enthalpy. A small apparent activation energy may result if the first of two
consecutive elementary reactions is in fact reversible.
In foods, we often have what may be calledreaction cascades, i.e., a
whole series of reactions, partly consecutive, partly parallel, with bifurca-
tions and with more than one reaction pathway leading to the same product.
Examples are nonenzymatic browning or Maillard reactions, as well as
several changes occurring during heat treatment. Chain reactions may be
involved as well, as in the formation of hydroperoxides during the
autoxidation of fats:
ROO?þRH?ROOHþR?
R?þO 2 ?ROO?Here? denotes a radical.
Simple reaction kinetics will never suffice, but one can try to elucidate
the various elementary reactions and then set up all the rate equations
(differential equations) and solve the whole set numerically. In fact, this
often is the only way to determine the reaction scheme with any confidence,
since only quantitative agreement between calculated and observed product
concentrations as a function of time guarantees its correctness. It is far
beyond the scope of this book to discuss even a simple example.
Catalysis. Many reactions are catalyzed, i.e., increased in rate, by a
compound in solution (homogeneous catalysis) or a group at the surface of
a particle (heterogeneous catalysis), where the catalyst is not consumed
itself. Examples are various hydrolyzing reactions, like the ester hydrolysis
mentioned above, that are catalyzed by Hþas well as OHions. In such a
case the reaction rate greatly depends on pH, though the ions themselves do
not appear as reactants in the overall reaction scheme. Ubiquitous in natural
foods are enzyme-catalyzed reactions. The simplest case leads to Michaelis–
Menten kinetics, but several complications may arise.
Negative catalysis may also occur, since several compounds are known
thatinhibitreactions. For example, some cations, notably Cu^2 þ, catalyze the
autoxidation of lipids; chelating agents like citrate may greatly lower the
activity of divalent cations, thereby decreasing the oxidation rate. Inhibition
of enzymes is frequently observed.
Compartmentalization. Be it of one or more reactants or of a
catalyst, compartmentalization often occurs in foods. A simple case is
oxidation, e.g., of unsaturated lipids in several foods, where the oxygen