CHEESE, YOGURT, AND SOUR CREAM
5.5 MORE MILK CURDLING
Now you know that the main components of the cheese curd are the casein proteins and that, somehow, the casein proteins that are soluble in milk become insoluble dur-ing cheese production. Your next questions might be: how is curd formation induced?
Is it the same or different in different types of cheeses? Does it matter how the curd is induced?
Curd formation is commonly induced in one of three ways: (i) by increasing the acidity of the milk, (ii) by increasing the temperature of the milk, or (iii) through the use of an enzyme commonly called rennet. Depending on the type of cheese being made, any one, two, or all three of these methods can be used to produce cheese of a particular type.
5.5.1 Acid‐mediated Curd Formation
As the title implies, in acid‐mediated curd formation, a cheesemaker makes the milk more acidic either through direct addition of an acid, like lemon juice, or by bacteria fermentation production of lactic acid. Some cheeses use acid‐mediated curd formation with another method, while it is the only method used for curd formation in many fresh cheeses, such as Chévre, cream cheese, goat cheeses, and ricotta. First, we will learn about how acid induces the formation of the cheese curd at a molecular level, and then we will go into some of the details about the different types of acid‐mediated curd formation processes and the resulting cheeses produced from each type.
5.5.2 The Molecular Basis for Acid‐mediated Curd Formation:
Why does Acid cause Milk to Curdle?
In order to induce a milk curd to form, you have to disrupt the casein micelles in some way. One way to cause this molecular disruption is by changing the pH of the milk. As you recall from Chapter 1, a protein’s structure and its characteristics are usually very sensitive to pH; when you change pH, you change the protonation state of acidic and basic amino acids within the protein. Generally speaking, a decrease in pH (a more acidic solution) will cause some amino acids to become more protonated, while an increase in pH (a more basic solution) will cause some amino acids to become more deprotonated (Fig. 5.10). Continue to raise the pH and the interaction between acidic and basic amino acids is lost as the amino groups lose their proton at pH greater than 9.
The native structure of a protein is often stabilized by a number of intermolecular interactions that occur between its component amino acid side chains. If an interaction depends upon an amino acid having a particular charge (positive or negative), as soon as you alter the pH of the solution, you will change the relative amount of the amino
acid that will have that charge, thereby potentially destabilizing the protein’s structure.
For example, if there is an interaction between a negatively charged amino acid (i.e., glutamate) and a positively charged amino acid (i.e., lysine) within a protein structure at a particular pH (Fig. 5.10), as soon as you decrease the pH of the solution, the inter-action might be less likely to occur or be weaker. Thus, some protein molecules might denature/unfold in the solution.
If we come back to relate this concept to the formation of cheese curds, milk has a pH of approximately 6.5. At pH 6.5, the casein and whey proteins are soluble in the milk. The caseins are largely folded and incorporated into the micelle structure described earlier with the negatively charged domain of κ‐casein at the surface, while the whey proteins have their compact, globular individual protein structures.
The negatively charged domain of κ‐casein is due to the presence of a high number of deprotonated glutamic acid (8) and aspartic acid (2) amino acids within the surface domain at pH 6.5.
As the milk pH decreases to a value of 5.5, the surface κ‐casein protein and other casein proteins begin to lose some of their negative charges due to protonation of negatively charged amino acids like aspartate and glutamate, which makes them neutral. This “neutralization” impacts the casein micelle in two ways. With loss of negative charges, the interaction between calcium and casein is reduced; thus some calcium phosphate is lost from the casein micelle structure. With less calcium, the submicelles that comprise the larger micelle complex are not as attracted to one another and dissociate from one another, and the micelle begins to fall apart.
As the milk pH continues to drop, the fragmented micelles become even less nega-tively charged and repulsion between one micelle fragment and another is reduced even further. The fragmented casein micelles, other free non‐micelle protein in the milk, and milk fats begin to aggregate to prevent exposure of the hydrophobic protein and fat components to the polar milk water. These large networks of protein and fat precipitate out of the milk liquid as a soft curd, fragile gel.
(a) (b)
FIGURE 5.10 Lactalbumin. Two types of whey proteins are (a) β‐lactalbumin and (b) α‐lactalbumin.
