• A Note on Milk

    by  • August 29, 2011 • Science • 0 Comments

    Given that the main ingredient of cheese is milk, it’s safe to say that milk choice is one of the most critical aspects of the cheesemaking process. Before we get in to the gritty details, let’s first discuss the general components of milk.

    Composition of Milk

    As I’ve said before, milk is considered an emulsion of fat globules and other proteins in water. In the most general sense, one can say that milk is composed of the following:

    • Water
    • Protein (caseins, endogenous enzymes)
    • Fat (saturated, unsaturated)
    • Carbohydrates (lactose, or milk sugar)
    • Calcium
    • Cholesterol
    • Fatty acids

    The milk from different species varies in its composition. For instance, cow milk contains 3.7 grams of fat per 100 grams, while water buffalo milk contains 9 grams of fat.

    Treatment of Milk

    Pasteuriization of milk was first recommended by Franz von Soxhlet in 1886. This process involves heating the milk to about 160°F, the intent of which is to kill any pathogenic microorganisms such as Mycobacterium tuberculosis.

    The normal pasteurization process extends the shelf life of milk because it also destroys the “good” bacteria such as Lactobacillus species. However, the heat treatment is not sufficient to kill every last bacterium, and that is why store-bought milk will eventually go sour after several weeks. That sourness is the result of lactic acid production by the remaining bacteria – the exact process we are trying to replicate in our cheese vats! We compensate for the much lower amount of endogenous bacteria by supplementing the milk with cheese cultures.

    Pasteurization does, however, have some additional irreversible effects on the milk itself that affect the cheesemaking process. The high heat not only kills the endogenous bacteria – it also inactivates the endogenous enzymes and dissociates some of the casein micelles. As described in the coagulation article, proper curd formation relies on these micelles – so when they are disrupted it is much more difficult to create a good gelatinous matrix. Therefore, the curds formed using pasteurized milk are much more fragile than those made with raw milk.

    For whatever reason, many organic dairies go beyond the normal pasteurization process and treat their milk by ultra-pasteurization. This is the absolute kiss of death for milk in the cheesemaking sense, because the heat involved (135°C/275°F) will wreak havoc on protein structure, inactivate enzymes, and kill 99.999% of all microorganisms present. Since the casein proteins hanging out on the surface of the casein micelles become unfolded from the high heat, they will no longer behave the way they do naturally. This means that the enzymes in the rennet can’t recognize the bits of the protein that are supposed to get chewed up, so they are unable to coagulate by any method we have in our coagulation arsenal. I’ve tried making cream cheese with ultra-pasteurized milk and it barely formed any curds. Curses! Thankfully, most ultra-pasteurized milk is (relatively) clearly marked on the label or on the carton. If it’s on the carton, the letters UP or UHT will be hanging around somewhere. I typically find it’s right on the label.

    Calcium Chloride to the Rescue

    When the casein micelles are disrupted in the pasteurization process, the calcium that was previously held in the gel matrix gets leeched out into the whey and we get crappy curd formation. We fix this problem by adding calcium chloride to the milk when we pour it in the cheese vat. So what is going on here?

    In unpasteurized milk, the calcium is present in clusters on the surface – the very ones that we alter (with heat, acid, or enzymes) to make the micelles interact with each other and form curds. The micelles look a little something like this:

    Image courtesy of University of Guelph Department of Dairy Science

    The blue dots in the above image refer to a complex of calcium on the surface of the micelles (in this case, it is calcium phosphate). In fact, this is the main mechanism by which newborn mammals get the normally insoluble calcium in to their bodies. So in milk, there are both soluble and insoluble forms of calcium floating around.

    When the heat gets turned up, the balance of insoluble and soluble calcium begins to shift toward more insoluble calcium. In addition, the proteins that are floating around in the whey begin to unfold – at which point they will start interacting with the proteins that are on the surface of the micelles. This reduces the reactivity of the micelles, and makes it so the micelle proteins will no longer interact with each other.

    Image courtesy of University of Guelph Department of Dairy Science

    Pasteurization thus makes for a looser curd network. When we add calcium chloride, the calcium balance begins to shift back toward the soluble forms of calcium, which makes the casein micelles very happy. This increases their surface reactivity, so we can get stronger curd formation.

    Better Milk = Better Cheese

    At the end of the day, if you want to make cheese you need good milk. For the absolute best results, you should try and get some raw milk. However, if you live in a state that does not allow the sale of raw milk, pasteurized milk works just great. Just make sure that you’re not using ultra-pasteurized milk! Supplement it with calcium chloride to stabilize the curd if you want (it’s not completely necessary), and you’ll be good to go.

    Interestingly, when making mozzarella, I’ve had the best results by using regular, pasteurized, store-bought milk in the plastic jug. When I tried it using expensive, unhomogenized and gently pasteurized milk, I ended up with delicious although runny mozzarella goo. It just goes to show that making cheese truly is an art and science – sometimes we just don’t know why things work the way they do!

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