Water Activity and Plasticization - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 23

contents below the BET monolayer value and probably also below T g . The retention
was high until the water content reached its maximum value. At higher relative
humidities, the amount of adsorbed water decreased and the rate of the volatile loss
increased. The results showed that crystallization of amorphous lactose resulted in
loss of both adsorbed water and encapsulated volatiles. It is obvious that the crys-
talline structure was not able to entrap volatile compounds.
The effect of glass transition on the rate of oxidation of methyl linoleate that
was encapsulated in an amorphous lactose matrix was studied by Shimada et al. 94
They observed that oxidation did not occur in encapsulated methyl linoleate. Lactose
crystallization was observed at temperatures above T g and the rate increased with
increasing T - T g . Shimada et al. 94 did not observe oxidation above T g until crystal-
lization released the encapsulated compound. Methyl linoleate that was released
from amorphous lactose became accessible to atmospheric oxygen and oxidized
rapidly. It is obvious that nonencapsulated lipids are susceptible to oxidation in low-
moisture foods. Encapsulated lipids in foods may become protected from oxidation,
but crystallization of the encapsulating matrix releases such compounds and causes
rapid deterioration.

B IOCHEMICAL S TABILITY

Enzyme activity has been found to be related to hydration. 95,96 At low water activities
enzymatic activity is generally not observed, as water cannot enhance diffusion of
substrates to enzyme molecules. 95 The water activity dependence applies both to
hydrolases and oxidases, unless the substrates are non-aqueous liquids allowing
changes to occur at low a w . 95 Obviously, enzymatic activity depends on diffusion of
substrates and products as well as enzyme molecules and it may depend on the
physical state of the material as well as a w . It has also been found that the limiting
a w for enzyme activity may decrease with an increase in temperature, probably
because of an increase in molecular mobility. 97
A change in heat capacity and an increase in motional freedom of enzyme
molecules have coincided with the onset of enzyme activity, 98 which suggests that
a relationship may exist between enzyme activity and glass transition. Silver and
Karel 99 studied the effect of water activity on sucrose inversion by invertase. The
rate of the reaction increased with increasing a w and the rate followed first-order
kinetics. The samples were freeze-dried and the sucrose was likely to exist in the
amorphous state. This was also noticed from the fact that the onset of hydrolysis
occurred at water activities below the suggested mobilization point of 0.81 a w for
crystalline sucrose. 78 Silver and Karel 99 observed a continuous decrease in the acti-
vation energy with increasing a w , which was concluded to suggest that the reaction
was diffusion-controlled. Drapron 97 stated that not only a w , but also the ability of
water to give a certain mobility to enzymes and substrates, is important to enzyme
activity. He assumed that the amount of water needed increases with increasing
molecular size due to impaired diffusion. However, lipase activity was not related
to the mobility provided by water. Interestingly, Drapron 97 pointed out that in

-amy-
lolysis the a w at which the reaction started was lower at 30°C than at 20°C. He
assumed that the mobility of the components increased with temperature.

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