Antioxidants in Fruits and Their Possible Anticancer Property

Fruits are good sources of natural antioxidants such as carotenoids, vitamins, phenols, flavonoids, dietary glutathionine, and endogenous metabolites. These antioxidants are capable of performing a number of functions including acting as free radical scavengers, peroxide decomposers, singlet and triplet oxygen quenchers, enzyme inhibitors, and synergists. Active oxygen species are generated as byproducts of normal metabolism. Increased levels of these active oxygen species or free radicals create oxidative stress, which leads to a variety of biochemical and physiological injuries often resulting in impairment of metabolism, and eventually, cell death. The different antioxidant components found in fruits provide protection against harmful free radicals and have been associated with lower incidence and mortality rates of cancer and heart disease, in addition to a number of other health benefits. This paper summarizes the antioxidant capacities of various fruits, and the factors which affect their antioxidant activities such as crop genotype variation and maturity, pre-harvest conditions, post-harvest handling and processing. Many attractive opportunities exist for enhancing the quantity and quality of essential nutrients present in fruits. Strategies for establishing a new research and production paradigm will be reviewed, such as improving selection criteria among different horticultural cultivars and improving pre-harvest conditions and post-harvest handling to enhance nutrient quality. Evidence will also be presented on the prevention and inhibition of tumor growth by fruit extracts and bioactive compounds isolated from fruits. INTRODUCTION In the past, the agricultural industry has focused on maximizing the quantity of fruits and vegetables produced for commercial markets. However, modern consumers are now interested in optimizing the nutritional composition of foods in order to promote health. Therefore, much attention has now been placed on the agricultural practices that will enhance the nutritional content of fruits and vegetables being produced today. This trend is likely to continue in the future. Fresh fruits and vegetables have been shown to provide not only the usual nutrients such as vitamins and minerals, but also healthpromoting antioxidants, free radical scavengers, and other cancer-fighting agents. Eating more fruits and vegetables has been associated with lowering the incidence and mortality rates of cancer and heart disease in addition to a number of other health benefits (Ames et al., 1993; Gey, 1990). Research is in full force to find the best genotypes, optimum cultural practices, and most desirable postharvest handling techniques to enhance and preserve the nutritional components in fruits and vegetables that are beneficial to our health. This paper summarizes the antioxidant capacities of various fruits and describes the factors affecting these antioxidant activities. In addition, the effectiveness of fruit extracts and bioactive compounds isolated from fruits on the prevention and inhibition of tumor growth are also discussed. ANTIOXIDANT CAPACITY OF FRUITS Fruits contain a wide range of flavonoids and phenolic acids. The main flavonoid subgroups in fruits are anthocyanins, proanthocyanins, flavonols, and catechins. Phenolic Proc. III IS on Trop. and Subtrop. Fruits Eds.: M. Souza and R. Drew Acta Hort. 864, ISHS 2010 464 acids present in fruits are glycosides of hydroxylated derivatives of benzoic acid and cinnamic acid (Macheix et al., 1990). Good sources of natural antioxidants are found in many fruits, most notably prunes, raisins, blackberries, black currants, blueberries, raspberries, strawberries, grapes, and pomegranates (Prior et al., 1998; Wang and Lin, 2000; 1996; Gil et al., 2000). Among the small fruits, black raspberries and blackberries have higher antioxidant activities than red raspberries, while strawberries generally have lower values of total antioxidants (Wang and Lin, 2000). These results correlate with the findings that black raspberries and blackberries contain high amounts of cyanidin glycosides, a strong antioxidant (Macheix et al., 1990), while strawberries contain pelargonidin 3-glucoside (Gil et al., 1997) and ascorbic acid, which are weak antioxidants. Among the anthocyanins, the relative antioxidant strength in preventing oxidation of human lowdensity lipoprotein is as follows: delphinidin > cyanidin > malvidin > pelargonidin (Satué-Gracia, 1997). EFFECT OF PRE-HARVEST FACTORS ON ANTIOXIDANT ACTIVITY OF FRUITS Genotypes and Maturity For many crops, a large variety of cultivars exist, and thus there is potential for genetic variability leading to varying amounts of antioxidant activity. For example, in blackberry the ‘Hull Thornless’ cultivar yields higher antioxidant values compared to the ‘Chester Thornless’, and ‘Triple Crown’ cultivars. For raspberry fruits, ‘Jewel’ (a black raspberry) has higher values compared to red raspberry cultivars (‘Autumn Bliss’, ‘Canby’, ‘Sentry’, and ‘Summit’) (Wang and Lin, 2000). Various species and cultivars of blueberry and bilberry contain different antioxidant capacities (Prior et al., 1998). Many phytonutrients are synthesized in parallel with the overall development and maturation of fruits and vegetables. Therefore, antioxidant capacity varies considerably with different levels of maturity. Blackberry, raspberry and strawberry fruits harvested during their ripe stage consistently yield higher antioxidant values than when harvested during the pink stage. Antioxidant activity decreases steadily in strawberry fruit from the green stage to the 50 percent red stage, which has the lowest antioxidant value. Beyond the 50 percent red stage, the antioxidant value steadily increases again with fruit maturity. Immature blueberries harvested at an early stage (immediately after turning blue) have lower oxygen radical absorbance capacity (ORAC) values and total anthocyanin content when compared to more mature blueberries harvested 49 days later (Prior et al., 1998). Both ascorbic acid and total carotenoids increased with maturation and ripening in ‘clingstone’ peaches (Kader et al., 1988). Climate, Temperature, and Light Pre-harvest conditions of fruits and vegetables such as growing temperature and light intensity can affect their content of vitamins, especially carotenoids, thiamine, ascorbic acid, and riboflavin. For example, strawberry grown under high temperature conditions significantly contain an enhanced content of flavonoids and antioxidant capacity. Strawberry grown in cool day and night temperatures generally have the lowest antioxidant capacity in fruit. One explanation for this difference could be related to different flavonoid concentrations (Wang and Zheng, 2001). Month-to-month variability in vitamin C content dependent on growing conditions has been documented in processed Florida citrus products such as orange and grapefruit juices (Lee and Coates, 1997). The composition of flavonols in red raspberry juice has also been reported to be influenced by cultivar, processing, and environmental factors (Rommel and Wrolstad, 1993). Agricultural Practices 1. Compost. Composts have been utilized in agriculture as a significant source of organic matter. Compost, as a soil supplement, significantly enhanced content of ascorbic acid