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CHEMISTRY

CHEMISTRY

In addition to the conventional nutritional compounds noted just above, PJ was reported to be comprised of 85.4% water, 10.6% total sugars, 1.4% pectin, 0.2–1.0% polyphenols, and organic acids.5,17,18 Other reported minor compounds include fatty acids, sterols, triterpenoids, and α-tocopherol.19 The quantitative profile of PJ constituents has been found to vary considerably though, depending upon the cultivar, geoclimatic factors, harvesting, processing, and storage conditions.20,21

The polyphenols are responsible for the astringent quality of the juice. All parts of the fruit contain polyphenols, including the peel, the inner membranes, and the arils; although the pericarp (peel and membrane) is reported to contain the highest concentrations.21

PJ contains 2 major classes of polyphenolic compounds: hydrolyzable tannins (HT) and flavonoids, with very small levels of condensed tannins. Figure 1 shows the chemical structures of the most abundant of these constituents.

The HT are the predominant polyphenols in PJ and include ellagitannins, gallotannins, and gallagoyl esters.21 The most abundant HT are the gallagyl tannins punicalagins (anomers A and B) and related tannins, which comprise roughly 63% of the constituent polyphenols.21 Other reported HT include pedunculagin, punicalin, gallagic and ellagic acid esters of glucose (16.8%), and ellagic acid derivatives (4.9%).19,21 Figure 2 shows a breakdown of the phenolic compounds in PJ.

The flavonoid component consists mainly of anthocycanidins, flavanols, and flavanol glucosides. The anthocyanidins are the pigments which imbue the juice with its ruby red color and include the compounds cyanidin; cyanidin-3-glucoside; cyanidin 3,5-diglucoside; cyanidin-3-rutinoside; delphinidin; delphinidin-3-glucoside; delphinidin 3,5-glucoside; pelargonidin-3-glucoside; and pelargonidin-3,5-glucoside (see Figure 3).20,22,23,24 Of these, cyanidin derivatives appear to be the most abundant, followed by delphinidin glucosides.21,25 Other flavonoid constituents include kaempferol, myricetin, rutin, narigenin, luteolin and luteolin glycosides, quercetin, and quercetin glucosides.26-29

The main condensed tannins found in the juice are catechin, procyanidin B1, and procyanidin B2.26 Gallocatechin and its derivatives have also been reported.30 Other minor phenolics include gallic acid, protocatechuic acid, catechin, and phloridzin.31

 

Figure 1. Chemical Structure of Phenolic Compounds in Pomegranate Juice

 

Citric acid is the most abundant organic acid in fresh PJ, followed by malic acid.31,32 Other reported minor organic acids include caffeic, chlorogenic, cinnamic, coumaric, ferulic, gallic, malic, oxalic, protocatechuic, quinic, succinic, and tartaric acid.26,31,32

Although the steroid hormone estrone (and to a lesser extent estriol, 17-β-estradiol, and testosterone) have been reported in pomegranate seed,28,33-37 these findings were not confirmed with definitive mass spectrometry and/or nuclear magnetic resonance analyses. More accurate evaluations utilizing high power liquid chromatography and gas chromatograph mass spectrometer did not detect any of these compounds in pomegranate seed or PJ38 or a pomegranate fruit extract (PFE).29

Similarly, the sterols β-sitosterol, camesterol, cholesterol, daucosterol, and stigmasterol, and the coumestan coumestrol have been reported in pomegranate seed,35,37,39 but these compounds have not been found in PJ.