Milk Structure

Structural elements of milkRaw milk quality provided an introduction to milk chemistry. Now we look briefly at milk physics to help understand how milk coagulation works. Refer to the figure on the right and review the following facts:

  • Milk is an emulsion with fat particles (globules) dispersed in an aqueous (watery) environment.
  • The fat globules do not coalesce and form a separate layer (oil off or churn) because they are protected by a membrane layer which keeps the fat particles separate from the water phase.
  • The principal group of milk proteins, the caseins, are not soluble in water and exist in milk as small particles (<300 nm) called micelles.

We can now define the following terms:

Milk: a dispersion of milk fat globules (fat particles) and casein micelles (protein particles) in a continuous phase of water, sugar (lactose), whey proteins, and minerals. 

Milk Plasma: what is left after you separate the fat globules; equivalent to skim milk for practical purposes.

Milk Serum: what is left after you take away both fat globules and casein micelles; equivalent to cheese whey for most practical purposes

Milk permeate: what is left after you take away fat globules, casein micelles, and whey proteins.

Coagulation is what happens when the casein micelles stick together. Because casein particles are hydrophobic (they hate water) their natural tendency is to aggregate (clump together). In normal milk, aggregation is prevented by two factors. If one of these factors is eliminated the micelles will aggregate and form a gel something like jello.

  • The first stabilizing factor is a 'hairy' layer of surface active protein, called kappa-casein (-casein), on the surface of the micelle. This layer helps prevent the micelles from getting close enough to stick together.
  • The second factor is a negative charge on the micelles. At the pH of milk the micelles are negatively charged so they repel each other.

So, basically there are two ways to coagulate milk; one is to remove the hairy layer from the micelles. That's called enzymic coagulation. The other is to neutralize the negative charge on the micelle. That can be accomplished by acidification or a combination of high temperature and acidification.

Acid coagulation

Acid milk gels can be formed by lactic bacteria or the use of acidifying agents such as glucono-delta-lactone (GDL is slowly hydrolysed to gluconic acid in the presence of water). Acid coagulation is used in the production of cottage cheese, bakers cheese and quark as well as other fermented milk products such as yoghurt, commercial butter milk, kefir etc. In the case of cottage cheese and quark a small amount of chymosin may be used (2 ml/1,000 hl) to make the curd more elastic and less subject to breakage (dusting).

Heat-Acid coagulation

This process permits recovery of caseins and whey proteins in a single step. The basic principle is that whey proteins which are normally acid stable, become sensitive to acid coagulation after heat treatment. This principle is exploited in the manufacture of ricotta cheese, Paneer and Channa, and in the manufacture of "co-precipitated" milk protein concentrates. The basic process for heat-acid coagulation is:

  • Heat milk or milk-whey blends to at least 80C for at least five minutes to completely denature (unfold) the whey proteins and encourage association of whey proteins with casein micelles.
  • Continue heating and acidify slowly with gentle agitation. The caseins and whey proteins will coagulate together and form either sinking or floating curds.