Functional edible oils enriched with pistachio and walnut extracts -

Functional edible oils enriched with pistachio and walnut extracts

Development of functional edible oils with potential health promoting effects, enriched with phenolic-rich extracts obtained from pistachio and walnut. A high phenolic content, 10860 mg/kg and 7030 mg/kg in walnut and pistachio kernels respectively, with a corresponding strong radical scavenging effect were found and were potential as bioactive ingredients.

Materials and methods

Pistachio and walnut samples (3 kg each) were collected and the oil was extracted using a screw employing a nozzle of 6 mm of diameter and a screw speed of 30 rpm. The screw press was first run empty for 10–15 min to raise the screw-press barrel temperature to the minimum required for extracting the oil (about 50 °C) using the electrical resistance ring attached around the press barrel. During the process the crude oil (approximately 1.5 L of each variety) is separated leaving the corresponding residual partially defatted cake. Then the oil was centrifuged at 5,000 rpm in order to remove the residual plant material and stored in amber bottles without headspace to protect them from light and the residual cakes were place in labelled pouches and vacuum packed to prevent any oxidative degradation. All samples were stored at 4 °C in darkness until analyzed.

Total Polar Phenolic (TPP) content was analyzed by standard method using FCR, Total antioxidant capacity (TAC) were evaluated by two different methods: 2,2-diphenyl-1-(2,4,6-trinitrophenyl)hydrazyl (DPPH) radical assay and the oxygen radical absorbance capacity (ORAC). Individual phenolic compounds were determined by an HPLC-DADESI-MS/MS method. Oxidative Stability was expressed as the oxidation induction period (hours) measured with the Rancimat apparatus using an oil sample of 3.5 g warmed to 100 °C, and an air flow of 10 L/h.

Results

Walnut and pistachio nuts are recognized for their high phenolic content, 10860 mg/kg and 7030 mg/kg respectively − 1.5 times higher in walnut than pistachio in this study. The TPP in defatted cakes obtained after the cold press extraction of the corresponding virgin nut oils was significantly higher, as expected due to the separation of the oil (a fat content of 52% and 62%, for Pistachio and walnut respectively) and consequent concentration of the components in the residual partially defatted cakes, since the phenolic components of walnut and pistachio are quite polar.

Individual phenolic compounds LC-MS characterization.

The main families of phenolic compounds found in pistachio kernel and its extract similar to TPP results, a much higher concentration in the lyophilized extract (14.19 g/kg) as compared to the kernel (2.61 g/kg) is observed. Almost 90% of the phenolic compounds identified are accounted by the flavanols group, both in kernel and extract, being procyanidins (6.26 g/ kg in the extract) more abundant than catechins (4.05 g/kg) and epicatechins (2.40 g/kg). Much lower amounts of anthocyanins (0.90 g/kg, mainly cyanidin-3-O-galactoside), flavonols (0.28 g/kg, mainly myricetin-3-galactoside, kaempferol-3-galactoside and quercetin-3-galactoside), flavanones (0.11 g/kg, mainly eriodictyol-7-Oglucoside) and phenolic acids (0.18 g/kg) were observed.

In the lyophilized pistachio residual defatted cake similar results were found, which presented a slightly higher concentration (e.g. 13.58 g/kg of flavanols and a total of 15.93 g/kg). The phenolic content in the residual cake (4.32 g/kg) is more concentrated than the kernel (2.61 g/kg) due to the separation of the oily phase during processing. The main phenolic families found in walnuts kernel and its extract are hydrolysable tannins (2.13 and 91.9 g/kg in the kernel and its extract respectively). Its most abundant compound is Di-HHDP glucose (Di-Hexahydroxydiphenoylglucose) with a concentration of 0.9 g/kg in the kernel. Glansreginin – a dicarboxylic acid derivate – follows with 0.43 g/kg. Flavanols is the second family in importance, with 27% of the total (0.85 g/kg in the kernel). As already observed for pistachio, all the phenolic compounds identified in the kernel also appear in the corresponding residual partially defatted cake although as expected more concentrated due to the separation of the oily phase (about 60% of total weight) during the extraction process.

Enrichment of the vegetable oils and their characteristics

Eight different edible oils were enriched using both the Pistachio and Walnut Kernels (PKE and WKE) and Cakes (PCE and WCE) phenolic-rich lyophilized Extracts. The edible oils used were the Virgin Pistachio Oil (VPO) and Virgin Walnut Oil (VWO) produced in this study as well as Refined Olive Oil (ROO), practically not containing phenolics (TPP 50 mg/kg and DPPH 0,4 mmol/kg). As expected, the direct addition of the nut extracts did not allow a satisfactory enrichment of the oil in the phenolic compounds extracted from the nuts due to their polarity. An enrichment yield of 64–72% was obtained in all cases. TPP concentrations of 340–570 mg/kg have been reached in the different edible oils. A great increase in the DPPH antioxidant activity increase (AOinc) was observed in all the oils prepared, which was much higher when walnut extracts were employed than pistachio extracts. The enrichment of ROO gave antioxidant activity close to the corresponding enriched VPO or VWO. The calculated increase in the antioxidant activity (AOinc) is much smaller simply because the DPPH of ROO (0.4 mmol/kg) was much higher than VPO and VWO (0.07 mmol/kg). Apparently, the antioxidant activity provided by the kernel extracts is higher than that obtained from the corresponding residual cakes, even if the TPP content is higher in the cake: e.g. 54 mmol/kg and 42 mmol/kg DPPH in VWO enriched with kernel and cake extracts respectively. The oxidative stability of the enriched oils was also measured using the Rancimat apparatus (100 °C, 10 L/h air). VWO was the most susceptible to oxidation (induction period of 5.3 h), as expected by its high unsaturated fatty acid profile (> 60% linoleic and > 12% linolenic acids); its enrichment with WKE and WCE greatly increased its oxidative stability (1.8–2.4-fold higher). The Rancimat stability of VPO and ROO was similar, 40.1 and 42.4 h respectively. VPO enriched with pistachios extracts increased their IP a 30–40% (up to 56.3 and 56.3 h). Finally, the oxidative stability of enriched-ROO gave much better results using walnut extracts (e.g. 121.8 h, 2.9-fold higher) as compared to pistachio’s (e.g. 68.8 h, 1.6-fold higher), showing once again that walnut phenolic extracts apparently possess higher antioxidant capacity than pistachio.

Conclusions

These novels developed functional vegetable oils characterized by a well-established bioactive activity and phenolic content and profile, may imply a good strategy to ensure an optimal intake of polyphenols, since the daily intake of bioactive phenolic is very variable due to different proportion of the different foods of our diet.

Reference:

Fregapane, G., Guisantes-Batan, E., Ojeda-Amador, R.M. and Salvador, M.D., 2020. Development of functional edible oils enriched with pistachio and walnut phenolic extracts. Food Chemistry, 310, p.125917.