Determination of a colour index for fruit of pomegranate varietal group “Mollar de Elche”
Highlights
► Determination of a colour index for fruit of pomegranate. ► A strong correlation between the colorimetric coordinates and the maximum temperature. ► The colorimetric maturity index (MIc) integrate the three colorimetric coordinates, L*, a* and b*. ► Relation between colorimetric maturity index, chemical maturity index and fruit development.
Introduction
Several studies have confirmed the excellent organoleptic and nutritional properties of pomegranates (Al-Said et al., 2009, He et al., 2010, Ozgen et al., 2008, Shwartz et al., 2009, Martínez et al., 2012). Moreover, the traditional importance of pomegranate fruit as a medicinal plant is currently the subject of renewed research because of its anti-carcinogenic, anti-microbial and anti-viral properties (Al-Maiman and Ahnad, 2002, Bell and Hawthorne, 2008, Kotwal, 2007, Reddy et al., 2007, Legua et al., 2012). Although knowledge about the importance of pomegranate in human nutrition has increased in recent years, the external colour of the fruit has not been studied in detail. Pomegranate maturity status is commonly assessed based on rind and juice colour and acidity (Cristosto et al., 2000).
Pomegranate acceptability by consumers and processors depends basically on a combination of several quality attributes, which include rind colour, sugar content, acidity, flavour, etc. (Al-Said et al., 2009). Some researchers have studied the correlation between rind colour parameters (L*, a*, b*, C* and hab*) and acidity, total soluble solids, citric acid and anthocyanin content (Dafny-Yalin et al., 2010). Manera et al. (2011) studied the correlation between pomegranate rind colour parameters and air temperature, and Manera et al. (2012) studied the relationship between air temperature and the degreening of lemon peel colour during maturation. Carreño et al. (1995) proposed an objective index for evaluating the colour of red grapes. Jiménez-Cuesta et al. (1981) determined a colour index for citrus fruit, while other authors studied the colour changes that take place during the post-harvest storage of several fruits (Arnal and Del Río, 2004, Shwartz et al., 2009, among others). The colour changes that occur during degreening treatments have also been studied in mandarin orange (Carvalho et al., 2006). All this underlines the enormous interest in, and potential of, colorimetry as an agricultural tool and explains our principal objective in this study, which is to determine the evolution of the external colour of the fruit from the young state to harvest, in order to develop a maturity index for the pomegranate varietal group “Mollar of Elche”, thus providing growers with an objective criterion for deciding the optimal time to harvest.
Section snippets
Plant material
The cultivar was selected according to four main criteria: sweetness, soft-seeds, large fruit size and high yield. The commercial variety “ME14” was selected from the “Mollar de Elche” varietal group, which is one of the most highly valued worldwide because of its outstanding flavour and high antioxidant capacity (Calín-Sánchez et al., 2011). It is the most widely grown varietal group in Spain. The selected plant material belongs to the main pomegranate gene bank of the EU, located at the
Evolution of fruit colour
The colorimetric coordinates L* (lightness), a* (green-red) and b* (yellow-blue) increased from the beginning of fruit development to mid-September, coinciding with the highest mean maximum temperatures (around 33 °C). At this temperature, the value of a* is approximately zero, the fruit being in veraison stage. From mid-September onwards, coinciding with a marked fall in mean maximum temperatures, a* rapidly increased until reaching a maximum during the first week of October, when it stabilised
Conclusion
The proposed colorimetric index (MIc = L*·a*·b*−1) is straightforward, non-destructive, integrates the three colorimetric coordinates L*, a* and b* and is intuitive, since negative values indicate green (unripe) fruit, the value 0 indicates veraison and positive values are associated with fruit maturity.
The index divides the period of fruit development and ripening into four phases, which run from the beginning of fruit development (MIc < −20) (very green immature fruit) to ready-for-harvest fruit
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