Dairy Chemistry And The Cheesemaking Process

For many cheese makers, there is an art to making cheese. To cheese manufacturers, it is commonly a routine, strictly controlled process. No matter how it is made, cheese is a complex entity in a constant state of change, which has been likened to an ecological community of living organisms in which microbiological activities affect and are influenced by chemical changes.

Production of cheese involves two interconnected phases: The first is to develop the desired composition and pH, and the second is to develop desired physical and flavor characteristics. The first phase is controlled through milk composition and manufacturing protocol, particularly rate and extent of acid development by the starter during the manufacturing process. The second phase is influenced by the first but is dictated by metabolism of a variety of microorganisms and by enzymatic and chemical reactions. This process is called ripening, curing, or maturation, and depending on the cheese variety, may take many months to complete.

In concept, manufacture of cheese is simple. In reality, it is a complex process governed by a series of interrelated chemical and physical phenomena. During cheese making, a coagulum is formed in which milk proteins (caseins) are clotted, entrapping the milk fat, water, and water-soluble components. Further manipulations of the coagulum (cutting, heating, stirring) and development of acid result in controlled moisture expulsion and desired physical and chemical changes of caseins. The resulting curd and whey mixture is separated, with curd being formed into blocks, wheels, or other shapes.

Development of desired flavor, body, and texture is brought about through a combination of the activity of specific introduced microflora and enzymes as well as naturally occurring or contaminating bacteria and enzymes. Part of the initial maturation process involves physical changes to the protein brought about through a decrease in pH, loss of calcium, and hydration of casein. Without the ripening process, it would be impossible to distinguish one variety of cheese from another except to note that different cheeses may have different physical characteristics.

Milk solids are composed of protein (casein and whey protein), milkfat, lactose, citric acid, and mineral salts (usually associated with the casein) collectively called ash. The composition of milk varies considerably between species and individual animals. It is affected by breed and genetics of the animal, feed, environmental conditions, lactation number, stage of lactation, and animal health. All of these factors can also influence cheese making and cheese characteristics. An average composition of cow milk is as follows: 87.6% water, 3.9% milk fat, 3.1% true protein (82% caseins, 18% whey proteins), 4.6% lactose, 0.7% ash (also see Chap. 1).

There are three basic ways to make cheese, but a given variety is made with only one method. All methods involve development of acid by a select group of lactic acid bacteria called the starter. All methods involve some means of concentrating the milk solids (mostly milkfat and protein) by expelling a portion of the aqueous phase of milk (serum or whey).

Rennet curd cheeses (most varieties) are made by clotting milk with a coagulating enzyme (all are proteolytic enzymes) such as chymosin (the most active ingredient in rennet). Acid curd cheeses (cottage, cream) are made with acidification of milk sufficient to cause casein to form a clot. Heat-precipitated curd cheeses (ricotta, queso blanco) are made with a combination of low pH and high heat to precipitate proteins (both casein and some whey proteins).

Fresh or nonripened cheeses such as cottage and mozzarella can be made by direct addition of acid (acetic, lactic, or citric). Cheeses made by this method are called direct acid cheeses (e.g., direct acid mozzarella).

A. Rennet Curd Cheese Manufacture

Rennet curd cheeses are those in which the coagulum is formed by activity of a coagulant, an enzyme mixture with particular proteolytic activity. Coagulants are commonly called rennets. Calf rennet is derived from an extract of calf stomachs, but there are other rennets derived from different sources: fungi, other animals, and some plants, especially thistles. All contain proteolytic enzymes, which, through their activity, help to destabilize casein micelles in milk, an event that subsequently transforms milk from a liquid to a semisolid (coagulum). Chymosin is the desired coagulating enzyme in calf rennet, but because of cost, demand, and the lack of calf stomachs, most chymosin used in the United States is produced by genetically engineered bacteria, yeasts, or molds. Fermentation-derived chymosin is highly purified (100% purity) and is used in liquid or tablet form. Chymosin is the preferred coagulant, because it has specificity toward one peptide bond in K-casein. Although chymosin hydrolyzes bonds in casein molecules at other sites when they are accessible, the specific site of hydrolysis that occurs during coagu lation is Pheios-Metioe. The nonspecific proteolytic activity of some other coagulants causes concern over excessive proteolysis, leading to a soft-bodied cheese, bitter flavor defects, and reduced cheese yield.

Caseins exist in complexes of discretely arranged molecules called micelles. There are four types of casein molecules, as1, as2, P, and K-caseins. The exact molecular arrangement of molecules is not known, but it is hypothesized that micelles are composed of groups of casein molecules linked together through various types of bonding, including calcium phosphate bridges, and most importantly electrostatic and hydrophobic interactions. A hydrophilic (and negatively charged) portion of K-casein molecules protrudes from the micelle surface, giving the micelle stability from spontaneous aggregation.

