Make It Yourself: Milk to Mozzarella I realize that not every AP Biology class may have been like mine, which gave students the opportunity to do a lot of hands-on biology. Very few others that I have met made cheese in their high school biology classrooms, to eat with Triscuts, tomatoes, and balsamic glaze, in the name of learning about enzyme function. So perhaps not everyone is aware that you can make mozzarella fairly easily in your own kitchen with a few simple ingredients from the store and Amazon. Or perhaps you re aware, but it sounds like too much of an undertaking. It s no microwave dinner, that s for sure, but if you understand a few of the basic processes and how the ingredients interact with one another, you can tell people you made your own cheese. To help guide any cheese lovers who want to try it out at home, I experimented with some milk types and processes to determine how they affect the final product so you don t have to learn the hard way. All cheese starts with milk, and the basic curdling process is relatively the same for all cheese types. What is done with the curd how much whey is squeezed out, how it s shaped, what aging process it is subjected to, the kind and method of bacterial introduction is what creates the hundreds of cheese varieties available. Milk is composed of whey, sugar, protein, and fat, each of which plays a different role in cheese coming together. Whey is the water component, a liquid formed by water, whey proteins, lactose, and minerals. This is the solution left behind when curds are formed and separated for cheese making. The primary milk sugar is lactose, which is metabolized when lactobacillus is added, producing lactic acid that drops the ph of the milk, a key step in the process. The major protein in milk is casein, which becomes the basis of cheese as these proteins coagulate to form curds. Lastly, there is fat, also known as butterfat, or the taste globules of milk. For reference, cream has about 80% fat content, while whole milk has about 3.5% fat, and most cheeses are made with whole milk. The basic chemical process of turning milk into cheese involves the coagulation of casein proteins, which traps fat in the resulting structure, leaving whey behind. The formed curds can then be molded, stretched, and aged. Cheese making requires adding two ingredients to milk while heating it: either lactic acid bacteria or citric acid to lower
the ph, and rennet, the chymosin enzyme that initiates curd formation. While different cheese makers may cite various reasons for acidifying milk with citric acid versus bacteria or vice versa, either is an option. The method of acidification changes the taste of the cheese. For simplicity s sake, I used citric acid; it s more readily available and doesn t involve the potential to mess up things with bacteria. For basic homemade American mozzarella, the process is fairly simple and shouldn t take more than 30 minutes from milk to cheese. A batch of milk is heated to 13 C, and then the citric acid is added. This drops its ph to a level that is optimal for enzyme function. After heating the milk to 31 C, the rennet (enzyme) is added with continual stirring. This enzyme cuts the hydrophilic k-casein proteins on the outside of casein micelles, exposing the hydrophobic caseins. They clump together in their hydrophobic state, which forms what we recognize as the curds. As the temperature of the mixture continues to rise, curds will form, and once the mixture hits 40 C, the curds can be removed from the whey. They re strained via cheese cloth and squeezed until as much whey as possible has been expelled. The curd ball is then alternately microwaved and kneaded; anywhere from 2-4 repetitions of this are necessary to finish squeezing out the whey and stretching the cheese. The cheese is salted during kneading, and once it feels stretchy and smooth, your homemade mozzarella is complete. To test out the variations caused by changing certain aspects of the cheese making process, I varied the type of milk used and then switched around the order of the steps. The original recipe, which served as my control for scientific purposes, was made with whole milk. This is the standard state of milk as produced by the cow. To vary the milk I tried using lactose-free milk. This milk was also 1% milk, so two variables were at play here. Finally, to vary the order of the cooking process, I added the rennet first and then the citric acid, altering the way the curds formed. Each trial was a good lesson in chemical interactions within food processes and helped me understand how certain elements of milk operate when altered by other compounds or heat. With the traditional homemade mozzarella, I followed the recipe and recorded the details of the process so I could compare my variations later. I used a half-gallon of whole milk and heated it on medium heat with a thermometer at hand. Watching the curds form was akin to watching cloud formations shift in and out of shapes, though a bit less pleasant. I will admit that the separated curds and whey don t form an appetizing mixture at first. The straining process definitely takes a few sets of hands to prevent spillage and handle the amount of curd, especially if you re making cheese from a half-gallon or more of milk. A lot of whey was squeezed out initially, and during the kneading process, it became clear that this cheese was starting to dry out. I tried to microwave it a little less, only getting it warm enough to continue stretching and kneading. It looked
