Correction: Ghani et al. Multi-Dimensional Antioxidant Screening of Selected Australian Native Plants and Putative Annotation of Active Compounds. Molecules 2023, 28, 3106.

In the original publication [...].

Multi-Dimensional Antioxidant Screening of Selected Australian Native Plants and Putative Annotation of Active Compounds.

Acacia implexa, Eucalyptus rossii and Exocarpos cupressiformis are native plants of Australia, which were used by the First Peoples for medicinal purposes. In this study, 70% aqueous ethanol crude extracts were prepared from A. implexa bark and leaves, E. rossii leaves and E. cupressiformis leaves, and partitioned via sequential extraction with n-hexane, dichloromethane (DCM), ethyl acetate and ethanol. The crude extracts and fractions were screened for antioxidant activity using a novel, high-throughput lipid-based antioxidant assay, as well as the aqueous ABTS (2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)) assay and the Folin-Ciocalteu test for total phenols. In the lipid-based assay, non-polar n-hexane and DCM fractions showed higher antioxidant activity against the formation of peroxides and thiobarbituric acid reactive substances (TBARS) than the other fractions, whereas the non-polar fractions were not effective in aqueous assays. This illustrates that the high potential of the lipid-soluble n-hexane and DCM fractions as antioxidants would have been missed if only aqueous-based assays were used. In addition, the potent antioxidant compounds were putatively annotated using liquid chromatography quadrupole time-of-flight mass spectrometry (LC-qTOF-MS). Gallic acid, (+)-catechin, (-)-epicatechin and tannins were found in most crude extracts.

Development of a Method Suitable for High-Throughput Screening to Measure Antioxidant Activity in a Linoleic Acid Emulsion.

An improved system for measuring antioxidant activity via thiobarbituric acid reactive substances and ferric thiocyanate assays is reported, on the basis of oxidation of a linoleic acid (LA) emulsion. Oxidation times were reduced from 20 h to 5 h by increasing the reaction temperature from 37 °C to 50 °C and with an acceptable precision of <10% coefficient of variation (CV). Antioxidants varying in polarity and chemical class-250 µM Trolox, quercetin, ascorbic acid and gallic acid-were used for method optimisation. Further reductions in reaction time were investigated through the addition of catalysts, oxygen initiators or increasing temperature to 60 °C; however, antioxidant activity varied from that established at 37 °C and 20 h reaction time-the method validation conditions. Further validation of the method was achieved with catechin, epicatechin, caffeic acid and α-tocopherol, with results at 50 °C and 5 h comparable to those at 37 °C and 20 h. The improved assay has the potential to rapidly screen antioxidants of various polarities, thus making it useful in studies where large numbers of plant extracts require testing. Furthermore, as this assay involves protection of a lipid, the assay is likely to provide complementary information to well-established tests, such as the 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay.

Measurement of antioxidant activity with the thiobarbituric acid reactive substances assay.

The thiobarbituric acid reactive substances (TBARS) assay is widely used to measure lipid oxidation and antioxidant activity in food and physiological systems. However, there has been no review (to our knowledge) that focuses exclusively on this test. This review presents an overview of the current use of the TBARS test in food and physiological systems, before looking at the various ways in which the assay is used in studies on antioxidant activity. As an antioxidant assay, the TBARS test may lack acceptable reproducibility, and long reaction times may preclude its adoption as a rapid screening method. Despite these potential limitations, there are features of the TBARS test that make it useful as a complement to popular screening tests such as Trolox equivalent antioxidant capacity. This review concludes with proposals for development of the TBARS test so that it can be used as a rapid and robust antioxidant assay.

Effect of processing conditions, prestorage treatment, and storage conditions on the phenol content and antioxidant activity of olive mill waste.

The impact of two- and three-phase processing systems and malaxation conditions on phenol content (both total and individual phenols) and antioxidant capacity of laboratory-generated olive mill waste (OMW) was assessed. Two-phase olive processing generated a waste with higher phenol content and antioxidant capacity. Using the two-phase system, both malaxation time and temperature affected the phenol content and antioxidant capacity. The effects of different prestorage drying treatments on phenol content and antioxidant capacity were also compared. Air drying and drying at 60 degrees C resulted in a substantial decrease in the phenol content and antioxidant capacity. Drying at 105 degrees C and freeze-drying produced less degradation. The phenol content and antioxidant capacity of OMW stored at 4 degrees C and of OMW preserved by 40% w/w ethanol and 1% w/w acetic acid and stored at 4 degrees C were monitored for 30 days and compared with those of OMW stored at room temperature. None of these storage conditions could prevent the rapid decrease in phenolic concentrations and antioxidant capacity, which happened within the first 24 h.

