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University of Melbourne (2017)

Potential nutritional effects of date (Pheonix dactylifera) seeds : a study on polyphenols

Sirisena, Sameera

Titre : Potential nutritional effects of date (Pheonix dactylifera) seeds : a study on polyphenols

Auteur : Sirisena, Sameera

Université de soutenance : University of Melbourne

Grade : Doctor of Philosophy (PhD) 2017

Résumé partiel
Date (Phoenix dactylifera L.) seeds are abundant by-products from the date fruit industry. Date seed extracts and powders have shown various beneficial bioactivities such as protection against neurodegeneration and oxidative stress. It has been suggested that the main compounds responsible for the high antioxidant and biological activities in date seeds are polyphenolic compounds. In addition, potential food applications developed from date seeds have shown promising results, which can ultimately contribute to the sustainability and value addition in the date fruit industry. This study investigated the chemical composition, starch digestive enzyme inhibitory activity and in-vitro bioaccessibility of date seed polyphenols.
Seeds of five date palm varieties grown in Australia (Medjool, Deglet nour, Barhee, Bou sthammi and Dayrie) were used in this study. Free (also called extractable) polyphenols (FPP) were extracted from seed powders using 50% aqueous acetone, and the residue was extracted with HCl-butanol to obtain the bound (also called non-extractable) polyphenols (BPP). The extracts were examined for their total phenolic content (TPC), total flavonoid content (TFC), total antioxidant capacity (TAC), and starch digestive enzyme inhibition potential in-vitro. Deglet nour variety showed the highest TPC of 4.16±0.22g Gallic Acid Equivalent (GAE)/100g dry weight, and Ferric Reducing Antioxidant Power (FRAP) of 1.58±0.23 mM Fe2+ Equivalent/g dry weight in the FPP extract. Date seed extracts (FPP and BPP) significantly (P<0.05) inhibited the activity of rat intestinal α-glucosidase in-vitro, and showed weaker inhibition against porcine pancreatic α-amylase. Notably, BPP extracts showed stronger α-glucosidase inhibition measured as IC50 (concentration required to achieve 50% inhibition) compared with FPP, which ranged from 0.39±0.02 mg GAE/mL (Deglet Nour) to 0.68±0.02 mg GAE/mL (Medjool). IC50 values for FPP ranged from 0.907±0.08 mg GAE/mL (Medjool) to 1.75±0.15 mg GAE/mL (Barhee-khalal). Consequently, the bound polyphenol fraction was therefore identified as the main source of polyphenols with inhibitory activities against α-glucosidase and α-amylase. The detected high amounts of BPP in the date seed powder suggested the potential use of this by-product as a functional food with anti-hyperglycaemic properties.
Date seed FPP and BPP profiles of Deglet Nour variety were investigated using HPLC and HPLC-MS methods. BPP extract was used in HPLC without further purification or separation. The FPP extract was further processed with Solid Phase Extraction using Supelco® Supel-select HLB cartridges to obtain a fraction free of polymeric material, facilitating better peak resolution. An Ultimate 3000 RS UHPLC coupled to a TSQ Quantiva triple-quadrupole mass spectrometer was used for the HPLC-MS analysis. Selective reaction monitoring (SRM) mode and the selected ion monitoring (SIM) modes were used for detection and analysis. Tentative HPLC-MS identifications revealed that FPP extract contained flavan-3-ol (proanthocyanidins) monomers, dimers, trimers and tetramers. Additionally, several glycosylated flavones and flavonols, simple phenolic acids as well as two A-type dimers were also detected. A-type proanthocyanidin dimers have not been reported in date seeds before. BPP extract was found to consist of flavan-3-ols of various degrees of polymerisation, in addition to 3 dihydroxybenzoic acid (DHBA) isomers. From those MS identifications several major compounds were confirmed by HPLC using an internal spiking method with commercially obtained standards, and subsequently quantified using their respective HPLC standard curves. The quantifications of FPP included procyanidin B1 (499.8±7.8 μg/g DSP), procyanidin B2 (288.6±6.1 μg/g DSP), catechin (167.6±2.1 μg/g DSP), epicatechin (39.44±0.39 μg/g DSP), and protocatechuic acid (1.77±0.22 μg/g DSP). One of the 2 A-type dimers was confirmed as procyanidin A2 (24.05±0.12 μg/g DSP). In the BPP extract, epicatechin (52.59±0.76 μg/g DSP), procyanidin B2 (294.2±3.7 μg/g DSP), and protocatechuic acid (2.138±0.025 μg/g DSP) were quantified. These findings have contributed to our knowledge of date seed phytochemicals and would provide understanding of their contribution to bioactivities.

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