Given that cheese is made in a mostly open, non-sterile environment, using non-sterile raw materials, and is exposed to or held at non-lethal, non-inhibitory temperatures, it is not surprising that all sorts of microorganisms can gain entry in cheese. Although the combined effects of moderately low pH, moderately high salt concentrations, and low Eh provide a barrier against some microorganisms, there are others, including spoilage organisms, and in rare cases, pathogens, that can grow in the cheese environment. Thus, microbial defects are not uncommon in cheese and can be responsible for significant economic loss. Of course, chemical and physical defects (e.g., oxidation, crystal formation, discoloration, mechanical openings) can also be responsible for decreased shelf-life or consumer rejection, however, they generally are less serious.
Microorganisms can cause several general types of spoilage in cheese. These include appearance defects, texture defects, and flavor and aroma defects. Appearance defects are those that may cause the consumer to reject a product based strictly on sight. Perhaps the best example is cheese contaminated with mold growth. Although there are many different fungi capable of growing on cheese, Penicillium spp. are the most common. Fungi are obligate aerobes, so growth occurs almost exclusively on the surface, where oxygen is available. However, mold growth on packaged cheese, even vacuum-packaged products, can also occur, provided there is enough oxygen present. Mold growth is more common in block cheese, and even though it can be trimmed prior to cutting and packaging, trimming is time-consuming and results in loss of yield. Since some fungi that grow on cheese belong to species known to produce mycotox-ins, there has been concern that mycotoxins could have diffused in the cheese and that trimming visible mold from the surface may not be sufficient to render the product safe. However, numerous studies have revealed that mycotoxins are not produced on cheese, even when the fungi is theoretically capable of synthesizing toxins. Presumably, the cheese environment restricts or inhibits expression of toxin biosynthesis pathways.
Appearance spoilage also occurs due to formation of undesirable pigments. The most common and best-studied example is the pink defect (or pink ring) that occurs around the exterior of Parmesan-type cheeses. The defect is worse after prolonged aging, and if the cheese is subsequently grated, a pink-to-brown color may occur throughout the grated cheese. Several factors contribute to the formation of the pink color, including oxygen and the presence of reducing sugars and free amino acids (i.e., reactant of the Maillard browning reaction). The starter culture, and the L. helveticus strains in particular, appears to be the main culprit, by virtue of its ability to produce oxidized pigments from the amino acid, tyrosine.
Finally holes or slits may form as a result of gas production. Several organisms are capable of producing gas (mainly carbon dioxide) in cheese, including yeast, coliforms, propionibac-teria, and clostridia. The clostridia, especially Clostridium tyrobutyricum, are more common in milk obtained from silage-fed cows and can produce enough CO2 to cause formation of large holes (>3 cm in diameter). However, the heterofermentative lactic acid bacteria are probably the most common cause of the slit defect. Relevant species include Lactobacillus fermen-tum and Lactobacillus curvatus. These bacteria are either naturally present in the milk or are normal contaminants in a cheese plant; thus, they may be particularly difficult to control, especially in raw milk cheese. Residual lactose or galactose in the cheese provides substrate for these anaerobic bacteria, whose growth is enhanced within the interior of large cheese blocks or barrels where the cheese is slow to cool and where low Eh conditions prevail.
The most common texture defects include slime, crystal or sandy mouth feel, and poor body. Polysaccharides that impart a slimy texture to the cheese can be produced by psy-chrotrophic Gram negative bacteria such as Pseudomonas and Alcaligenes, as well as by yeast and lactic acid bacteria. Crystals that form on cheese are usually comprised of calcium lactate and are mainly a physical-chemical problem, caused by low solubility of calcium lactate salts at the cheese surface. However, pediococci, lactobacilli, and other non-starter lactic acid bacteria may contribute to this problem by converting L(+) lactate to its isomer, D(—) lactate, which is less soluble. Tyrosine-containing crystals can also form in dry cheeses (e.g., Parmesan and Romano).
Flavor defects can be caused not only by contaminating organisms, but also by the starter culture itself. If the culture is given the opportunity, it may produce too much lactic acid, causing an acid defect. As described earlier, certain peptides generated by starter culture proteinases are bitter-tasting, and unless further metabolized, cause a bitter defect. Het-erofermentative, non-starter lactic acid bacteria produce ethanol and acetic acid in addition to CO2.Acetic acid imparts a highly objectionable vinegar-like sour flavor to the cheese. These bacteria may also hydrolyze triglycerides, releasing free fatty acids that contribute fruity flavors into the cheese. Production of hydrogen sulfide and other sulfur-containing compounds, ammonia, and other volatiles by bacteria and fungi, are also common flavor defects.
Minimizing the entry of spoilage organisms into the cheese milk and subsequently into the cheese is one of the most important steps in preventing spoilage. In most cases, the cheese will only be as good as the milk from which it was made.The use of high quality milk, application of good sanitation practices, and proper cheese manufacture are essential. Pasteurization of milk, even for aged cheese, is now common, not only to inactivate pathogens (see below), but also to kill potential spoilage or ganisms. Some specific spoilage problems can be addressed by the use of chemical preservatives, either added to the milk or applied to the cheese. For example, mold growth can be inhibited by sorbic acid (usually in the form of potassium sorbate salts) or the antibiotic natamycin (also known as pimaricin). In some European countries, but not in the United States, lysozyme and sodium nitrate can be added to milk to control clostridia.
Although spoilage of cheese by microorganisms is a constant concern, the presence of pathogens in cheese occurs only infrequently, and rarely does foodborne disease result (Box 5-4). However, due to the serious public health consequences, as well as potential economic loss due to recalls and liability, attention to food safety is an absolute requirement. Just a single positive test for L. monocytogenes or E. coli O157:H7 may initiate a product recall. Thus, the goal is not just to prevent the growth of pathogens, but also to reduce or eliminate their very presence in cheese. In the United States, this means more and more cheese is being made from pasteurized milk, and exposure to environmental sources of contamination is minimized (e.g., via the use of enclosed vats). Most cheese manufactures have also adopted Hazard Analysis Critical Control Points (HACCP) plans that are designed to anticipate and prevent food safety problems.
Another type of foodborne disease that occasionally occurs in cheese is caused by the presence of the biogenic amines histamine and tyra-mine, but is unrelated to foodborne pathogens. In sensitive individuals, these amines (mainly histamine) can cause headaches, nausea, and cramps and other gastrointestinal symptoms. These compounds are produced from the amino acids histidine and tyrosine, respectively, via specific amino acid decarboxylases. Bacteria that produce these enzymes and form these amines include Lactobacillus buchneri and other lactobacilli, lactococci, and enterococci. Aged cheese, particularly aged Cheddar, Gouda, and Swiss, are more likely to contain biogenic amines due to the higher concentration of free amino acid substrates that accumulate during more extensive proteolysis. Because the bacteria responsible for amine production are ordinarily present as raw milk contaminants, pasteurization effectively minimizes this problem.
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