Milk Solids

Milk solids divide in two

The solids that come from milk — the milk solids — divide first in two. On one side stands fat; on the other, everything else taken together — the milk solids-not-fat (solidi del latte non grassi, MSNF). The MSNF is three things: protein, lactose, and minerals.

In a formulation — the amounts and proportions of the mix — the two are handled on separate scales. Fat mainly carries the feel in the mouth and the melt; MSNF mainly carries the frame and the holding of air. Different jobs, so the amounts are decided separately.

Fat — the carrier of corpo and melt

Milk fat gives gelato corpo and resistance to melting, and it carries flavor compounds across the tongue. Its melting point is lower than that of most other fats. It melts smoothly near body temperature, and that maps directly onto the melt in the mouth, and onto easy digestion.

Inside the mix, as Chapter 1 showed, the fat exists as an emulsion of fine fat globules. Each globule is wrapped in a membrane of lipoproteins; the membranes repel one another, and the dispersion holds. The smaller the globules, the more of them there are and the better the membranes work — which is why homogenization matters.

In the cold of aging (maturazione), part of the fat inside each globule crystallizes. Not all of it hardens; much of the interior stays liquid. This partly solidified state improves whipping, and it lays the ground for holding air bubbles stably through the mantecazione that follows. The mechanical shear of freezing with agitation then partly breaks the emulsion. The exposed fat builds a network around the air bubbles — the structure of gelato, supported from within. This is exactly where the emulsifiers do their work (Chapter 4).

Fat has a ceiling. Past about 8 percent of the formulation (some say 10–12), the fat phase separates more easily when homogenization is not thorough. The outer frame of "up to 12 %" in the table of Chapter 5 assumes that homogenization holds this separation down.

There is a second cost, and it works on the structure itself. Too much fat lets the captured air escape. A moment ago, the exposed fat stood on the supporting side, building its network around the bubbles. At high proportions, the same fat is thought to switch sides. It changes the surface tension of the mix, breaks the walls of the bubbles, and releases air already taken in. Overrun stops growing, and what growth remains stops serving quality. Fat that clusters to excess, and unevenly, also takes longer to melt away in the mouth. The supporter, in excess, becomes the obstacle.

Fat is also the most expensive of the raw materials, and its calories are about 2.25 times those of protein or carbohydrate. In a frozen dessert eaten for its coolness, excess fat lets the heaviness arrive ahead of the satisfaction, and pushes the next bite away. It is not a material where more is better.

Milk solids-not-fat — building the frame

The lead actor in MSNF is the protein. Milk proteins bind water strongly and give the mix viscosity; they help the intake of air during freezing (overrun), and they stabilize the emulsion. The "frame" of gelato comes from here.

Milk proteins divide into two broad kinds. One is casein, present in milk as calcium caseinate, which coagulates with acid when the pH falls to around 4.5. In a strongly acidic mix — one built around sharply acid fruit — the clumped casein can be a cause of sandiness. The other kind is the whey proteins (lactalbumin and lactoglobulin), and these denature with heat. That is why they concern the design of pasteurization temperatures (Chapter 6).

Approfondimento — deep diveWhat is casein? — The micelle and how acid sets it↓ Go deeper

Egg proteins work inside the same frame. They emulsify, stabilize, bind, and foam — a many-sided function that complements the milk proteins in crema formulations.

The ceiling called lactose

Raise the MSNF and the frame grows stronger — but a constraint is built in. MSNF always brings lactose with it. Lactose is the only sugar in milk, and it makes up about half of the MSNF (more than 50 % in skim milk powder).

Lactose dissolves poorly. At 15 °C, one part of lactose needs six parts of water. Under the same conditions two parts of sucrose dissolve in one part of water, so lactose dissolves about one twelfth as well. Push the MSNF too high, and this reluctant sugar passes saturation in the unfrozen phase, then grows into coarse crystals during storage. The result is the defect felt as sand on the tongue — sandiness (sabbiosità), one of the structural defects listed in Chapter 1. Most of it comes from this excess crystallization of lactose.

The ceiling on MSNF therefore does not come from the demands of the frame. It comes from elsewhere — and there are two of them. One is the solubility of lactose, just seen. The other is that skim milk powder itself has a limit to how much water it can hold. Whichever of the two is stricter becomes the ceiling of that formulation.

What matters is that neither is a fixed number. The ceilings move with how much water the formulation leaves. So they are calculated, every time — the procedure is the business of Chapter 5. In practice the MSNF most often lands within about 8 to 12 percent of the total weight. But that is the outcome in typical formulations, not the ceiling itself. Where more solids are wanted regardless, there is the lactose-hydrolyzed product, with most of its lactose converted by enzymes to dextrose and galactose. It adds frame without the risk of sandiness.

And both of these are ceilings. MSNF also has a floor.

With too little, the protein cannot hold the water that needs holding. The water left unbound stays free, freezes, and becomes ice crystals. The mouth then meets a rough, gritty feel — the same "sandiness" as the excess side, with a different substance behind it. Excess builds crystals of lactose; shortage builds crystals of ice.

MSNF is pinched from above and below. Above runs the line where lactose fails to dissolve and crystallizes. Below runs the line where water goes unheld and ice grows. Fall out on either side, and the symptom reads the same — grittiness on the tongue. So when sandiness appears, the first question is whether the MSNF is high or low (Chapter 13).

Approfondimento — deep diveWhy does sandiness happen? — Supersaturation and the crystallization of lactose↓ Go deeper

Where the dairy ingredients stand

The same "adding milk solids" divides differently between fat and MSNF, depending on the ingredient chosen. The representative ingredients, sorted on those two axes:

What matters is that the ingredients allow fat and MSNF to be added separately. More resistance wanted without raising fat — skim milk powder or condensed skim milk. Fat wanted without adding water — anhydrous milk fat. How this distribution is assembled as numbers is the business of Chapter 5.

One consequence follows. A formulation whose only added fat is butter grows thin in MSNF. In the making of butter, the protein and the lactose leave with the whey. That is what the butter row means by "almost no protein or lactose remains". So the literature says: where butter is the only source of fat, make up the loss with skim milk powder. Fat was being added, and the frame was being lost — here is the road that breaks through the floor from below.