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Phytol >• Pyropheophorbide

FIGURE 9.2 Chlorophyll degradation. (Adapted from van Boekel.4)

The food quality of fruits and vegetables is associated with freshness and color and is one of the most important factors for consumers. The green color in these plant materials is produced by chlorophyll and has been used as an indicator of health and ripeness. However, after crop harvesting, chlorophylls are degraded with a rate depending on plant material and processing conditions. Chlorophyll degradation may occur within few hours or over several weeks.2

In plant tissue, chlorophyll is released from its protein complex followed by phytyl elimination and possibly pheophytinization. This degradative process is also observed in foods by effect of processing and the type of product depends on the severity of the treatment (Figure 9.2).4 Degradation proceeds by oxidation of the ring structure to chlorins and ultimately by formation of colorless end products. Additionally, a loss of chlorophyll has been reported in frozen peas associated with lipoxygenase activity and fat peroxidation.1

Different methodologies have been used to preserve the green color of fruits and vegetables. Techniques employed are pH control, use of salts, control of the thermal treatment, use of modified atmospheres, and combinations of these, among others (Table 9.1).5-22

The transformation of chlorophylls into pheophytins is a simple process; weak acids are enough to induce the loss of the magnesium atom from the porphyrinic ring and its replacement by hydrogen. In vivo, this transformation has been associated with an enzymatic demetallation. The loss of phytyl group is another common reaction by which chlorophyll is converted to chlorophyllide. Hydrolysis of this group may proceed by acid or alkali catalysis; moreover, the reaction may proceed in vivo by effect of the chlorophyllase enzyme. The acidification would give rise to the demetalled pheophorbide. These reactions are observed for chlorophyll a as well as for chlorophyll b. Thus, the preservation of Mg in the chlorophyll structure is important for the product quality of green plant foodstuffs. At this respect, alkaline treatments (the Blair process) have been implemented to prevent pheophytinization and to improve the shelf life of chlorophyll-containing products. Another approach is online pH control during food preparation (Table 9.1).1222

Chlorophyll complexes with zinc (Zn) or copper (Cu) are more stable than Mg-chlorophyll. In particular, Zn complexes are of greater interest because of the toxic nature of Cu ions. Thus, the replacement of Mg for one of these metals has

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