The perception of complex textures in food is strongly related to the way food is processed during eating, and is modulated by other basic characteristics, such as taste and aroma. An understanding at the colloidal level of the basic processes in the mouth is essential in order to link the composition and structure of food products to more complicated texture attributes.

by Ton van Vliet

Sensory perception of food
This has traditionally been thought of as resulting from the combination of taste and odour (which together are termed ‘flavour’) with some simple mechanical properties such as viscosity, firmness and hardness. The main reason consumers choose a certain food product is the expected sensory perception together with its satiating action. During sensory perception all five senses are used, with additional information coming from temperature, pain and irritation receptors. However, it is clear that sensory perception is not simply a result of the detailed chemical composition of a product and the threshold values for taste and odour perception, but that the structure of the food product is also of major importance. The same applies to the perception of texture, where it is the interplay of several physical and chemical parameters along with the processing of the food in the oral cavity that determines texture perception.

The importance of texture
Studies of consumers have shown  that texture is the most important quality of foods together with flavour. In general, this is more important for solid foods than for liquid ones, being a critical quality in products such as crisps, celery and meat, and only of minor importance for beverages such as apple juice. A common definition of texture is “all the mechanical attributes of a food product perceptible by means of mechanical, tactile, and where appropriate, visual, and auditory receptors”. This all-encompassing definition shows that texture is a multi-factorial quality and not a single entity. Moreover, in contrast to other sensory food qualities (e.g. taste and colour) there are no single or specific receptors for texture because of its multi-parametric nature.

Colloidal aspects of food structure
Texture is perceived through the initial product characteristics and through the structural changes that occur during the processing of food in the mouth [Figure1]. The deforming of softer semi-solid foods, the wetting and mixing with saliva and the adhesion to, and coating of, the oral surfaces are all dependent on the colloidal properties of the food. The fragmentation and aggregation processes of food particles at the colloidal level are, therefore, crucial in modulating the transfer rate of taste and odour components to the taste buds and the nose. One can conclude that the sensory perception of food is determined largely by its physical and chemical properties at this mesoscopic scale. In view of the very different ways in which hard solid, soft solid and liquid foods are processed in the mouth these three categories of food will be discussed separately.

Hard solid food products: perception of crispiness
Hard solid foods (e.g. breakfast cereals, dry toast, expanded starch products, and several vegetables and fruits) are defined by sensory attributes such as ‘hardness’, ‘brittleness’, ‘crispness’, ‘crunchiness’ and ‘splinter formation’.

Crispness and fracture events
Crispness represents a complex texture, the perception of which is dependent largely on the audible acoustic emissions generated by multiple, low work fracture events of the food during the biting process. Low fracture strain is an indication of brittle behaviour and, as humans can only distinguish sound pulses at time intervals of at least 5-10 ms, the associated acoustic emissions imply high crack speeds which result in fast growing cracks that are initiated at different places at separated times. Essentially, the crack must also stop or the product would fracture in one crack event lasting less than one millisecond. This requires the presence of crack stoppers, such as the pores in the cellular structure typical of dry crispy products, at distances of approximately a few hundred micrometres. Based on these parameters of fracture behaviour and fracture force (an indicator of hardness), it was possible to deduce rough estimates of the maximum and minimum sizes of the pores and the beam/film thicknesses for a dry ‘crispy’ food, which were all of the order of 25 to 750 µm, i.e. mesoscopic size levels. The effect of the structure of toasted rusk rolls on crispiness is shown in figure 2, where the coarse product had more pores with a larger size (cross section > 0.02 mm), fewer with a smaller cross section and was evaluated as being ‘crispier’. The same was found for the sensory attributes ‘amount/loudness of sound’, ‘snapping’, ‘hardness’ and ‘splinter formation’. These results also show that products can only be made coarser to a limited extent before becoming too hard.

The effect of water
From Figure 2 it is also clear that an increase in water content of the products causes a decrease in ‘crispness’. The original idea was that loss of ‘crispness’, as a result of water acting as a plasticiser, is governed by its effect on the glass-rubber transition that takes place with increasing water content. However, it has often been observed that ‘crispness’ is already affected at water contents below the midpoint of the glass-rubber transition range. A more convincing relationship has been seen between low field NMR proton T2 relaxation times of ‘crispy’ model bread crusts as a function of water activity (aw) and the decrease in ‘crispness’. Here, the transition aw coincides with the starting point of the sensory loss of ‘crispness’ and suggests that it is the increased mobility of small molecules such as water and/or small parts or side chains of the macromolecules that causes loss of ‘crispness’.

Semi-solid food products
This category of foods covers a large variety of products such as desserts, sauces, soft fruits, meat and bakery products and examples of their sensory attributes are ‘firm’, ‘soft’, ‘crumbly’, ‘creamy’, ‘sticky’, ‘juicy’, ‘spreadable’, ‘thin’ and ‘thick’. Significantly, the oral processing of soft foods includes pressing the food between the tongue and palate, involving fracture and/or yielding of the product, accompanied by saliva secretion. Hence, rheology, fracture mechanics, surface adhesion, water holding, wetting and lubrication are all important for analysing the relationship between products characteristics, oral processing and texture perception.