Changes in virgin olive oil quality during low-temperature fruit storage.

'Frantoio' olive fruits were stored at low temperature (4 +/- 2 degrees C) for 3 weeks to investigate the effect of postharvest fruit storage on virgin olive oil quality. Volatile compounds and phenolic compounds explained the changes in sensory quality that could not be explained with quality indices (FFA, PV, K232, and K270). Increases in concentrations of ( E)-2-hexenal and hexanal corresponded to positive sensory quality, whereas increases in ( E)-2-hexenol and (+)-acetoxypinoresinol were associated with negative sensory quality. Volatile and phenolic compounds were also indicative of the period of low-temperature fruit storage. Oleuropein and ligstroside derivatives in olive oil decreased with respect to storage time, and their significant ( p < 0.05) change corresponded to changes in bitterness and pungency. ( Z)-2-Penten-1-ol increased during low-temperature fruit storage, whereas 2-pentylfuran decreased. Changes in volatile compounds, phenolic compounds, quality indices, and sensory notes indicated that virgin olive oil quality was lost within the first week of low-temperature fruit storage and regained at 2 weeks. This research suggests that low-temperature olive fruit storage may be beneficial, with a possibility of increasing oil yield and moderating the sensory quality of virgin olive oils. This study demonstrates that deeper insights into virgin olive oil quality changes during low-temperature fruit storage may be gained by studying volatile and phenolic compounds in addition to quality indices and physical appearance of the fruit.

Chemical screening of olive biophenol extracts by hyphenated liquid chromatography.

Chemical screening using reversed phase HPLC-photodiode array detection (RPLC-DAD) and RPLC-electrospray ionisation mass spectrometry (RPLC-ESI-MS) is widely applied as an approach to streamline natural products research. The full potential of this approach is demonstrated in this paper by application to the chemical screening of olive products including olive mill waste (OMW). Out of 100 biophenols previously reported in olive products, the on-line RPLC-DAD-ESI-MS was able to confirm the presence of 52 compounds in OMW. This included a number of simple phenols, flavonoids and secoiridoids. By careful examination of the combined DAD and ESI-MS data, extra information was elucidated including: the site of glycosidation on the phenol ring of hydroxytyrosol; the identity of the other luteolin-glucoside isomer as luteolin-4'-O-glucoside; identifying rutin rather than the previously reported hesperidin (and the reasons for possible mis-assignment); and the detection of diastereomers of 4-hydroxyphenylethyl alcohol-deacetoxy elenolic acid dialdehyde (4-HPEA-DEDA) and 3,4-dihydroxyphenylethyl alcohol-deacetoxy elenolic acid dialdehyde (3,4-DHPEA-DEDA).

Effect of added caffeic acid and tyrosol on the fatty acid and volatile profiles of camellia oil following heating.

Camellia oil is widely used in some parts of the world partly because of its high oxidative stability. The effect of heating a refined camellia oil for 1 h at 120 degrees C or 2 h at 170 degrees C with exogenous antioxidant, namely, caffeic acid and tyrosol, was studied. Parameters used to assess the effect of heating were peroxide and K values, volatile formation, and fatty acid profile. Of these, volatile formation was the most sensitive index of change as seen in the number of volatiles and the total area count of volatiles in gas chromatograms. Hexanal was generally the dominant volatile in treated and untreated samples with a concentration of 2.13 and 5.34 mg kg(-1) in untreated oils heated at 120 and 170 degrees C, respectively. The hexanal content was significantly reduced in heated oils to which tyrosol and/or caffeic acid had been added. Using volatile formation as an index of oxidation, tyrosol was the more effective antioxidant of these compounds. This is contradictory to generally accepted antioxidant structure-activity relationships. Changes in fatty acid profiles after heating for up to 24 h at 180 degrees C were not significant.

