Rheological Challenges for the Food Industry

Several key challenges for rheologists working in the food industry are presented. These range from pure material science problems (complex yield behaviour, fracture mechanics of “soft” materials, interfacial rheology) and interactions with solid surfaces (wall slip) to the probably biggest challenge: the effects of rheology on processes inside the human body. Finally, the need for enhanced teaching of advanced rheological concepts in the field is highlighted. INTRODUCTION Designing and controlling rheological properties of food and nutrition products is a key to their commercial success: these properties crucially affect product manufacture, distribution, convenience and sensory experience. Since the range of relevant rheological properties can be very broad and only well understood through advanced physical concepts, it is a challenge to achieve this task within the constraints of the industry. Food structures contain many types of soft matter structures, e.g. amphiphilic molecules, polymers, molecular complexes, micelles, colloidal particles, particulate gels, polymer gels and so forth and “managing rheology” in foods from a purely phenomenological (as opposed to microstructural) perspective not very promising. The complex microstructure brings about specific challenges for the understanding of rheological behaviour of foods itself and for interaction with the different environments they experience during their life time. SEVEN RHEOLOGICAL CHALLENGES 1. Characterisation, theory and use of wall slip phenomena It is now quite well accepted that slip or “apparent slip“ phenomena of liquid and semi-solid materials near solid surfaces are ubiquitous in food products. This should come as no real surprise, since many food products are highly concentrated disperse systems (hard or soft suspensions, emulsions or foams), which are known to exhibit this type of behaviour. However, the practical consequence in most cases is that one merely tries to eliminate slip phenomena when performing rheometry, e.g. by using roughened surfaces, vane geometries and other special tools. This in itself is not trivial to do, but even where it is successful, it is only a first step. During processing and consumption of foods (e.g. flow from a bottle or tube, use of a spoon to dose a product, in-mouth perception), the slip characteristics may significantly affect, or even dominate the perceived behaviour and thus be vital to successful products. Likewise, pumping of “slippery” materials can not be optimally handled by simply Rheological Challenges for the Food Industry