The degradation rates of some sterols in a heated sediment

Samples of a lacustrine sediment characterized by a high content of desmethylstanols, Δ 5-, Δ 5,22-desmethylsterols and 4-methylstanols were heated from 12 to 48 hours at 175°C under anoxic conditions in order to measure the thermal degradation rates of the various free sterols. The results show that: i) the thermal degradation of sterols under these conditions follows first order kinetics; ii) desmethylstanols and Δ 5-desmethylsterols degrade with similar rate constants; iii) Δ 5,22-desmethylsterols are less stable than desmethylstanols; iv) 4-desmethylstanols are significantly more stable than desmethylsterols with regard to thermal degradation; and v) the fact that the steroidal ketones do not seem to be degraded is due to the balance of their oxidative formation from the sterols and their degradation.