Chlorophyll Degradation in a Spinach System at Low and Intermediate Water Activities

Chlorophyll degradation in blanched freeze-dried spinach puree was studied in model systems as a function of water activity (a_w), pH, temperature and also in the presence of glycerol, a water-binding agent. For a_w > 0.32 the most important mechanism of chlorophyll degradation is the transformation into pheophytin. The reaction can occur at very limited water concentration and has a first order dependence on pH, water activity and pigment concentration. The addition of glycerol increases the rate of degradation of chlorophyll. At temperatures higher than 56.7°C and a_w= 0.11, the energy of activation increased, indicating different mechanisms. For temperatures below 38°C the rate tended to increase instead of decreasing with decreasing temperatures because of changes in the sorption isotherm (increase of water content) caused by the shift in the temperature.