Glass Transition Temperature in Gelato — Tg' Explained
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If gelato that tasted right on day one turns icy by day five, the cause is almost always storage temperature relative to the glass transition temperature of the unfrozen serum. That single threshold decides whether ice crystals stay frozen or quietly grow.
A blast chiller running at −40 °C: cold enough to drive the serum into the glassy state before storage.
What Glass Transition Means in Frozen Desserts
Quick reference. Tg' is the temperature at which the unfrozen, sugar-rich serum in ice cream becomes a glass — non-crystalline but solid. For typical ice cream and gelato, Tg' sits between −32 °C and −34 °C.
Figure 1 — Where common storage temperatures sit relative to Tg' of typical ice-cream serum.
Glass transition is a thermodynamic concept that applies to amorphous (non-crystalline) materials. In a gelato mix, water freezes out into ice crystals as temperature drops, leaving behind an increasingly concentrated solution of sugars, MSNF and proteins. This residual serum eventually becomes so concentrated and viscous that it can no longer crystallise — instead it locks into a glassy, immobile state.
That locking transition is Tg' (read "T-g-prime"), the glass transition of the maximally freeze-concentrated serum. Goff and Hartel report a typical Tg' of −32 °C for commercial ice creams, with variations from −31 °C to −36 °C depending on sugar profile and dry matter. The lower the molecular weight of the dominant sugar, the lower Tg' moves.
Above Tg': Why Gelato Slowly Turns Icy
Storage at any temperature above Tg' leaves molecular motion in the serum. Water molecules can still migrate, and migration drives Ostwald ripening: small ice crystals melt at their high-curvature surfaces and the released water re-freezes on the surfaces of larger crystals. Over weeks, the average crystal size grows from 30 µm to over 80 µm — the threshold the tongue reads as "icy."
This is exactly the mechanism behind a gelato that goes icy after a few days in a home freezer. At −18 °C — a typical domestic setting — ice cream is roughly 14 °C above its Tg', and recrystallisation is fast.
Same recipe, same churn, different storage. The right-hand sample lived three weeks at −15 °C; the left, one week at −30 °C.
Below Tg': The Glassy State
Drop temperature below Tg' and the serum transitions into a true glass. Molecular motion essentially stops on human timescales — diffusion coefficients fall by orders of magnitude — and ice grain growth halts. Industrial frozen-dessert plants exploit this for shipping by hardening product at −35 °C to −40 °C and shipping in chambers held below −30 °C.
The relevant practical thresholds:
| Stage | Typical T | Position vs Tg' | Stability |
|---|---|---|---|
| Service in vetrina | −14 °C | far above | unstable, must turn over weekly |
| Home freezer | −18 °C | well above | days to weeks |
| Display freezer | −20 °C | above | slow degradation |
| Tg' boundary | ≈ −32 °C | — | transition |
| Bulk transport | −30 °C | borderline | usable for short windows |
| Hardening tunnel | −40 °C | below | indefinite (months) |
Practical Storage Rules for Pros
The professional implication is direct: any gelato you do not plan to sell within seven days should never sit at service temperature. Hardening immediately in an abbattitore or blast chiller drops the core below Tg' and arrests recrystallisation. Pulling pans from hardening to vetrina then becomes a one-way trip with a known shelf-life clock.
Three rules from Marshall, Goff and Hartel's "Ice Cream" (7th edition) hold reliably:
- Time above Tg' is cumulative — every hour at service temperature shortens shelf life.
- Temperature cycling (in and out of the freezer) accelerates recrystallisation faster than steady warm storage.
- The smaller the initial ice crystals, the more headroom you have above Tg' before the texture reads as icy.
Initial crystal size is set in the mantecatore — the colder the extraction temperature and the faster the dasher, the smaller the crystals.
How Sugar Choice Shifts Tg'
Tg' is sensitive to the molecular weight of the dominant sugar. Lower-MW sugars depress Tg' more:
| Sugar | MW (g/mol) | Approx Tg' shift vs sucrose |
|---|---|---|
| Fructose | 180 | −4 °C |
| Dextrose / glucose | 180 | −3 °C |
| Sucrose | 342 | baseline |
| Trehalose | 342 | +1 °C |
| Maltodextrin DE 20 | ~900 | +3 °C |
A sorbet sweetened mostly with fructose runs a Tg' near −37 °C and is much more demanding to hold — it must be stored colder to stay stable. A custard gelato sweetened with sucrose plus a touch of maltodextrin runs a higher Tg' (closer to −30 °C) and tolerates warmer storage better.
This is one reason artisanal Italian gelato is traditionally built on sucrose with limited dextrose — the sugar selection guide walks through the trade-offs in detail.
Tg' vs PAC — Two Different Concepts
People sometimes confuse glass transition with PAC (anti-freezing power). They are different.
PAC measures how much water stays unfrozen at a given temperature — it controls scoopability. Tg' measures the temperature at which the remaining unfrozen serum becomes a glass — it controls long-term stability. A recipe can have a perfect PAC for the vetrina at −14 °C and still degrade quickly if Tg' is unusually low.
Use PAC calculation for scoop hardness; use Tg' thinking for storage planning.
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