Tribology of multi-component food systems: Lubrication behavior and sensory perception

With an increasing population, more awareness of food selection and greater focus on the health benefits of foods, the demand for personalized foods for different consumer groups is increasing. This triggers the effort to develop novel foods and also reformulate foods with beneficial characteristics. One example is the development of nutritional dairy-based beverages, which could be used as nutritional supplements for people who may suffer from malnutrition, like the elderly population. Such beverages contain multiple components e.g. proteins, fat, and carbohydrates, which all influence the texture perception, which is a critical factor contributing to food palatability and consumer acceptance. Texture perception and other relevant sensorial characteristics of liquid foods like dairy-based beverages depend on their specific composition and system structure. Such multi-component systems can have rather complex structures, and the link between the structural features of such systems and sensorial attributes is not yet well understood.

Tribology and rheology are useful tools to gain understanding of the link between food structure and sensory characteristics. Rheology of food systems has been discussed for a long time, but tribology has been introduced in food science only few decades ago, and many researches on lubrication are ongoing. Understanding the tribological properties has challenges, since food systems are rather complex, as they contain multiple ingredients and the interactions between them have an influence on their structure. Therefore, in this thesis, we focused on dairy-based beverages and explored the contribution of the individual components and interactions between them to the physical properties and perception of such complex systems. 

We found that the lubrication properties of the single component dairy systems such as protein dispersions, polysaccharide solutions and emulsions were mainly related to two types of lubrication mechanisms: particle lubrication and film lubrication. Large particles such as whey protein isolate aggregates (WPA), micellar casein isolates (MCI), and starch granules are considered to provide particle lubrication. The efficiency of such particles is largely determined by the particle characteristics. Particles with spherical shape, high stiffness, large size and high-volume fraction tend to provide good lubrication, whereas non-spherical particles generated high friction. Proteins in non-aggregated form, polysaccharides, and starch molecules, i.e. amylose, mainly offered film lubrication. The lubrication efficiency is largely dependent on the molecular characteristics since they determine spreading efficiency and film formation (coverage) on the interacting surfaces. Especially for polysaccharide lubrication, conformation played a critical role, and polysaccharides with high stiffness and naturally high charge density showed good lubrication capacity. Interestingly, for emulsion droplets, both mechanisms can occur, depending on the properties of the oil droplets. This is mainly related to the affinity of the emulsifiers to the surfaces, and the stability of the oil droplets under shear. If the emulsion droplets (i.e. soy lecithin stabilized  emulsion, SE) readily coalesce and form oil patches/film on the surface under shear, oil film lubrication is the dominating mechanism. In comparison, if emulsion droplets (i.e. whey protein isolate stabilized emulsion, WE) remain intact as stable particles with limited coalescence, these droplets lubricate by rolling/ ball-bearing mechanism.

In the mixed systems i.e. binary and multi-component systems, the lubrication mechanism could be altered due to the co-existed components, leading to unexpected effects compared to the lubrication functionality of the individual components. For example, large protein particles (WPA/MCI) had limited effect on the lubrication of emulsions droplets (WE/SE), independently of the emulsion lubrication mechanism. However, when small protein molecules, i.e. whey protein isolate, were added, the friction coefficients increased. In addition, in the mixed systems, the structure of the systems play an important role in determining lubrication behavior. We observed that system with a more heterogeneous structure gave higher friction coefficients. This was specifically the case when the presence of polysaccharides led to oil droplet clustering, due to either repulsive or attractive interactions between polysaccharides and droplets. This clustering led to an increase in the friction coefficient and jamming effects. Such an increase in friction was also observed in systems consisting of components that individually provided good lubrication. For example, the presence of amylose increased friction for large swollen granules, which were good lubricants on their own. However, the addition of polysaccharides could also lead to a decrease in the friction coefficient when mixed with less efficient lubricants. For example, amylose decreased the friction coefficients of small stiff starch. Similar effects were observed when pectin was added to non-spherical WPA particles. These results show that in single systems, film formation could be more efficient than particle lubrication. However, in mixed systems, the structure of the systems had larger effects on the lubrication properties.

Additionally, we found that structural differences also had a large effect on lubrication properties and sensory perception for multi-components systems. The clustering of particles or oil droplets increased friction and negatively affected sensorial attributes. Systems with a homogeneous structure and lower friction coefficients provided more creamy, thick, and fatty sensations. Dynamic friction parameters, like the slope of frictional curves in different defined regimes were also correlated to specific sensory attributes, such as slipperiness, dryness, and mouthcoating. This means that in the systems consisting of binary or multiple components, not only the molecular charactistics or particle properties matter, but also the interactions between components and the resulting structure.

In conclusion, the thesis contributes to gaining understanding of the key factors affecting the lubrication properties of complex systems and the role of the individual components. The knowledge obtained from this thesis can be applied to adjust the palatability of beverages, but also provides basic insights for the research on new lubricating materials in other fields, like mechanical engineering, material science and biological engineering.