14.4 FORMATION OF A GAS PHASE
Liquid–gas transitions are common during food processing or storage, but
they often occur at the boundary of a condensed phase, as mentioned in
Section 14.1. In some situations, a gas phase forms inside a liquid or
solidlike material. Examples are the formation of CO 2 bubbles in beer after
the release of the pressure in the bottle or can; slow formation of CO 2
bubbles in a bread dough due to sugar fermentation by yeast cells; the
formation of ‘‘eyes’’ in some types of cheese by a similar mechanism; and the
change of a whipping cream that is under high pressure in an aerosol can
into a foam when the cream leaves the can through a nozzle.
It may be useful to recall a few relations about gases. The first is
Henry’s lawabout the equilibrium distribution of a substance over gas and
liquid phases. It reads
pA¼kHmA ð 14 : 17 ÞwherepAis the partial pressure of component A in the gas phase andmAits
mole fraction in the liquid phase. The magnitude of the Henry constantkH
depends for every A on the solvent (liquid phase) and on the temperature. It
is assumed that the solute behavior is ideal, i.e., the activity of A in the liquid
phase equalsmA. We further recall theideal gas law,pV¼nRT. In the
following, we assume ideal behavior; Eq. (14.17) then implies that the
solubility of a gas, say air, in a liquid is proportional to its pressure. The
solubility of air and most other gases decreases with increasing temperature.
For N 2 O and CO 2 , the gas law does not hold precisely at high pressures.
Moreover, CO 2 can form H 2 CO 3 upon dissolution in water, and this may
dissociate and form salts, like CaCO 3 ; in other words, its solute behaviour
may be far from ideal.
Nucleationof gas bubbles is notoriously difficult, and the following
calculation may explain it. Assume that a gas embryo of 2 nm radius is
formed in a liquid at atmospheric pressure. The interfacial tension will
generally be about 0:07 N?m^1. The Laplace pressure in the embryo will
then be about 2 60 : 07 = 2? 10 ^9 ¼ 7? 107 Pa, which equals 700 bar. The
supersaturation ratio of the gas should then be about 700 for such a small
bubble to survive, and that is very unlikely to be the case. In a beer bottle the
pressure is a few bar, in an aerosol can with N 2 O about 7 bar. Moreover, the
number of gas molecules inside the embryo would be about 560 (try to make
the calculation), more than could possibly associate by chance. Homo-
geneous nucleation can therefore not occur. By similar reasoning, it can be
derived that heterogeneous nucleation on a surface, as depicted in Figure
14.5b, frames 1 and 2, is not possible either, even if yis quite small.