Chapter 11 377 Temperatures Ranging 100°C to 170°C: The processes occurring at temperatures above 100°C came to the interest of cosmetic scientists with the increasing use of curling and straightening irons. Milczarek et al. claimed that the physical transformation in hair structure occurring as a result of annealing between 70°C and 180°C is an increase in fiber crystallinity.42 This effect was said to be similar to wool fiber strengthening, which has been observed after short- term heat treatments ranging 130‑150°C. According to the same paper, longer heating times can also cause destabilization of the alpha-helical component, as detected by mechanical stress-strain or relaxation measurements. Differential Scanning Calorimetry (DSC) was offered as experimental proof of this so-called “toughening transition,” which was subsequently demonstrated to be the result of evaporating residual water in the keratin fiber.43 It appears from this work that heating keratin fibers in the range 100‑170°C does not contribute in any way to an increase in strength. Using low-angle X-ray diffraction, it was shown that heating wool to temperatures ranging from 170°C to 235°C leads to the formation of amide cross-links.44 Earlier research also provides evidence for the formation of cross-links as a result of heat application. This same process could also be responsible for a decrease in urea-bisulfite solubility, and a loss in moisture regain45 which can be observed as a result of short thermal treatments of wool at temperatures ranging from 110°C to 230°C. The mechanism for cross-linking within the cortex at temperatures in the range 140‑170°C is now considered to be based on the reaction between lysine, or glycine residues with glutamine, as illustrated below: R-CH2-CH2-CO-NH2 + NH2-XàR-CH2-CH2-CO-NH-X + NH3 where X is the alkyl residue of lysine or glycine, and R is for the glutamine residue in a polypeptide chain. The reaction should release ammonia, and so the detection of this gas is a clear indication of cross-linking. Work published recently46 on the