phase is not yet present; nevertheless, a phase transition occurs readily, as is
explained in Section 14.2.2.
Transitionsinside a liquid phasecan be of various kinds. The whole
liquid can freeze, or—in the case of a solution—the liquid can separate into
a less concentrated solution and a phase made up by the solute. The latter
phase may be either solid (crystals), a liquid (droplets), or a gas (bubbles). A
liquid!liquid transition is rare in foods, except when it concerns aqueous
polymer solutions.
Although deposition of material—especially water—from agas phase,
be it as a liquid or a solid, frequently occurs in foods, it seems never to
involve a phase transition, since the other phase is already present.
Note A stable dispersion of small solid or liquid particles may also
show a kind of phase separation when conditions in the liquid are
changed in such a way that attractive forces between the particles
become dominant. A separation into a condensed phase (high
volume fraction of particles) and a very dilute dispersion would
then result. The interfacial tension between these phases is very
small, e.g., a fewmN?m^1. Conditions for this to occur are (a) that
the particles are about monodiperse and of identical shape and (b)
that the attractive forces do not become large (because that would
lead to fractal aggregation; see Section 13.2.3). Since these
conditions are rarely met in food systems, we will not further
discuss the phenomenon. Nevertheless, phenomena like depletion
flocculation (Section 12.3.3) show some resemblance to a phase
separation.Question
Would you consider the precipitation of protein from a solution, e.g., due to a
change in pH, a (first-order) phase transition?
Answer
No. Precipitation generally involves the formation of irregularly shaped aggregates,
and a layer of these aggregates forms due to sedimentation. A phase transition will
only occur if protein crystals are formed.