The effect of MAP on the nutritional quality of fresh fruits and vegetables

During the last few years many studies have demonstrated that fruit and vegetables are rich sources of micronutrients and dietary fibre. They also contain an immense variety of biologically active secondary metabolites that provide the plant with colour, flavour and sometimes antinutritional or toxic properties (Johnson et al, 1994). Among the most important classes of such substances are vitamin C, carotenoids, folates, flavonoids and more complex phenolics, saponins, phytosterols, glycoalkaloids and the glucosinolates.

The nutrient content of fruit and vegetables can be influenced by various factors such as genetic and agronomic factors, maturity and harvesting methods, and postharvest handling procedures. There are some postharvest treatments which undoubtedly improve food quality by inhibiting the action of oxidative enzymes and slowing down deleterious processes. Storage of fresh fruits and vegetables within the optimum range of low O2 and/or elevated CO2 atmospheres for each commodity reduces their respiration and C2H4 production rates (Kader, 1986; Kader, 1997). Optimum CA retards loss of chlorophyll, biosynthesis of carotenoids and anthocyanins, and biosynthesis and oxidation of phenolic compounds. In general, CA influences flavour quality by reducing loss of acidity, starch to sugar conversion, and biosynthesis of aroma volatiles, especially esters. Retention of ascorbic acid and other vitamins results in better nutritional quality, including antioxidant activity, of fruits and vegetables when kept in their optimum CA (Kader, 2001). However, little information is available on the effectiveness of controlled atmospheres or modified atmosphere packaging (CA/MAP) on the nutrient retention during storage. The influence of CA/MAP on the antioxidant constituents related to nutritional quality of fruits and vegetables, including vitamin C, carotenoids, phenolic compounds, as well as glucosinolates will be reviewed here.

16.7.1 Vitamin C

Vitamin C is one of the most important vitamins in fruits and vegetables for human nutrition. More than 90% of the vitamin C in human diets is supplied by the intake of fresh fruits and vegetables. Vitamin C is required for the prevention of scurvy and maintenance of healthy skin, gums and blood vessels. Vitamin C, as an antioxidant, reduces the risk of arteriosclerosis, cardiovascular diseases and some forms of cancer (Simon, 1992). Ascorbate oxidase has been proposed as the major enzyme responsible for enzymatic degradation of l-ascorbic acid (AA). The oxidation of AA, the active form of vitamin C, to dehydroascorbic acid (DHA) does not result in loss of biological activity since DHA is readily reconverted to l-AA in vivo. However, DHA is less stable than AA and may be hydrolysed to 2,3-diketogulonic acid, which does not have physiological activity (Klein, 1987) and it has therefore been suggested that measurements of vitamin C in fruits and vegetables in relation to their nutritional value should include both AA and DHA.

The vulnerability of different fruits and vegetables to oxidative loss of AA varies greatly, as indeed do general quality changes. Low pH fruits (citrus fruits) are relatively stable, whereas soft fruits (strawberries, raspberries) undergo more rapid changes. Leafy vegetables (e.g. spinach) are very vulnerable to spoilage and AA loss, whereas root vegetables (e.g. potatoes) retain quality and AA for many months (Davey et al, 2000). Fruits and vegetables undergo changes from the moment of harvest and since l-AA is one of the more reactive compounds it is particularly vulnerable to treatment and storage conditions. In broad terms, the milder the treatment and the lower the temperature the better the retention of vitamin C, but there are several interacting factors which affect AA retention (Davey et al, 2000). The rate of postharvest oxidation of AA in plant tissues has been reported to depend upon several factors such as temperature, water content, storage atmosphere and storage time (Lee and Kader, 2002).

