From being an integral component of fancy garden parties to an ingredient in your sandwich, cheese is an extraordinary delicacy. The processes that cheese-makers undergo, starting with milking either some sort of animal, are strenuous and time-consuming. The milk must be pre-cured into something called curds, which is then aged for weeks to years, depending on the type of cheese, before it ends up in your homes. There are also many different types of cheeses, all of which depend on the internal molecular structure of the milk from which it is produced from as well as manufacturing processes that affect its chemical makeup. Nonetheless, form and flavor are one of the two most important things to consider in making cheese, and we are going to tell you just how to make the perfect batch.
Cheese starts off as a milk conglomeration, or heterogeneous mixture, of lipids and proteins. The milk is prepared for curdling through a number of different ways, all including the addition of a culture ofLactococci, Lactobacilli, or Streptococci bacteria. Yuck! Do not fear because the types of bacteria used for cheese-making are strictly either isolated from either the dairy environment or plant material. They are the good kind of bacteria needed in the body to function, also known as probiotic strains. The bacteria cultures are fermented to convert lactose in the milk to lactic acid.
Figure 1. (from left to right) Lactococci, Lactobacilli, or Streptococci bacteria.
Once the milk is pre-cured, it is inoculated with the lactic acid bacteria and rennet. Rennet is a complex of enzymes, one of which being rennin. This specific enzyme converts uncoagulated protein in milk called caseinogen into coagulated and insoluble protein, called casein, with a two-stage process. Firstly, rennin splits a bond in the amino acid chain of kappa-casein, a mammalian milk protein. A glycopeptide is removed from the soluble casein, producing para-casein. This process causes an imbalance in intermolecular forces in the milk, causing hydrophilic macropeptides to be released into the whey. The second step requires colloidal calcium phosphate to bridge with a casein micellar structure to create a curd substance. Colloidal Calcium phosphate, also known as CCP, bridges hundreds of submicelles that form the casein micelles. The bonds are either covalent or electrostatic. The micelles that contain a lot of kappa-casein take a position on the surface whereas those with less stay in the interior. This new layer prohibits further accumulation of micelles.
Figure 2. Micellar structure consists or a hydrophobic tail and a hydrophilic head suspended in an aqueous solution.
The role the lactic acid plays in the cheese-making process is to denature milk proteins to coagulate. Milk contains about 80% casein and 20% whey protein, in which the casein has a negative charge. The H+ concentration turns the negatively charged casein micelles into neutral. At a pH of 4.7, the milk reaches a point where all charges are neutral, known as the isoelectric point. Since the micelle formation calls for hydrophilic ends to converge and the hydrophilic ends to be on the outer edges of the molecule, large molecules of casein proteins are suspended in the milk water. This creates a precipitate called acid casein, as seen in cottage and cream cheese.
Most cheeses use rennet to curdle. It sets the cheese into a stronger, rubbery gel, compared to the delicate curds made by acidic coagulation. Rennet also allows curdling to occur at lower acidity which will be a huge benefit when flavor-making bacteria is introduced. The rennin coagulum will have a fluffier texture than the acid precipitate. Rennet is used in the manufacturing of most cheese currently.
Figure 3. Curds are solid particles and whey is a liquid that is extracted from the milk-cheese process.
To watch a video on how cheese is made from milk, please click here.
Now to actual manufacturing and processing. The unique form and flavor of a cheese is dependent on choice of milk, the bacteria strains, salt, brining, and most importantly, aging. Firstly, the animal from which the milk is extracted from, for example goat, sheep, or cow, affects the flavor of the cheese due to their difference in structure and composition. There are different protein structures, fatty acids, and butterfat contents in the milk that differentiate the cheeses from one another. For example, cow’s milk is enriched with cream for soft cheeses, increasing their fat content. The fat content allows for different reactions during the cheese-making process, in which there is a greater amount of moisture in the cheese that gives it its softer texture. This is because the enzymes cannot break the fats into its smaller counterparts, which slows coagulation. The manufacturers from the Cabot Creamery in Vermont share that the “longer the cheese ages, the longer the enzymes have to break the fats and proteins down into their basic building blocks (fatty acids from the milk fat; amino acids and peptides in the proteins).” The structure of the milk cheeses contributes to aging times as well. Goat’s milk cheeses mostly cannot be aged as long as cow’s and sheep’s milk cheeses, for it will be dry and crumbly compared to the usual soft texture of cheese.
The different types of bacteria, as well as how they are treated during the cheese-making process affects the flavor of the cheese. Bacteria strains can be blended for a desired flavor. This is due to the bacteria’s acid tolerance, spore forming ability, and fermentative metabolism. If a bacteria strain cannot handle the H+ concentration needed to neutralize the negatively-charged casein micelles, its isoelectric point will be more basic, thus resulting in a softer cheese. Spore forming ability is also called a bacteria’s CO2-forming ability. Curd acidification is accompanied by the production of CO2, which results in the “holes” in cheese. This affects the texture and form of the cheese and the taste, for aerated cheese melts easier in the mouth and satisfies the taste buds more than firmer cheese would.
Figure 4. Swiss cheese has a high spore-forming ability.
Brining is a process that preserves the cheese. Also known as salting, after whey is extracted from the curds, the reason for brining is to slow down or stop the bacteria from converting lactose to lactic acid, as well as contributing to taste. Most of the lactose is also extracted due to brining, increasing the cheese’s shelf life. The cheese curds are formed into their desired shapes before brining. They are then cooled to the temperature of the brine, to prevent the increase of rate of salt absorption. The brine has a pH of about 5.2 and through the process of osmosis, the cheese will stabilize its pH and absorb some of the salt from the brine solution. Cheeses will form their own brine due to surface moisture, creating a protective covering around the cheese.
Figure 5. The higher pH level, the higher the viscosity of the milk-cheese mixture, resulting in a firmer cheese.
Lastly, and frankly the most popular process in the cheese industry, is aging. During aging, microbes and enzymes transform the texture of the cheese as well as intensify flavor. This is due to the breakdown of casein proteins and milk fat, resulting in a complex mix of amino acids, amines, and fatty acids. Aging is possible in cheese because of the bacteria that is present, all of which are alive. The bacteria have stopped undergoing lactic fermentation, but continue to break down the proteins and fat molecules in the milk cheeses due to the enzymes in the bacteria. The longer cheese is left to age, the firmer and more intense cheese. The cheese would be firmer since the moisture from the cheese would evaporate and the molecules would have time to coagulate.
This video shows a time lapse of cheddar cheese aging from 1-9 months. As you can see, the cheese darkens as it ages, and the cheese-makers rotate and flip the cheese every once in a while to make sure that it ages evenly. You can also see that the longer the cheese ages, the more pores begin to appear on the surface as well as the firmer the cheese gets.
Now you know that there are many processes that happen to turn milk into edible, tasty cheese. First, the milk has to be pre-cured, so that the enzyme rennin and lactic acid contribute to the breaking down of protein and fat bonds. The milk cheese then has to coagulate into what’s called curds, being formed and shaped to prepare for aging. Before aging, the cheese must be brined and the temperature of the cheese mixture must be at the perfect temperature to maintain its moisture content, as to prevent over-salting. Lastly, the cheese is aged to intensify flavors and allow for more breaking down of molecules to make the cheese firmer. So, the next time you decide to delight yourself with the wonder that is cheese, think of all the chemical processes that it had to undergo (and of course the live bacteria that you are eating), and how that tasty snack had to pay such an incredible price to become such a treat.