A process for preparing a conjugated linoleic acid (CLA) material that is enriched in the cis 9, trans 11 (rumenic acid) CLA isomer. The process involves subjecting a material containing at least 75 weight % CLA moieties to an enzymatic conversion, wherein the enzyme has the ability to discriminate between the cis 9, trans 11 and trans 10, cis 12 isomers. The enzyme is advantageously a lipase derived from Candida rugosa. The resulting CLA product stream is distilled to separate the free fatty acid fraction from the glyceride fraction. The recovered free fatty acid fraction contains about 55 weight % to about 70 weight % of the cis 9, trans 11 isomer (rumenic acid), and has a weight ratio of cis 9, trans 11 isomer to trans 10, cis 12 isomer of at least 3.5:1. The material enriched in rumenic acid may be used in foods, particularly infant formulas, or in food supplements or in pharmaceutical compositions.
