424 Novel food packaging techniques Peroxidases (POD; EC 1 11. 1.7. )could also be involved in browning although to a lesser extent, due to low availability of H2O2 within the plant cell (Artes et al, 1998; Sanchez-Ferrer et al, 1995). Lipoxygenase(EC 1. 13.11)and lipase(EC 3.1.1.3) have been considered as the main causes for the breakdown of some vegetables like cucumbers. The reaction between lipoxygenase and lipids substrates generates hydroperoxides that are related to senescence an scald induction. Fatty acid radicals induced by peroxidase can react with cell components leading to further breakdown. Particularly, bleaching of B-carotene and chlorophyll a occurs as a consequence of lipoxygenase catalysed reactio Browning could easily be evaluated by colorimetric methods. For example visual scores for browning of cut lettuce were well correlated with hue(values decreased as browning occurred)and a", although correlation with b* was lower while it was not significant with L', Hue angle values decreased as browning occurred( Peiser et al., 1998) 20.5 Colour stability and maP To remain competitive in the fruit and vegetable market, suppliers must offer products with an optimal overall quality. Thus, the entire chain from producers and processors to retailers must be increasingly sensitive to consumer requirements, particularly as they relate to colour. In fact, perception of sweetness, sourness and flavour intensity was highly correlated to skin colour as has been reported for sweet cherries; full dark red cherries, measured by both visual and colourimetry, had higher consumer acceptance than full bright red Crisosto et al, 2002) Atmospheres with reduced O2 and/or elevated CO2 concentrations are known to extend the storage life of fruit and vegetables. MAP can bring the lowering of respiratory activity and ethylene production, delay in ripening and softening, limiting weight losses and reduced incidence of physiological disorders and decay-causing pathogens(Ahvenainen, 1996, Artes, 2000b). As MAP slows the rate at which energy reserves are used it can be applied in combination with chilling storage for improving shelf-life of fruit and vegetables. At the same time, MAP affects biochemical reactions related to pigment synthesis and degradation(Artes, 1993 and 2000a), although responses to MAP depend on the kind of fruit or vegetable. In addition to this, the effect of respiratory gases on the metabolic behaviour of plant materials depends on temperature of application due to its influence on solubility of these gases. The effects of low O2 and/or high CO2 on colour changes in packaged fruit and vegetables will be examined using several examples recently reported 20.5.1 Low oxygen effects It has been observed that the activity of tyrosinase, responsible for mushroom browning, is dependent on O2 concentration MAP induced higher L values andPeroxidases (POD; EC 1.11.1.7.) could also be involved in browning although to a lesser extent, due to low availability of H2O2 within the plant cell (Arte´s et al., 1998; Sa´nchez-Ferrer et al., 1995). Lipoxygenase (EC 1.13.11) and lipase (EC 3.1.1.3) have been considered as the main causes for the breakdown of some vegetables like cucumbers. The reaction between lipoxygenase and lipids substrates generates hydroperoxides that are related to senescence and scald induction. Fatty acid radicals induced by peroxidase can react with cell components leading to further breakdown. Particularly, bleaching of -carotene and chlorophyll a occurs as a consequence of lipoxygenase catalysed reactions. Browning could easily be evaluated by colorimetric methods. For example, visual scores for browning of cut lettuce were well correlated with hue (values decreased as browning occurred) and a*, although correlation with b* was lower while it was not significant with L*, Hue angle values decreased as browning occurred (Peiser et al., 1998). 20.5 Colour stability and MAP To remain competitive in the fruit and vegetable market, suppliers must offer products with an optimal overall quality. Thus, the entire chain from producers and processors to retailers must be increasingly sensitive to consumer requirements, particularly as they relate to colour. In fact, perception of sweetness, sourness and flavour intensity was highly correlated to skin colour as has been reported for sweet cherries; full dark red cherries, measured by both visual and colourimetry, had higher consumer acceptance than full bright red (Crisosto et al., 2002). Atmospheres with reduced O2 and/or elevated CO2 concentrations are known to extend the storage life of fruit and vegetables. MAP can bring the lowering of respiratory activity and ethylene production, delay in ripening and softening, limiting weight losses and reduced incidence of physiological disorders and decay-causing pathogens (Ahvenainen, 1996; Arte´s, 2000b). As MAP slows the rate at which energy reserves are used it can be applied in combination with chilling storage for improving shelf-life of fruit and vegetables. At the same time, MAP affects biochemical reactions related to pigment synthesis and degradation (Arte´s, 1993 and 2000a), although responses to MAP depend on the kind of fruit or vegetable. In addition to this, the effect of respiratory gases on the metabolic behaviour of plant materials depends on temperature of application due to its influence on solubility of these gases. The effects of low O2 and/or high CO2 on colour changes in packaged fruit and vegetables will be examined using several examples recently reported. 20.5.1 Low oxygen effects It has been observed that the activity of tyrosinase, responsible for mushroom browning, is dependent on O2 concentration. MAP induced higher L* values and 424 Novel food packaging techniques