Practical applications of this testing are relatively straightforward: lubricants that provide low amounts of friction between sliding steel surfaces are useful to reduce friction from surface-surface contact, such as in mechanical pivots or sliding pistons.
Likewise, rubber for car tyres needs to provide sufficient friction for the tyres to grip the road, but not so much friction that rotation of the wheel requires significantly more energy and results in rapid wear of the tyres. More recently, tribology has been used in food research to study food friction behaviour.
The most common tribological apparatus involves an indenter or ball that slides against a flat surface, with or without inclusion of a lubricant between the surfaces. One of the major objectives of food tribological research is to relate food sensory texture to mechanical friction measurements. A recent review of rheological and tribological contributions to food texture proposed that oral behaviour of food during the initial stage of oral processing is dominated by bulk rheological behaviour but tribological (thin-film) behaviour dominates during the later stages of mastication. Thus, oral evaluation of foods involves sensation of both mechanical and friction behaviour, as well as compositional and physical properties (e.g. pH, particle size, fat content, and moisture content). This reasoning has led to the hypothesis that measuring food friction behaviour may yield information on food texture that is not provided by traditional mechanical testing and improve understanding of food structure-texture relationships.
Is tribology the Holy Grail of rheological testing? Will it be able to replace descriptive sensory analysis, the current gold standard for determining food texture? The most probable answer to both of these questions is no. The human mouth is a complex environment in which many physicochemical changes occur and multiple textural attributes are evaluated simultaneously. In fact, the capabilities of sensation and evaluation in the oral cavity rival the most modern food research lab facility: no single test or instrument is capable of simultaneously evaluating mechanical and friction behaviour, pH, volatile composition, fat and moisture content, and structure, as well as the changes in these properties over a range of timescales. However, combining the results of multiple tests gives a clearer picture of food texture and the mechanisms behind it. Although tribological testing may not be able to completely describe food texture, using tribology to evaluate food friction behaviour provides additional information on the role of friction in food texture, particularly when saliva is added during tribological testing. While not the Holy Grail of instrumental measurements, tribology is an additional tool for measuring the complex relationships between food physicochemical properties, structure, and texture.
Scientists from the University of Queensland have been investigating tribology and its growing use in the study of food oral processing and sensory perception. Studies on “soft-tribology” have emerged to provide knowledge and tools to predict oral behaviour and assess the performance of foods and beverages. This study has shown that there is a now a comprehensive set of fundamental literature, mainly based on soft contacts in the Mini-traction machine with rolling ball on disk configuration, which provides a baseline for interpreting tribological data from complex food systems. Tribology-sensory relationships do currently exist. However, they are restricted to the specific formulations and tribological configuration used, and cannot usually be applied more broadly. A modified TA.XT2 Texture Analyser with a ‘plate on three balls’ configuration has been used to measure friction in an oral processing context. With a careful and rigorous formulation/experimental design, the researchers envisage tribological tools to provide insights into the sensory perception of foods in combination with other in vitro techniques such as rheology, particle sizing or characterisation of surface interactions. Read more
Researchers from Zhejiang Gongshang University have been investigating the oral tribology of saliva-tea compound mixtures, correlating sweet aftertaste perception and friction coefficient. Sweet aftertaste (Huigan) is a sensation perceived after drinking tea, and lasts in the mouth and throat, leading to salivation for an extended period of time. The study aimed to reveal the underpinning mechanisms of Huigan and its influencing factors. A tribology approach was applied in conjunction with sensory analysis and other physiology assessments. They used their TA.XTplus Texture Analyser to manufacture an in-house tribometer setup for lubrication studies. The Huigan intensity perceived by the sensitive group was found to be highly correlated with the friction coefficient measured, especially at sliding speeds lower than 0.5 mm/s. Read more
Scientists from Wageningen University & Research have been investigating the friction behaviour of intact soft solid foods and food boli. The structure of soft solid foods changes dynamically throughout its consumption; food is reduced in size, mixed with saliva and enzymes, and a cohesive mass is formed. During this process, the mechanical properties of food change, as does the perception. In order to know how processing and composition of foods affect sensory perception, the dynamic changes in the rheological and tribological behaviour of foods have to be studied. They used their TA.XTplus Texture Analyser to perform compression tests on soft solid food gels and sausages. The decrease in friction by saliva was found to be boli particle dependent. Read more
Whilst not performing tribology tests, researchers from Wageningen University have been investigating strategies to compensate for undesired gritty sensations in foods. The study aimed to investigate whether the addition of macroscopic particles or fat to quark can be used to compensate for negative texture sensations such as grittiness by either psychological or physical mechanisms. They used their TA.XTplus Texture Analyser to characterise the mechanical properties of granola and peach gel pieces using a cutting test. The addition of crunchy granola pieces or fat was a useful strategy to shift and increase dominance of positive and liked attributes, leading to an increase of overall liking, although negative sensations such as grittiness caused by microparticles were still perceived. This approach could be used to compensate for undesired texture sensations in different types of foods, such as high protein foods. Find out more
There is a Texture Analysis test for virtually any physical property. Contact Stable Micro Systems today to learn more about our full range of solutions.
For more information on how to measure texture, please visit the Texture Analysis Properties section on our website.
The TA.XTplus texture analyser is part of a family of texture analysis instruments and equipment from Stable Micro Systems. An extensive portfolio of specialist attachments is available to measure and analyse the textural properties of a huge range of food products. Our technical experts can also custom design instrument fixtures according to individual specifications.
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