Freezing Point Depression in Gelato — The Science of PAC

Every spoonful of well-made gelato sits at a specific equilibrium: roughly 60–70 percent of the water in the mix is frozen, the rest stays liquid as a concentrated sugar syrup. That ratio is what defines texture. The physics that controls it is freezing point depression, and the recipe variable that tracks it is PAC.

The cream-like spoon resistance of -14 °C gelato is a precise balance of frozen water and concentrated sugar syrup.

What "Freezing Point Depression" Means

Quick reference. Adding sugar to water lowers its freezing point. The molar concentration matters more than the weight. PAC quantifies this for gelato.

Figure 1 — Freezing point of an aqueous solution drops as sugar concentration rises, with fructose and dextrose depressing it more than sucrose per gram.

Pure water freezes at 0 °C. Dissolve sugar in that water and the freezing point drops below zero. The amount of drop depends on how many solute molecules you dissolved per unit of water — chemists call this the molality of the solution, and Raoult's law plus the cryoscopic constant of water (1.86 °C·kg/mol) lets you predict it.

For a gelato maker, the practical consequence is this: in a mix at -14 °C in the showcase, the unfrozen serum has concentrated until its freezing point matches -14 °C. Any free water with a higher freezing point has already turned to ice. The unfrozen serum keeps the gelato deformable on the spoon. Without it, you get a brick of pure ice.

This is why an unsweetened cream simply freezes hard. There is not enough sugar in the system to keep a liquid phase at typical serving temperatures. Conversely, an over-sugared mix never hardens — too much liquid serum, no structural ice phase.

Why Sugar Type Matters

A mole of fructose depresses the freezing point exactly as much as a mole of sucrose. But fructose has a molar mass of 180 g/mol versus sucrose's 342 g/mol — almost half. So 100 g of fructose contains nearly twice the molecules of 100 g of sucrose, and therefore depresses the freezing point nearly twice as much per gram.

Two batches of the same recipe — left, balanced PAC 250 reads icy at -14 °C; right, PAC 280 reads creamy.

This is the entire reason gelato recipes care about dextrose, fructose, and inverted sugar versus plain sucrose. They are not just for "flavor" or "anti-crystallisation". They are powerful freezing point depressants per gram. The standard reference table:

These values come from molality-derived calculations published in Goff & Hartel's Ice Cream, 7th ed. (Springer 2013, Chapter 11). They are the basis of every PAC calculator in commercial use.

PAC Defined

PAC (Potere Anti-Congelante, "anti-freezing power") is a unit-free index. The convention sets sucrose at PAC = 100 and scales everything else proportionally. Multiply each ingredient's mass by its relative PAC, sum the contributions, divide by total mix weight × 10, and you have the recipe PAC.

A mix at PAC 240 means it has the equivalent freezing point depression of 24 percent sucrose by total weight. At PAC 280, equivalent to 28 percent sucrose. The Italian rule of thumb is linear: divide PAC by -20 for gelato (by -25 for sorbetto) to get the display temperature the mix is built for. PAC 240 suits a -12 °C case and PAC 280 a -14 °C case, so for a -14 °C showcase a milk gelato wants PAC around 280.

Every recipe is a balance equation: total solids, fat, MSNF, and PAC all moving together.

How PAC Shapes Texture

A 1 kg milk gelato mix typically contains 580 to 640 g of water. At PAC 270 and -14 °C, roughly 60–65 percent of that water is frozen — about 380 g of ice. The remaining 220 g of unfrozen water carries all the dissolved sugars, milk proteins, and stabilisers as a viscous concentrated syrup.

That unfrozen phase is what gives gelato its plasticity. Pull the recipe to PAC 320 and the frozen fraction at -14 °C drops to about 50 percent, leaving more serum. The result is too soft to scoop. Push to PAC 220 and frozen fraction rises to 75 percent — the gelato turns crumbly and icy because there is no longer enough serum to glue the ice crystals together.

This is part of why American ice cream, served colder at around -14 to -16 °C after storage at -18 to -20 °C, feels harder than gelato served at -11 to -13 °C even at similar sugar levels. Lower temperature freezes more water. The texture difference is largely a serving temperature decision.

Working PAC Targets by Style

Quick reference. For a display case at -11 to -14 °C, Italian milk gelato runs PAC 220 to 300 (a crema base at 240 to 260 suits -12 to -13 °C; chocolate and nut bases sit higher, up to 300) and sorbetto PAC 280 to 340. Divide PAC by -20 (gelato) or -25 (sorbetto) to estimate the display temperature. American ice cream is served colder, around -14 to -16 °C.

Within crema base, the choice between sucrose, dextrose, and inverted sugar tunes both PAC and POD (Potere Dolcificante, sweetening power) independently. A common Italian split: 100 g sucrose + 30 g dextrose + 25 g inverted sugar per kilo gives a balanced PAC of ~250 with POD ~165. Pure sucrose would need 180 g to deliver the same POD but only reach PAC 180.

Interaction with Other Solids

Sugars are the primary lever, but they are not alone. Lactose from milk (about 4.7 g per 100 g whole milk) contributes ~6 PAC per kilo of finished gelato. Salts from milk proteins contribute another 3–5. Alcohol — used in adult recipes like rum raisin or marsala gelato — is a powerful freezing point depressant; 1 g of ethanol depresses freezing point as much as about 7.4 g of sucrose, the ratio of their molar masses. Work in grams of ethanol, not in degrees on the label: a 40% ABV spirit is only about 33% ethanol by weight.

This is why a tiramisu gelato or rum raisin needs careful balancing. Swap 30 g of the water in a 1 kg mix for Marsala wine (Marsala DOC is 17–18% ABV minimum, about 14 g of ethanol and 11 g of sugar per 100 g) and you add roughly 37 PAC units. A recipe balanced for PAC 260 without the wine ends up at 297 with it. Not catastrophic, but enough to shift the spoon feel from firm to soft.

When PAC Goes Wrong

The two clearest symptoms of mis-balanced PAC are visible at the spoon. Under-PAC: the gelato reads coarse and icy, particularly at the rim of the pan where air exposure accelerates ice crystal growth. Over-PAC: the gelato weeps free liquid on the surface within 30 minutes of display, and a scoop slumps within 2 minutes at ambient temperature.

Standard correction matrices appear in every gelato handbook. Under by 10–20 PAC: add 5–10 g dextrose per kilo. Over by 10–20 PAC: reduce inverted sugar by 5–10 g, replace with sucrose at equal mass. Always recompute POD after the change — a switch from 10 g inverted sugar to 10 g sucrose drops POD slightly, which a trained palate may register as less sweet.

Related Concepts

• Water activity and shelf life
• PAC Calculator
• POD Calculator
• Sugar Selection Guide
• Sugar Substitution Calculator
• How to Balance a Gelato Recipe
• How to Balance a Sorbetto Recipe
• Custard Base Balance Guide
• Total Solids Calculator
• Pasteurization Deep Dive
• How to Make Professional Gelato
• Dextrose Monohydrate vs Anhydrous: The Hidden PAC Error
• Maltitol vs Sorbitol in Gelato
• Semifreddo vs Gelato: Structure, Sugar and Serving