Changes in volatile and phenolic compounds with malaxation time and temperature during virgin olive oil production.

Virgin olive oils produced at wide ranges of malaxation temperatures (15, 30, 45, and 60 degrees C) and times (30, 60, 90, and 120 min) in a complete factorial experimental design were discriminated with stepwise linear discriminant analysis (SLDA) revealing differences with processing conditions. Virgin olive oils produced at 15 and 60 degrees C for 30 min showed the most significant (p < 0.01) differences. Discrimination was based upon volatile and phenolic compounds detected in olive oils, peroxide value (PV), free fatty acids (FFA), ultraviolet (UV) absorbances, and oil yield. There were different discriminating variables for processing conditions illustrating the dependence of virgin olive oil quality on malaxation time and temperature. Volatile compounds were the dominant discriminating variables. Common oxidation indicators of olive oil (PV, K232, and K270) were not among the variables that significantly (p < 0.01) changed with malaxation time and temperature. Variables that discriminated both malaxation time and temperature were hexanal, 3,4-dihydroxyphenyl ethyl alcohol-decarboxymethyl elenolic acid dialdehyde (3,4-DHPEA-DEDA) and FFA, whereas 1-penten-3-ol, E-2-hexenal, octane, tyrosol, and vanillic acid significantly (p < 0.01) changed with temperature only and Z-2-penten-1-ol, (+)-acetoxypinoresinol, and oil yield changed with time only. Virgin olive oil quality was significantly influenced by malaxation temperature, whereas oil yield discriminated malaxation time. This study demonstrates the two modes of hexanal formation: enzymatic and nonenzymatic during virgin olive oil extraction.

Discrimination of storage conditions and freshness in virgin olive oil.

Virgin olive oil samples stored in the light at ambient temperature, in the dark at ambient temperature, and at low temperature in the dark for 12 months both with and without headspace were separated into recognizable patterns with stepwise linear discriminant analysis. The discrimination with variables volatile and phenolic compounds, free fatty acid (FFA), peroxide values, K232, and K270 revealed a departure of stored oil from freshness and showed significant (p < 0.01) differences between storage conditions. Virgin olive oil stored at low temperature had characteristics closest to fresh oil while oil stored in the light showed the largest departure from freshness. Parameters that exclusively and significantly (p < 0.01) discriminated storage conditions were identified as potential markers of the storage condition. In the presence of oxygen, hexanal was a marker of storage in the light, FFA was a marker for dark storage, and markers of low-temperature storage were acetic acid and pentanal. In the absence of oxygen, octane was the marker for storage in the light whereas tyrosol and hexanol were markers of virgin olive oil stored in the dark, with no marker indicative of low-temperature storage. E-2-Hexenal, K232, and K270 were identified as markers of virgin olive oil freshness.

Investigation of Australian olive mill waste for recovery of biophenols.

Olive mill waste is a potential source for the recovery of phytochemicals with a wide array of biological activities. Phytochemical screening of hexane, methanol, and water extracts revealed a diversity of compounds, perhaps overlooked in previous studies through intensive cleanup procedures. Methanol and water extracts contained large amounts of biophenols, and further testing of polar extraction solvents, including ethyl acetate, ethanol, propanol, acetone, acetonitrile, and water/methanol mixtures, highlighted the latter as the solvent of choice for extraction of the widest array of phenolic compounds. Stabilization of the resulting extract was best achieved by addition of 2% (w/w) sodium metabisulfite. Quantitative data are reported for nine biophenols extracted using 60% (v/v) methanol in water with 2% (w/w) sodium metabisulfite. Six compounds had recoveries of greater than 1 g/kg of freeze-dried waste: hydroxytyrosol glucoside, hydroxytyrosol, tyrosol, verbascoside, and a derivative of oleuropein.

Discrimination of olive oils and fruits into cultivars and maturity stages based on phenolic and volatile compounds.

