Week 9: Phase Transitions in Cooking

What happens inside a youtiao as it deep-fries — a moving moisture front and a crust crossing its glass transition, closing Stage 3.

26 April 2027

Outline#

• Recap: Module 7 gave us transport, Module 8 gave us rate. This lecture’s transition is neither — it’s a material crossing a state boundary, at a rate set by how fast Module 7’s transport gets it there.
• Following a youtiao through frying as two zones with a moving boundary between them: an outer crust drying and vitrifying while a moist core lags behind, the boundary sweeping inward as frying continues.
• Defining the glass transition itself: not melting, not a chemical change, but an amorphous solid’s mobility collapsing (or unlocking) as it crosses a temperature- and moisture-dependent threshold.
• In-class calculation: given two youtiao crusts at the same temperature but different measured moisture contents, use the Gordon-Taylor equation to work out which one has already crossed T_g and vitrified — the two variables the model links, applied to a comparison rather than a definition.
• Closing Stage 3: transport, kinetics and phase transition are now all on the table as three coupled, separately-modelled processes. Stage 4 fits models to data using exactly these three, then designs with them.