Changes in physical properties, chemical and elemental composition and antioxidant capacity of pomegranate (cv. Ruby) fruit at five maturity stages

Abstract

This study was conducted to investigate the physical and compositional changes as well as antioxidant properties of pomegranate fruit (cv. Ruby) at five distinct maturity stages between 54 and 139 days after full bloom (DAFB). Physico-chemical properties such as fruit mass, size, juiciness, colour, total soluble solids (TSS), pH, titratable acidity (TA), individual organic acids and sugars, and phenolic composition were investigated. Mineral element concentrations were determined using inductively coupled plasma optical emission spectrometry (ICP-OES) while total antioxidant capacity was measured by DPPH scavenging activity (DPPH) and ferric ion reducing power (FRAP). Results showed that major compositional changes in fruit are developmentally regulated. Significant increases in total soluble solids (TSS), sugars (glucose and fructose) and anthocyanins composition, coupled with significant decline in titratable acidity (TA), organic acids and total phenolics occurred with advancing maturity. Principal component analysis (PCA) showed that fruit at advanced maturity stages (132 and 139 DAFB) were characterized by intense peel and aril pigmentation and better juice quality. PCA results also showed that peel and aril colour attributes and indices of sugar/acid ratio (TSS/TA and BrimA index) could be useful measures of fruit maturity and ripeness for ‘Ruby’ pomegranate cultivar, and therefore might be suitable for the development of reliable fruit maturity index to assess fruit optimal maturity.

Introduction

For most fruit, advancing maturity corresponds to a number of coordinated physiological, biochemical, and structural processes that result in changes in size, colour and flavour, making the fruit desirable for consumption (Moing et al., 1998, Nunes et al., 2009, Opara, 2000). Pomegranate fruit is usually consumed as fresh arils, or as processed products, mostly juice. The edible part of the fruit (aril) contains considerable amounts of acids, sugars, polyphenol and important minerals (Al-Maiman and Ahmad, 2002).

Optimum fruit maturity is crucial for maintaining high total soluble sugar (TSS) content, good colour and overall fruit quality. Fruit external skin colour is not a good indicator of the degree ripening or readiness for consumption of pomegranate (Holland et al., 2009); hence, it is imperative that fruit maturity status of pomegranate cultivars is assessed based on a combination of parameters that determine its physical and sensory attributes. Recent studies have shown that fruit cultivar and maturity status influence the antioxidant activity and other physicochemical properties of pomegranate such as aril yield, pH, total soluble solids (TSS), total sugars, titratable acidity (TA), total phenolics and anthocyanins as well as mineral elements composition (Al-Maiman and Ahmad, 2002, Al-Said et al., 2009, Opara et al., 2009, Shwartz et al., 2009). During pomegranate fruit maturation, significant changes in organic acids, sugars and phenolic composition have been reported by various authors (Ben-Arie et al., 1984, Gil et al., 1995, Melgarejo et al., 2000, Poyrazoglu et al., 2002, Al-Maiman and Ahmad, 2002, Mirdehghan and Rahemi, 2007). According to the authors, fruit chemical, phenolic and antioxidant properties are also influenced by cultivar, growing region and degree of fruit maturity at harvest.

Commercial production of pomegranates is fairly new and increasing rapidly in South Africa and the cultivar ‘Ruby’ is one of the most widely grown in the country and globally (Holland et al., 2009, Kumar, 2009). Although changes in maturity indicators during fruit growth and development have been described in a number of pomegranate cultivars grown in different agro-climatic regions (Ben-Arie et al., 1984, Gil et al., 1995, Al-Maiman and Ahmad, 2002, Shwartz et al., 2009), there is a dearth of scientific information on the pomegranate cultivars grown in South Africa. The objective of the present study was to investigate the changes in physical and compositional attributes, as well as the antioxidant capacity of pomegranate fruit (cv. ‘Ruby’) at different fruit maturity stages. Knowledge of the changes in fruit maturity attributes during the time course of fruit growth and development would be useful in efforts to develop objective maturity indices for harvest and postharvest management.