EFFECT OF MEDIUM-CHAIN FATTY-ACIDS IN MOLD RIPENED CHEESE ON THE GROWTH OF LISTERIA-MONOCYTOGENES

Listeria monocytogenes and List. innocua were isolated from commercial soft ripened and blue-veined cheeses manufactured in Prance, mainly f rom Brie cheese made from unpasteurized milli. Fire isolates mere List . monocytogenes serotype 1/2 and two were List. innocua. Examination o f Bleu d'Auvergne cheese with the cryoscanning electron microscope sho wed that many conidia spores were present in the blue veins in close c ontact with the cheese surface. There were few conidia spores in the B rie, mostly on the outside of the cheese but not in contact with the s urface. High concentrations of free dodecanoic (lauric) acid (1.77-2.5 0 g/kg cheese) and tetradecanoic (myristic) acid (2.54-6.38 g/kg chees e) were found in the veins of the blue cheese, but concentrations in t he white regions were much lower. Free lauric and myristic acids were not detected in the Brie cheeses. There was no difference in the overa ll fatty acid composition of the fat in the surface ripened and blue-v eined cheeses, although higher concentrations of free medium-chain fat ty acids were found in a blue cheese compared with a surface ripened c heese. The pH and fat content were higher in regions with obvious fung al growth, the blue veins of Fourme d'Ambert and the rind of Brie. Fre e lauric acid dissolved in butteroil inhibited multiplication in broth at pH 7.0 of a test strain of List. monocytogenes isolated from Bleu d'Auvergne. Some inhibition was seen with hexanoic, octanoic, decanoic and tetradecanoic acids. We suggest that the presence of localized co ncentrations of free medium-chain fatty acids (dissolved in the fat) i n the blue veins of blue mould ripened cheese could act as natural pre servatives and inhibit the growth of listerias in conditions where (if present), one would otherwise expect them to grow.