Eggplant has a high antioxidant capacity (Cao et al., 1996), and this is attributed to its high content in phenolic compounds. The main class of phenolics in eggplant includes hydroxycinnamic acid conjugates (Whitaker and Stommel, 2003) and, of these, chlorogenic acid (5-O-caffeoylquinic acid and its isomers) typically accounts for 70% to 95% of total phenolics in eggplant fruit flesh (Stommel and Whitaker, 2003). The beneficial effects on health of chlorogenic acid and related compounds present in minor quantities in eggplant are numerous, and apart from their potent antioxidant activity, they also include free radical scavenging and antitumoral activities (Sawa et al., 1998; Triantis et al., 2005).
The selection of eggplant accessions with an increased concentration of phenolics as a way to develop new varieties with improved nutritional quality was suggested by Stommel and Whitaker (2003). These authors studied the concentration of hydroxycinnamic acid conjugates in the fruit flesh in a core collection of eggplant and found considerable variation, with a difference of almost fourfold among the varieties with highest and lowest concentrations. This indicates that there are ample possibilities for developing new commercial varieties with an increased concentration of phenolic compounds.
However, a drawback of increasing the concentration of these antioxidants in eggplant is that the oxidation of phenolics causes the browning of the fruit flesh after its exposure to the air, and this may lead to a reduction in the apparent quality (Macheix et al., 1990). When a fruit is cut, either for home consumption or for industrial processing, the destruction of fruit cellular compartmentation allows the orthodiphenolic substrates (hydroxycinnamic acid derivatives) to be accessible to polyphenol oxidases, which catalyze their oxidation to quinones, which react nonenzymatically with O2, sulfhydryl compounds, amines, amino acids, and proteins to give brown-colored compounds (Ramírez et al., 2002). The activity of polyphenol oxidases varies among eggplant varieties (Dogan et al., 2002), which suggests that selection for a combined high concentration of phenolics and low browning should be feasible. Other factors, like intracellular pH, which affects the activity of the polyphenol oxidase (PPO) enzymes (Concellón et al., 2004; Dogan et al., 2002), or the presence of ascorbic acid in the fruit flesh tissues, which prevents the oxidation of orthodiphenols (Macheix et al., 1990), might also have a role in the modification of the browning process in eggplant. Given the preference of consumers and the industry for varieties with white flesh and a low degree of browning (Prohens et al., 2005), new varieties with a high content of phenolics should also have a moderate flesh browning.
Apart from the variation existing in the eggplant types traditionally grown in Europe, America, western Asia, and Africa, there are other varieties corresponding to eastern Asian materials that have different plant and fruit characteristics (Daunay et al., 1997; Hallard, 1996; Lester and Hasan, 1991) and that could be sources of variation for a high content of phenolics, low browning, or both. Also, the gboma eggplant (Solanum macrocarpon) and scarlet eggplant (S. aethiopicum), which are cultivated species related to S. melongena (Sakata and Lester, 1997), mainly grown in Africa, but also in other parts of the world, like the case of scarlet eggplant in southern Italy (Polignano et al., 2004), could be useful for the genetic improvement of eggplant. Both species hybridize with the common eggplant (Ano et al., 1991; Bletsos et al., 2004), and might represent genetic resources of interest for the improvement of the quality of the common eggplant and for other traits like resistance to diseases (Gisbert et al., 2006). All these materials contain important genetic diversity for concentrations of hydroxycinnamic acid conjugates (Stommel and Whitaker, 2003).
During this investigation we determined the variation in phenolic concentration and fruit flesh browning in a collection of eggplant from different varietal types, the relationship between both traits, and the effects of other physicochemical traits on browning of eggplant. The use of a set of hybrids and their respective parents also allowed us to obtain information on the heritability of phenolic concentration and browning-related traits in eggplant. This is of interest for breeding new eggplant varieties that combine an improved nutritive value with a low degree of browning.
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