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  • Total content of hydroxytyrosol and tyrosol in virgin olive oils: method optimization, validation and application
    Publication . Silva, Aline Boatto da; Rodrigues, Nuno; Cruz, Rebeca
    Olive oil consumption increased worldwide and recognized beneficial health effects are largely related to the presence of minor phenolic compounds that promote the oxidative stability in olive oil, influence its organoleptic properties and contribute to the claim on the protection of blood lipids from oxidative stress, particularly hydroxytyrosol and its derivatives (e.g. oleuropein complex and tyrosol). However, there is no international regulation for their specific analysis, with a myriad of methods and conditions published over the years, most of them requiring increasingly sophisticated equipment. Many procedures have emerged over the years to simplify their determination, including the pre-hydrolysis of the bound forms of hydroxytyrosol and tyrosol derivatives followed by simpler quantification by high performance liquid chromatography. Thus, the present work aimed to internally optimize and validate a simple, high-throughput and reliable analytical method for the estimation of these compounds, with a particular focus on the hydrolysis conditions. For this purpose, an all- in-one acid hydrolysis/extraction method and subsequent analysis by chromatography was selected from the literature. The effectiveness of hydrolysis over time was tested with standard and samples, indicating that there is an effective need for a hydrolysis time of 6 h for completeness, neither with signs of significant degradation of the released phenolic alcohols nor of the internal standard. The chromatographic conditions were also optimized under isocratic elution, allowing for 6 min runs. For method validation, linearity was verified (0.1 - 19.0 μg mL-1 and r2 = 0.9999 for both hydroxytyrosol and tyrosol) and low limits of detection and quantification were obtained for hydroxytyrosol (6.5/21.8 ng or 8.2/27.3 mg/kg of olive oil) and tyrosol (4.6/15.2 ng or 5.7/19.0 mg/kg of olive oil). Precision was assessed and all results showed a relative standard deviation below 1%. The method also showed excellent accuracy with recoveries above 99%. The method was applied to 103 olive oil samples from centenarian olive trees from the Côa Valley region, with variable total contents of hydroxytyrosol and tyrosol derivatives between 7.6 and 36.3 mg/20 g of olive oil, with results above the limit established for the use of the health claim, which demonstrates that the studied oils are of great interest.
    2022Dissertação de mestrado Acesso aberto Ver mais