On the temperature reversibility of the viscoelasticity of acid-induced sodium caseinate emulsion gels

Viscoelastic properties of acid-induced sodium caseinate emulsion gels have been investigated using a controlled shear stress rheometer. Gelation was introduced by addition of acidulant glucono-delta -lactone (GDL) at three different temperatures (5, 25 and 45 degreesC). It was found that the gelation temperature has a significant effect on the rate of gelation and on the dynamic moduli of the emulsion gels. The rheology of these emulsion gels was investigated over the temperature range 5-45 degreesC. The viscoelasticity of the emulsion gel prepared at 45 degreesC was temperature reversible, suggesting that the temperature change only affects the strength of physical bonding within the network and not the gel microstructure. In contrast, the temperature-dependent viscoelasticity of the emulsion gel prepared at 5 degreesC exhibited a highly irreversible character. This implies significant structural reorganization of the network during the heating stage from 5 degreesC. Analogous temperature irreversibility has been observed in emulsion electrophoretic mobility measurements and in solution surface tension measurements of the corresponding caseinate systems at pH values near the isoelectric point of the protein. (C) 2001 Elsevier Science Ltd. All rights reserved.