Extraction of Polyphenols from Olive Leaves Employing Deep Eutectic Solvents: The Application of Chemometrics to a Quantitative Study on Antioxidant Compounds

Stability study and validation of a liquid chromatography tandem mass spectrometry method for the quantitative analysis of polyphenols in fish feed ingredients

The perception of polyphenols as a safe, healthy, and sustainable solution for replacing synthetic antioxidants has been an important factor for their rapid growing in the global food market. Therefore, it is essential to use reliable methods for their quantification in commercial products intended for animal or human consumption. The purpose of this study is to evaluate the performance of some solvents used for the extraction of selected polyphenols, explore their stability under different experimental conditions, and validate a liquid chromatography tandem mass-spectrometry method for their quantification in commercial fish feed ingredients by using the standard addition method. The regression models for gallic acid, hydroxytyrosol, catechin, oleuropein, carnosol and carnosic acid were linear in the range 0-30 μg/mL, limit of detection and quantification around 0.03 and 0.1 μg/mL, respectively, and accuracy within ± 15 % of the nominal concentrations. The method was successfully applied to the determination of specific polyphenols in commercial fish feed ingredients supplemented with polyphenols from olive and rosemary extracts.

Olive Oil Waste as a Source of Functional Food Ingredients: Assessing Polyphenolic Content and Antioxidant Activity in Olive Leaves

Around two million tons of olive oil are produced in Europe annually, with Portugal being among the top five European olive oil-producing countries. Olive oil production results in a substantial amount of waste in the form of olive leaves. These discarded olive leaves contain valuable phenolic compounds with antioxidant, anti-inflammatory, hypoglycaemic, neuroprotective, and antiproliferative properties. Due to their richness in polyphenols with health-promoting properties, olive leaves can be considered a potential functional food ingredient. Thus, sustainable practices for reusing olive leaf waste are in demand. In this study, the polyphenolic content in olive leaves from different Portuguese locations was determined using HPLC-UV-Vis after defining the best fit-for-purpose liquid extraction strategy. The differences in the in vitro antioxidant activity in these samples were determined by several methodologies based on radical scavenging (against 2,2'-azino-bis-3-ethylbenzthiazoline-6-sulphonic acid (ABTS), 2,2-diphenyl-2-picrylhydrazyl (DPPH), and peroxyl radical (ORAC)) and on reducing properties (cupric-reducing antioxidant capacity (CUPRAC), and Folin-Ciocalteu assay (FC)), to unveil the relationship between the profile and quantity of polyphenols with antioxidant mechanisms and their capacity. At last, the stability of extracted compounds upon lyophilization and exposition to surrogate biological fluids was assessed, envisioning the future incorporation of olive leaves extracted compounds in food products.

Alternative Assisted Extraction Methods of Phenolic Compounds Using NaDESs

A renewed understanding of eco-friendly principles is moving the industrial sector toward a shift in the utilization of less harmful solvents as a main strategy to improve manufacturing. Green analytical chemistry (GAC) has definitely paved the way for this transition by presenting green solvents to a larger audience. Among the most promising, surely DESs (deep eutectic solvents), NaDESs (natural deep eutectic solvents), HDESs (hydrophobic deep eutectic solvents), and HNaDESs (hydrophobic natural deep eutectic solvents), with their unique features, manifest a wide-range of applications, including their use as a means for the extraction of small bioactive compounds. In examining recent advancements, in this review, we want to focus our attention on some of the most interesting and novel 'solvent-free' extraction techniques, such as microwave-assisted extraction (MAE) and ultrasound-assisted extraction (UAE) in relation to the possibility of better exploiting DESs and NaDESs as plausible extracting solvents of the phenolic compounds (PCs) present in different matrices from olive oil components, such as virgin olive pomace, olive leaves and twigs, virgin and extra virgin olive oil (VOO and EVOO, respectively), and olive cake and olive mill wastewaters (OMWW). Therefore, the status of DESs and NaDESs is shown in terms of their nature, efficacy and selectivity in the extraction of bioactive phytochemicals such as secoiridoids, lignans, phenolic acids and alcohols. Related studies on experimental design and processes' optimization of the most promising DESs/NaDESs are also reviewed. In this framework, an extensive list of relevant works found in the literature is described to consider DESs/NaDESs as a suitable alternative to petrochemicals in cosmetics, pharmaceutical, or food applications.

White rice enrichment with phenols upon cooking in olive leaf infusion: a preliminary study

BACKGROUND

White rice is poor in health-promoting phytochemicals; therefore, the production of a phenol-enriched commodity is highly desirable. Recent findings on its enrichment via cooking in plant extracts are promising, yet studies employing aqueous extracts of olive leaves (OLs), containing well-recognized bioactive phenols (e.g. oleuropein) are absent. In addition, little is known about the levels of phenols that are maintained after rice drying and rehydration, an important aspect for the future design of 'ready-to-eat' functional rice.

RESULTS

The examination, for the first time, of white rice adsorption capacity of phenols from OLs upon cooking in infusions containing different levels of phenols, after freeze-drying and rehydration, showed the following: (i) the total phenol content, the antioxidant activity (assessed via 2,2-diphenyl-1-picrylhydrazyl radical and ferric reducing antioxidant power assays), the oleuropein and luteolin-7-O-glucoside levels increased dose dependently; (ii) upon rehydration, the average decrease of total phenol content and antioxidant activity values was significantly lower when an exact volume of water was used compared with an excess (~10% versus 63%). A similar trend was observed for oleuropein (36% versus 83%) and the luteolin-7-O-glucoside (24 versus 82%) levels; (iii) the dried enriched kernels were less bright with a hay-yellow hue (CIELab coordinates).

