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 occurring 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; Villamiel et al., 2006). The deterioration rates of the non-enzymatic browning are closely related to temperature and water activity. In most cases, the discolouration rates increase with water activity and processing temperature (Goula et al., 2006; Labuza and Tannenbaum, 1972; Maskan et al., 2002; McMinn and Magee, 1997; Rapusas and Driscoll, 1995; Topuz, 2008).
How to browse
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 Arrhenius-type equation. However, the browning rate decelerates at high water activity values 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 reported that activation energy of the browning deterioration can be a function of moisture 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.
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