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The structures of gelatin gels have been studied by holographic relaxation spectroscopy (HRS) with fluorescein as a doped tracer. An HRS spectrum with double peaks has been observed. It has been experimentally proven that this “anomalous” HRS spectrum is related to the structures of the gelatin gels. It is shown that two kinds of gel networks are formed when a gelatin solution is cooled at a rate of 2°C/min to a temperature below the gel temperature. Microviscosity inside one kind of network is about four times lower than inside the other one. One with lower microviscosity is called a “coarse” network, and it is formed through aggregation of a collagen-like, triple-stranded helix. The other is named a “fine” network and it is constructed simply by entanglements between polymer chains. The structure of gelatin gel can be considered as a blend of these two networks. By controlling the cooling speed, the structure of gelatin gels can be dominated by either “coarse” or “fine” gel networks.

A new method of combining temperature ramp with holographic relaxation spectroscopy (TR-HRS) to investigate thermoreversible gels is reported. The gel melting temperature, gel content, and mobility of the gelatin molecules inside the gel networks can be simultaneously measured in one TR-HRS experiment.