Chlorophyll degradation mechanism

YAMAUCHI N, HAMAGUCHI s and AGATA K (1980), Physiological and chemical studies on ascorbic acid of fruits and vegetables. Vn. Mechanism of chlorophyll degradation and action of ascorbic acid in the inhibition of yellowing in harvested parsley leaves , J Japan Soc Hort Sci, 49, 414 20. 

Water activity has been shown to have a definite influence on the rate of degradation of chlorophyll in freeze-dried, blanched spinach puree [73]. At 37°C and an higher than 0.32, the most important mechanism of chlorophyll degradation was conversion to pheophytin. At lower than 0.32, the rate of pheophytin formation in spinach was low. The rate of chlorophyll-a transformation was 2.5 times faster than chlorophyll-. The study of the degradation of chlorophyll as a function of a . pH, and temperature in a spinach system... 

The kinetics of chlorophyll-a transformation was studied as a function of time at different water activities at 38.6°C (Figme 29.8) [74]. For > 0.32 the most important mechanism of chlorophyll degradation was the transformation into pheophytin this had a first-order dependence on pH, water activity, and pigment concentration. 

The steps successive to protoporphyrin are unique to each branch of the pathway. Those unique steps were classified into the siroheme branch, the heme branch that includes phyrochromobilin, the chlorophyll branch, and the chlorophyll cycle that refers to the interconversion steps of chlorophyll a and b. The chlorophyll cycle was put into a separate category because this cycle is regulated by a distinct mechanism from those of that control the other branches. Moreover, the chlorophyll cycle has a unique feature distinct from the other branches. Not only is the chlorophyll cycle a part of the biosynthetic pathway but the latter two reactions of the cycle are also part of the chlorophyll degradation pathway. 

In vegetable foods subjected to lactic acid fermentation, activity of the enzyme chlorophyllase is very often favored during the process, and dephytylated chlorophyll derivatives are found [133,138-141]. The degradation mechanism proposed is as follows ... 

Carbon monoxide emissions from the terrestrial biosphere are small, but forest fires produce 0.02 Pg C/yr. Degradation of chlorophyll is dying plant material seems to be the largest CO-producing mechanism at 0.04-0.2 Pg C/yr (Freyer, 1979). 

Fig. 8.5 A proposed mechanism for the degradation of chlorophylls in processed and stored vegetables, a = induced by heat.

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