Kinetic studies of saffron (Crocus sativus L.) quality deterioration

Kinetic studies on carotenoid loss and changes in other quality attributes of saffron were carried out under varying conditions of water activity (alpha(w)) and temperature. The water sorption isotherms of saffron powders were also determined. Within the a(w) range tested (0.11-0.75), both Brunauer-Emmett-Teller (BET) and Guggenheim-Anderson-Deboer (GAB) sorption models fit the data well and gave estimates for the monolayer values between 1.8 and 5.2 g of H2O/100 g of solids, depending on temperature and the equation used. The glass transition temperature (T(g)) of freeze-dried saffron carotenoid extracts, as detected by calorimetry (DSC), was very sensitive to the moisture content of the system; water plasticizes the amorphous matrix and thereby increases the mobility and reactivity of the reactants. Saffron carotenoid (mainly crocins) degradation followed first-order-like reaction kinetics, and it was strongly dependent on temperature and a(w). The dependence of the rate constant (k) for decoloration on a(w) was different from that for other typical nonpolar carotenoids, resembling more the kinetic responses of water-soluble pigment degradation and nonenzymatic browning reactions. Carotenoid loss occurred only at temperatures above the T(g), presumably because of limited molecular diffusion in the matrix at sub-T(g) temperatures. However, testing of the applicability of a Williams-Landel-Ferry (WLF)-governed dependence of k on temperature was not successful. Activation energies, calculated from Arrhenius-type plots, gave an average value of approximately 20 kcal/mol, typical of other deteriorative processes in food materials. At ambient temperature, an intermediate water activity (a(w) approximately 0.43-0.53) seemed to favor the development of aroma constituents (safranal) while maintaining a relatively low degradation rate for the carotenoids.