DNA tool detects drop in 'good' bacteria for cheese

Cheese makers could improve the end product as new DNA tools lead
French scientists to identify that refrigerating raw milk for as
little as 24 hours can cut levels of bacteria responsible for the
development of desirable qualities of raw-milk cheeses and in
parallel up the levels of undesirable contaminants and foodborne
diseases.

Precise descriptions of microbial dynamics, up until now not possible, occurred as a result of new molecular approaches based on direct analyses of DNA (or RNA) in its environment without 'microbial enrichment'.

"Until recently, the bacterial community of raw milk was described by classical microbiological methods, which are generally long and tedious, and allow only a partial inventory of the bacterial microflora," say the researchers based at the French food security organisation, the Agence Française de Sécurité Sanitaire des Aliments, as well as other government-funded food laboratories.

Using these new methods, the researchers analysed bacterial populations in fresh raw milk and raw milk that had been refrigerated for 24 hours.

"Many of the species identified after refrigeration were present in the initial sample. However, the relative proportions of bacterial were clearly altered by refrigeration," say the researchers.

In the fresh samples, the scientists report that the dominant bacterial population was Lactobacillus lactis, a species of bacteria that is commonly used as a starter culture for many cheeses.

After refrigeration, the researchers found a significant decrease in L. lactis population. In addition, they detected increases in a variety of other bacteria, including the common food pathogen, Listeria monocytogenes.

"The results obtained are of interest not only for their contribution to the knowledge on the bacterial flora of raw milk samples but also because they describe the consequences of a simple process, milk refrigeration, on the quality of dairy products and its impact on health," report the scientists.

Full findings are published in the September 2004 issue of Applied and Environmental Microbiology, 70. 9: 5644-5650.