Gel microstructure
As soft solids often derive their structure from a protein/polysaccharide mixture, the textural properties ‘watery’, ‘spreadable’ and ‘crumbly’ were investigated in a study of cold set whey protein isolate (WPI) gels. The different polysaccharide concentrations in the gels resulted in strongly varying microstructures and sensory properties [Table 1]. The following microstructures can be distinguished in the mixed protein-polysaccharide gels; homogeneous, coarse stranded, bicontinuous and protein continuous gels [Figure 3]. Protein continuous gels contain large droplets with a size of the order of 10 µm. Coarse stranded gels and bicontinuous gels both contain pores weaving through the protein gel network with protein strands of 1-3 and 3-15 µm thick, respectively.

Watery
Defined by panellists as ‘a wet feeling in the mouth’, this texture attribute is associated with the gels that exude liquid on compression. Results presented in table 1 show a clear relationship between the microstructure of the gels, liquid release and the ‘watery’ grading. Typically, coarse stranded and bicontinuous gels contain pores with a large cross section allowing extensive liquid release and were graded as more watery, reflecting the theory that liquid flow through a porous material is proportional to the pore diameter squared and decreases with increasing volume of the matrix material (protein strands).

Spreadable
Defined as ‘sample spread between tongue and palate’, this characteristic was associated with coarser gels that fractured into a larger number of particles and therefore covered a larger area. The tentative explanation is that coarse stranded gels exhibit a more yielding type of fracture (observed using a relevant instrument).

Crumbly
Defined as ‘sample falling apart in (small) pieces upon compression between tongue and palate’. Interestingly there was no relationship between ‘crumbly’ and the number of particles formed, the polysaccharide type of the mixed gels or the microstructure of the gels. However, a correlation was observed between the ‘crumbly’ score and the shape of the force versus strain curve after the gels were fractured. Gels were more ‘crumbly’ if there was a relatively strong decrease in the force after fracture. If the decrease in force was only relatively minor, indicative of a more yielding type of fracture, ‘crumbly’ scores were low.

Liquid food products
These vary from simple Newtonian fluids (with constant viscosity) such as water, wine and clear fruit juices to strongly non-Newtonian fluids, where viscosity depends on the deformation rate, such as yogurt drinks and tomato juice. Important texture characteristics include ‘thickness’, ‘sliminess’, ‘creaminess’, ‘fattiness’ and ‘roughness’. Oral processing primarily involves the tongue pushing small portions of the liquid in the direction of the throat, followed by swallowing.

Textural properties of protein stabilised emulsions
Drinks such as milk, yogurt and several soy-based drinks are, in essence, protein stabilised oil-in-water emulsions. Protein stabilised emulsion droplets strongly affect texture perception both by their effect on the viscosity of the product and by their interactions with the oral environment. For products with a low volume of emulsion droplets the latter effect is the most important. In general, the following mechanisms affect the colloidal stability of emulsion droplets in the oral cavity: (i) droplet aggregation; (ii) adhesion of droplets at oral surfaces; (iii) coalescence of emulsion droplets and (iv) oil spreading at oral surfaces.

Emulsion droplet aggregation
This occurs on the mixing of thin liquid emulsions with saliva and considerably increases the viscosity of the mixture. Since saliva is present at the oral surfaces, droplet aggregation will immediately occur at these surfaces leading to a viscous layer consisting of aggregated droplets and components of saliva. As a consequence the mechanoreceptors located on the tongue surface will experience a viscosity higher than that of the bulk of the original emulsion.

Adhesion
Adhesion to the tongue surface of this viscous layer is possibly the explanation for the sensation of a ‘soft’ or ‘velvety coating’ after tasting liquid emulsions. Emulsions that aggregate with saliva in a reversible manner are perceived as creamy and fatty while irreversible aggregating, lysozyme stabilised emulsions were perceived as ‘dry’, ‘rough’ and ‘astringent’, probably because the lubricating salivary proteins are depleted from the saliva. However, ingredients can affect several texture qualities at the same time. For instance, emulsified fat not only increases the viscosity of products, but also has an separate important effect by improving lubrication resulting in higher ‘creaminess’ ratings. Thickeners (e.g. polysaccharides), however, will primarily increase viscosity leading to higher ‘thickness’ ratings.

Coalescence and oil spreading at oral surfaces
This makes the tongue surface more hydrophobic and improves the retention of emulsified fat, which is associated with the sensory perception of ‘creaminess’ and a ‘velvety, smoother’ mouth-feel. This also creates a substantial after-feel of fat-related qualities after swallowing of the bulk of the emulsion.

Concluding remarks
A profound understanding of many aspects of colloid science has been shown to be essential for understanding the relationship between product composition, oral processing and texture perception. It has also been demonstrated that the colloidal aspects of food structure affect texture perception differently in different food stuffs. Knowledge of the interplay between product properties, oral physiology, sensory science aspects and the colloidal and chemical processes taking place during oral processing is, therefore, essential for creating the desired texture perception.

References
For a full reference list see: Van Vliet T, Van Aken GA, De Jongh, HHJ, Hamer RJ. Adv Colloid Interface Sci 2009; 27.

The author
Ton van Vliet 1,2,*,
1 TI Food and Nutrition
P.O. Box 557,
6700 AN Wageningen
The Netherlands
2 Wageningen University and
Research Centre,
Department of Agrotechnology and
Food Sciences,
P.O. Box 8129,
6700 EV Wageningen, The Netherlands
e-mail: ton.vanvliet@wur.nl