like a small, pale ball of bread dough when I decided to stop, even though the cheese hadn t reached the 42 C that the recipe stipulated. This was no Italian or artisanal mozzarella, but it wasn t bad for a first-time attempt. With a salty, tangy taste and firm texture like American string cheese, it was satisfying enough. I blame the microwave process it seemed to dry out the cheese and I can t imagine that any cheese producer would dream of using a microwave. It was great from a convenience standpoint, but in traditional methods, the mozzarella is kneaded and stretched in a bath of hot water. Lactose is not an essential component in the transformation from milk to cheese, but I was curious how the cheese making process might be affected in the absence of this sugar. Lactose-free milk contains lactase, the enzyme that breaks down the sugars into their absorbable molecules, which is what lactose intolerant people are missing in their gastrointestinal systems. While the presence or lack of lactose does not seem to have an effect on the coagulation of casein, it does pose a problem if bacteria is being used to lower the ph of the milk. In the bacterial acidification process, lactic acid bacteria are added to the milk instead of citric acid, and when they consume the lactose, they produce lactic acid that drops the ph of the milk. No lactose for food means an environment unsuitable for the enzymes to do their job. Since I was using citric acid, this wasn t a concern; however, lactose-free milk only comes in a 1% variety, which I believe factored into this mozzarella production trial. The differences in results began after the heating process when I drained the whey from the curds. There seemed to be much more liquid straining out, and the cheese was not easy to remove from the cheesecloth. Squeezing did not form the curds into a ball they retained a feta-like consistency and did not clump, even during attempts to knead it. It looked like a cross between cottage cheese and feta, not exactly an appealing texture for a ball of mozzarella. It still had that salty plain flavor of low quality mozzarella or string cheese, but was much firmer and crumbly in texture. It had a smaller mass than the whole milk cheese (a 96 g ball compared to a 190 g ball though we did loose some of the curds due to spillage). After doing some additional research into the fat content of milk when cheese making, I believe the 1% milk is to blame. During the coagulation process, the clumping casein micelles trap the milk s fat globules, incorporating them into the blobs that will become the cheese product. If the milk has a lower fat content, there is less to incorporate into the curds, resulting in a smaller volume of cheese with less flavor, as the butterfat also contributes to the pleasant taste of cheese. With less fat, the cheese has a higher water content, which could potentially explain the lack of clumping during kneading. Proper cheese making also comes down to following instructions; doing the steps out of order can ruin the final product. I switched up the order of ingredient additions, mixing in the rennet first, then the citric acid later. When only the enzyme was added to the heating milk, no reaction occurred because the natural ph of the milk (6) is not acidic enough for the enzyme to function. Once the citric acid was added to the milk and rennet mixture at 40 C, curds formed rapidly, within 10 to 15 seconds. The curds were much larger than in the previous two batches, and formed a larger ball when strained into the cheesecloth. It was very squishy and sticky, and in the microwaving and kneading process, more whey was squeezed out and the cheese came together. It was still curdy,
but moist enough that the curds stuck to one another, a bit like congealed cottage cheese. Again, not very appetizing, and it s texture was bizarrely soft and not at all smooth. A possible explanation for this result is the quick coagulation that occurs when the citric acid is added. The casein structure that forms the curds traps more water as the micelles come together quickly, which ultimately affects the final texture of the cheese and would account for the squishy feel of this mozzarella ball. My mixed results with lactose free and 1% milk, and changing the order of the process seems to indicate that altering a cheese recipe does not produce great results. Of course, there are certain alterations that are necessary later on in the process: molding the cheese in different ways, aging it for varying amounts of time, using different strains of bacteria for additional flavor. But in this basic recipe to form solid cheese from milk, the chemical content and interactions are essential. Perhaps this explains why cheese makers are so exact, even when they are changing variables. One misstep and your cheese has an odd texture or unpleasant aroma. Everything matters: milk quality, butterfat content, and reading the instructions. I can see why it was such a good hands-on experiment for my AP Biology class not only did it teach us about enzyme function and protein interactions, it taught us how to follow scientific procedures carefully. Many processes in food production can be this precise, because that s what creates consistent and delicious results. Rennet before citric acid cheese Whole milk cheese ( control recipe) Lactose-free 1% milk cheese
Sources Karlin, Mary. More About Milk, Artisan Cheese Making at Home. Ten Speed Press, 2011. http://www.artisancheesemakingathome.com/cheesemaking-milk.html Milk and Cream. Home Cheese Making. New England Cheese Making Supply Company. http://www.cheesemaking.com/learn/faq/milk.html Milk Structure. Cheese Making Technology. Ontario Agricultural College, University of Guelph. https://www.uoguelph.ca/foodscience/book-page/milk-structure