Professor Chen Jianjian, Zhejiang Gongshang University, et al.: Food sensory science research: challenges and possibilities

Sensory perception is not exclusive to humans, and many animals (or even some plants) have this ability
.
The five sensory organs of human beings have integrated a set of all-round, multi-dimensional and coordinated accurate perception system, so that people can perceive the color, form and distance of the external world through vision, perceive the chemical properties and composition of the external world through smell and taste, perceive external sound waves through hearing, and then judge external events from a distance, and perceive the physical and mechanical properties
of the external environment through touch.
Human sensory perception of food is based on this system, supplemented by the reward mechanism formed in the long-term evolution process (its brain mechanism is still poorly understood), forming a preference judgment
about food.

Food sensory science is generally considered to be an independent discipline
under the genus of food science.
Although systematic research on food senses has been carried out for more than half a century and has achieved many landmark results, the principles of food sensory perception are still poorly
understood.
The relevant perceptual principles of physics, physiology, psychology and brain neuroscience are still unclear, and many related fields still need further research
.
Chen Jianjian from the School of Food and Biological Engineering of Zhejiang Gongshang University and Liu Yuan from the College of Agriculture and Biology of Shanghai Jiao Tong University will start from some basic concepts of food senses, discuss the challenges and possible breakthrough directions of food sensory science research, throw bricks and lead jade, link colleagues in food sensory science research, and promote the rapid development
of food sensory science in China.

1.
Whether the senses are the nature of food

The scientific and accurate nature of the term "sensory properties" is debatable, and there are three reasons
for this question.
First of all, food senses are people's eating experience of food materials, which has obvious subjectivity and individual differences
.
Secondly, if "sensory properties" are a property of food, then they should have values and dimensions like other material properties, and the subjective description or strength and preference values given through sensory experience do not have dimensions
in most cases.
Third, "sensory properties" do not have the objective measurability
of ordinary material properties.
There are two types of material properties, one is strength properties, and its measurement value is independent of the quantity of the material itself, such as the specific gravity of the material; The second is the breadth nature, the measurement value of which is directly related to the amount of material, such as the volume of the
material.
Both should have specific, measurable characteristics
.
The measurement of material properties can be performed in different experimental environments using different technical methods, but the results obtained should be unique and reproducible
.
No matter who uses what instrument, the results should be consistent
.
However, the "sensory nature" obviously does not have this characteristic, and it varies
from person to person.

The peculiarity of the senses is that it is both related to the nature of the food and lacks uniqueness
.
The author believes that the senses are people's response to external stimuli, the cognition and experience of the properties of materials, so it may be more appropriate
to call it sensory experience.
Sensory experience is strongly influenced by people (physiology and psychology), time, environmental conditions, etc.
, and is characterized by
clear subjectivity and variability.
This is why
the results of sensory analysis tend to be normally distributed.
Figure 1 shows the distribution of sensory cognition in the population, which is a typical Gaussian distribution, such as the preference for a certain food, generally like and very dislike the minority, distributed at both ends of the Gaussian curve, and most of the population's sensory preferences are distributed in the middle
.
The evaluation of sensory properties also corresponds to the characteristics of
the Gaussian distribution.
This is also the basic basis
for sensory analysis experiments in personnel selection, training and data processing.

2.
Whether the sensory experience of food can be objectively measured

In the design, development and production of food, there is an urgent need for industry to connect
the sensory experience of food with consumer preferences.
Therefore, a lot of energy and material resources are invested in the research and development of various measuring devices and instruments and the creation of various new methods, such as food texture analyzers, electronic noses, electronic tongues, etc.
are typical examples
.
Obviously, such devices still stop at the characterization and measurement of the properties of food materials, rather than the human sensory experience
.

In order to better express the appreciation of delicious food and discern the subtle differences in the sensory experience of different foods, humans have created a rich vocabulary to describe the unique sensory properties and pleasant feelings
of food.
And these words are descriptive, regional and developmental, and some can only be understood but not communicated
.
For example, brittleness is a characteristic sensory quality of certain solid foods, which gives consumers a chewable, relaxed and pleasant feeling, which is deeply loved
by consumers.
There are 3 most common words in Chinese to describe the brittleness of such foods: "crispy", "crispy" and "loose"
.
These 3 crispy descriptors can be used alone or often in combination, such as "crunchy", "crispy", "crispy", etc
.
Although the intrinsic sensory meanings are similar, there are very subtle differences
.
Table 1 lists the main material characteristics of these three sensory words, from which it can be seen that although the texture senses of "crisp", "crispy" and "loose" are very similar, there are clear and subtle differences, which are reflected in their corresponding food material characteristic properties, including their microstructure properties, mechanical properties, density, void shape, void size, water/oil content and other factors
.
However, it is clear that none of these physical and structural properties can directly correspond to the sensory characteristics of "crisp", "crispy" and "loose", so it is difficult to quantify directly
with instruments.
The only way to predict sensory experience is to use major material property measurements or multiple combinations of material properties, but the combination of physical models corresponding to sensory experiences is not set in stone and is likely to vary
from person to person.

