In-Depth Chemical Characterization of Punica granatum L. Seed Oil

Abstract

Fruit seeds belonging to the pomegranate cultivar "Granata" were subjected to extraction and oily component analysis, with the aim of obtaining information about their composition. The presence of conjugated isomers of linolenic acid (CLNA isomers) in the oily phase extracted from the seeds gives a high added value to this part of the fruit, which is too often considered and treated as waste. The separated seeds were subjected to a classic Soxhlet extraction with n-hexane or extraction with supercritical CO₂, assisted by ethanol. The resulting oils were evaluated by ¹H and ¹³C-NMR and AP-MALDI-MS techniques. Differences in the triacylglycerols composition, with particular regard to punicic acid and other CLNA content, were studied in depth. Results showed the prevalence of punicic acid in the triacylglycerol mixture up to the 75%, with clear preponderance in the extract by supercritical fluids. Consequently, other CLNA isomers are, altogether, two-fold less represented in the supercritical extract than in the Soxhlet one. The two oily residues were subjected to solid phase extraction (SPE) and to HPLC-DAD analysis for the polyphenolic isolation and characterization. In addition to HPLC analysis, which showed different content and composition, DPPH analysis to evaluate the antiradical potential showed that the extract obtained with supercritical CO₂ was much more active.

Keywords: 1H and 13C-NMR analysis; AP-MALDI-MS analysis; HPLC-DAD analysis; pomegranate seed oil.

Figure 1

Amplification of the TAG region in the 820-960 mass range of the AP-MALDI mass spectrum.

Figure 2

¹³C NMR diagnostic olefinic region related to transesterified Soxhlet oil, 1a, (A), transesterified scCO₂ oil, 2a (B).

Figure 3

Example chromatograms of polyphenols fraction. (A): Sample 1b (SPE Soxhlet oil), polyphenolic profile at 280 and 360 nm; (B): Sample 2b (SPE CO₂ oil) polyphenolic profile at 280 and 360 nm. Identified peaks: 1. Coumaric acid; 2. Carvacrol; 3. Thymol; 4. Quercetin-3-D-galactoside.

Figure 3

Example chromatograms of polyphenols fraction. (A): Sample 1b (SPE Soxhlet oil), polyphenolic profile at 280 and 360 nm; (B): Sample 2b (SPE CO₂ oil) polyphenolic profile at 280 and 360 nm. Identified peaks: 1. Coumaric acid; 2. Carvacrol; 3. Thymol; 4. Quercetin-3-D-galactoside.

Figure 4

Antiradical activity of Soxhlet oil, 1 and scCO₂ oil, 2, compared to a mediated result obtained from 45 samples of EVOOs. Results are reported as GAE mg/g oil.