Physical Chemistry of Foods

2.2.5 Deviations from Ideality

In foods, we often have situations in which concentrations are markedly
different from activities, or in other words, the activity coefficients may be
far from unity. This may have important consequences for partition
equilibria, for reaction equilibria, and often also for reaction rates. Below
some important causes for deviations from ideality are listed.

1. Not all species are reactive. This is theoretically fairly trivial, but
   the practical implications may be considerable. An example is the presence
   of a reducing sugar, for instanceD-glucose, in various forms. Here we have

a-glucoseÐopen chain formÐb-glucose

and only in the open-chain form, which may be less than 1%of the glucose
present, can the sugar participate in Maillard reactions or other reactions
involving the aldehyde group. Another example is an organic acid, here
denoted by HAc, which dissociates according to

HAcÐHþþAc

The dissociated form Ac
can react with cations, whereas the undissociated
form HAc may be active as an antimicrobial agent. The activities of each
depend not only on the overall concentration but also on the dissociation
constant (which depends on temperature), the pH, the presence and
concentration of various cations, etc. It may be argued in these cases not
that the activity coefficient is (much) smaller than unity but that we should
take the concentration of the species involved in the reaction only. The
result is, of course, the same, and we may speak of an apparent activity
coefficient.

2. High concentration. At high concentrations of a solute, its activity
   coefficient nearly always deviates from unity. This may be for two reasons.
   First, the solvent quality affects the activity coefficient, and the effect
   increases with increasing solute concentration. Solvent quality depends on
   the interaction energy between solute and solvent molecules; this is further
   discussed in Section 3.2. A poor solvent tends to increase and a good solvent
   to decrease the activity coefficient of the solute (and thereby, for instance, to
   increase its solubility).
   Second,volume exclusionoccurs, which always causes an increase in
   the activity of a solute if the solute molecules are larger than the solvent
   molecules. At high concentration the amount of solvent available to the
   solute is effectively less than the nominal amount, which means that the
   solute concentration is effectively higher. This is easiest envisaged for
   spherical molecules of radiusr; such a molecule takes up a volume equal to