Important Microorganisms

There are many species of pathogens or spoilage microorganisms to be considered, in and on cheeses. However, many microorganisms are not found in cheese or will not grow during the cheese process. Absence of other microorganisms is controlled by veterinarian authorities in the primary (at the farmhouse) production. Among others Brucella, Mycobacteria, and Tuberculosis are under veterinarian control in most countries.

A. Clostridia

Clostridia are widespread in nature and occur in raw milk. Only very few cases of illness due to C. botulinum can be attributed to cheese; thus, the major concern is spoilage due to C. tyrobutyricum, which causes late blowing of hard or semihard cheeses. Late blowing occurs when the number of C. tyrobutyricum in the cheese milk exceeds 10-50 spores per liter and the pH is 5.2 or higher (15). During late blowing, lactate is converted into butyric acid, carbon dioxide, and hydrogen; spoiling is characterized by extreme eye formation, split defects, and off-flavors. Prevention of spores in cheese milk can be achieved, to some extent, by bactofugation (16), but bactofugation is not adequate to prevent late blowing. Double bactofugation or microfiltration is, however, sufficient to prevent late blowing. If it is not possible to bactofugate or microfiltrate, the addition of nitrate or lysozyme is an alternative, but the legal amounts allowed of these compounds may not be sufficient to prevent late blowing. An effective alternative is to cool the cheese down below 8°C, at which temperature the spores will not develop.

Normally, E. coli should not occur in cheeses, although 10-1000 E. coli per gram can be allowed from time to time, depending on the cheese type. The major concerns are the pathogenic E. coli types. These are divided into enterohemorrhagic (EHEC), entero-pathogenic (EPEC), enteroinvasive (EIEC), and enterotoxigenic (ETEC) (17). They can cause serious disease and have been reported to cause foodborne diseases in at least five outbreaks (18-20). Most concern is with the EHEC E. coli H7:O157 that was involved in an outbreak in cheese produced from raw milk. In order to prevent pathogenic E. coli, it is crucial to pasteurize the cheese milk. This will assure that the pathogens are not present in the milk, although postcontamination may occur. Good manufacturing practice is normally sufficient to prevent such contamination. If postcontamination does occur, it is important to prevent microbial growth. The activity of the primary starter should be controlled to assure a fast pH drop to below 5.5, which will inhibit E. coli growth. If hard cheeses are produced, the water activity should be held as low as possible, because growth of pathogenic E. coli does not occur at a water activity below 0.96. Such conditions are present in some blue-veined cheeses such as Danish Blue and Roquefort.

C. Salmonella

As for pathogenic E. coli, only a few cheese-related Salmonella outbreaks have been reported (21-23). Because Salmonella are very heat and salt sensitive, they are not likely to grow in cheese. Thus, prevention of their contamination of cheese milk is crucial. Properly pasteurized milk is sufficient to eliminate Salmonella, and the same precautions as described for control of E. coli should be taken. Fast acidification and good manufacturing practices along with maintaining low water activity as is possible is usually enough to produce safe Salmonella-free cheeses.

D. Listeria

Listeria is widespread in nature and can be found in up to 50% of milk samples from raw milk silotanks, depending on geographical and seasonal variations. Listeria monocytogenes, a gram-positive pathogen, has caused a few outbreaks of disease. Two of these outbreaks have caused higher rates of mortality (24,25). In 1985, 48 people died due to consumption of a Mexican-style cheese (25). The reason for the contamination was a leak in the pasteurization equipment, resulting in a mixing of raw and pasteurized milk. In the mid 1980s, 34 people died due to consumption of Vacherin Mont d'Or from Switzerland (24).

Listeria is difficult to control due to its relatively high heat stability (D10 at 69 °C is about 15 sec). The temperature range for growth is 0-45°C, at pH 4.4-9.5, and up to 10% NaCl. In order to prevent growth, it is important to pasteurize efficiently, which means ensuring that the cheese milk has been heated to at least 72 °C for 15 sec. In the above mentioned disease outbreaks, the products had characteristics that favored growth of L. monocytogenes. In the Mexican-style cheese, for example, only a weak acidification took place and the NaCl content was low. In the case of the Vacherin Mont d'Or, insufficient hygiene, coupled with a rise in pH and a low NaCl content also favored growth of L. monocytogenes (24). It is also worth noting that bacteriocin-producing starters and surface ripening cultures may inhibit the growth of L. monocytogenes (12). The infectious dose of Listeria is high compared to Salmonella and pathogenic E. coli, which makes it easier to control in the products. But, on the other hand, Listeria is more likely to grow in the final product, depending on the type of cheese, because of its high resistance to low pH, NaCl, and low temperature.

E. Staphylococcus aureus

S. aureus is associated with milk because of its close association with cows. It is relatively salt tolerant but sensitive to pH. The infectious dose is high, about 105 per milliliter. Disease arises from heat-stable toxins, which means that even though no living S. aureus may be detected, the toxins may still be present (26). Thus, it must be assured that the number of S. aureus transferred from cow to product does not exceed 105/mL, which is assured by a good manufacturing practice on the farms, including ensuring cooling in milk tankers and raw milk silotanks. Because S. aureus does not grow below about 8°C, the holding temperature should be held below 8°C, especially if the milk is stored for a long time in a silotank. S. aureus will grow only in cheeses with low acid content or if the cheese surface pH rises during curing. Good hygiene is normally enough to assure either the absence or low numbers of S. aureus in or on cheeses.

F. Others

Other microorganisms may cause spoilage or diseases in addition to the ones discussed above. Yeast and heterofermentative lactobacilli may cause gas production or off-flavors in cheeses, although the problem is generally easily solved by cleaning. In both cases, spoilage is due to heavy contamination in the dairy environments, and normally it is easy to control the environments to prevent heavy contamination. There is also the possibility of mold growth, which may be a problem on the surface of cheeses. Formation of molds should be avoided—partly because the damage to the product is severe and partly because formation of molds in some cases leads to formation of mycotoxins. Packaging in a modified atmosphere and in a material that creates a high oxygen barrier will prevent growth of molds. The risk from growth of molds is the formation of mycotoxins, but normally if molds become apparent, the cheese will be destroyed. Mycotoxins are not able to penetrate the entire cheese but are normally located in the outer rind, about 0.5-1.0 cm in depth, depending on the cheese type and water activity.

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