FTIR Spectroscopy and DFT Calculations to Probe the Kinetics of β-Carotene Thermal Degradation

The thermal degradation of β-carotene in air was investigated. The sample was heated at different temperatures (90, 100, 115, and 130 °C) for periods of up to 8 h to perform a complete kinetic study, the product analysis having been carried out via infrared spectroscopy in attenuated total reflectance mode coupled to density functional theory (DFT) calculations. The kinetics of this thermal degradation process was found to follow a first-order scheme, with rate coefficients varying from k_90 °C = (2.0 ± 0.3) × 10^-3 to k_130 °C = (11.0 ± 0.7) × 10^-3 min^-1, the experimental activation energy having been calculated as (52 ± 1) kJ mol^-1. This E_a value is close to the DFT energies corresponding to a C_15-15' or a C_13-14 cis-trans isomerization, followed by the formation of a carotene-oxygen diradical, which was characterized for the first time. Comparison between the experimental and calculated infrared data confirmed the C_15-15'- cis rupture as the predominant reaction pathway and retinal as the major degradation product.