REACTION AT LIMITED WATER CONCENTRATION. 2. Chlorophyll Degradation

SUMMARY— Chlorophyll degradation was studied as a function of water activity (A_w) in freeze‐dried blanched spinach puree and in model systems containing cellulose‐citrate buffer and chlorophyll a. At 37°C and A_w higher than 0.32, the most important mechanism of chlorophyll degradation is conversion to pheophytin. The rate of chlorophyll a transformation is 2.5 times faster than for chlorophyll b. The relation between A_w and log time for 20% chlorophyll degradation is linear and the chlorophyll/pheophytin ratio is useful for storage prediction as a function of A_w (down to A_w 0.32). At A_w lower than 0.32, the rate of pheophytin formation in spinach is low. New products are formed which can interfere with spectrophotometric determination of chlorophylls and pheophytins. The possibility of removal of Mg from the porphyrin ring of chlorophyll a was shown in model systems even at A_w below BET (Brunnauer‐Emmet‐Teller) monolayer coverage.