Milk Chemistry

Cheese ultimately gets its start from milk. Understanding the chemistry of milk is the first step to understand cheese.

Milk?

I know what many of you are thinking – “Listen here buddy, I want to learn about cheese. Get outta here with this milk bullshit.”

Normally I’d agree with you. But considering the multitude of future topics I have lined up covering the area of cheese chemistry, I wanted to have a post dedicated to the basics of milk chemistry. Cheese is, after all, the controlled decomposition of milk. If one wishes to understand what makes cheese tick, starting with milk is the way to go. This post will be focused on the composition of cow’s milk. All the illustrations in this post are from a milk infographic I made a few years ago. It’s available for download at the bottom of the post.

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Basics

Milk is an emulsion of milkfat globules in an aqueous (watery) environment. The aqueous (watery) portion of milk contains a variety of substances including lactose (milk sugar), protein (casein and whey), minerals, and certain vitamins. This section is an overview of what’s in milk. The following sections will dive into more detail.

Composition

At its heart cheese making is the process by which we remove water from milk, concentrating the fat and protein. Water makes up the vast majority of milk. All the “stuff” that isn’t water is referred to as “solids”. Of the solids, the major components are lactose (milk sugar), fat, and protein (casein and whey). Other minor constituents like vitamins and minerals will be discussed in a separate post at a later date.

Structure

Why is milk white? It all has to do with the structure of milk. I mentioned earlier that milk is an emulsion. That means if you were to zoom in on milk, you’d see a bunch of little fat blobs distributed in milk’s water phase. If you were to zoom in even more, you’d see a bunch of casein micelles distributed in milk’s water phase. Casein micelles are discussed in more detail below.

These two substances, fat globules and casein micelles are what give milk its color. Light is scattered (deflected/reflected) when it hits the fat globules and casein micelles and gives the opaque white color milk is known for. In the case of skim milk, many of the fat globules have been removed. The casein micelles scatter light the most in the blue range of the spectrum. This is why skim milk can sometimes appear slightly blue in color.

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Sugar

Lactose is the sugar found in milk. Lactose is a disaccharide, which means it is made up of two monosaccharides (simple sugars): glucose and galactose. Our bodies can’t directly metabolize lactose. Many of us produce an enzyme, lactase, which splits the lactose molecule. Those who are lactose intolerant lack the necessary amount of the lactase enzyme in their digestive system. Instead, bacteria in their colon ferment the lactose which causes much of the discomfort associated with lactose intolerance.

In cheese, the bacterial cultures added during the production process consume lactose and produce lactic acid. This process happens more and more over extended periods of time. With enough age, all the lactose has been converted into lactic acid. For this reason, many aged cheeses can be safely consumed by those who suffer from lactose intolerance.

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Fat

The fat portion of milk exists in globular structures. These milk fat globules are complex structures with multiple layers and membranes. The homogenization process breaks up the larger fat globules into smaller ones. These smaller globules stay distributed in the watery phase of milk better and are less likely to rise to the top and form a “cream line”. Fat soluble vitamins (A, D, E, and K) and their precursors (e.g. beta-carotene) are also found in this portion of the milk.

Milk fat, on its own, has a yellow hue due to pigments such as beta-carotene that the cow gets from eating grass. Dispersed in the watery environment, most of that yellow hue is diminished and milk appears as white. During the cheese making process, the fat in concentrated and the light yellow color can be seen in many cow’s milk cheeses. Another way to see the color of milk fat is to simply freeze milk. When you're freezing the milk, the water in the milk is forming ice crystals. These ice crystals tie up the water; meaning you are basically concentrating the fat and protein in the milk. As the fat becomes more and more concentrated, you see its natural yellow color more and more. Another way to think about it is that the ice crystals are allowing you to see the fat with less obstruction, since water really dilutes any color that would be there, ice is letting you see the fat that's already there more clearly.

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Protein

Milk contains two main types of proteins: casein and whey. Casein proteins are the ones of most interest in the cheese making process. Whey proteins won’t be discussed here and will be covered elsewhere. As mentioned earlier, casein proteins exist in structures called micelles. These micelles are made up of different types of casein proteins. The main types are alpha s-1 (αs1), alpha s-2 (αs2), beta (β), and kappa (κ). Kappa casein is of most interest during the cheese making process. κ-casein forms a “hairy” surface around the casein micelle. These hairs prevent the casein micelles from sticking together. In order to make cheese, those casein micelles must stick together and aggregate in order to form curd from milk.

Cheese makers utilize rennet in order to get milk to “clot”, coagulate, and form curd. Rennet is made up of enzymes that are originally found in calf stomach. These enzymes act live a razor and shave off all the κ-casein hairs. Without the hairs, the micelles can now aggregate, forming the backbone of cheese structure! Addition of acid can also cause this aggregation to happen, but that is to be discussed some other time.

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For More Information

• Properties of Milk and Its Components
• Wikipedia - Milk
• Milk chemistry - An introduction