Water Activity and Plasticization - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 20

Nonenzymic Browning
Nonenzymic browning is a series of condensations that can be considered to be
bimolecular. 18 The initial reactants of nonenzymatic browning in foods are often a
reducing sugar and an amino acid or amino group. The reaction produces flavors in
foods during processing, but it decreases food quality during storage.
Eichner and Karel 81 studied the effect of mobility, viscosity, and the glassy state
on the rate of browning in glucose-glycine-glycerol-water model systems. They found
a decrease in the browning rate especially at low water contents when the amount of
glycerol in the system was low. The decrease was assumed to result from decreased
mobility of the reactants and reaction products when the sugar solution was reported
to be in the glassy state. Eichner and Karel 81 found that the addition of glycerol
improved the mobility of the reactants as a result of plasticization and the rate of the
reaction was increased. Flink et al. 82 studied the browning rate of nonfat milk powder,
which was humidified at 0, 11, and 32% RH at 37°C and stored at various temper-
atures. They observed that the rate of browning was low below a critical temperature,
above which the rate of the reaction increased substantially. They also observed that
the browning rate was dependent on water content and the critical temperature for
the reaction decreased with increasing initial a w . The results of the study of Flink
et al. 82 suggested that the rate of browning at temperatures below T g was low and the
increase in browning rate above the critical temperature occurred as a result of
plasticization and increasing molecular mobility above glass transition.
Nonenzymic browning rates have been reported for several foods that are likely
to exist in the glassy state at low a w or water contents and to exhibit increasing
browning rates above some critical a w values or water contents. Karmas et al. 76
derived T g values for cabbage, carrots, onions, and potatoes and analyzed their
browning rates as a function of T - T g . The results showed that nonenzymic browning
was not likely to occur below T g . Browning occurred above a critical T - T g , which
was dependent on water content. It would be expected that the true rate constant of
the browning reaction is affected by both water content and temperature. A material
with a high water content has a low T g and browning may occur at a relatively low
temperature. However, the true rate constant decreases with decreasing temperature,
which also decreases the observed rate constant.
Karmas et al. 76 reported browning data as a function of temperature for several
model systems that had various initial water activities at room temperature. Arrhenius
plots for the materials were nonlinear with two changes. These changes were
observed to occur in the vicinity of T g and at about T g + 10°C above T g . Karmas
et al. 76 found that the activation energies for the reaction below T g (30 to 90 kJ/mol)
were lower than above T g . The activation energies above T g (65 to 190 kJ/mol) were
typical of the nonenzymic browning reaction. However, those within the glass
transition temperature range (250 to 400 kJ/mol) were substantially higher than
values commonly obtained for the reaction. Karmas et al. 76 pointed out that the step
change in the Arrhenius plots was similar to those found for diffusion in polymers.
Roos and Himberg 75 found that browning in food models occurred at temperatures
below T g which agreed with the results of Karmas et al. 76 The rate of browning
increased both with increasing temperature and increasing T - T g .