Knowledge

Solid fats are one of the most difficult stains to remove at low temperatures. Mechanical action is beneficial for stain removal, but the potential and limitations of such an essential part of washing are not known. Fabric abrasion has been studied for the first time in a systematic and controlled manner using a tribometer. The efficiency of cotton–cotton abrasion, to simulate the rubbing of clothes, was studied in the absence of detergents using models of liquid and solid oils (hexadecane, octadecane, and undecanoic acid) and real fats (lard and buttermilk fat). In model oils, abrasion is not very effective at any temperature, whereas in typical fats abrasion significantly improves cleaning in a wide range of temperatures. The different behavior is caused by the temperature-dependent solid fat content of lard and butter. Fluorescence microscopy is introduced as a novel methodology for the quantification of the fat content in soiled fabrics.

Keywords: Fabrics, Fats, Fouling, Cleaning, Fluorescence microscopy, Imaging

The impact of food industry on societal aspects grows dynamically, thereby posing new challenges for food developers to comply to consumer demands. Consumers expect new and more sophisticated products that are tuned to their very specific needs, like health and life-style. This means that food products must be fully programmable to meet these market demands. The taste, smell, and texture perception of food are the most important properties that influence the consumer’s experience. To provide control over these features the physics behind them during oral manipulation and structure breakdown must be understood. Most desirably, the physical properties can be tightly linked to attributes of oral perception. In this thesis the texture of semi-solid and liquid (model) food is studied using Quantitative Descriptive Analysis. Despite the quantification of sensory scores, it remains difficult to translate these into food physical characteristics in an absolute mode. For that reason, in parallel a physical study is performed using mainly tribology to provide quantitative description of the mechanical processes in the oral environment.

Therefore, the experimental setup is adapted such to mimic oral conditions by employing soft surfaces. Next to more complex commercial products, like milk, the samples studied include aqueous dispersions of protein aggregates or polysaccharides, and emulsion filled gels. These components may contribute to fat-feel in low or no-fat dairy products and provide a major uncertainty in predicting the sensorial restraints of the product. In this thesis we investigate how various factors influence the friction of above-mentioned systems and how this friction can be understood at a more fundamental level. In addition, the surface effects are studied in detail to determine how sensitive the choice of the oral surface analog is on the friction and what the most important surface properties are that are relevant to better predict sensory attributes. In this thesis we present an overview of correlations obtained between different sensory attributes and frictional data. We show that oral perception can be translated into physical quantities using friction.

The best correlations are obtained for physical conditions that resemble most the oral environment, i.e. speed below 50 mm/s, shear rate of about 50 s-1 and soft, rough surface. For instance a low friction coefficient correlates well with the creamy perception of the consumer. This finding may contribute to develop healthy foods with a full fat creamy taste. Moreover, samples with low friction coefficient can be perceived as soft and velvety, while high friction results in rough texture perception. Some attributes, like filmy or slimy, correlate better with viscosity, as their perception is determined by the bulk properties of a sample rather than interaction with oral surfaces. In a few cases the correlation with friction coefficient could not be established, e.g. for sticky or powdery attributes. The latter one, however, shows a good correlation with particles size if they are present in the solution. This thesis presents a comprehensive picture of the relation between physical quantities and sensory attributes and provides a step forward towards a better control over oral perception of food.

The salivary conditioning film (SCF) that forms on all surfaces in the mouth plays a key role in lubricating the oral cavity. As this film acts as an interface between tongue, enamel and oral mucosa, it is likely that any perturbations to its structure could potentially lead to a change in mouthfeel perception. This is often experienced after exposure to oral hygiene products. For example, consumers that use dentifrice that contain a high concentration of sodium bicarbonate (SB) often report a clean mouth feel after use; an attribute that is clearly desirable for oral hygiene products. However, the mechanisms by which SB interacts with the SCF to alter lubrication in the mouth is unknown. Therefore, saliva and the SCF was exposed to high ionic strength and alkaline solutions to elucidate whether the interactions observed were a direct result of SB, its high alkalinity or its ionic strength. Characteristics including bulk viscosity of saliva and the viscoelasticity of the interfacial salivary films that form at both the air/saliva and hydroxyapatite/saliva interfaces were tested. It was hypothesised that SB interacts with the SCF in two ways. Firstly, the ionic strength of SB shields electrostatic charges of salivary proteins, thus preventing protein crosslinking within the film and secondly; the alkaline pH (≈8.3) of SB reduces the gel-like structure of mucins present in the pellicle by disrupting disulphide bridging of the mucins via the ionization of their cysteine's thiol group, which has an isoelectric point of ≈8.3.

The effect of β-lactoglobulin (β-LG) at pH 3.5 and 7.0 on lubricating property of saliva as related to astringency perception was investigated using tribology. Saliva was adsorbed onto surfaces of a rotating poly dimethylsiloxane (PDMS) ball and disc to form a film under conditions that mimic the rubbing contacts in the oral cavity (Bongaerts, Rossetti, & Stokes, 2007) and the lubricity of saliva films upon exposure to astringent compounds was measured. While addition of non-astringent β-LG at pH 7.0 slowly increased friction of saliva film between tribopair surfaces, β-LG at pH 3.5 rapidly increased the friction coefficients of saliva, similar to other astringent compounds (epigallocatechin gallate and alum). This supports the hypothesis that astringency of β-LG arises from the loss of lubrication of saliva which is in agreement with the well-accepted astringency model of polyphenols. Increasing β-LG concentration at pH 3.5 (0.5–10% w/w) caused a rapid increase in friction coefficient; however, at the highest protein concentration, the friction coefficient, although higher than observed for water, was below the values observed for the lower protein concentrations. This suggests that static tribology testing is different from the dynamic in-mouth system such that a simple relationship between friction and sensory astringency cannot be found for all conditions.

Keywords: Astringency, Saliva, Lubrication, Whey proteins, Tribology