Microbes Aid Raw-Milk Cheese Safety

Interest in cheeses made from unpasteurized cheeses is growing as consumers embrace traditional, artisan foods. But as their popularity rises, so do questions about the safety of these products. However a recent study indicates the naturally occurring population of microbes may provide protection against Listeria.

The July/August issue of Dairy Science and Technology published a paper from the 4th International Dairy Federation Dairy Science and Technology Week, in Rennes, France that investigated whether microbial diversity can help guarantee the microbial safety of raw milk cheeses. The Raw Milk Cheesemakers Association describes raw milk cheese as: Cheese produced from milk that, prior to setting the curd, has not been heated above the temperature of the milk (104°F, 40°C) at the time of milking and that the cheese produced from that milk shall be aged for 60 days or longer at a temperature of not less than 35°F (2°C) in accordance with U.S. FDA regulations.

Researchers found that microbial consortiamultiple interacting microbial populationsfrom the surface of raw milk cheeses can self-protect against Listeria monocytogenes. The study showed, 10 complex microbial consortia among 34 tested from the surfaces of raw milk Saint-Nectaire cheeses were particularly effective for reducing the growth of L. monocytogenes on cheese surfaces compared to a commercial ripening culture, despite the high pH values on the surfaces.

The scientists selected one of these consortia (TR15) and propagated it on cheese surfaces to create a collection of strains consisting of lactic acid bacteria, Gram-positive and catalase-positive bacteria, Gram-negative bacteria and yeasts. On the exterior of uncooked cheeses, selected microbial colonies consisting of combinations of several isolates from this collection displayed less antimicrobial activity against L. monocytogenes than the naturally-occurring complex consortium TR15. The results showed that in the microbial dynamics of the raw-milk cheeses, TR15 differed from that of the defined consortia. TR15 cheeses had the highest levels of cultivable lactobacilli and leuconostocs and contained high levels of Marinilactibacillus psychrotolerans, Carnobacterium mobile, Arthrobacter nicotianae or A. arilaitensis, Arthrobacter ardleyensis or A. bergerei and Brachybactrerium sp.

While results were positive, the researchers concluded that further investigation is needed to gain a better understanding of the microbial interactions involved in inhibiting L. monocytogenes.