Occurrence of 12-methyltridecanal in microorganisms and physiological samples isolated from beef

12-Methyltridecanal (MT) smelling tallowy, beef-like was formed from plasma logens when beef was boiled. To clarify the origin of MT, its concentration was determined by a stable isotope dilution assay in bacteria and protozoa isolated from the rumen of bovine animals as well as in the plasma, erythr ocytes, and other physiological samples. The highest amounts of MT were fou nd in bacteria followed by protozoa. The MT content of the erythrocytes was small. The results support the hypothesis that microorganisms are the main source of MT of which a small amount is resorbed by the animal and transpo rted to the muscular tissue where MT is incorporated into plasmalogens.