Carnosic acid (top two images) is a diterpene with an abietane skeleton; rosmarinic acid (bottom two) is a caffeic acid ester. Both are found in the rosemary plant (Rosmarinus officinalis), each also occurs in other plants.
But carnosic and rosmarinic acids have something else in common: They are meat preservatives. Rosemary extract was originally added to meat as a flavoring agent, but people noticed that it kept meat from spoiling. It was later discovered that the acids inhibit the free-radical reaction that oxidizes fats and oils and causes spoilage. Neither acid is responsible for the rosemary flavor.
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