Advances and challenges in soft tribology with applications to foods

The last few decades have witnessed exciting progress in the understanding of soft material mechanics. Many of these advances have been inspired by, and have broad ramifications in the field of food science. One particular aim of food science is to get a better understanding of the physico-chemical mechanisms that are relevant in sensory perception and oral processing. It is recognized that not only rheological properties but also frictional properties are relevant in these processes. The frictional phenomena relevant for sensory perception can be understood by means of tribological measurements. The foods assessed are typically soft, hydrated and heterogeneous; measuring and understanding frictional properties of such materials is a challenge. Yet, also in the field of soft solid tribology, significant steps forward have been made, which now make it possible to do well controlled studies of even realistic food tribology scenarios. In this brief review, we provide a summary of recently developed experimental methods. We discuss challenges including the system dependence of a frictional measurement, and opportunities, such as mimicking in-mouth conditions by including human saliva and using tribo-pairs with similar properties to the oral surfaces. These advances lead to progress on the path towards a complete understanding of oral processing and sensory perception.