5.5.3 How is Milk made more Acidic?
There are two mechanisms whereby milk can be made more acidic. The easiest way that you can make milk more acidic is simply by adding an acid! There are many acids that are used in cooking and are effective enough to instigate the curdling pro-cess, such as lemon juice, citric acid, or vinegar. If you have ever eaten ricotta (often used in lasagnas), mascarpone (an Italian soft cheese used in the dessert Tiramisu), or pannier (a cheese common in South Asian cuisine), you have eaten cheeses that are made by the direct addition of an acid to milk. What is really neat about these cheeses is that you can easily prepare them in your own kitchen and that they don’t really melt! They simply get drier and stiffer when you heat them up. The chemical explanation behind this phenomenon is the aggregation of the casein proteins in the
BOx 5.2 MASCARPONE: ACID‐CURDLED CHEESE
Mascarpone is an Italian fresh cheese from the Lombardy region, made by curdling milk cream with citric acid or acetic acid. The whey is removed from the casein curds without any type of pressing; thus the cheese has a high moisture content and is thick and soft. Because it is made from cream, it has a very high fat content ranging from 60 to 75%. The texture of mascarpone ranges from smooth, creamy, to buttery, depending on how it is processed during cheese-making. Making the cheese is so simple that many people easily make their own mascarpone at home.
Recipe:
• Five hundred milliliters of whipping (36%) pasteurized cream (not ultrapas-teurized)
• One tablespoon of fresh lemon juice
Bring 1 inch of water to a boil in a wide skillet. Reduce the heat to medium‐
low so the water is barely simmering. Pour the cream into a medium heat‐
resistant bowl, and then place the bowl into the skillet. Heat the cream, stirring often, to 190°F/89°C. It will take about 15 min of delicate heating. Add the lemon juice and continue heating the mixture, stirring gently, until the cream curdles. All that the whipping cream will do is become thicker, like sour cream. The back of your wooden spoon will have a thick layer of the cream, and you will see just a few clear whey streaks when you stir. Remove the bowl from the water and let it cool for about 20 min. Meanwhile, line a sieve with four layers of dampened cheesecloth and set it over a bowl. Transfer the mix-ture into the lined sieve. Do not squeeze the cheese in the cheesecloth or press on its surface (be patient; it will firm up after refrigeration time). Once cooled completely, cover with plastic wrap and refrigerate (in the sieve) overnight or up to 24 h.
presence of acid. When the acid curd is heated, the casein aggregates have strong interactions; the first thing that is disrupted upon exposure to heat is the milk water.
The water evaporates and the casein proteins become more concentrated and dried out, rather than stringy and more liquid‐like (which requires the presence of water).
Because of this trait, acid‐curdled cheeses retain shape upon exposure to heat and can be fried.
The second mechanism used to make milk more acidic is more indirect and uti-lizes the process of bacteria fermentation that you first learned about in Chapter 3.
Since you may not think about bacterial fermentation on a regular basis, we will highlight a few of the key points here, as it relates to milk coagulation in the cheesemaking process. In fermentation, bacteria utilize the enzyme lactate dehydrogenase to convert the glycolysis products of pyruvate and NADH to lactic acid and NAD+ (Fig. 5.11).
Fermentation is critical for survival of the organism, as it regenerates the NAD+ necessary to produce ATP through glycolysis. However, the lactic acid that is pro-duced also makes the environment in which the bacteria are growing more acidic!
Thus, if you allow bacteria to grow in milk that is being used to produce cheese, you have an internal source of acid, the lactic acid that is produced by the growing bacteria. Moreover, the lactic acid produced doesn’t just affect the formation of the milk curd, but the presence and amount of lactic acid generated also impact the taste and aroma of a cheese. Depending on the cheese being made, two strains of bacteria may be used: a starter bacteria (such as lactobacteria) to begin the acid production and a second strain of bacteria, commonly called a nonstarter, or ripening bacteria (Fig. 5.12). The finishing bacteria will vary depending on the type of cheese being made. Some will produce gas to make Swiss cheese, and others will produce other flavors as the bacteria eat protein, fat, and sugars pro-ducing new flavorants. The common characteristic of finishing bacteria is they can thrive in a lower, more acidic pH produced by the starting cultures. Many of these bacterial strains can handle the higher heats used to complete the cheese production.
Protein Protein
Protein Protein
No ionic interaction maintaining protein structure
Acidic pH (pH <5) HN
HN O C
C H CH2
NH3+ CH2 CH2 CH2 CH2 C = O OH
O
O C O
HC
–H+
Ionic interaction between protein chains
Neutral pH (pH 6.5–7.5) Protein H Protein
N C H NH3+ CH2 CH2 CH2 CH2 O C
O
Protein H Protein N
O C CH2 C = O O–
–H+ H O
C
Basic pH (pH >9)
No ionic interaction maintaining protein structure
Protein H Protein N C H NH2 CH2 CH2 CH2 CH2 O C
O
Protein H Protein N
O C CH2 C = O O– H O C
FIGURE 5.11 The impact of pH on ionic interactions and protein structure.