Converting a liquid into a solid

Water removal and the concentration of protein and fat occur via a combination of biochemical, biological, and physical-chemical events. Many of these events happen at nearly the same time and often have complex effects on one another. For example, exposure of cheese curds to both high temperature and low pH enhances removal of water from the curd (a phenomenon known as syneresis). But if the temperature is too high, the microorganisms that produce the acid that lowers the pH will be inactivated, resulting in poor syneresis (and poor quality cheese). If, on the other hand, too much acid is produced, then significant mineral loss (specifically calcium) will occur, making the cheese crumbly (which may or may not be a good thing, depending on the cheese).

To further complicate the situation, removing water from the curd also removes the solutes dissolved in the curd.Thus,the amount of lactose in the curd is reduced as the curds become dryer, and less substrate will be available later for the culture.The point is that each process step has consequences (hopefully, intended) not only on other cheese making activities, but also on the overall properties and quality of the finished cheese.

The conversion of liquid milk into a solid mass of cheese is done via coagulation (or precipitation) of milk protein. Milk, as noted above, contains about 3.3% protein. Of the protein fraction, about 80% is casein (2.5% of the milk), and the remaining 20% are known collectively as whey proteins. For most cheeses, the protein portion consists almost entirely of casein. When milk coagulates and the coagulated material is separated, the soluble whey proteins are released into the water or whey fraction. The casein matrix not only contains some water (and whatever solutes are dissolved or suspended in the water phase), but also a large portion of the lipid fraction that was originally present in the milk, depending on how coagulation occurs.

There are three ways the initial coagulation step is accomplished. First, milk can be coagulated by acids produced by lactic acid bacteria, based on the same principle used for yogurt and other cultured dairy products (Chapter 4). When the milk pH reaches 4.6, casein is at its isoelectric point and its minimum solubility, and therefore it precipitates. However, unlike the process used for cultured products, the coagu-lum or gel is not left intact, but rather is then cut into die-sized curds. After separation from the whey and cooking and washing steps are completed, the acid-precipitated curds are comprised almost entirely of casein and water. In fact, pure casein and caseinate salts (made by neutralizing acid casein), both of which are of considerable commercial importance, are made via the acid precipitation method. In cheese making, of course, the curds are then further processed, resulting in products such as Cottage cheese and farmers' cheese. It is important to realize that casein coagulates at pH 4.6 whether acidification occurs via fermentation-generated acids or simply by addition of food-grade acids direct into the milk. In fact, the latter process is preferred by some producers of Cottage, cream, and other acid-coagulated cheeses, due in part to the ease of manufacture and the elimination of starter cultures as an ingredient. However, because theses products are not fermented (and, therefore,they must be labeled as acid-set),their manufacture will not be considered further in this text.

The second and most common way to effect coagulation is by the addition of the enzyme chymosin (or rennet). This enzyme hy-drolyzes a specific peptide bond located between residues '05 (a methionine) and '06 (a phenylalanine) in k casein. The hydrolysis of this bond is sufficient to cause a part of k casein (the glycomacropeptide fraction) to dissociate from the casein micelle, exposing the anionic phosphates of 0-casein. Thus, the remaining casein micelle becomes sensitive to calcium-mediated precipitation (Box 5-'). In contrast to acid-precipitated casein, the coagulated casein network formed by chymosin treatment traps nearly all of the milkfat within the curd. Most of the cheeses manufactured around the world rely on chymosin coagulation. It is worth emphasizing that even though chymosin, alone, is sufficient to coagulate milk, lactic starter cultures are also absolutely essential for successful manufacture of most hard cheeses.The lactic acid bacteria that comprise cheese cultures not only produce acid and reduce the pH, they also contribute to the relevant flavor, texture, and rheological properties of cheese, as described later.

Until relatively recently, most chymosin was obtained from its natural source, the stomachs of suckling calves after slaughter,

Box 5—1. Casein Chemistry and Chymosin Coagulation

Casein,the main protein in milk, consists of several subunits, including as1, as2, p, and K.They are assembled as complexes called micelles that contain thousands of casein subunits, along with calcium phosphate, that are held together primarily via hydrophobic and electrostatic interac-tions.Although some researchers have suggested that casein is organized in the form of sub-micelles, how casein micelles actually exist in milk is uncertain (Lucey, 2002; Lucey et al., 2003). Moreover, it also interesting to understand how these micelles form into a gel when exposed to the enzyme chymosin. Over the past forty years, several models have been proposed that describe the casein micelle structure and how gel formation occurs.

One simplified model suggests that the hydrophobic aand p subunits are located within the interior core of the micelle, and are surrounded by k casein, as shown below.Accordingly, a part of the kappa casein molecule (called the glycomacropeptide) protrudes from the micelle, providing steric hindrance between neighboring micelles (Figure 1). Since this section of the k casein is negatively charged, electrostatic repulsion between casein micelles also prevents micelles from coming into contact with other micelles. However, once this barrier is removed (via the action of the enzyme chymosin; see below), glycomacropeptide is released and the micelles can interact with one other.There will still be a small negative charge on the micelles; however, soluble cationic calcium serves to neutralize the charge.This allows the micelles to come into contact with one another, resulting in formation of a gel. Importantly, this gel will entrap fat, forming the basis of cheese manufacture (Figure 2).

Casein Isoelectric Point
Figure 1. Structural model of the casein micelle, showing individual submicelles. Adapted from Walstra, 1999.

Box 5—1. Casein Chemistry and Chymosin Coagulation (Continued)

whey protein

Casein Micelle Structure

Figure 2. Chymosin-coagulated cheese matrix model. Casein is envisioned as surrounding fat globules; small amounts of whey protein may also be present. Adapted from Hinrichs,

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Responses

  • Jarmo
    What is isoelectric point for fermented milk?
    6 years ago
  • harriet
    Is casein a liquid or solid?
    5 years ago

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