The rheological and structural properties of fish collagen cross-linked by N-hydroxysuccinimide activated adipic acid

Fish skin collagen, with a concentration (6 mg/ml) higher than its critical aggregation concentration (similar to 0.5 mg/ml), was cross-linked using N-hydroxysuccinimide activated adipic acid (NHS-AA). Dynamic rheological measurement indicated that the storage modulus, loss modulus, complex viscosity and dynamic denaturation temperature increased while the loss tangent decreased with increasing the degree of cross-linking (DC). In addition, collagens with different DC exhibited a significant difference in their activation energies. It was found that collagens with DC in three intervals (DC < similar to 25%, similar to 25% < DC < similar to 50% and DC > similar to 50%, respectively) possessed remarkably different rheological behaviors each other. That is, collagens with DC < similar to 25% exhibited a liquid-like behavior, while collagens with DC > similar to 25% tended to exhibit a solid-like behavior, and especially a sudden loss of flow was observed for collagen with DC > similar to 50%. Similar trends were observed from the features in differential scanning calorimetry (DSC) thermograms of collagen. That is, denaturation temperature (T-d) was slightly increased and the endothermic peak became broader as DC < similar to 25%, T-d was increased gradually as DC was further increased, while a sudden increase in T-d was observed and the endothermic peak became sharp for collagen with DC > similar to 50%. The three-step changes in the rheological behaviors and DSC thermograms of collagens might be due to the unique aggregation behavior of collagen in solution. (C) 2012 Elsevier Ltd. All rights reserved.