Dairy Foods

Cheeses are derived from the milk of cows, goats, and other ruminant animals. The textural characteristics of cheeses are determined, to a large extent, by the methods of curd manufacturing, but the flavors and aromas of cheeses are developed primarily in the ripening process. Cheese ripening is an enzymatic process involving enzymes endogenous to the milk, to the ruminants from which the milk is taken, and to microorganisms that are an essential part of the cheese-making process.

Milk protein is composed primarily from a set of proteins collectively called casein. These proteins form an as sociation in milk to form micelles that remain suspended. These suspended micelles, along with fat globules and calcium phosphate, give milk its characteristic opaque white color. The first step in making cheese is the formation of a curd. Usually, the curd results from treatment of whole milk with acid or with enzyme to precipitate the casein fractions. Traditionally, this curd formation was accomplished using rennin (EC 3.4.23.4), an enzyme from the stomach of ruminants. The enzyme has a restricted specificity such that the hydrolysis proceeds to the extent necessary to precipitate the milk protein without causing extensive further protein hydrolysis. A commercially available substitute for rennin derived from microorganisms is called rennet. Other commercially available proteases may be used, but many cause too much hydrolysis, resulting in the formation of "bitter peptides" that contribute undesirable flavor characteristics. Protease treatment accomplishes the concentration of the milk protein into a dense curd, but it also initiates reactions that contribute to flavor development. This flavor development is enhanced by protease enzymes from the milk itself, primarily plasmin (EC 3.4.21.7), as well as enzymes endogenous to microorganisms used in the cheese process. These microorganisms may be bacterial and/or fungal depending on the specific cheese.

A number of processes occur during ripening, all enzyme mediated, which contribute to textural and flavor characteristics. They can be described by three categories of enzymatic activity. A more detailed description of each of these stages may be found in Reference 4. First, probably the most important enzymatic process involves proteolysis of the milk protein by rennin, plasmin, and microorganism-derived enzymes. Proteolysis results in the formation of peptides, amino acids, thiols, and other compounds, all of which contribute significantly to the flavor of the cheese.

A second lipolytic enzymatic process also contributes to the essence of cheeses. Although some lipolysis may lead to a perceived rancidity in some cheeses, the free fatty acids released by lipase (EC 3.1.1.3) activity do influence flavor positively as well. For the most part, the lipases involved are endogenous to microorganisms, although there is some evidence for participation of milk lipases as well. Esterases (EC 3.1.1.1) are also thought to contribute to the process by hydrolyzing mono- and diglycerides to ester compounds that are known to provide various flavor and aroma attributes.

A third process involves glycolysis, in which the lactose in milk is enzymatically converted to lactic and other acids through microorganism metabolism. The acids contribute significantly to the character of the cheese. In modern cheese making, these enzymatic reactions are accelerated by the addition of exogenous esterases and lipases to produce stronger flavors in a shorter period of time while providing addition control over the process.

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