Water Activity and Plasticization - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 16

behavior. Iglesias and Chirife 39 studied crystallization kinetics of sucrose in amor-
phous food models in the presence of polysaccharides. Amorphous sucrose adsorbed
high amounts of water, which was lost rapidly. Materials with added polysaccharides
showed a fairly rapid water uptake. However, the loss of adsorbed water occurred
more slowly and the rate decreased with an increase in the polysaccharide content
suggesting an inhibitory effect on crystallization.

CHEMICAL AND MICROBIAL STABILITY

Kinetics of chemical reactions and quality changes is an important determinant of
food shelf life. The rate of a chemical reaction defines the change of concentration
at a given time. Extended shelf life can be based only on slow reaction rates and
manipulation of rate affecting factors to a desired level. The order of a chemical
reaction is defined by Eq. (1.10), which states that the change in concentration, C,
of a chemical compound (or quality factor) during a chemical reaction at time, t, is
defined by the initial concentration, the reaction rate constant, k, and the order of
the reaction, n.

dC
dt

kC n −= (1.10)

The reaction rate constant is a measure of the reactivity and defines the change
in concentration of the reactant or quality factor as a function of time.

T EMPERATURE D EPENDENCE OF R EACTION R ATES
AND Q UALITY C HANGES

An empirical approach in studies of temperature-dependent kinetics of reaction rates
and quality changes is determination of the rate, k T , at a temperature, T, and the rate,
k T+10 , at T + 10 which allows definition of the ratio of the rates known as the Q 10
value. The Q 10 value defines that an increase in temperature by 10° increases the
rate by the Q 10 factor.
The temperature dependence of reactions and changes affecting shelf life of
foods often follow the Arrhenius-type temperature dependence. The Arrhenius rela-
tionship is given in Eq. (1.11), where k is the rate constant, k 0 is the frequency factor,
E a is activation energy, R is the gas constant, and T is absolute temperature. The
Arrhenius relationship defines that a plot of k against 1/T gives a straight line with
the slope E a /R.

E
RT

a

−

kke = (1.11)

0

The Arrhenius relationship is probably the most important relationship used to
model temperature dependence of various quality changes in foods. However, there
are important factors that may result in deviation from the Arrhenius kinetics as well
as cause unexpected changes in product shelf life. According to Labuza and Riboh 71