Thermostability of Oxytetracycline, Tetracycline, and Doxycycline at Ultrahigh Temperatures

Abstract

The thermostability parameters of three tetracycline antibiotics at high and ultrahigh temperatures (110–140 °C) as well as the influence of treatment medium pH and water activity on their thermotolerance have been investigated. The thermal degradation of the three antibiotics followed a first-order reaction kinetic within the 1.5–2 log₁₀ cycles investigated. A linear relationship was observed between the log of the D_T values and the treatment temperature. The temperature dependence of the D_T values was similar for the three molecules (z = 28 ± 2 °C). D_T values of doxycycline were approximately 1.5 and 3 times higher than those of tetracycline and oxytetracycline, respectively. Changes in the treatment medium pH (7.0–4.0) and water activity (0.99–0.93) scarcely varied the antibiotics’ thermal stability. Only when doxycycline was heat-treated at pH 4.0 did its thermal resistance increase by 3 times. The thermostability parameters obtained would allow the effect of different cooking and sterilization procedures to be estimated. Whereas low-temperature−long-time treatments (conventional sterilization) would destroy >98% of the initial concentration of the residues of the three antibiotics, high-temperature−short-time treatments (UHT) would leave unaltered residues in the 50–90% range.