Note Assuming DS{ to be negligible, this also yields
DH{&136 kJ?mol^1. Cf. Table 4.2.
These examples may suffice to illustrate that we may have very slow
reactions in foods. Here a brief summary is given of the possible causes for
slow reactions.
- The (effective) concentration of reactants is small, which is
especially important in second-order reactions. It may be due to
small total concentrations, to compartmentalization or immobi-
lization, or to a complicated cascade of reactions with several
‘‘side-tracks’’ that consume reactants for other reactions. - The activity coefficient(s) may be small. See Section 2.2.5.
- The activation free energy is large. If this means that also the
activation enthalpy is large, as will often be the case, the reaction
rate is strongly temperature dependent. - A suitable catalyst is missing or unavailable, or inhibiting
substances are present. - Diffusion is very slow, because of a very high viscosity. This is
often the case in low-moisture products. See also Section 8.4.2.
4.5 RECAPITULATION
Most foods are not in thermodynamic equilibrium. For several possible
reactions, the reaction free energy is large and negative, suggesting that the
reaction would be fully completed in a very short time, while nevertheless
the reaction proceeds slowly. The composition of a reaction mixture then is
not thermodynamically controlled but kinetically. In some cases, a steady
state rather than an equilibrium state is attained, and it may be useful to
distinguish these.
Reaction kinetics, or at least the mathematical formulas describing it,
depend on the order of the reaction. Orders of 0, 1, and 2 are mostly
considered. Reaction order, however, is an empirical number, mostly not
equal to the molecularity of the reaction. It has to be determined
experimentally, and noninteger values may be observed; moreover, order
may change in the course of the reaction or may vary with conditions such
as temperature.
The classicalrate theorydue to Arrhenius proceeds on the Maxwell–
Boltzmann distribution of the velocity, and thereby the kinetic energy, of
molecules or particles; their average kinetic energy equalsð 3 = 2 ÞkBT.Iftwo
molecules collide with a kinetic energy larger than anactivation energy Ea
for a reaction between them to proceed, they are assumed to react. The