Ong, Law - Hygrothermal Properties of FVFs
2.2.2. Browning Reactions
Colour is one of the important quality attributes for dried food product. Although the
optical property is often an assessment of the physical appearance of the product, the
colour development is in fact the results of various chemical and biochemical reactions.
Browning reaction, in either positive or negative way, is an important phenomena oc-
curring in food during processing and storage. In brief, the major reactions leading to
browning can be grouped into enzymatic phenol oxidation and non-enzymatic browning.
Enzymatic browning is often catalyzed by the enzymes polyphenol oxidase (PPO), where
the phenolics constituents are oxidized to quinones in the enzymatic reaction and then
further polymerized to melanoidins (brown pigment) that has high molecular weight.
On the other hand, non-enzymatic reactions are referring to Maillard reaction (reaction
between carbonyl and amino compounds), caramelization, ascorbic acid browning, lipid
browning and pigment destruction (Perera, 2005; Prachayawarakorn et al., 2004; Villa-
miel et al., 2006). The deterioration rates of the non-enzymatic browning are closely re-
lated to temperature and water activity. In most cases, the discolouration rates increase
with water activity and processing temperature (Goula et al., 2006; Labuza and Tannen-
baum, 1972; Maskan et al., 2002; McMinn and Magee, 1997; Rapusas and Driscoll, 1995;
Topuz, 2008).
Generally the rates of degradation follow the zero or first order kinetics while the
dependence of degradation rate constant on temperature can be described by Arrhe-
nius-type equation. However, the browning rate decelerates at high water activity val-
ues because of dilution effect on reactants concentration. The same phenomenon occurs
at low water activity because solute mobility is limited below the monolayer. Browning
reaction is at maximum when water activity value is in the range of 0.5 to 0.8 in dried
and partial dried foods (Leung, 1987; Villamiel et al., 2006). In addition, it has been re-
ported that activation energy of the browning deterioration can be a function of mois-
ture or water activity (Labuza and Tannenbaum, 1972). Followings are some examples
from literature to show the different order of browning kinetics (Error! Reference source
not found. (a) and Error! Reference source not found. (a)), the Arrhenius model for the
browning rate constant (Figure 2.1. (b) and Figure 2.2. (b)) and the effects of water
activity on the browning kinetics and activation energy (Figure 2.3).