3.
The main challenges of current food sensory science research

Multiple stimuli of the senses and their interactions

It has been concluded that people can produce a corresponding sensory experience of individual sensory stimuli, forming famous sensory laws
such as Weber, Fechner, and Stevens.
However, the reality is that multiple stimuli often exist at the same time during the eating process, resulting in the interaction of multiple stimuli, which in turn produces sensory experiences
that do not directly correspond to individual stimuli.
Many sensory science researchers have put forward different hypotheses about the interaction process of multiple sensory stimuli, and there are many different views
.
For example, does the interaction stimulate the interaction first or stimuli independently and then the signal interaction? Is it interaction at the receptor stage or independent stimulation signals interacting in the brain, or both, co-existing? As shown in Figure 2, sensory stimuli D1, D2, and D3 may produce corresponding sensory experiences G1, G2, and G3, but more often produce sensory properties Ga, Gb, and Gc
that do not directly correspond to the stimulus.

Properties and function of thin layers of oral fluid

The thin layer of oral fluid refers to the mucus layer adsorbed on the surface of the mouth, which plays the role of wetting and protecting the tissues on the surface of the mouth, wrapping sensory receptors including taste, smell, and touch, and becomes the third phase between the food ball and the surface of the mouth when eating, and becomes the third phase between the de facto food ball and the surface tissue of the human body (Figure 3).

During eating, broken food particles form a cohesive esophagus moistened and wrapped in saliva, which is separated
from the oral surface (tongue surface with papillae shown in Figure 3) by a thin layer of oral fluid.
The thin layer of oral fluid is mainly composed of saliva components, but there are increasing indications that the thin layer of oral cavity is very different
from saliva in terms of composition, microstructure, interface and colloidal properties.

At present, the preliminary understanding of the thin layer of oral fluid mainly includes two aspects: First, the lubrication function, which is also the reason why
oral friction research has received a lot of attention in recent years.
The thin layer of oral fluid can eliminate the pain caused by the friction between the tongue and other oral surfaces during speech, so that the tongue can be deformed freely and produce a variety of wonderful sounds and tones
.
For diet, the thin layer of oral fluid plays a role in reducing the friction between the food ball and the oral surface to reduce the wear and burning pain
of the tongue surface.
Second, the thin layer of oral fluid should also ensure the effective control of the tongue on the food ball to ensure effective chewing and safe swallowing
.
Theories and experiments on the soft friction between thin layers of saliva and oral cavity have been reported
in a series of literature in recent years.

Mechanism of posterior nasal olfactory senses

The existence of posterior nasal smell perception has been confirmed by many studies and is generally accepted
by the academic community.
The importance of the posterior nasal olfactory senses also lies in its potential impact on the taste senses, with increasing evidence that nasal aroma sensations during eating can strongly influence or alter taste sensations
in the mouth.
Therefore, accurate understanding of the post-nasal olfactory mechanism is not only of great significance for improving the theory of food flavor, but also has a realistic guiding role
in optimizing the sensory design of food products.
Many studies assume that the transport of odor molecules during post-nasal sniffing is simply a migration process of material from the mouth to the nasal cavity by breathing into the lung cavity (Figure 4A).

If odor molecular delivery is so simple, then there is every reason to believe that odor molecular stimulation in the nasal cavity should have a strong "yes-no" "rhythm" with lung cavity breathing (Figure 4B).

That is, when exhaling, the odor molecules released in the mouth enter the nasal cavity with the respiratory airflow of the lungs, resulting in an aroma sensation; When inhaling, air is inhaled from outside the body, and there should be no odor perception
at this time.
However, personal experience shows that there is no such rhythmic switch of the posterior nasal smell senses, but a continuous and continuous sense
.
This indicates a certain buffering or relaxation of odor components during the posterior nasal olfactory sensory process
.
What is the mechanism behind this phenomenon? Is it influenced by the physical mechanism of molecular migration? Or is it the cause of physiological relaxation of nasal odor receptors? One possible explanation is that the adhesion and trapping of odor molecules by the nasal mucosa causes olfactory stimuli to relax, but this remains to be experimentally verified
.

Development of biomimetic sensing technology