The effect of controlled atmospheres on the ascorbate content of intact fruit has not been extensively studied. The results vary among fruit species and culti-vars, but the tendency is for reduced O2 and/or elevated CO2 levels to enhance the retention of ascorbate (Weichmann, 1986; Kader et al, 1989). A reduction in temperature and of O2 concentration in the storage atmosphere have been described as the two treatments which contribute to preserve vitamin C in fruits and vegetables (Watada, 1987) and so Delaporte (1971) and others observed that loss of AA can be reduced by storing apples in a reduced oxygen atmosphere. However, Haffner et al (1997) have shown than AA levels in various apple cul-tivars decreased more under ultra low oxygen (ULO) compared to air storage. On the other hand, increasing CO2 concentration above a certain threshold seems to have an adverse effect on vitamin C content in some fruits and vegetables. It has been reported that the effect of elevated CO2 on AA content varied among commodities and was dependent on CO2 level and storage temperature and duration (Weichmann, 1986). Bangerth (1977) observed accelerated AA losses in apples and red currants stored in elevated CO2 atmospheres. Vitamin C content was reduced by high CO2 concentrations (10-30% CO2) in strawberries and blackberries and only a moderate to negligible effect was found for black currants, red currants and raspberries (Agar et al, 1997). Storage of sweet pepper for 6 days at 13°C in CO2 enriched atmospheres resulted in a reduction in AA content (Wang, 1977). Wang (1983) noted that 1% O2 retarded AA degradation in Chinese cabbage stored for 3 months at 0°C. He observed that treatments with 10 or 20% CO2 for 5 or 10 days produced no effect, and 30 or 40% CO2 increased AA decomposition. Veltman et al (1999) have observed a 60% loss in AA content of 'Conference' pears after storage in 2% O2 + 10% CO2. There were no data available to show whether a parallel reduction in O2 concentration alleviates the negative CO2 effect. Agar et al (1997) proposed that reducing O2 concentration in the storage atmosphere in the presence of high CO2 had little effect on the vitamin C preservation. The only beneficial effect of low O2 alleviating the CO2 effect could be observed when applying CO2 concentrations lower than 10%.

In fresh-cut products, high CO2 concentration in the storage atmosphere has also been described to cause degradation of vitamin C. Thus, concentration of 5,

10, or 20% CO2 caused degradation of vitamin C in fresh-cut kiwifruit slices (Agar et al, 1999). Enhanced losses of vitamin C in response to CO2 higher than 10% may be due to the stimulating effects on oxidation of AA and/or inhibition of DHA reduction to AA (Agar et al, 1999). In addition, vitamin C content decreased in MAP-stored Swiss chard (Gil et al, 1998a) as well as in potato strips (Tudela et al, 2002). In contrast, MAP retarded the conversion of AA to DHA that occurred in air-stored jalapeno pepper rings (Howard et al, 1994; Howard et al, 1998). Wright and Kader (1997a) found no significant losses of vitamin C occurred during the post cutting life of fresh-cut strawberries and persimmons for 8 days in CA (2% O2, air + 12% CO2, or 2% O2 + 12% CO2) at 0°C. In studies of cut broccoli florets and intact heads of broccoli CA/MAP resulted in greater AA retention and shelf-life extension in contrast to air-stored samples (Barth et al, 1993; Paradis et al, 1996). Retention of AA was found in fresh-cut lettuce packaged with nitrogen (Barry-Ryan and O'Beirne, 1999). They suggest that high levels of CO2 (30-40%) increased AA losses by conversion into DHA due to availability of oxygen in lettuce (Barry-Ryan and O'Beirne, 1999). This fact has also been shown in sweet green peppers (Petersen and Berends, 1993). The reduction of AA and the relative increase in DHA could be an indication that high CO2 stimulates the oxidation of AA, probably by ascorbate peroxidase as in the case of strawberries (Agar et al, 1997) and of spinach (Gil et al, 1999). Mehlhorn (1990) demonstrated an increase in ascorbate peroxidase activity in response to ethylene. High CO2 at injurious concentrations for the commodity may reduce AA by increasing ethylene production and therefore the activity of ascorbate peroxidase. Ascorbate oxidase from green zucchini fruit, which catalyses the oxidation of AA to DHA, has been found to be unstable and to lose activity below pH 4 (Maccarrone et al, 1993). This could partially explain the lower DHA content of the strawberries (pH 3.4-3.7) and the higher DHA content of the persimmons (pH 5.4-6.0) (Wright and Kader, 1997a) as well as the tendency of some vegetables at pH near to neutral to lose AA during storage (Gil et al, 1998b).

In conclusion, the loss of vitamin C after harvest can be reduced by storing fruits and vegetables in atmospheres of reduced O2 and/or up to 10% CO2 as Lee and Kader (2002) have reported. CA conditions do not have a beneficial effect on vitamin C if high CO2 concentrations are involved, although the concentrations above which CO2 affects the loss of AA must be estimated for each commodity (Kader, 2001).

