Unit 2a. Milk Structure and Composition

Proximate Analysis

Proximate analysis of a food is the percentage distribution of the principle solid components, namely fat, protein, carbohydrates, ash, and moisture. In milk, these components are arranged in different phases, giving milk their characteristics white color, liquid texture, etc., as well as its capacity to be transformed into cheese, yogurt, etc.

Milk fat is the most diverse of all fats because some fatty acids from feed are incorporated as is into milk fat, others are modified in the rumen before transportation to the udder, and others are synthesized in the udder. Milk fat is mostly present in the form of fat globules packed inside the Mil Fat Globule Membrane (MFGM).

Milk proteins are comprised of caseins and whey proteins. Caseins are defined as the fraction of milk proteins that coagulates and precipitates from milk acidified to pH 4.6. Caseins are the principal structural component in most cheeses. Whey Proteins are the residual proteins in whey after caseins are separated by renneting or acidification. Lactose is the only carbohydrate in milk. It is a saccharide composed of glucose and galactose. The most important role of lactose is to be a substrate for lactic acid bacteria in cheese and other fermented dairy products. The principal component of ash is calcium phosphate salts which are associated with caseins. The principal functional role of these salts is to modify the casein and thereby help determine its functionality in dairy products. Of course, the ash includes Ca along with a long list of other nutritionally important minerals.

Milk Structure

Figure 1 depicts big fat globules compared with the much smaller casein micelles swimming in serum containing whey proteins (in many colours) and dots representing minor components: sugars, vitamins and minerals.  The scales are not precise, only representative.

Figure 1. The structure of Milk. Slide the images to appreciate the different fractions in milk.

The drawings are mere representation and are not in scale. To have a better idea of the scales, review the figure above by clicking on the hot spots marked.

 Figure 2. Microstructure of milk on scale.

First, at the macro level (bottom middle) milk in a glass appears turbid. That tells you that milk is loaded with particles, like the clear water on a mud puddle goes turbid when you step in it. ​The next figure, following on the left, is a confocal micrograph  in raw milk magnified about 5000 fold, showing in red a range of sizes of fat globules,  wrapped in membranes that keep them discreet and separate from one another (MFGM). The third image on the top shows the relative sizes of the MFG and sorrounding casein micelles (in black). As we will study later, casein micelles are the principal structural material in a milk gel (such as cheese curd). The last picture on the lower right, shows a scanning electron micrograph of a single casein micelle sorrouded by K-casein, which protudes like hair.

Milk (and milk gels) is white because it scatters light trying to pass through it (a true solution would not scatter light), and the scattering particles are both the fat globules and the casein micelles. Milk serum, however, is transparent, it no longer scatters light. It contains the serum/”whey” proteins, lactose, dissolved minerals, vitamins, etc., all in solution.

Figure 3. A casein gel (cheese curd) is separating from the clear yellowish whey (mostly on the right)

Milk Proteins

• There are three main categories of milk proteins: caseins (78%), serum proteins (19%), and miscellaneous proteins (2.7%).

• Caseins are insoluble phosphoproteins, of which there are four main types: αs1-casein, αs2-casein, β-casein and κ-casein. These proteins are all integrated into the casein micelle, a colloidal particle in milk of about 100 nm diameter.
• Serum proteins are smaller than the caseins and are globular, folded proteins. In milk, they are correctly referred to as “serum” proteins as they are soluble in the serum phase, and they can be extracted easily from milk using membrane separation techniques to make native serum proteins. But, during cheesemaking, they remain in the aqueous, liquid whey after curd formation, hence when isolated from whey, they are known as “whey” proteins. There are some slight differences.
• There are ~ 12 different serum proteins, although β-lactoglobulin and α-lactalbumin are the most prevalent.

The following table summarizes the main proteins in milk

 Table 1. Main Proteins in Milk

 It is very important to notice that, from 33.g g/kg of protein in milk around 80% is casein. This has huge economical and functional implications in cheesemaking. While why proteins are a small fraction (<20%) they also have important implications in cheese making, specially when heat treatments (such as pasteurization or cooking) is involved. The group of proteins under miscellaneous (<1% of all milk proteins), can also have important implications specially related to ripening and spoilage of cheese.

Table 2. Some properties of caseins that are important to cheese making

 The different caseins arrange themselves into nanometric spheres known as casein micelles (Figure one on the right). Casein micelles “float” freely in milk, without sticking to each other because of the different properties of the micelles. We will study this in more detail later. While right know the properties of each individual casein might not mean much for some of you, later we will realize how important those properties are for milk and cheese making.

Table 3. Some properties of whey proteins that are important for cheese making. Various sources.​

As shown in table 3, most whey proteins are sensible to heat (they denaturate). That means that their structure and functionality change with heat treatment unlike caseins, which remain mostly unaltered by heat treatments. This property is very important for any dairy product that involves a heat treatment, which is most of all dairy products including cheese.