On the surface, there are few things more boring than a plain old glass of milk. But of course, being cheese people, we see nothing but a world of potential there. While what milk eventually becomes is the object of our affection, milk in its liquid state tells a story all its own and reveals how our favorite cheeses get to be themselves and develop their own unique character.
In order to understand how differences in milk equate to differences in cheeses, outlining how the milks of different species are unique from one another provides a great jumping off point.
Milk is, essentially, a whole bunch of solid nutrients packaged into a convenient delivery system: water. Liquid milk is about 90% water by weight, which enables it to be ingested easily by a newborn animal and also provides them with the hydration that they need. The other 10% is where the nutrition lives, and as cheese folks this is the part that we’re most concerned with. That 10% consists of proteins, fats, minerals and sugars, all of which have their own unique role to play when milk gets turned into cheese.
Arguably the most important component at play here is the protein. Protein is what makes up the physical structure of the cheese and, when it is broken down by enzymes during aging, contributes the most distinctive flavors to the cheese. The main protein in milk, known as casein, exists as tight bundles of smaller protein particles (called micelles) held together with calcium. These micelles can be pictured as a sort of koosh-ball shape: a single particle surrounded by tons of tiny filaments. All of these projections on the outside are known as kappa caseins, and carry a negative charge, which causes them to repel one another when they collide. This is what enables the solids in milk to be suspended evenly throughout the liquid component, giving milk its opaque appearance. When a cheesemaker makes cheese, they are essentially working to undo this negative charge on the micelles in order to cause the proteins to stick together, forming a curd and pushing out water (aka whey) in the process.
In addition to the proteins, milk contains an abundance of fat, which is of the utmost importance to the cheesemaking process. The fats in milk are referred to as butterfats, which consist of a specific type of fat called triglycerides. These molecules, called globules, are made up of several smaller fatty acids (basically just chains of carbon atoms) stuck together and (continuing with the toy comparisons) are shaped like tiny beach balls, with a thin negatively charged membrane surrounding the outside. When curd is formed, the globules are swept up into the coagulating proteins and trapped within the curd structure, trapped in a sort of protein net.
The rest of the solids in milk consist of minerals (most of which is the calcium holding those protein micelles together) and lactose, the sugar component of the milk. Lactose provides much needed energy that the newborn animal will use to grow rapidly, but in the cheesemaking process lactose is important mostly in that it is turned into lactic acid by starter culture bacteria, which prepares the milk to be curdled.
So, with the basics in mind, how does cow’s milk differ from goat’s milk, and how do these differ from sheep’s milk?
Since humans settled down and started farming, cows have been selectively bred to be the most efficient and productive milk machines that they can be. Cows are able to produce up to 5 gallons of milk per day, and are able to be milked about 300 days out of the year.
The milk that they produce is very balanced, with solids making up about 12.7% of the overall volume of their milk, and with fat and protein taking up 3.7% and 3.4%, respectively. The consistency of their milk, combined with the sheer volume that they can produce throughout the year, makes cows the animal of choice for most cheesemakers looking to produce cheese on a larger scale year-round.
Sheep, on the other hand, represent the opposite extreme. Their milk is far and away the the richest in solids, with proteins, fats and other solids making up 19.3% of the overall volume of the milk (with a whopping 7.4% being pure fat). This translates to a much higher cheese yield from the milk, meaning that, say, 10 gallons of sheep’s milk will make more cheese than 10 gallons of cow’s milk.
While this might make the humble sheep sound like quite the efficient cheese machine, a ewe will produce far less milk per milking that a cow or a goat. In fact, over their entire lactation cycle (meaning the amount of days per year that the animal is producing milk), a sheep will only give about 12% of the milk that a cow would over the same amount of time. This, combined with the fact that, like goats, sheep are seasonal breeders and can only be milked for about 180 days per year, means that a sheep produces a comparatively tiny amount of milk. And while the milk that they do produce makes quite a bit of cheese, a farmer raising sheep will still end up with only a fraction of the cheese yield at the end of the day than they would if they were milking cows.
Right in the middle of the road we find our friends the goats. Goats’ milk is very similar in composition to cow’s milk, with 12.4% overall solids and comparable amounts of fats and proteins. However, like sheep, goats are also finicky seasonal breeders and only produce milk for about 8 months out of the year. During those 8 months, goats produce quite a bit more milk than sheep do per milking, meaning the overall volume of milk produced during their lactation cycle will be more than a sheep but less than a cow.
With all of these differences in mind, there is of course the questions of just why cheese made from each species milk taste different from one another. While the base ingredients are all the same (protein, fat, minerals), the makeup of each of these components varies greatly between types of animals. For instance, sheep’s milk contains fat globules that are massive compared to the size of the fats in goats milk. When cheese is made from sheep’s milk, this will lead to the fats dominating the flavor more, giving a stronger herbal, earthy and peppery flavor once these break down during aging.
Similarly, the chemical makeup of these elements (not just the size) can determine differences in flavor as well. Goat’s milk, for example, contains a higher proportion of a particular type of fatty acid in its fat globules that is quite volatile and breaks down very quickly and easily during aging. This leads to the distinctively “goaty” aroma that is so familiar in an aged goat cheese, and explains why that particular flavor can’t be found elsewhere.
Finally, even within species, the milk composition (and therefore the flavor of the cheese) can differ greatly between breeds. Holstein cows (the ubiquitous black and white cows that dot the highways of the USA) produce a relatively mild milk due to the balanced nature of the nutrients, while Jersey cows (a breed developed in the British Isles for buttermaking) produce a milk much higher in butterfat, making for a more aggressively and distinctively flavored cheese.