundecanoic acid (Ph-11:0), 12-phenyldodecanoic acid (Ph-12:0), 13-phenyltridecanoic acid (Ph-13:0), along with one unsaturated phenyldecenoic acid (Ph-10:1) isomer. Preliminary results indicate that the aromatic fatty acids may have been formed exogenously in the rumen of the cows. The total amount of the aromatic fatty acids was estimated at 0.15 mg/g butter fat, which corresponds with an average daily intake of *5 mg per day in Germany and *4.4 mg per day in Europe.

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