Effect of thermal sterilization on ferulic, coumaric and cinnamic acids: Dimerization and antioxidant activity

Background: Some phenolic compounds, such as ferulic acid and p-coumaric acid, exist in the form of free acids, in fruits, rice, corn and other grains. Thermal treatment (121 °C at 15-17 psi) for different times on ferulic, p-coumaric and cinnamic acids as well as equimolar mixtures of these acids was investigated. Results: Ferulic and p-coumaric acids underwent decarboxylation, yielding dimeric products formed through their corresponding radical intermediates, while cinnamic acid was recovered unreacted. High-performance liquid chromatography analysis showed no cross-dimerization when equimolar mixtures of pairs of hydroxycinnamic acids were treated under the same conditions. Dimers were characterized as (E)-4′,4″-(but-1-ene-1,3-diyl)bis(2′-methoxyphenol)) (dimer of 4-vinylguaiacol) and (E)-4,4′-(but-1-ene-1,3-diyl)diphenol) (dimer of 4-vinylphenol) by nuclear magnetic resonance and mass spectrometry. Conclusion: Sterilization by thermal processing produced dimers of ferulic and coumaric acid. The antioxidant activity of these dimers was greater than that of the respective hydroxycinnamic acids. These results may be relevant for fruits and grains that contain hydroxycinnamic acids and undergo sterilization processes such as canning.