Influences Of Microbiological Quality And Milk Composition On Cheese Quality

The microbiological quality and composition of milk play an integral part in the quality of the cheese made from it. Cheese can be made from grade A or grade B milk, but cottage, cream, and mozzarella cheeses must be made from grade A milk only. The bacterial count of grade A milk, as determined by a standard bacterial count or loop count, cannot exceed 100,000/mL at the time of receipt or collection. The bacterial count of grade B milk cannot exceed 300,000/mL (Wisconsin Administrative Code). Processors often pay premiums for low bacterial count milk as an enticement to farmers to produce high-quality milk. In practice, processors have recorded that milk from greater than 90% of producers has a bacterial count of less than 20,000/mL. The bacteria found in the milk arise from contamination (especially from air and biofilms on equipment) or from the animal itself (see Chap. 2).

The level of contamination is reflective of the cleanliness of the entire milking operation, including that of the animal before milking. Clostridia and lactic acid bacteria generally originate in silage and other feeds and are concentrated in feces. High levels of clostridia in silage indicate poor lactic acid fermentation (Stadhouders and Spoelstra, 1990). Feces can get on the udder, and if the udder is not cleaned, milk can become contaminated. Improper cooling rates or final holding temperatures of milk result in high numbers of bacteria reflective of an environment conducive to microbial growth. Most bacteria in milk are, not surprisingly, psychrotrophic bacteria and they are the contaminants likely to grow at the low temperature at which milk must be stored (not to exceed 7°C for grade A and 10°C for grade B within 2 h after milking). Pseudomonas spp. are usually the dominant psychrotrophic organisms found in milk. Although these bacteria are easily killed by pasteurization, they produce lipases and proteases, which are not totally inactivated by this heat treatment (Griffiths et al., 1981). The enzymes are active in milk and can cause bitterness (protein hydrolysis) and rancidity (milk fat hydrolysis) in products made from milk if the level of activity is high enough (Cousin, 1982). Milk may be held for 2 days (legally) after receipt at the factory and microbial counts will undoubtedly increase. It is growth of Pseudomonas sp. during refrigerated milk storage that concerns the cheese maker.

A more important cause of rancidity in milk and cheese is activity of endemic animal lipases (milk lipase). The level of activity of this enzyme is increased in milk obtained from animals with mastitis (udder infection). In this instance, lipase activators and somatic cells are secreted from blood into milk. Somatic cells are used as an indicator of cow health and limits have been set by individual states (not to exceed 750,000/mL) (Wisconsin Administrative Code). Milk from mastitic animals has decreased casein content, the major protein found in milk, although the total amount of all proteins (whey proteins increase) may decrease only slightly, if at all (see Chap. 1).

The composition, quality, and amount of cheese produced are greatly affected by the casein content of milk. The other proteins, collectively called whey proteins, are water soluble and contribute much less to cheese yield. The lower the casein content of milk, the lower the yield of cheese. Cheese makers do not routinely directly measure casein in milk, because the test is expensive and takes too long to complete. Instead, they use fast, inexpensive, automated tests to measure total protein. Casein content is calculated by multiplying the percentage of total protein by 0.82. In mastitic milk, however, the amount of casein as a percentage of total protein decreases. Cheese makers cannot predict this value. Rather a high somatic cell count indicates that the casein content of milk may be reduced. Consequently, the cheese maker commonly pays premiums for low somatic cell count milk.

IV. MILK PRETREATMENT: CLARIFICATION,

STANDARDIZATION, AND HEAT TREATMENT

All milk received by the cheese plant is first tested for the presence of antibiotics. Milk containing antibiotics must be dumped (liquid manure or landspread) even though, if diluted with other milk, a negative test could be obtained. Raw milk, as the cheese maker receives it, is almost universally filtered to remove extrane ous matter (straw, hay, and large clumps of bacteria). The Code of Federal Regulations establishes fat (milk fat content by weight of the cheese solids or fat in the dry matter [FDM]) and moisture limits for some cheeses. These values are called the standard of identity. The casein to milkfat ratio in milk determines the FDM of cheese, whereas moisture is controlled by the manufacturing process. The use of whole milk almost always results in cheese with an FDM of at least 50%. To manufacture cheeses with a lower FDM, such as part-skim mozzarella or Swiss cheese, milkfat is removed or skim milk is added to whole milk. The process of manipulating the composition of milk is called standardization and is becoming more popular for all cheese types because of economic considerations and a desire for uniformity of cheese composition and cheese yield.

