Acoustic emission measurement and analysis .
Acoustic emission measurement of oral processes give valuable information about the way the food and personal care products interact with the surfaces of the skin, hair and inside the mouth. The physical measurement gives absolute and reproducible quantifiers that are closely related to the mechanical stimuli that excite the tactile sensory receptors which signal the sensory perception of the consumer. The main advantage of the technique is that it directly relates the physical interaction of the product with the body and is much easier to interpret and can often replace extensive sensory paneling during product development. In 2013 the method has been awarded with an IFT food expo innovation award.
The sensation of touching a material is highly dependent on the physical properties of the contact area. This contact can for example be adhesive (sensed as stickiness or tackiness), roughened by asperities (such as particles, the surface texture of the skin, tongue papillae), wetted by a layer of sebum, sweat or saliva or modified by an absorption layer of emulsifiers, proteins or fat, altering the interaction and lubrication in the contact area. Moreover, food entering the mouth will change its properties by being masticated, warmed to body temperature and mixed with saliva and salivary enzymes. As a consequence, textural and tactile perception will be highly dependent on the interaction of the product with the body. The physical properties of the product while it is sensed are typically not directly related to the native physical properties.
The sensation of touch (tactile sensation) is commonly measured by sensory paneling, which is time-consuming, expensive and subject to individual variations in body surface properties and differences in judgement of the sensation and the sensory descriptor terminology. More reproducible measurements of the physical effects can be obtained from instrumental measurements (e.g. rheometers, tribometers), but these operate at non-biological surfaces and under non-physiological conditions.
However, the same forces that produce the tactile stimuli at the body surfaces also produce acoustic effects that can be recorded and analyzed. These acoustic emission measurements can performed in real time and in vivo.
The basic principles of the method have been described in two scientific publications.
One publication introduces the use of acoustic measurement for the assessment of tribological (surface frictional) properties. Because the acoustic signal contains relevant information about force variations rather than constant forces, this acoustic signal is more closely related to the sensitivity of the mechanoreceptors that are involved in texture perception.
The other publication describes the tribological behavior of soft interacting surfaces, such as skin and tongue surface against substrates, skin palate and teeth, which is relevant for understanding the role of viscosity, surface roughness and particles in determining the crossover between the boundary and hydrodynamic friction regimes, which is sensed as a transition between perceived roughness and smoothness. Tapping the tongue or skin gives sound signals that relate to tongue or skin adhesive properties, which are affected by moistness, viscosity and lipids or emulsifiers deposited on the surfaces and the properties of the surface that is tapped (palate, food, skin, textile, wood etc.).
Two main measuring modes have been developed so far: skin touch and oral texture.
A microphone attached to a finger records the sound produced by rubbing and tapping the finger against any substrate. The signal shows large differences between surfaces and moisture conditions, directly related to the adhesive and frictional properties of the contact, which can be analyzed from the sound produced. Similarly, the microphone can also be attached to for example a comb or scissors to measure hair conditions.
In this application the microphone placed against the front teeth or against the lip, picking up sound transmitted through skull bones from both the soft mucosal oral surface of the tongue, palate and gums and the hard surfaces of the teeth. The sound signal can be analyzed in various ways, such as frequency analysis, total energy released, the time dependency of the change in signals between products, total time the product is held in the mouth, and retention and recovery after swallowing.
Soft mucosal tissues. To measure frictional properties of the tongue surface, the tongue surface is rubbed against the palate or against a solid food bolus in the mouth, producing sound signals caused by friction. This friction is affected by the presence (moist mouth) or absence (dry mouth) of saliva, fluid viscosity, the presence of an adherent layer of fat and particles. These aspects can be analyzed from the sound signal and correlated to sensory properties such as smoothness, dryness, grittiness, creaminess. Tapping the tongue against the palate or a solid food produces a sound that is caused by detachment of the two surfaces and hence the sound signal relates to the adhesiveness of the contact. These relate to the sensation of stickiness or tackiness and is affected by the properties of the tongue coating ort the presence of a pellicle.
Teeth. Tapping the teeth produces sounds that strongly change upon the presence of a dental coating or plaque, which can be quantified directly. Biting on food produces a sequence of sound bursts (first bite and subsequent masticatory chews) that gives information about aspects such as first snap, crispiness and crunchiness, the gradual loss of crispiness and crunchiness due to the moistening by saliva and reduction in particle size and the number of chews until swallowing. These physical parameters gives detailed information about the aspects involved in crispy perception.
The short video demonstrates the application of the method on a small child eating a rice cracker. The sound, which is normally recorded only, could be heard by her and made her dance to her own sound.
The methods are readily available to help you assess and instrumentally quantify differences in tactile sensory attributes between your products. The results can be used directly for mechanistic interpretation of the oral processing and oral interactions that underlie the sensory perception. This can be used to direct further improvements of your product. Examples of sensory attributes and applications are:
- fatty mouthfeel
- dry mouthfeel, dry mouth
- rough tongue
- tongue coating
- afterfeel sensations
- sensations of mouth freshness and tooth plaque
- crispiness, crunchiness
- loss of crispness during oral processing
- time-intensity variations of textural attributes
- skin touch of fabrics, paper, wood finishes, etc.
- skin moisturizers
- skin creams
- shampoo, hair conditioners
- eye moisteners
The method might also be applicable to quantify friction in (artificial) joints, mucosal rubbing in the pharynx during swallowing.
This figure shows the acoustic spectra measured for rubbing the fingertip against the cheek and neck, either dry or moistened with various substrates.
This figure shows line voltage amplitudes raw data measured during rubbing of the tongue against palate and changing the food material in the mouth. The sequence clearly shows differences in friction sounds produced by the samples. The smaller the amplitude, the smoother the sensation.
This figure shows the roughening effect of black coffee and the smoothening effect of creamer in coffee. The smoothest mouthfeel is obtained for creamer alone, an effect that persists for several minutes after intake of the creamer.
This figure shows a sequence of sound bursts caused by first bite and chewing a rice cracker. The sound bursts can be analyzed for changes in frequency spectrum and loudness during mastication, giving information about crispy and crunchiness and the rate by which crispiness and crunchiness disappear due to diminution of the particles and the interaction with saliva (moistening and starch breakdown by salivary amylase). These characteristics vary strongly between products and can be used to quantify their differences.
Acoustic emission measurement of rubbing and tapping contacts of skin and tongue surfaces in relation to tactile perception,
Food Hydrocolloids 31 (2013) 325-331. http://www.sciencedirect.com/science/article/pii/S0268005X12002858
Modelling texture perception by soft epithelial surfaces.
Soft Matter 6 (2010) 826–834 http://pubs.rsc.org/en/content/articlepdf/2010/sm/b916708k
Textural perception of liquid emulsions: role of oil content, oil viscosity and emulsion viscosity
Food Hydrocolloids 25(4) (2011) 789–796. http://www.sciencedirect.com/science/article/pii/S0268005X10002304
Food colloids under oral conditions.
Current Opinion in Colloid & Interface Science 12 (2007) 251-262. http://www.sciencedirect.com/science/article/pii/S1359029407000702