proportion of collisions of sufficient energy increases with T, and the
reaction rate would be proportional to expðEa=RTÞ. This is a useful
relation to describe temperature dependence, but it is insufficient in other
respects; for instance, it is difficult to fit into reactions of order
(molecularity) unity. The theory of theactivated complexproceeds on the
basis of an activation free energy, i.e., including an entropy term. It often
allows us to make quantitative predictions of the reaction rate. The
denaturation of proteins, which is mostly at the root of, and rate determining
for, the inactivation of enzymes and the killing of microorganisms, is
extremely temperature dependent, because of the very large activation
enthalpyð&EaÞ; the also very large positive activation entropy then causes
the reaction to proceed at a measurable rate. Most ‘‘simple’’ chemical
reactions occurring in foods have a much weaker temperature dependency.
Chemical reaction kinetics proceeds on the (often implicit) assumption
that the reaction mixture is ideally mixed, and does not consider the time
needed for reacting species to encounter each other by diffusion. The
encounter rate follows from the theory ofSmoluchowski. It turns out that
most reactions in fairly dilute solutions follow ‘‘chemical’’ kinetics, but that
reactions in low-moisture foods may be diffusion controlled. In the
Bodenstein approximation, the Smoluchowski theory is combined with a
limitation caused by an activation free energy. Unfortunately, the theory
contains several uncertainties and unwarranted presumptions.
Several furthercomplicationsmay arise. A number of consecutive
reactions is very common in foods, and then kinetics may become very
complicated. There may be a whole cascade of reactions and moreover some
components formed may react in various ways, causing the reaction scheme
to be branched. In such cases, uncoupling often occurs, i.e., the reaction
mixture obtained (relative proportion of reaction products) depends on
conditions like temperature. Several reactions can be catalyzed, notably by
enzymes, and enzyme activity strongly depends on conditions like
temperature and pH. Inhibitors, e.g., of enzymes may further complicate
matters. In many foods, reactants, catalysts, or inhibitors are compartmen-
talized, which often causes a decrease in reaction rate.
BIBLIOGRAPHY
The textbooks mentioned in Chapter 1 all give information on chemical kinetics.
Especially the text by P. W. Atkins discusses the topic in detail. Specialized books are
K. J. Laidler. Chemical Kinetics. Harper and Row, New York, 1987.
and
H. Maskill. The Physical Basis of Organic Chemistry. Oxford Univ. Press, 1985.