At the normal pH of milk (6.6-6.7), micelles carry a net negative charge because of the nonprotonated amino, carboxyl, and phosphate groups on caseins. Through electrostatic repulsion and stearic hindrance via the ''hairs'' of K-casein, micelles are stable (show no tendency to flocculate or gel) and remain as individual entities. Activity of the coagulant removes the protruding, hydrophilic region on the K-casein molecule. This eliminates stearic hindrance and reduces the negative charge at the micelle surface. With loss of these barriers, micelles begin to come together (clot formation). Ionic calcium (added as CaCl2 or released from micelles through acidification of milk) allows adjacent micelles to aggregate through hydrophobic and electrostatic interactions. Eventually (20-30 min), casein micelles form a continuous network of aggregates called the clot or coagu-lum. Milkfat, water, and water-soluble components (serum) are entrapped within the casein network. Undenatured whey proteins are water soluble and do not participate in forming the network but are trapped in spaces (pores) that form between aggregates of micelles.

Once the desired firmness of the coagulum has been reached, it is cut into small cubes or pieces (curd). The firmer the coagulum when cut and the larger the curd particles, the higher the moisture content of cheese. After the coagulum is cut, casein molecules continue to interact and squeeze out serum trapped between them, and with exogenous pressure, curds shrink and become firmer. This process is called syneresis and is enhanced by lowering the pH, increasing the temperature of curd (cooking process), and stirring the curd. Therefore, the rate of acid development by the starter has a great influence over moisture content of cheese and control over the rate of acid development is key to successful cheese manufacture. Body (soft to firm) texture (grainy to smooth), melt, stretch, chewiness, oil release during baking, casein hydration, and color of cheese are directly controlled by pH. In addition, growth and metabolism of microorganisms and flavor development are strongly influenced by pH.

Each variety of cheese has a desired rate and extent of acid development, which if not met or compensated for, may result in too much or too little moisture or too high or too low pH, creating undesirable physical and flavor characteristics in cheese. At the proper time, curd is separated from whey and treated appropriately as dictated by the variety of cheese. Curd may be continuously stirred as whey is being removed or it may be allowed to mat. Curd may be salted first and then formed into the desired shape or formed first and then salted by placing the cheese into brine. Pressing of blocks, cylinders, or wheels of cheese removes trapped whey from the cheese and helps individuals curds to fuse, forming a solid mass of cheese. Not all cheeses require pressing. The unripened cheese is then ready for maturation. Camembert and surface-ripened cheeses (Limburger) will be inoculated with specific microorganisms at this time.

B. Acid Curd Cheeses

Acid curd cheeses do not rely on activity of a coagulating enzyme to clot milk. Instead, milk is acidified by direct addition of acid or through lactic acid developed by starter bacteria. At a pH of approximately 5.2, caseins in milk begin to flocculate and eventually gel as the pH decreases. Gelation is the consequence of acidification-induced physicochemical changes to caseins. At neutral pH, casein micelles remain as individual entities and are unable to interact or form aggregates. This is, in part, caused by charge repulsion (micelles are negatively charged). In addition, hydrophilic regions of K-casein molecules protrude from the micelle core and prevent hydrophobic cores of adjacent micelles from interacting (stearic repulsion).

As the pH is lowered, the calcium-phosphate complex disintegrates and some casein molecules dissociate from micelles. There is also a reduction of the net negative charge on casein molecules, an increase in hydrophobic interactions, and it is thought that the protruding portion of casein molecules falls back onto the casein micelle core. The net result is that micelles and solubilized casein molecules begin to form aggregates, eventually leading to formation of a continuous network of aggregates and visible gel (pH ~4.95). In cottage cheese, the gel is cut into small cubes at a pH of 4.65-4.75. Serum (whey) is immediately expelled from the curd.

In cream cheese manufacture, the gel is stirred at pH 4.4-4.8 rather than cut as in cottage cheese, and whey is removed by centrifugation. Traditionally, clotted milk was put into bags of cheesecloth and hung to filter out serum. A low pH of cheese tends to produce a grainy or gritty product. Separated cheese is packaged (cold-pack cream cheese) or processed. Hot-pack cream cheese is made by blending cold-pack cream cheese with cream, whole milk, salt, stabilizers, and skim milk solids and heating the mixture to (72-74°C). The homogenized blend is packaged hot. Microbiologically induced defects are similar to those in cottage cheese but are less likely to occur, because the cheese is packaged hot.

C. Acid-Heat Coagulated Cheese

The premise for manufacture of acid-heat coagulated cheeses is to heat milk to 78-80°C and then acidify milk by direct addition of citric, acetic, or lactic acid to the desired pH (5.8-5.9 for ricotta, 5.2-5.3 for queso blanco). Milk for queso blanco can also be first acidified by lactic acid bacteria (Lactococcus spp.) and then heated. Heating of the milk (ricotta milk is usually a mixture of sweet whey, whey protein concentrate, and milk) causes coagulation and flocculation of caseins and whey proteins. In ricotta cheese manufacture, proteins and entrapped fat are removed or filtered from the remaining serum and drained until packaged. In queso blanco cheese manufacture, curds are allowed to settle and whey is drained. Curds are then salted and pressed. Both cheeses are consumed fresh, and because denatured whey protein forms a network with the casein, the cheeses resist melting during frying or baking. Because of the high-heat treatment under acidic conditions, survival of bacteria other than spore formers is minimal, but contamination during packaging is of concern. Microbiologically induced defects are comparable to those of cottage cheese. Most defects are caused by growth of Pseudomonas sp., yeasts, and molds.

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