16.7.2 Carotenoids

Carotenoids form one of the most important classes of plant pigments and play a crucial role in defining the quality parameters of fruit and vegetables. Their role in the plant is to act as accessory pigments for light harvesting and in the prevention of photo-oxidative damage, as well as acting as attractants for pollinators. The best documented and established function of some of the carotenoids is their provitamin A activity, especially of b-carotene. a-Carotene and b-crytoxanthin also possess provitamin A activity, but to a lesser extent than does b-carotene. Many yellow, orange or red fruit and root vegetables contain large amounts of carotenoids, which accumulate in the chloroplast during ripening or maturation. In some cases, the carotenoids present are simple, e.g. b-carotene in carrot or lycopene in tomato, but in other cases complex mixtures of unusual structures are found, e.g. in Capsicum. Carotenoids are found in membranes, as microcrystals, in association with proteins or in oil droplets. In vivo, carotenoids are stabilised by these molecular interactions, that are also important in determining the bioavailability of the carotenoids. Plant materials do not contain vitamin A, but provide carotenoids that are converted to vitamin A after ingestion. Provitamin A carotenoids found in significant quantities in fruits may have a role in cancer prevention by acting as free radical scavengers (Britton and Hornero-Mendez, 1997). Lycopene, although it has no provitamin A activity, has been identified as a particularly effective quencher of singlet oxygen in vitro (Di Mascio et al, 1989) and as an anticarcinogenic (Giovannucci, 1999). Carotenoids are unstable when exposed to acidic pH, oxygen or light (Klein, 1987). The effect of controlled and modified atmospheres on the carotenoid content of intact fruits has not been well studied. Modified atmospheres including either reduced O2 or elevated CO2 are generally considered to reduce the loss of provitamin A, but also to inhibit the biosynthesis of carotenoids (Kader et al, 1989). Reducing O2 to lower concentrations enhanced the retention of carotene in carrots (Weichmann, 1986). The carotene content of leeks was found to be higher after storage in 1% O2 + 10% CO2 than after storage in air (Weichmann, 1986).

Few studies on the effect of CA storage on the provitamin Acarotenoid content of fresh-cut products have been published. Wright and Kader (1997b) found for sliced peaches and persimmons, that the limit of shelf life was reached before major losses of carotenoids occurred. Low changes in carotenoids have been observed in minimally processed pumpkin stored for 25 days at 5°C in MAP (Baskaran et al, 2001). Petrel et al (1998) found no changes on the carotenoid content of ready to eat oranges after 11 days at 4°C in MAP (19% O2 + 5% CO2 and 3% O2 + 25% CO2). In addition, the content of b-carotene in broccoli florets increased at the end of CA storage (2% O2 + 6% CO2) and remained stable after returning the samples to ambient conditions for 24h (Paradis et al, 1996). Lutein, the major carotenoid in green bean tissue, also showed an accumulation after 13 days of CA storage (1% O2 + 3% CO2) and in these conditions retained carotenoids up to 22 days at 8°C (Cano et al, 1998). However, Sozzi et al (1999) have observed that CA of 3% O2 and 20% CO2 both alone and together with ethylene prevented total carotenoid and lycopene biosynthesis on tomato. After exposing the fruits to air, total carotenoids and lycopene increased but were in all cases significantly lower than those which were held in air.

16.7.3 Phenolic compounds

There is a considerable evidence for the role of antioxidant constituents of fruits and vegetables in the maintenance of health and disease prevention (Ames et al,

1993). Epidemiological studies show that consumption of fruits and vegetables with high phenolic content correlates with reduced cardio- and cerebrovascular diseases and cancer mortality (Hertog et al, 1997). Recent work is also beginning to highlight the relation of flavonoids and other dietary phenolic constituents to these protective effects. They act as antioxidants by virtue of the free radical scavenging properties of their constituent hydroxyl groups (Kanner et al, 1994; Vinson et al, 1995). The biological properties of phenolic compounds are very variable and include anti-platelet action, antioxidant, antiinflamatory, antitumoral and oestrogenic activities, which might suggest their potential in the prevention of coronary heart diseases and cancer (Hertog et al, 1993; Arai et al, 2000).

In the last few years there has been an increasing interest in determining relevant dietary sources of antioxidant phenolics and red fruits such as strawberries, cherries, grapes and pomegranates have received considerable attention due to their antioxidant activity. However, storage under CA/MAP conditions has been focused on keeping the visual properties and few studies have been made on the effect on the nutritional quality. Generally an increase in phenolics is considered a positive attribute and enhances the nutritional value of plant product. However, many secondary metabolites typical of wild species of fruits or vegetables have toxic effects although they are not considered here. In addition, the organoleptic and nutritional characteristics of fruit and vegetables are strongly modified by the appearance of brown pigments. Oxidative browning is mainly due to the enzyme polyphenol oxidase (PPO) which catalyses the hydroxylation of monophenols to o-diphenols and, in a second step, the oxidation of colourless o-diphenols to highly coloured o-quinones (Vamos-Vigyazo, 1981). The o-quinones non-enzymatically polymerise and give rise to heterogeneous black, brown or red pigments called melanins decreasing the organoleptic and nutritional qualities (Tomas-Barberan et al, 1997; Tomas-Barberan and Espin, 2001).

