Jianshe Chen, Jingang Shi
In physics, material properties are divided into two main categories, intensity properties and breadth properties, based on their expressive characteristics. Strength properties are those that are not related to the quantity of a material, such as temperature, density, hardness, consistency, colour, etc. Extensive properties, on the other hand, refer to properties that are related to the quantity of the material, such as weight, volume, content of a substance, etc. So are the sensory properties of food (or sensory properties as they are often called) intensity properties or breadth properties? This is a fundamental question in sensory science and is worth considering.
It is clear from many facts that food sensory as an essential property of food materials has a distinct intensity property characterised by the intensity of its experience independent of the quantity or volume of the food material. For example, when we taste the sweetness of a drink, it has little to do with whether we take a large sip or a small one; consumers are able to accurately judge the strength of the drink's sweetness (concentration of sweetener) based on their own experience. Another example is when we judge the hardness or brittleness of a food product, or the viscosity or consistency of a drink, which also reflects the typical characteristics of the strength property, and is not necessarily related to the volume of food in the mouth.
On the other hand, consumers also largely characterise the intensity attribute when expressing or describing the sensory experience of food. Consumers often qualitatively describe food products on a scale between "very sweet to not sweet", "very hard to very soft", "very crunchy to not crunchy", "very thick to very thin " with different levels of qualitative description. Where quantification of scores is required, they are also expressed as dimensionless relative values for relative comparison, independent of the volume or weight factors of the food.
When analysed in the light of the two points above, the senses of foodstuffs undoubtedly belong to the attribute of intensity (or intensity nature), and this, too, is in line with the evolutionary view that, as land animals with little physical advantage, the senses evolved to serve the
purpose of quickly determining the palatability and quality (density) of food. As for the quantity of nutrition, humans determine and regulate it through the homeostatic system of the brain, rather than the sensory dimension.
But there is a great paradox in the experience of the food senses. Many studies have confirmed the existence of the so-called Super taster in real life. These super tasters have more or more sensitive taste buds than the average person, and therefore have a higher sensitivity to taste perception than the average person, or an ultra-low sensory threshold (see Figure 1). This suggests that the intensity of the physiological information acquired by the brain for sensory stimulation is positively related to the number of taste buds in addition to the concentration of the stimulus, a relationship that defies the basic principles of the intensity property and is characterised more by the breadth property. Thus, it is hypothesised that food sensory is a process whereby sensory stimuli with intensity properties are transformed by the breadth of physiological information (information flow = concentration x number of receptors x flow rate (attention)) and then expressed as intensity properties.
Figure 1 The difference between the taste superheroes and the average consumer is that the former have a very rich distribution of taste buds on the surface of the tongue, allowing them to taste the maximum amount of taste stimuli. This shows that the physiological information acquired by the brain about sensory stimuli is characterised by its breadth property.
According to the authors, the first step as a sensory process is the stimulation of sensory receptors (including taste buds, mechanical receptors, temperature receptors, nociceptive receptors, etc.) by the food material. What the sensory receptors perceive should be the intensity properties of the food, including chemical properties, mechanical properties, microstructure, etc. are all intensity properties. In contrast, there is an intensity to breadth transition from the intensity stimuli of the sensory receptors to the physiological or neural signals received by the brain. The strength of the neural signal in the brain directly determines the strength of the sensory stimulus, which is generally expressed in brain neuroscience research in terms of blood flow or oxygen consumption in the brain, and its intensity magnitude, under the memory of the intensity of attention (coefficient) set for food pleasures or hazards, then directly depends on the number (or sensitivity) of sensory receptors and is typically characterised by its breadth nature. Little is known about the mechanisms of conversion of sensory interpretation or sensory expression of brain signals, but it is clear that consumers' sensory descriptions should be based on the intensity of the brain's neural signals and characterised by intensity properties. For example, the intensity of the brain response observed using functional magnetic resonance techniques is expressed both in terms of the blood flow (oxygen consumption) to the brain region observed. As shown in Figure 2, by imaging observations of target brain regions, sensory physiological information has been represented by a haemodynamic response signal (BOLD) characterised by an intensity property, the magnitude of which is positively correlated with the intensity of the consumer's fat sensation.
Figure 2 Functional magnetic resonance observation of the association between the hemodynamic response (BOLD) in the middle orbitofrontal cortical layer and anterior cingulate cerebral cortex of the network and subjective ratings of qualitative pleasure values. (Rolls, E. T. (2011). The neural representation of oral texture including fat texture. Journal of Texture Studies, 42, 137-156. https://doi.org/10.1111/ j.1745- 4603.2011.00296.x)
From the above analysis, it is reasonable to conclude that the senses, as a complex process of human response to external physical and chemical stimuli, contain multiple internal information conversion mechanisms. Firstly, the material properties (or stimuli) corresponding to sensory properties are intensity properties, and the expression of sensory properties is also clearly characterised by intensity properties, but there may be a conversion from intensity to breadth properties at the stage of conversion from receptor stimuli to neural signals in the brain. As illustrated in the schematic diagram in Figure 3, food stimuli are detected as intensity properties by different sensory receptors (Ix ), and their separate physiological signals are pooled in the target area of the brain to form integrated sensory physiological information (Q = SkIx ), which is then expressed as intensity properties (S). The whole process moves from stimulus to cognitive analysis to sensory expression, three relatively independent processes that involve a transition from intensity to breadth to intensity. Psychophysical research has cleverly simplified this complex process with the well-known Stevens' law, S = k (I/I )on , which disregards
the intermediate process of sensory information conversion and successfully correlates sensory stimulus intensity and sensory strength directly, and is widely used in sensory practice (Chen, J., Tian, S., Wang, X., Mao, Y., Zhao, L. ( 2021). The Stevens law and the deviation of sensory perception. Journal of Future Foods, 1, 82-87).
Figure 3 The intensity property of the food stimulus crop is perceived by the receptor, and its perception by the brain is based on the breadth property (the amount of information received from the receptor), which is ultimately expressed in the form of the intensity property, Q representing the sensory- physiological information formed by the integration of information from independent receptor stimuli. The psychophysical process of perceived stimulus (I) → cognition → sensation (S) can be described by the classical Stevens' law, S = k (I/I ) n , .
Of course, there are also perceptions that do not have the intensity- breadth conversion described above. A simple example is the heat- sensing receptors on the surface of the skin that are used to directly perceive the degree of hot or cold of the substance they are in contact with. However, when the temperature is outside our comfort range and produces a sensation of discomfort or even pain, the breadth characteristic becomes apparent.
It is important to note that taste receptors and stimuli are not just one-to-one correspondences. A current research hotspot in sensory research is the study of a wide range of receptors responding to multiple stimuli. Human perception of stimuli is an integrated process of top- down anticipation and bottom-up cognition, and interactions between different kinds of stimuli through the nervous system can also have an impact on the intensity of perception.
About the author
Jianshe Chen, Director of the Institute of Food Oral Processing and Sensory Sciences at Zhejiang Gongshang University, PhD supervisor, and Member of the International Academy of Food Science and Technology.
Jingang Shi, Chairman and Chief Product Architect at EPC Natural Products Co., Ltd., and part-time PhD Supervisor at School of Food Science and Biological Engineering, Zhejiang Gongshang University.
