Phase Distribution of Mixed Biopolymer Gels in Relation to Process Conditions
The influence of various process parameters on the microstructure and rheological properties of pure and mixed gels of gelatin and whey proteins has been investigated. Gelatin is a cold-set protein and whey proteins are a group of thermo-set globular proteins. Depending on the charge present, whey proteins can form two types of gel structures. Particulate networks, with network strand dimensions in ~.my.m, are formed at pH-values within the isoelectric region (roughly between pH 4 and 6) and fine-stranded networks, with dimensions in nm, are formed above and below the isoelectric region. The microstructure has been characterised by light- and electron microscopy and further quantified by image analysis. The rheological properties have been characterised by viscoelastic measurements and tensile tests.
Phase-separated, bicontinuous mixed gels are formed at pH-values within the isoelectric region of the whey proteins, when conventional heating and cooling are used as the gelling technique. The bicontinuous morphology is unaffected by changes in the polymer mixing ratio, while the rheological properties shift, i.e. at a low whey protein concentration, the mixed gels follow the properties of gelatin and vice versa. The bicontinuous morphology is also unaffected by shear in the initial stages of gel formation of the whey proteins, but the homogeneity is affected. A mixed gel with a broader pore size distribution in the whey protein network, and thereby larger domains of the gelatin phase, is created by the shear. When image analysis is used to quantify both pure and mixed networks, it is evident that the shear induces inhomogeneities. In relation to the rheological properties, suspensions sheared under controlled conditions form gels with storage moduli twice as high as those for unsheared gels. Using a combination of temperature and high-pressure processing as the gelling technique, the order of gel formation between the pure polymers is changed compared to that followed when using temperature only. In accordance with the changes in the order of gel formation, the bicontinuous morphology is shifted to a gelatin continuous morphology.
Phase-separated bicontinuous mixed gels are formed at pH-values above the isoelectric region, when conventional heating and cooling are used as the gelling technique. As the mixing ratio is varied, a shift in rheological properties takes place, while no corresponding changes are found in the microstructure. When shear is used as a processing parameter for the mixed gels composed of fine-stranded whey proteins, no significant effects are found either in the microstructure or in the rheological properties. When the order of gel formation between the pure polymers is changed, using a combination of high-pressure processing and temperature for the gel formation, the bicontinuous morphology is shifted to a complex coacervate structure, composed of one single network structure. The complex coacervate shows a significantly higher storage modulus than the bicontinuous mixed network.