Chapter 11 383 both folded and unfolded states. Only if the proton binding affinities differ between the two states will pH have any effect on stability. Interestingly, modification of the denaturation process by mechanical and chemical loading can change the kinetics, but not the activation energy of the process. This situation suggested here to be due to a change in frequency factor, or entropy of activation, which is associated with the configurational entropy in the protein molecules themselves, and not their heat-labile bonds. A wide range of activation energy values were reported in the literature to identify protein denaturation. Values less than 10 kcal/mol are typically associated with simple diffusion processes, while values in the 10–30 kcal/mol range can involve enzyme controlled metabolic processes including membrane transport. The activation energies for protein denaturation can range from as low as 25 kcal/mol to as high as 200 kcal/mol, depending on the temperature and pH conditions. However, there are also arguments that protein denaturation usually only occurs for activation energies above 100 kcal/mol. This argument has been important in identifying protein denaturation as critical to thermal injury processes. Heat and water are known to have a destructive effect on keratin fibers. This fact was demonstrated by several authors who investigated heating wool above 100°C, in the presence of air, and found increasing damage to the fibers with increasing exposure time and temperature. Heating keratins in the absence of air has been reported with ambiguous results. Specifically concerning the presence of water, a recent work47 concludes that heat-straightening of wet hair damages the hair fiber, whilst the same action on dry hair keeps the fiber damage under control. In this respect, the authors show that heat-protection sprays with volatile ingredients, which help remove water from hair, leads to improved efficiency of the products. All investigations suggest that damage to the keratin fibers increases with temperature, exposure time and pH value of the examined material. The damage is quantifiable as color change, change in tensile strength, change in solubility, or amino acid