entropy (this is further discussed in Section 7.2). Consequently, the reaction
can proceed despite the very large activation enthalpy, which, on the other
hand, causes the reaction rate to be extremely dependent on temperature.
According to Table 3.1, breaking of a covalent bond would typically
take 200kBTat room temperature, equivalent to 500 kJ?mol^1. If breaking
of such a bond would be prerequisite for a given reaction to proceed, the
activation energy would roughly equal that quantity. In such a case, any
compensation by an increase in entropy (conformational or contact
entropy) would be small, and the activation free energy would be at least
450 kJ?mol^1. That implies that such a reaction would never occur, unless
the temperature is extremely high. In the present case, we calculate
k& 10 ^68 s^1 at room temperature andk& 10 ^5 s^1 at 1000C. In other
words, ‘‘simple’’ chemical reactions (i.e., those that do not involve a large
activation entropy) that occur at ordinary temperatures, at a perceptible
rate, must have a fairly small activation enthalpy, and their rate cannot be
strongly temperature dependent. For most of themQ 10 is 2 to 3, whereas it
ranges from 10 to 150 for protein denaturation. Some examples are given in
Table 4.2.
This difference has important practical implications for the food
technologist. Many foods are heat-treated to ensure microbiological safety
and to enhance keeping quality. In nearly all cases, the desired properties are
the result of heat inactivation of enzymes: enzymes may themselves cause
spoilage, but they are also essential for microbial (and all other) life,
implying that irreversible inactivation of some of their enzymes kills the
TABLE4.2 Typical Examples of the Temperature Dependency of Reactions
Type of reaction DH{ðkJ?mol^1 Þa Q 10 at 100 8 CMany chemical reactions 80–125 2–3
Many enzymatic reactions 40–60 1.4–1.7
Hydrolysis, e.g. 60 1.7
Lipid autoxidation 40–100 1.4–2.4
Maillard reactions 100–180 *2.4
Protein denaturation 150–600 4–200
Killing of microbes 200–600 6–175
Killing of bacterial spores 250–330 9–17
aIn most cases an apparent average activation enthalpy, since it concerns a number of
consecutive reactions.