Structure and Scaling Behavior of Aging Rennet-Induced Casein Gels Examined by Confocal Microscopy and Permeametry

A study is presented on the structure of rennet(-induced) casein or skim milk gels at three pH values (5.3, 6.0, and 6.65) and temperatures (20, 25, and 30 °C). The structure was examined by confocal scanning laser microscopy and permeametry. Deconvolution was applied to the microscopic images. A fractal scaling analysis has been applied to the images and permeametry results. In this analysis, the fractal dimensionality ( D f ), lower cutoff length ( R 0 ), and apparent pore size ( P ) of the linear scaling regime were calculated from the microscopical data. The D f and apparent pore size were also calculated from the permeametry data. During aging of the gels, a coarsening of the structure was observed; the pore size increased and the clusters became more compact. This was reflected in the fractal parameters: R 0 and P increased during gel aging. Their values are generally high (0.5−1.5 and 5.0−15 μm, respectively) compared to data obtained by computer simulations. The D f value is also high (∼2.2−2.6), which is an indication of slow aggregation or rearrangements during aggregation. The gel aging effects are probably mainly due to rearrangements such as particle fusion and strand fracture, which rates increase with increasing temperature, and even more pronouncedly, with decreasing pH.