Physical Chemistry of Foods

different from those in a heat exchanger, for instance the times needed

for heating up and for cooling may differ. (c) Occasionally a greater

contamination may occur.

This implies that a producer will mostly remain on the safe side,

and heat for e.g. 35 s. However, in many products longer heating

impairs quality, for instance flavor, and a compromise may be needed.

4.2 CHEMICAL EQUILIBRIUM

In principle, any chemical reaction isreversible. Thus if we have A?B, we
also have B?A. The first reaction may have a rate constantk 1 , the reverse
onek
1. We therefore have for the rate at which A is transformed

d½AŠ

dt

¼k 1 ½AŠ
k
 1 ½BŠ¼



k 1 
k
 1

½BŠ

½AŠ



½AŠð 4 :5aÞ

If we start with A only, it will be changed into B, but the apparent rate
constant (the factor between parentheses) will become ever smaller, since [B]
increases. When the right hand side of [4.5a] has become zero, i.e., at infinite
time, equilibrium is obtained, and it follows that the equilibrium constant,
K:½BŠ?=½AŠ?, equalsk 1 =k
1. Equation (4.5a) yields upon integration and
some rearrangement

½AŠ
½AŠ?

½AŠ 0 
½AŠ?

¼expð
ktÞð 4 :5bÞ

where the rate constantk¼k 1 þk
1. A good example of such a reaction is
the ‘‘mutaroration’’ of reducing sugars like glucose and lactose, i.e., the
transmutation of theaanomer into thebanomer and vice versa.
Referring to Section 2.2, we observe that at equilibrium the chemical
potentials of A and B must be equal. This leads to

m^7 AþRTlnaA¼m^7 BþRTlnaB

and, consequently, to

aA

aB

¼exp



m^7 A
m^7 B

RT



¼exp



DG

RT



ð 4 : 6 Þ

where, of course,aA=aB¼ 1 =K:DGis the standard free energy (per mole)
for the transition of A to B. Since DG¼DH
TDS, we have two
contributions.
DH&U, the net molar bond energy, values of which are
tabulated in reference books. Generally,DSis made up of two terms. The