Biomacromolecules 2001, 2, 1148-1154
1148
Compatibility of Gelatin and Dextran in Aqueous Solution Marijke W. Edelman† and Erik van der Linden Food Physics Group, Department of Agrotechnology and Nutrition Sciences, Wageningen University, Bomenweg 2, 6703 HD Wageningen, The Netherlands
Els de Hoog and R. Hans Tromp*,† NIZO Food Research, Kernhemseweg 2, 6718 ZB Ede, The Netherlands Received April 12, 2001; Revised Manuscript Received August 22, 2001
The temperature-composition phase diagrams of aqueous solutions of gelatin and dextran, which show liquid/liquid phase segregation, were explored at temperatures above the gelation temperature of gelatin. The compositions of the coexisting phases were found to show practically no dependence on temperature between 40 and 80 °C. Also, the total polymer concentration at which phase separation occurred was found to be nearly independent of temperature. These observations suggest an entropy-driven phase separation. An explanation in terms of depletion, reversible clustering, and subsequent transient network formation of gelatin at temperatures well above the temperature of gelation is suggested. Phase separation is found to be accompanied by strong fractionation of the molar mass distribution in the two phases. Introduction As is commonly the case for mixtures of biopolymers, gelatin and dextran are incompatible in aqueous solution. At sufficient concentrations [typically above 3% (w/w) of both components], segregative phase separation takes place, resulting in gelatin-rich and dextran-rich domains. Such phase-separation processes are often the basis for structuring processed food. In the literature, a large quantity of experimental data is available on the compatibility of food biopolymers.1 Because of the polydispersity and the tendency to form gels above a certain concentration and below a certain temperature, quantitative descriptions of biopolymer phase separation are relatively rare.2,3 Gelation, if it occurs, prevents the phase separation from proceeding beyond a certain point. At this point, the system usually consists of microscopic phase regions that do not necessarily have the composition characteristic of thermodynamic equilibrium. The work presented here is part of an effort to provide a detailed description of the equilibrium phase diagram of mixing of an aqueous mixture of two biopolymers above the gelation temperature of both components. The gelatin/dextran system was chosen because it is experimentally accessible and it is representative for gelling and phase-separating mixtures of biopolymers. Moreover, it enables one to study the kinetics of phase separation in the absence and in the presence of gelation.4 As a consequence, the complicated interplay between phase separation and gelation can, in principle, be unraveled. Above the gelation temperature (about 30 °C) of gelatin, the segregation appears to proceed like “normal” liquid-liquid phase separation in contrast with the case where that gelation of gelatin takes * Corresponding author. E-mail:
[email protected]. † Affiliated with the Wageningen Centre for Food Sciences (WCFS), Diedenweg 20, 6703 GW Wageningen, The Netherlands.
place (below 30 °C). In the latter case, the thermodynamic driving force for segregation becomes stronger, at least partly because the molecular weight of gelatin effectively increases through aggregation. At the same time, a viscosity difference between the gelling gelatin domains and the nongelling dextran domains is established. This viscosity difference probably has a profound influence on the segregation kinetics.5 Eventually, gelation will halt the process of segregation, leaving the system in a kinetically frozen, metastable state. Recently, the crucial role of the presence of an disorder-order transition of one the polymers in driving phase separation was demonstrated6,7 to be not unique for gelatin-containing biopolymer mixtures. Here, we are principally interested in answering the questions of why gelatin and dextran are incompatible aboVe the gelation temperature of gelatin and to what extent the phase diagram of mixing can be understood in terms of a disorder-order transition, as is the case below the gelation temperature of gelatin. Commonly, a difference in solvent quality is assumed to play an important role in biopolymer compatibility.1 However, almost 30 years ago, it was suggested that self-aggregation of gelatin was a driving force for segregation from other polymers, especially near the isoelectric point (IEP) of gelatin.8 This would imply a deviation from the predictions of mean-field theory, as such predictions do not take into account concentration fluctuations. However, self-aggregation of sodium caseinates as a determining factor for phase separation in mixtures with polysaccharides could be explained qualitatively9 by the classical condition for phase instability ∂2Gmix ∂2Gmix ∂c22
∂c12
( )
∂2Gmix 2
