Load- and velocity-dependent friction behavior of cow milk fat

Milk fat can influence the friction between oral surfaces and simultaneously contribute to the sensory perception of food products. The aim of this study was to gain insight into the friction mechanism of low- and high-fat cow milk. Here, we have measured the friction of milk (0.3% and 3.5% fat) between sliding hydrophobic (soft) surfaces, at different loads and velocity decay rates, using a pin-on-disk instrument. Results show that friction coefficient of 0.3% low-fat milk and 3.5% high-fat milk was lower than that of water, and the difference was two orders of magnitude in some cases. Low- and high-fat milk show a shear thinning effect; that is, the friction coefficient increases as the sliding velocity is decreased. The friction coefficient of 0.3% fat milk was lower than that of 3.5% fat milk at 1.5 N load, and this behavior reversed as the load was increased to 6.5 N. We hypothesize that this switch is due to a complex interaction between fat molecules and casein in the adsorbed surface layers formed after shear thinning. More adsorbed casein from low-fat milk increases the binding of casein with water molecules (through the hydrophilic tail) to reduce the friction at low load, whereas more adsorbed fat molecules in the case of high-fat milk reduce the friction at high load. Furthermore, as the velocity decay rate is increased from 0.003 to 0.005, low-fat milk still maintains the low friction compared with high-fat milk. This is attributed to casein’s (the hydrophilic tail) ability to quickly recover and restore water molecules at the interface. Overall, the synergistic effect of casein and fat molecules, depending on the fat content in milk, is vital to the friction mechanism. Hopefully, this study will be useful in dairy food product research.