The use of internal friction measurements for the study of ultra- and nanofiltration of modified curd whey

A technique is proposed for the experimental determination of internal friction in food materials, as well as a device based on a torsion pendulum with laser registration of the angle of rotation, which ensures the minimum relative displacement of the layers of the material under study in the measurement process. The definition of internal friction is based on finding the attenuation of a torsion pendulum connected to a cylinder immersed in the medium under study. As an example of the application of the method to food systems, the dependence of internal friction on viscosity in solutions of sucrose and glycerin is determined. This dependence is linear for sucrose solutions and non-linear for glycerol solutions, which may be due to the interaction of hydrated molecules in solutions. Examples of the application of internal friction measurements in the study of membrane concentration processes are given. The method of internal friction was used as an auxiliary rheological method in a comprehensive study of the process of deposition of concentrated substances on membranes during ultra- and nanofiltration of whey. Thus, it has been shown that, despite the thinness in comparison with the membrane, the polarization layer makes a significant contribution to the internal friction of the membrane-layer system. This leads to a sharp decrease in the flow of curd whey filtrate through the membrane, compared with the flow of water under the same conditions. The dependence of internal friction on the concentration in the curd whey that underwent electroflotation treatment was determined, as well as the thermal isomerization of the lactose contained in it. Using such curves, the relationships between the permeability coefficient Lp * in the Kedem–Kachelsky equation and the parameters of the nanofiltration process of given food systems can be found.