What a Casein Micelle Is — The Gelato Textbook

Casein, the chief of the milk proteins, does not drift as a lone molecule. It floats through milk as an assembly called a micelle. Know that structure and it becomes clear why casein sets with acid and holds out against heat.

Advanced Chapter 3 set casein as one of the two main milk proteins. It coagulates with acid when the pH falls toward 4.5, and that gives sandiness. Advanced Chapter 4 counted that acid coagulation among the constraints on the design of a formulation. But what does casein actually look like? Why does it set with acid, and hold out against the heat of pasteurization? The answer is that casein builds itself into an assembly called a micelle.

Casein does not drift alone

Casein makes up some eight tenths of the milk proteins. But in milk it is not dissolved molecule by molecule. Several kinds of casein — named α_s, β, κ and so on — gather into one particle. It measures about 150 nanometers in mean diameter, and it is the casein micelle (micella caseinica).

There is one more storey in how it is built. On the picture most widely accepted, casein molecules first form small lumps of about 8 to 10 nanometers across, called submicelles. Those submicelles are then bound to one another through clusters of calcium phosphate, and the 150-nanometer micelle is the result. Molecule → submicelle → micelle: two storeys. Take the calcium away and the micelle falls apart into submicelles — which is what bears out the role of that glue.

This size settles how milk looks. The micelle sits at the top end of the size called colloidal (Advanced Chapter 1), and it scatters light. Milk and a mix look white and cloudy because countless casein micelles are scattering the light — nothing more than that.

Fig. L3-9-1　The casein micelle, in outline. Inside is a gathering of casein and calcium phosphate; the surface is covered by water-loving hairs of κ-casein. These hairs raise the repulsion between micelles and keep the dispersion.

The hairs of κ-casein guard the dispersion

Why do the micelles not stick to one another, but stay dispersed in the milk? The key is the surface of the micelle. Of the several caseins, κ-casein covers that surface, pushing its water-loving part out toward the water. Those chains standing out make what may be called a layer of hairs.

The stability comes clear if the micelle is taken as a charged colloidal particle. Between particles, two forces act at once. A van der Waals attraction pulls them together; an electrostatic repulsion pushes them apart, because their charges share a sign. Draw the sum of the two against distance, and a hill appears partway — an energy barrier. Unless that hill can be crossed, the particles cannot come close and gather.

At the pH of milk, around 6.6, the casein micelle carries a negative charge overall. That charge holds the hill of repulsion high and keeps the micelles a set distance apart. The water-loving tail of κ-casein carries this charge, and it also keeps its neighbors off by sheer bulk. Milk is stable as a colloid because of this surface. The emulsifiers and stabilizers of Advanced Chapter 4 work at the boundary. In the same way, κ-casein is a player that guards dispersion at the boundary.

Why it sets with acid

With that in hand, the reason for acid coagulation is plain. The stability of the micelle rested on the hill of repulsion raised by surface charge. But when sharply acid fruit brings the pH of the mix down, that negative charge is canceled away. Less charge, lower hill. And when the pH reaches the isoelectric point of casein, around 4.6, the surface charge is effectively gone and the hill with it. Nothing holds the micelles back now: they draw close as the van der Waals attraction pulls, and they gather. That is acid coagulation.

So acid coagulation is not a matter of breaking the micelles. It is a matter of taking away the hill of repulsion that held them apart. Casein that has gathered, met by the tongue, reads as grit — as sandiness. Advanced Chapter 3 said that sharply acid fruit put with milk can turn gritty; this is what that is, at the scale of molecules.

Fig. L3-9-2　How acid coagulation works. As the pH falls, the negative surface charge is canceled, the repulsion that held the micelles apart is lost, and they gather. This is what the grit in a sharply acid mix is.

Why it holds out against heat

By contrast, the casein micelle is remarkably strong against heat. Advanced Chapter 3 said that of the milk proteins, the whey proteins denature with heat. Casein, for its part, does not denature in the range of pasteurization temperatures. That difference too comes from structure.

β-lactoglobulin, the chief of the whey proteins, holds a neatly folded three-dimensional structure of β-sheets and α-helices. Inside it there is a free SH group, left over without a partner. When heat unfolds the structure, that free SH group is exposed and begins to throw S−S bonds across to neighboring molecules. Molecule sewn to molecule becomes a net — and that is what coagulation by heat is.

Casein, on the other hand, is a gathering of soft chains with no set fold to begin with. It has no structure that heat can unfold and break. In the range of pasteurization temperatures, then, the casein micelle does not in the main coagulate. What pulls the trigger is not heat but acid.

Holding gelato up from within

The casein micelle is not merely a particle that turns things white. It holds water and lends the mix viscosity. It settles on the boundaries of fat globules and air bubbles, steadying the emulsion and the foam (Approfondimento 5 and 8). And as a milk protein it builds the frame of the gelato (Advanced Chapter 3). The stabilizers and emulsifiers of Advanced Chapter 4 are guards of the boundary "added from outside". The casein micelle is the guard of the boundary that milk "holds from the start". The two work together at the boundary, and the structure of gelato is kept.