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The initial coagulation is achieved by using an added enzyme, rennin, or by lactic acid fermentation, or both, depending on the desired character of the final product. The green cheese is then ripened via the addition of certain microorganisms that will accomplish the breakdown of the protein, lipids, and sugars to the small molecular weight components that contribute to the cheesey flavors found in the final product.

Flavor Attributes of Cheese by Families. The less cheesey flavored cheeses are those with the least amount of ripening: fresh soft low-fat, higher fat cream/Neufchatel, and the whey/ricotta type. Their flavors are primarily due to the components contributing to milk, cream, and butter flavor. They should be low in flavor intensity or they would be considered to have a flavor defect.

The next group in terms of flavor strength consist of the Swiss (Emmentaler), German (Muenster), and Dutch (Gouda) cheeses. The importance of peptides was recognized in studies with Swiss cheese (30). Results indicated that the small peptide that interacted with calcium and magnesium gives the cheese a sweet flavor, with the small free peptides and amino acid contributing a slight brothy-nutty flavor. An important reaction in Emmentaler is the conversion of pyruvic acid to propionic acid by propioni-bacterium. Propionic acid is found in Emmentaler in levels of up to 1%, and it is considered the basis of the sweet taste of that cheese (31). Gouda has a defect called catty taint that is due to the formation of 4-mercato-4-methylpen-taone-2 (32).

Cheddar cheese is by far the most consumed cheese in the United States. It is produced by a unique process, ched-daring, introduced in 1857 to repress the growth of spoilage organisms during cheese-making. Cheddaring involves the piling and repiling of blocks of warm cheese curd in the cheese vat for ca 2 h. During this period the lactic acid increases rapidly to a point where coliform bacteria are destroyed.

The basic taste of cheddar is due to the peptide fraction and, in particular, to the umami and bitter components. The development of acetic acid during ripening contributes to the sharpness of aged cheddar. The release of the acetic acid and amino acids continue during the ripening process. The medium chain fatty acids (C6 to C18) that are released during the ripening process have also been shown to be important in cheddar cheese flavor character (33).

Many other studies over a 10-year period indicated that it is necessary to produce the right level of acidity to provide a reducing environment in the finished cheese so that there may be a release of sulfur-containing compounds. The lost of these compounds, in particular, methyl mercap-tan, results in a cheese with little cheddar character (3436). Methanethiol has also been shown to be a significant factor in aged cheddar (37-41). Methanethiol has a very fecal-like aroma with a threshold of detection reported at 0.02 ppb (42).

It has been suggested that acetylpyrazine and 2-meth-oxy-3-ethylpyrazine are important in aged cheddar (43). The possible microbial origins of pyrazines became evident when it was reported that dimethylpyrazine and 2-methoxy-isopropylpyrazine were isolated from milk con-

Table 6. Summary of the Flavor Chemistry of Dairy Products

Product

Process

Chemistry

Major flavor contributors

Milk Defects

Cream Butter

Cultured products

Yogurt Cheeses

Swiss types Cheddar types

Blue types

Homogenization and mild heat

Prolonged cooking

Oxidation

Sunlight exposure

Rancidity

Fat separation

Further fat separation

Fermentation

Fermentation

Salting

Aging

Fermentation Cheddaring

Fermentation

Natural components

Maillard reactions

Caramelization

Oxidation

Sulfur reactions

Lipolysis

Enzymolysis

Enzymolysis

Lactic acid formation

Proteolysis Propionibacteria Lactic acid fermentation Maillard reactions Pénicillium

Balance of fats, salts, and lactose (milk sugar)

Aldehydes and pyrazines

Sulfur-containing components

Aldehydes

Methanethiol

Butyric acid

Methyl ketones (low level) diacetyl and lactones

Diacetyl and butyric acid

Acetaldehyde

NaCl

Peptide formation

Propionic acid

Lactic and acetic acid

Acetyl pyrazine and other pyrazines

Methyl ketones (high amounts)

taminated by Pseudomonas taetrolens (44). Further alkylpyrazines have been isolated from mold ripened cheese using Penicillium caseicolum cultures (45). The overall flavor of good cheddar is a balance between the free fatty acids and the other flavor components noted above.

The blue cheese family has been found to contain from 13 to 37 times the amount of free fatty acids found in cheddar. This is due to the lipolysis of the fat by the lipases generated from the use of the mold Penicillium roqueforti (46). Roquefort is made from sheep milk and does not have the high levels of free fatty acids seen in the blue cheese made from cow's milk. Part of the flavor difference between the two is due to the lower concentration of propionic acid (4:0) in Roquefort and the relative larger amount of C8:0 and C10:0 free fatty acids.

The major aroma character of the blue cheeses is due to the high levels of methyl ketones found in the cheese. These components have been isolated in milk and are assumed to be the product of the free fatty acid. The importance of the methyl ketone in creating an imitation blue cheese flavor has been well reported (47-52).

Secondary alcohol may be produced from the methyl ketones when they are generated in high amounts driving the equilibrium from the methyl ketones to the secondary alcohols. Although the secondary alcohols have a similar flavor profile to the methyl ketones, their intensity and quantity in the cheese is less, therefore, making only a small contribution to the complete flavor (52).

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