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A capacidade de absorção do radical de oxigênio (ORAC) é um método que mede a capacidade antioxidante em amostras biológicas e alimentos.
O método ORAC baseia-se na inibição da oxidação induzida por radical peroxil, iniciada por decomposição térmica de azo-compostos.

Antes da medição do ORAC, as amostras sólidas são extraídas com acetona e água (na proporção de 50:50). Se necessário, as amostras líquidas devem ser filtradas. Em seguida, o extrato é misturado com fluoresceína (molécula fluorescente que serve como “sonda”).

Em seguida, é adicionado Apph (dicloridrato de 2,2'-azobis (2-amidinopropano)), resultando na formação de radicais peroxil. Esses radicais oxidam a fluoresceína em uma molécula não fluorescente. Isto é observado medindo a fluorescência decrescente em intervalos de tempo de 1 ou 2 minutos.

A medição da fluorescência continua até que toda a fluoresceína seja oxidada (geralmente 10 a 60min). Os antioxidantes da amostra atrasam ou retardam a deterioração da fluorescência.

O antioxidante de referência usado como padrão no teste ORAC (com o qual os outros antioxidantes são comparados) é o Trolox (ácido 6-hidroxi-2,5,7,8-tetrametilcroman-2-carboxílico).

O ensaio ORAC mede a capacidade antioxidante de:

• antioxidantes que atrasam a reação entre radical e fluoresceína (visível como um atraso) - a reação começa mais tarde.
• antioxidantes que retardam a reação entre radical e fluoresceína (visível como uma diminuição mais lenta da fluorescência) - a reação evolui lentamente.

Isso é possível analisar porque o curso da reação é rastreado até à oxidação completa da fluoresceína.

Este ensaio é um método amplamente utilizado, pois utiliza como fonte um radical biológico relevante e é o único método que combina o tempo de inibição e o grau de inibição em uma única quantidade. Além disso, é ótimo para medir alimentos e suplementos que contêm ingredientes complexos.

Abaixo, representados em uma tabela, estão alguns valores ORAC que foram encontrados para diferentes alimentos.





Referências
1. Ronald L. Prior, Ha Hoang, Liwei Gu, Xianli Wu, Mara Bacchiocca, Luke Howard, Maureen Hampsch-Woodill, Dejian Huang, Boxin Ou And Robert Jacob. Assays for Hydrophilic and Lipophilic Antioxidant Capacity (oxygen radical absorbance capacity (ORACFL)) of Plasma and Other Biological and Food Samples. J. Agric. Food Chem. 2003, 51, 3273−3279.
2. Aurelia Magdalena Pisoschi, Aneta Pop, Carmen Cimpeanu, Gabriel Predoi. Antioxidant Capacity Determination in Plants and Plant-Derived Products: A Review. Oxid Med Cell Longev. 2016; 2016: 9130976.
3. Y. Zhong, F. Shahidi. Handbook of Antioxidants for Food Preservation. Food Science, Technology and Nutrition. 2015, 287-333.
4. Barba, F.J., Esteve, M.J., Tedeschi, P. et al. A Comparative Study of the Analysis of Antioxidant Activities of Liquid Foods Employing Spectrophotometric, Fluorometric, and Chemiluminescent Methods. Food Anal. Methods 6, 317–327 (2013). 
5. Ou B, Hampsch-Woodill M, Prior RL. Development and validation of an improved oxygen radical absorbance capacity assay using fluorescein as the fluorescent probe. J Agric Food Chem. 2001 Oct;49(10):4619-26.
6. Ayse Karadag, Beraat Ozcelik, Samim Saner. Review of Methods to Determine Antioxidant Capacities. Food Anal. Methods (2009) 2:41–60.
7. David B. Haytowitz and Seema Bhagwat. USDA Database for the Oxygen Radical Absorbance Capacity (ORAC) of Selected Foods, Release 2. 2010.



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