Week 9 Module: Phase Transitions in Cooking

Research question#

A youtiao (油条, fried dough stick) goes into hot oil as a soft, moist dough and comes out with a rigid, crisp shell around a still-soft interior. No new ingredient is added during frying. What physical change, exactly, is responsible for the shell being rigid when the interior a millimetre away is not?

Specimen youtiao (油条), fried dough stick, crust mid-formation

Where this sits#

Closing module of Stage 3, Transport, Kinetics and Transformation. Module 7 gave the transport mechanics (heat and moisture moving through a food object); Module 8 gave the kinetics of thermally-driven change. This module combines both to explain a moving internal boundary, and introduces the glass transition concept that Module 11 turns into a full design space.

The model: a moving moisture front and a glass transition#

As the dough fries, heat conducts inward (Module 7) while water at the surface evaporates rapidly into the surrounding oil. Water loss is fastest where the temperature is highest — near the surface — so a boundary forms and moves inward over time: outside it, the dough has lost most of its water and is hot; inside it, moisture content is still close to the raw dough’s. This is structurally the same “moving boundary” idea as the gelatinisation front in a thick dumpling wrapper, but driven by moisture loss rather than heat alone.

Losing water does more than dry the crust out — it changes which physical state the starch-water crust matrix occupies. An amorphous (non- crystalline) food solid has a glass transition temperature, T_g, below which it is a rigid, brittle “glass” and above which it is a soft, deformable “rubber.” Critically, T_g is not fixed: water acts as a plasticiser, and T_g depends strongly on how much moisture remains, in a relationship the Gordon-Taylor equation approximates:

T_g (mixture glass transition temperature, K)

the T_g of the dry-solid/water mixture at a given composition.

w₁, w₂ (mass fractions)

of dry solid and water respectively; w₁ + w₂ = 1.

T_g1, T_g2 (K)

glass transition temperatures of the pure dry solid and of pure water (water's own T_g is far below 0 °C).

K (Gordon-Taylor constant)

a material-specific fitting parameter reflecting how strongly the two components' free volumes mix.

As the crust loses water (w₂ falls), its T_g rises. Once the crust’s falling moisture content pushes T_g above the crust’s actual temperature, the crust vitrifies, moving from a rubbery amorphous state into a rigid glassy amorphous state — this is the instant it becomes crisp. The soft interior, still at high moisture, has a T_g far below its temperature and stays rubbery.

States

glass ⇌ rubber

Model

Gordon–Taylor

Plasticiser

water lowers T_g

What you measure and analyse#

Using a supplied moisture-vs-depth profile at a fixed frying time, and Gordon-Taylor parameters for a starch-based dough, you compute T_g at each depth and identify the depth at which T_g first exceeds the local temperature — locating the crust/core boundary from the model rather than by eye.

Activity#

Two youtiao are fried for the same total time, one in oil 15 °C hotter than the other. Using the moving-front and glass-transition framing above, predict which one has a thicker crust, and separate your reasoning into the transport effect (Module 7: how fast the temperature and moisture fronts move) and the thermodynamic effect (this module: at what moisture content T_g is crossed).

References#

• Gordon, M. & Taylor, J.S. (1952). “Ideal Copolymers and the Second-Order Transitions of Synthetic Rubbers. I. Non-Crystalline Copolymers.” Journal of Applied Chemistry 2(9), 493–500.
• Roos, Y.H. Phase Transitions in Foods, 2nd ed. Academic Press.