Controlled atmospheres and modified atmosphere packaging (MAP) can directly influence the phenolic composition as reflected in the changes observed in anthocyanins. Carbon dioxide-enriched atmospheres (>20%) used to reduce decay and extend the postharvest life of strawberries induced a remarkable decrease in anthocyanin content of internal tissues compared with the external ones (Gil et al, 1997). Holcroft and Kader (1999) related the decrease in strawberry colour under CO2 atmosphere, with a decrease of important enzyme activity involved in the biosynthesis of anthocyanins, phenylalanine ammonialyase (PAL; EC 4.3.1.5) and glucosyltransferase (GT; EC 2.4.1.91). A moderated CO2 atmosphere (10%) prolongs the storage life and maintains quality and adequate red colour intensity of pomegranate arils (Holcroft et al, 1998). However, the arils of pomegranates stored in air were deeper red than were those of the initial controls and of those stored in a CO2 enriched atmosphere.

Modified atmospheres can also have a positive effect on phenolic-related quality, as in the case of the prevention of browning of minimally processed lettuce (Saltveit, 1997; Gil et al, 1998b). In addition, modified atmosphere packaging of minimally processed red lettuce (2-3% O2 + 12-14% CO2) decreased the content of flavonol and anthocyanins of pigmented lettuce tissues when com pared to air storage (Gil et al, 1998b). The increase of soluble phenylpropanoids observed in the midribs of minimally processed red lettuce after storage in air was avoided under MAP. When minimally processed Swiss chard was stored in MAP (7% O2 + 10% CO2), no effect was observed on flavonoid content after 8 days cold storage when compared to that stored in air (Gil et al, 1998b). In addition, the total flavonoid content of fresh-cut spinach remained quite constant during storage in both air and MAP atmosphere (Gil et al, 1999).

Abnormal browning frequently occurs when fruits are stored in very low oxygen atmospheres. Extended treatment in pure nitrogen enhances the appearance of brown surfaces in fruits, which then rot rapidly when they are returned to air (Macheix et al, 1990). These observations are probably the result of cell disorganisation under anaerobiosis, but may also be related to variations in phenolic metabolism.

There is a decrease in all phenolic compounds (e.g. anthocyanins, flavonols, and caffeoyl tartaric and p-coumaroyl tartaric acids) in both skin and pulp of grape berries rapidly brought under anaerobiosis in CO2 enriched atmosphere (Macheix et al, 1990). Anaerobiosis generally appears to be harmful for the fruit products formed, with the frequent appearance of unwanted browning or loss of antho-cyanins. In contrast, this treatment becomes necessary in the case of removal of astringency from persimmom fruit by means of an atmosphere of CO2 or N2. These treatments result in the production of acetaldehyde, and deastringency is due to the insolubilisation of kaki-tannin by reaction with the acetaldehyde (Haslam et al, 1992).

16.7.4 Glucosinolates

Brassica vegetables, such as cabbage, Brussels sprouts, broccoli and cauliflower are an important dietary source for a group of secondary plant metabolites known as glucosinolates. The sulphur-containing glucosinolates are present as glucosides and can be hydrolysed by the endogenous plant enzyme myrosinase (thiogluco-side glucohydrolase EC 3.2.3.1). Myrosinase and the glucosinolates are physically separated from each other in the plant cell and therefore hydrolysis can only take place when cells are damaged, e.g. by cutting or chewing (Verkerk et al, 2001). The hydrolysis generally results in further breakdown of glucosinolates into isothiocyanates, nitriles, thiocyanates, indoles and oxazolidinethiones. Glu-cosinolate degradation products contribute to the characteristic flavour and taste of Brassica vegetables. Glucosinolates and their biological effects have been reviewed in detail (Rosa et al, 1997). Indol-3-ylmethylglucosinolates, which occur in appreciable amounts in several Brassica vegetables, are of interest for their potential contribution of anticarcinogenic compounds to the diet (Loft et al, 1992) and so broccoli has been associated with a decreased risk of cancer based on several beneficial properties such as the level of vitamin C, fibre and glu-cosinolates. The glucosinolate content in Brassica vegetables can vary depending on the variety, cultivation conditions, harvest time and climate. Storage and processing of the vegetables can also greatly affect the glucosinolate content.

Processes such as chopping, cooking and freezing influence the extent of hydrolysis of glucosinolates and the composition of the final products (Verkerk et al, 2001).

There are a few reports describing the effects of storage on the glucosinolate content; for instance the storage of white and red cabbage for up to five months at 4°C which does not seem to affect the levels of glucosinolates (Berard and Chong, 1985). However, there is still little information about the influence of CA/MAP on total or individual glucosinolate content of Brassica vegetables but an increase in total glucosinolate content was reported in broccoli florets when stored in air or CA while the absence of O2 with a 20% CO2 resulted in total loss (Hansen et al, 1995).

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