Extraction of phenolics from pomegranate residues: Selectivity induced by the methods
Graphical Abstract
Introduction
Pomegranate is a fruit widely grown in various countries from the Middle East, Far East, America, North Africa and South Europe [1]. Aside from its organoleptic and nutritional properties, pomegranate has antioxidant properties due to its high content in phenolics, anthocyanins and vitamin C [2]. In the last two decades, the demonstration of pomegranate healthy potential has led to a marked increase in its consumption especially as juices. In the few cases where juices are produced by squeezing arils only, peels and seeds come as separated residues that can be exploited as individual sources of phenolics and pomegranate oil, respectively. Actually, they are mostly produced by squeezing the whole fruits [3], an operation that releases huge amounts of residues that come as a mix of peels (73%) and seeds (27%) [3]. This residue is often disposed of without economic value though it contains the same bioactive compounds as the fruit edible parts. The recovery of these compounds of high potential for food, cosmetic or pharmaceutical applications [4] or the utilization of the residue as a source of functional fractions [5] would improve the eco-sustainability of the pomegranate processing industries [6].
The health potential of pomegranate fruits and extracts pertains to a wide range of biological properties generally attributed to the high content in polyphenolic components [7], [8] though no distinction is made between the various phenolics (anthocyanins, proanthocyanidins, hydrolysable tannins). However, the two major hydrolysable tannins of pomegranate, namely ellagic acid (EA) and punicalagin (PU), have shown distinct individual biological activities [1], [8], [9]. Among others, punicalagin has neuroprotective and chemotherapeutic effects [10], [11] and ameliorates obesity and obesity‐induced inflammatory responses[12] whereas ellagic acid exhibits antioxidant, anti-inflammatory, anti-diabetes properties and offers protection of liver [13], [14]. Thus, it would be worth implementing selective extractions in order to tune the extract composition upon the sought health outcome.
The extraction of antioxidant compounds from pomegranate by-products is extensively reported in the literature. However, most studies are carried out on peels [4], [8], [15], [16], [17], [18], [19] and only two report extractions from the whole residue [20], [21], with one in which peels and seeds were separated prior to their extraction [20]. Extractions are usually performed by maceration in aqueous and/or organic solvents [4], [15], with occasionally a technological assistance by ultra sound, high pressure or enzymes [7], [16], [18], [22], [23], [24], but very few studies report extraction by supercritical CO2 technology [16], [20], [24]. Regarding the extract characterization, studies mainly focus on the total phenolic content (TPC) that is an indicator of the antioxidant potential. On the other hand, pomegranate fruits are rich in carbohydrates like glucose and fructose [2]. Monitoring the extracts sugar content, a characteristic rarely addressed in literature, is therefore relevant.
The objectives and originality of the work were thus i) to compare various extraction techniques in terms of yields and extract composition, ii) the use of a whole pomegranate residue made of peels and seeds pomace, and iii) to address the extraction selectivity of ellagic acid and punicalagin. On the basis of conventional maceration extractions, the objective was to evaluate the possible added-value of adding scCO2 especially in terms of selectivity among extracted phenolic compounds. Various methods employing generally recognized as safe (GRAS) solvents were therefore implemented and compared, namely maceration in ethanol, in water and in their mixture, and extraction by scCO2:ethanol and pressurized ethanol.
Section snippets
Material
Fresh pomegranate fruits (Punica granatum L.) of El Gabsi variety were obtained from a local market in Gabes, Tunisia. Pomegranate fruits having approximately the same size (≈8 cm diameter) were collected at full maturity and stored at 4 °C until use (maximum storage time was 14 days).
Ethanol for extraction (ethanol 96%) and solvents for HPLC (HPLC grade methanol 99.8%, formic acid 98%) were purchased from Atlantic Labo (Bruges, France). Deionized water with a resistivity of 15 mΩ.cm (ELGA,
Yields and kinetics data
Table 1 reports the specific yields obtained for different compounds or class of compounds when using the various techniques.
Conclusion
In this work, extractions were performed on a pomegranate residue made of peels and aril pomace to represent an industrial juice processing waste. Several extraction techniques employing different GRAS solvents and under different pressure conditions were implemented. For the first time, a systematic analysis of sugar extraction yields and measurement of sugar contents in extracts was performed. All extractions gave large extraction yields, up to 61%, due to the high extraction of sugars even
Declaration of Competing Interest
The authors declare no conflict of interest.
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Acknowledgments
The authors gratefully acknowledge Gabes University, Tunisia, for the B. Khoualdia's 2-months scholarship at CBMN in France.
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