Listeria in cheese

Microorganisms of the genus Listeria are widely distributed in nature, although with a limited number of species of medical and veterinary significance.

Listeria monocytogenes is an important human pathogen that can cause serious illnesses, with high mortality rates in susceptible individuals such as the elderly and people with immuno-compromised conditions.

It is a bacterium that is common in the environment and can be found in agricultural and food-processing settings, where it tends to persist once established. It might affect only the gastrointestinal tract, but may invade other parts of the body, potentially causing septicemia, meningitis, encephalitis, spontaneous abortion, and stillbirth.

Consumption of contaminated dairy products has been associated with cases and outbreaks of human listeriosis. Dairy products are good substrate for L. monocytogenes growth, as they contain proteins, lactose and trace elements. Outbreaks have been strongly associated with cheese consumption.

Fresh cheeses, which have been linked to human listeriosis outbreaks in the United States, are considered high-risk foods for L. monocytogenes contamination. Fresh cheeses are usually protected from contamination by refrigeration and usually packed with paper or shrink-wrapped with polyethylene or polypropylene with potential risk of proliferation of aerobic mesophilic bacteria, coliforms, molds or yeast.

Although L. monocytogenes is effectively inactivated by pasteurization, the prevention of post-processing contamination of milk products is of particular importance to dairy industries.
Listeria in cheese

Blue mold disease or wet rot disease

The most prevalent mold and the one causing the most damage is the ‘blue mold’, otherwise known as ‘Penicillium expansum’. It first appears usually as a white cottony growth on fruits, cheese, cured meats, vegetables, jellies, wine tanks, leather left in dark closets, and on other article favorable to its growth.

Blue mold is the most economically important postharvest disease of apples. The disease, also known as soft rot or wet rot, causes substantial losses in the United States, Canada, England and many other countries.

The blue mold can be controlled, but great care must be taken if it is to be completely eliminated. Its spores can be killed by heating to 82 °C, and growth is prevented with many chemicals.

However acquired resistance by Penicillium italicum and Penicillium digitatum to fungicides used on citrus fruit has become a matter of concern in recent years.

The term blue mold may also be applied to Penicillium roqueforti, a mold used in producing the taste and characteristic blue-green inclusions in Roquefort and other blue cheeses. Blue cheeses are typified by a semisoft texture and blue mold growing throughout the curd.  

Penicillium roqueforti produces the mottled blue color and laws gives rise to fatty acid ketones which contribute to the characteristics sharp, peppery flavor of these cheeses.

Mold growth is encouraged during the ripening period which can be from 3 to 10 months at cool, moist, cavelike conditions of about 4 ° C and 90% RH.
Blue mold disease or wet rot disease