Effect of water activity and temperature on the stability of creatine during storage

Creatine degradation to creatinine, which has no biological activity, in combinations of glycerol and pH 4.0 buffer solutions followed first-order kinetics up to a point where degradation started to level off, generally beyond the first half-life. Practical data are reported for a wide range of water activity (a_w) values (0.31–0.983) at 4°C, 23°C, and 35°C. Creatine degradation did not exhibit a dilution effect, that is a decrease in rate about an a_w of 0.7, as is found for both microbiological growth and chemical reactions in semisolid food matrix systems. The temperature dependence obeyed the Arrhenius relationship with an energy of activation of about 20 kcal/mol at a_w ≥ 0.68 increasing to 23 kcal/mole below that a_w. In addition, a semilog plot of half-life as a function of a_w at each temperature follows a predicted straight line. The pH and assumed liquid viscosity increase through increased glycerol concentration were not able to completely explain the decrease in rate of degradation as a function of a_w. Furthermore, we confirmed that creatine stability in the crystal form is very high as long as it does not reach the deliquescence state.