Gelatin gels are well-known for their ability to form nonpermanent, physical gels at room temperature and for their numerous applications in photographic and food industries. The difficulties in understanding the processes of gelation and the properties of the physical gels is partly due to the nature of the molecules that exhibit this state: these are often biopolymers. Concerning gelatin, a regain of interest appeared recently, in relation with the development of new types of moleculessmammalian gelatins were almost exclusively considered so far in the literature. In this series of two papers we investigated gelatin samples from various origins, mammalian and fish gelatins, with the aim of comparing their properties to the reference samples generally used in photographic and food applications. The samples were characterized by their imino acid composition, which indeed varies according to the biodiversity of the species from which they are extracted. The physical and chemical aspects are mainly reported in this paper, while the companion paper deals with the rheological properties. The influence of the thermal treatments, of the gelatin concentration, of the molecular weight, and of the solvent (aqueous and mixed solvents) was put in evidence and finally blending of samples was achieved. Optical rotation measurements were mainly performed; they allow us to fully characterize the development of the triple helices, which depends on the parameters mentioned before. A systematic comparison of the temperatures of helix formation and melting was undertaken. The results are discussed in the context of the models of helix-coil transitions often used for proteins and polypeptides and sometimes for polysaccharides. † This article is part of the special issue of Langmuir devoted to the emerging field of self-assembled fibrillar networks.

The rheological properties of gelatin gels from various sources (fish and mammalian) were followed in the course of the sol-gel transition and during gel maturation, by performing dynamic measurements under very small deformations. Different concentrations and molecular weights were investigated. The solvent was mainly water, but mixed solvents containing water and glycerol were also considered. Blends of gelatins from fish and mammalian in aqueous solutions were prepared and rheologically characterized. A systematic comparison was established between these measurements and those presented in paper 1 of the series. The comparison between the storage modulus and the concentration of helices leads to a master curve valid for all the samples investigated. This master curve is analyzed in terms of the percolation regime, near the threshold and in terms of a homogeneous network far from the threshold. In the percolation regime, a critical exponent of 2 is found for the storage modulus versus the distance to the threshold, in agreement with previous results. The theoretical models for rigid networks are briefly presented. The fully developed network appears as an entangled assembly of rigid rods (triple helices) connected by flexible links. This study shows that gelatin networks under small deformations do not behave like rubber-like networks, where long random coils are connected by local cross-links or entanglements.

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