Olive oil and fruit samples from six cultivars sampled at four different maturity stages were discriminated into cultivars and maturity stages. The variables-volatile and phenolic compounds-that significantly (p < 0.01) discriminated cultivars and maturity stage groups were identified. Separation by stepwise linear discriminant analysis revealed that Manzanilla olive cultivar was separated from cultivars Leccino, Barnea, Mission, Corregiola, and Paragon, whereas cultivars Corregiola and Paragon formed a cluster. The volatile compounds hexanol, hexanal, and 1-penten-3-ol were responsible for the discrimination of cultivars. All maturity stages were discriminated, with the separation of early stages attributed to oil phenolic compounds, tyrosol and oleuropein derivatives, whereas the volatile compounds (E)-2-hexenal, hexanol, 1-penten-3-ol, and (Z)-2-penten-3-ol characterized the separation of all maturity stages and in particular the late stages. Hexanol and 1-penten-3-ol characterized the separation of both cultivars and maturity stages.

Analytical approaches to the determination of simple biophenols in forest trees such as Acer (maple), Betula (birch), Coniferus, Eucalyptus, Juniperus (cedar), Picea (spruce) and Quercus (oak).

Analytical methods are reviewed for the determination of simple biophenols in forest trees such as Acer (maple), Betula (birch), Coniferus, Eucalyptus, Juniperus (cedar), Picea (spruce) and Quercus (oak). Data are limited but nevertheless clearly establish the critical importance of sample preparation and pre-treatment in the analysis. For example, drying methods invariably reduce the recovery of biophenols and this is illustrated by data for birch leaves where flavonoid glycosides were determined as 12.3 +/- 0.44 mg g(-1) in fresh leaves but 9.7 +/- 0.35 mg g(-1) in air-dried samples (data expressed as dry weight). Diverse sample handling procedures have been employed for recovery of biophenols. The range of biophenols and diversity of sample types precludes general procedural recommendations. Caution is necessary in selecting appropriate procedures as the high reactivity of these compounds complicates their analysis. Moreover, our experience suggests that their reactivity is very dependent on the matrix. The actual measurement is less contentious and high performance separation methods particularly liquid chromatography dominate analyses whilst coupled techniques involving electrospray ionization are becoming routine particularly for qualitative applications. Quantitative data are still the exception and are summarized for representative species that dominate the forest canopy of various habitats. Reported concentrations for simple phenols range from trace level (<0.1 microg g(-1)) to in excess of 500 microg g(-1) depending on a range of factors. Plant tissue is one of these variables but various biotic and abiotic processes such as stress are also important considerations.

Bioactivity and analysis of biophenols recovered from olive mill waste.

Biophenols have attracted increasing attention during the past few years due to their biological activities and natural abundance and are potential targets for the food and pharmaceutical industries. Olive mill waste (OMW) is rich in biophenols and typically contains 98% of the total phenols in the olive fruit, making value addition to OMW an attractive enterprise. The phenolic profile of OMW is complex, yet this complexity has not been fully exploited in the valorization of the waste. Most work on the bioactivity of OMW has focused on antioxidant and antimicrobial activities. The analytical techniques used to identify and quantify active biophenols are also reviewed.

LC-MS investigation of oxidation products of phenolic antioxidants.

Two oxidation systems were examined for the oxidation of three groups of phenolic antioxidants; five cinnamic acids, two benzoic acids, and two phenols characteristic of olive fruits. Periodate oxidation, which is reported to produce products similar to polyphenol oxidase, was contrasted with the reactivity of the Fenton system, an inorganic source of hydroxyl radicals. Reaction products were identified as various quinones, dimers, and aldehydes, but the nature of the products differed between the two oxidation systems. Structure-activity effects were also observed for the different phenols. All cinnamic acids in this study reacted with the Fenton reagent to produce benzaldehydes as the main products, with the exception of 5-caffeoylquinic acid. In contrast, periodate oxidation gave no reaction with some of the cinnamic acids. Quinone formation was observed for the two compounds, caffeic acid and 5-caffeoylquinic acid, possessing o-hydroxy groups. Caffeic acid was unusual in that dimer formation was the main initial product of reaction. Benzoic acids were readily oxidized by both systems, but no identifiable products were isolated. Oleuropein was oxidized by both oxidants used in this study, resulting in quinones in each system, whereas little or no oxidation of tyrosol was observed. This highlights the importance of conjugation between the alkene double bond and the hydroxy group. The results question the validity of many existing methods of testing antioxidant activity.