Water in Dairy Products - Advanced Dairy Chemistry - page 502

dependence. Moreover, based on the ''universal coefficients'', WLF kinetics
predicted a more rapid increase in crystallization rate with temperature than
was actually observed (Hartel, 1998). The same observation had been
reported (Simatos and Blond, 1991, 1993) concerning the crystal growth
rate in frozen beef, as well as other processes of deterioration in frozen
foods. The discrepancy was particularly striking, when one considered that
the decrease in viscosity as temperature increased above T g 0 , while initially
induced by the glass transition, was strongly amplified by dissolution (melt-
ing) of ice in the freeze-concentrated phase. It was further shown that the
dilution of reactants resulting from the dissolution of ice in the freeze-con-
centrated phase could partly compensate the effect of decreasing viscosity on
reaction kinetics, but that this effect was not significant enough to explain the
observed discrepancy. It was then suggested that the discrepancy could be
solved using T g values much lower than T g 0 (Simatos and Blond, 1991).
These observations have been confirmed by recent studies. The tempera-
ture dependence of various chemical and enzymatic reactions in the tempera-
ture range around or just above T g 0 (or T 2 ) can be characterized by apparent
activation energies between 50 and 150 kJ/mol: oxidation rate of ascorbic acid
in starch hydrolysates (Biliaderis et al., 1999), tyrosinase activity (Manzocco
et al., 1999), formaldehyde production in fish extracts with maltodextrins
(Herrera and Roos, 2001), colour changes and chlorophyll degradation in
green beans (Martins and Silva, 2002), colour and ascorbic acid changes in
peas (Giannakourou and Taoukis, 2003). Although fairly high for chemical
reactions in food systems, these figures are lower than those usually observed
for dynamic properties immediately above the glass transition range. Apparent
activation energy for recrystallization in ice creams was in the same range
(120 kJ/mol) (Hartel, 1998). Actually, WLF behaviour should be considered
only for systems the chemical composition of which is constant over the period
of time and temperature range considered. For food system applications in the
temperature range where melting of ice occurs, the reference temperature
should not be T g 0 , but rather the T g values relevant to the actual concentration
of the freeze-concentrated phase at the storage temperature T s (Figure 11.6).
This approach was used to model the alkaline phosphatase activity in
frozen sucrose solutions (Champion et al., 1997a). The rate constant was
assumed to vary with viscosity according to the expression derived by Atkins
(1998) for diffusion-controlled bimolecular reactions:

k app ¼ 8RT
3

(24)

The viscosity of the freeze-concentrated phase at T was predicted from the
WLF equation, with C 1 and C 2 determined from viscosity data for sucrose