Physical Chemistry of Foods

other hand,k 15 k 2 , any U that is formed will almost immediately be
converted into I, and½UŠ&0, again leading to a simple first-order reaction,
now with rate constantk 1.
For the reactions just considered, k 1 is likely to be far more
temperature dependent thank 2. An example is shown in Figure 4.6 for
the thermal inactivation of plasmin, a proteolytic enzyme. At low
temperature, the unfolding reaction N?U is very slow, and the second
reaction, albeit slow, is much faster than the unfolding. Consequently, the
overall reaction rate is determined by the unfolding, the rate constant of
which strongly depends on temperature (see Section 4.3.3). At high
temperature, unfolding is extremely fast, and the rate will be determined
by U?I, which is far less temperature dependent. At intermediate
temperatures, the rate constant given is a pseudo-first-order rate constant,
and more elaborate kinetic studies would be needed to describe properly the
overall reaction.
From this example, we can draw a few general conclusions. The first is
aboutuncoupling. At relatively high temperature, a high concentration of U
is formed, whereas at lower temperatures this is not so. In the more general
case of A?B?C, the ratio of the concentrations of the products B and C
formed will vary with temperature, if the consecutive reactions exhibit
different temperature dependencies. An example is potatoes becoming sweet
when storing them near 0 8 C. Broadly speaking, potatoes exhibit two
consecutive reactions: the hydrolysis of starch, leading to the formation of
sugar, and the conversion of sugar into CO 2 and H 2 O by respiration. At
room temperature, the latter reaction is the fastest, leaving the sugar
concentration low. At low temperature, both reactions are slowed down, but
the respiration more so than the hydrolysis, and sugar accumulates.
Numerous other examples could be given.
The second point concernschanging reaction order. The order may
change in the course of the reaction, for instance because one of the
reactants or intermediates becomes consumed, thereby leading to a different
mix of products and another reaction step dominating the order. This may
make it difficult to predict the extent of a reaction after various reaction
times from only a few analytical data, the more so since the relation may
vary with temperature.
This brings us to the third point, i.e., there isnot a single activation
energyfor the temperature dependence of a sequence of reactions. Again,
the curve for inactivation of plasmin (EC 3.4.21.7) in Figure 4.6 is a good
example, since such a situation is fairly general, although mostly not as
extreme. It may also be noted that the accuracy of the Arrhenius relation
becomes questionable if the activation energy is small: the rate equation has
Tin the preexponential factor, andEadiffers byRTfrom the activation