CONCLUSION

White rice enrichment with biophenols from OLs, a by-product of olive tree cultivation, was successful using a simple approach. Despite leaching upon freeze-drying/rehydration, sufficient amounts were maintained to obtain a functional rice that could serve as an alternative dietary source of OLs phenols to non-traditional olive tree product consumers or those refraining from sodium and fats. © 2023 Society of Chemical Industry.

Green Extraction of Antioxidant Compounds from Olive Tree Leaves Based on Natural Deep Eutectic Solvents

Agri-food industries generate a large amount of waste that offers great revalorization opportunities within the circular economy framework. In recent years, new methodologies for the extraction of compounds with more eco-friendly solvents have been developed, such as the case of natural deep eutectic solvents (NADES). In this study, a methodology for extracting phenolic compounds from olive tree leaves using NADES has been optimized. The conditions established as the optimal rely on a solvent composed of choline chloride and glycerol at a molar ratio of 1:5 with 30% water. The extraction was carried out at 80 °C for 2 h with constant agitation. The extracts obtained have been analyzed by high-performance liquid chromatography coupled to tandem mass spectrometry (HPLC-MS/MS) in MRM mode. The comparison with conventional ethanol/water extraction has shown that NADES, a more environmentally friendly alternative, has improved extraction efficiency. The main polyphenols identified in the NADES extract were Luteolin-7-O-glucoside, Oleuropein, 3-Hydroxytyrosol, Rutin, and Luteolin at the concentrations of 262, 173, 129, 34, and 29 mg kg^-1 fresh weight, respectively.

Phytochemicals Derived from Agricultural Residues and Their Valuable Properties and Applications

Billions of tons of agro-industrial residues are produced worldwide. This is associated with the risk of pollution as well as management and economic problems. Simultaneously, non-edible portions of many crops are rich in bioactive compounds with valuable properties. For this reason, developing various methods for utilizing agro-industrial residues as a source of high-value by-products is very important. The main objective of the paper is a review of the newest studies on biologically active compounds included in non-edible parts of crops with the highest amount of waste generated annually in the world. The review also provides the newest data on the chemical and biological properties, as well as the potential application of phytochemicals from such waste. The review shows that, in 2020, there were above 6 billion tonnes of residues only from the most popular crops. The greatest amount is generated during sugar, oil, and flour production. All described residues contain valuable phytochemicals that exhibit antioxidant, antimicrobial and very often anti-cancer activity. Many studies show interesting applications, mainly in pharmaceuticals and food production, but also in agriculture and wastewater remediation, as well as metal and steel industries.

Antioxidant and Antimicrobial Activity of Hydroethanolic Leaf Extracts from Six Mediterranean Olive Cultivars

Phenolic profiles, antioxidant, and antimicrobial activities of hydroethanolic olive leaf extracts from six Mediterranean olive cultivars (Croatian: Lastovka, Levantinka, Oblica; Italian: Moraiolo, Frantoio, Nostrana di Brisighella) were investigated. As expected, various distributions of phenolic levels were observed for each cultivar and the total phenolic content showed high variability (ranging from 4 to 22 mg GAE/g of dry extract), with the highest amount of phenolics found in the Oblica sample, which also provided the highest antiradical (ORAC) and reducing activity (FRAP). The screening of individual compounds was performed by HPLC-PDA-ESI-QTOF-MS and the main detected compounds were oleuropein, hydroxytyrosol, oleoside/secologanoside, verbascoside, rutin, luteolin glucoside, hydroxyoleuropein, and ligstroside. While the antioxidant activity of the samples was relatively high, they showed no bactericidal and bacteriostatic activity against E. coli and S. Typhimurium; weak activity against Staphylococcus aureus, Bacillus cereus, and Listeria innocua; and inhibitory effects against Campylobacter jejuni at 0.5 mg dry extract/mL. The obtained results support the fact that olive leaf extracts, and especially those from the Oblica cultivar, could potentially be applied in various industries as natural preservatives and effective and inexpensive sources of valuable antioxidants.

Extraction of Polyphenolic Antioxidants from Red Grape Pomace and Olive Leaves: Process Optimization Using a Tailor-Made Tertiary Deep Eutectic Solvent

In the framework of introducing green strategies for food processing, the industrial orientation has shifted towards the replacement of conventional petroleum-based solvents with alternative eco-friendly ones. On this basis, the objective of this study was to synthesize a novel, tertiary, food-grade deep eutectic solvent, composed of glycerol, citric acid, and L-proline (GL-CA-Pro), and to test it as a solvent for the extraction of polyphenols from agri-food waste biomass. After an initial screening on various common residual materials (apple peels, lemon peels, orange peels, red grape pomace, olive leaves), evidence emerged that indicated GL-CA-Pro was more effective than other DESs commonly used for polyphenol extraction. Furthermore, extracts from red grape pomace (RGP) and olive leaves (OLL) were shown to contain higher level of total polyphenols and increased antioxidant activity. Process optimization for those two materials with the response surface methodology revealed that the major difference pertained to the extraction time. In addition, for both materials, GL-CA-Pro was shown to provide higher total polyphenol yields (53.25 and 42.48 mg gallic acid equivalents per g of dry mass, respectively) compared to water and 60% aqueous ethanol. However, the chromatographic analyses for OLL suggested aqueous ethanol was a more suitable solvent for some principal polyphenolic constituents. The RGP extract produced with GL-CA-Pro exhibited significantly stronger antioxidant effects compared to the aqueous and hydroethanolic extracts, but the outcome for the OLL extracts was diversified. It was concluded that GL-CA-Pro is a very efficient solvent for RGP polyphenols, but its efficiency regarding OLL was comparable to that of aqueous ethanol.