A. Heat Treatment

Heat treatment given milk before cheese making varies from country to country, cheese maker to cheese maker, and cheese to cheese. Pasteurization of milk is a legal requirement in the United States for fresh cheeses such as cottage, mozzarella, and reduced-fat varieties. It is based on a 9-log destruction of Coxiella burnetti. Cheeses made from unpasteurized milk must be held for 60 days at a temperature not less than 1.7°C (Code of Federal Regulations, 1995). It is thought that pathogens will die out during this time period because of acidic conditions in cheese and growth of nonstarter lactic acid bacteria. However, this may not be true, especially if the level of contamination is high. Manufacturers who do not pasteurize milk use another heat treatment (65-70°C for 16-20 s), but the trend is toward pasteurization. A main argument against pasteurization is that cheeses made from pasteurized milk tend to have a milder flavor (the flavor takes longer to develop or the flavor is atypical of raw-milk cheese). Research into development of flavor in cheese may provide means to overcome this perceived obstacle, but the question of safety of raw-milk cheeses remains. Pasteurization is not a guarantee of safety, because milk or cheese can be contaminated after the milk has been pasteurized. When cases of illness can be attributed to consumption of cheese containing pathogens (a rare event), often the cheese is manufactured under poor hygienic conditions, is a fresh cheese, is made from unpas-teurized milk, or the rate and extent of acid development were curtailed (Johnson et al., 1990a). The rate of acid development is critical (as well as contamination in the first place), since some bacteria, especially coliforms, will not grow well at low pH and higher acid cheeses. It is not uncommon to find coliform bacteria in washed curd cheese varieties (lower in acid content—baby Swiss, reduced-fat varieties) or in cheeses where the acid development was slow (especially because of phagic infection).

The effectiveness of pasteurization in killing bacteria in milk depends on initial microbial numbers, composition (fat and sugar), and thermoresistance of individual microorganisms. The thermal death time of bacteria is logarithmic. This implies that within a given population of a single strain of microorganism, some individuals will survive pasteurization and other individuals will be killed. By definition, thermoduric microorganisms survive pasteurization, and by convention, thermoduric bacteria are classified as being thermoduric based on the potential for individual bacterial cells within a population to survive pasteurization. Genera containing thermoduric species include Microbacterium, Micrococcus, Bacillus spores, Clostridium spores, Streptococcus, Coryneform, Enterococ-cus, and Lactobacillus. Some of these bacteria are responsible for a variety of cheese defects (Hull et al., 1992), such as excessive softening of cheese, splits and cracks, off-flavors, and abnormal color. Thermoduric bacteria may colonize in the regenerative section of the pasteurizer. Indeed, a solution to keep numbers of thermoduric microorganisms low is to clean and sanitize the pasteurizer more often.

Although rarely used in the United States, a specially designed centrifuge called a Bactofuge (bactofugation) is used to remove most of the bacterial cells and spores (empirically 98%) from milk. Two streams of milk result from bactofugation, the ''cleaned'' milk and the bactofugate containing bacterial spores and cells. If used, the bactofugate is heated to 130°C for a few seconds, but the milk is pasteurized. The two fractions are then recombined. Bactofugation is used in Europe in lieu of sodium nitrate in controlling outgrowth of Clostridium tyrobu-tyricum spores, whose metabolism results in gassy, rancid cheese. The use of sodium nitrate in cheese is not permissible in the United States.

After heat treatment, milk is cooled to the temperature conducive for optimal starter activity and pumped into specially designed vessels called vats. Cheese vats vary in size, with the larger vats holding as much as 22,700 kg and the smaller commercial vats holding approximately 4500-6800 kg. Vats are generally double walled to permit controlled indirect heating of milk. If starter is used, it can be added while milk is being pumped into the cheese vat or after the vat is filled. The temperature of milk at the time starter is added is determined by the type of cheese to be made, type of starter, and the desired temperature at the time of coagulant addition, but it is generally between 31 and 34°C.

B. Starters

The strains and balance of strains of bacteria used in starters is often dictated by tradition as much as it is by manufacturing protocol and desired cheese characteristics. The choice of starter depends on the desired rate and extent of acid development (pH) during manufacture, proteolytic activity of the strains, flavor (and gas formation if desired), and conditions encountered during manufacture and storage such as pH, acidity, salt, and temperature profiles. Mesophiles are sometimes used to manufacture mozzarella (non-pasta filata type) and Swiss varieties instead of the traditional thermophilic starters. In these instances, a lower cook temperature is used and the resultant cheese is generally higher in moisture and may have a slightly different flavor profile (more acid, less buttery). The amount of starter used is based on the rate of acid development desired by the manufacturer and is dictated by cheese variety, but it is influenced by strain and how the culture was propagated (conditions of growth such as media, pH control, and age). This is an important concept, because amounts of starter listed in literature for cheese manufacture can be misleading (e.g., use of 1% w/w starter grown with no pH control may be equivalent to using 0.2% w/w starter grown with pH control). Additional information about starter cultures is given in Chapters 6, 7, and 8. The use of artisinal cultures is not common in the United States. These cultures are mixtures (unknown composition) of several genera, species, and strains of lactic acid bacteria. They may contain lactococci, lactobacilli, leuconostocs, streptococci, and enterococci and probably give the cheese special flavor characteristics.

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