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Professor Zhang Yuyu of Beijing Technology and Business University, et al.: Research on influencing factors and analysis methods of posterior nasal aroma perception

 Last Update: 2022-11-04
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Aroma is one of the important sensory qualities of food and determines consumer acceptance
.
Aroma perception is the sum of people's chemical stimulation, physiological responses and psychological effects on aroma compounds, and is the result of the
interaction between people and food.
Aroma perception includes two paths, the anterior nasal cavity and the posterior nasal cavity, and does not interfere with each other, the anterior nasal path refers to the perception of the aroma in the environment through the nostrils, providing early warning signals to the external environment and food edible information; The posterior nasal pathway emphasizes the food aroma perceived by the food during the chewing process (oral processing), provides information on food acceptance, sensory enjoyment, and also reflects human behavioral characteristics
.

Pu Dandan, Shaan Yimeng, Zhang Yuyu* from Beijing Key Laboratory of Food Flavor Chemistry, Beijing Technology and Business University, elaborated on the concept, basic process, main influencing factors and analysis methods of posterior nasal aroma perception, aiming to understand the mechanism of posterior nasal aroma perception and provide theoretical guidance and disease early warning for the development of healthy and delicious foods
.

1.
The difference in aroma perception between the anterior nasal cavity and the posterior nasal cavity

Genetic variation and dietary culture differences

Olfactory receptors are the main receivers of human aroma perception of food and external environment, and genetic variations of olfactory genes have an important impact
on aroma cognition and sensitivity in the process of human evolution and environmental selection.
The genetic sequence diversity of olfactory receptors has been shown to contribute to food preferences, and Eriksson et al.
found through genome-wide association analysis of 14 604 Europeans that the olfactory receptor gene cluster rs72921001 located on chromosome 11 has an olfactory receptor OR6A2 gene sensitive to aldehyde aroma, resulting in rejection
of coriander.
Food culture is inevitably related to genetic variation, and food culture determines the preference for aroma perception, and preference will promote genetic variation, which in turn will feedback and regulate aroma perception
.
There are significant diversity and differences in food cultures in different countries and regions, such as Westerners believe that pulses should be classified as savory foods, but in Asia, although soy products can be used as savory foods, they are mostly classified as sweet foods
.
Gotow et al.
confirmed that cultural differences drive consumers to enhance their perception of the nasal aroma of familiar foods, which is also accompanied by enhanced
food taste perception.

Differences in aroma delivery routes

Nasal aroma perception inhales aroma molecules from the external environment into the nasal cavity through the nostrils, transmits them to the olfactory epithelium, and binds to olfactory receptors, thereby forming anterior nasal aroma perception
.
Posterior nasal aroma perception is mainly derived from the aroma released during food oral processing, and the aroma molecules in the mouth are transmitted to the back of the mouth through the respiratory airflow, and then transmitted to the posterior nasal cavity area through the oropharynx and nasopharynx, where it binds to the olfactory receptors of the olfactory epithelium and finally forms the posterior nasal aroma perception (Figure 1).

The anterior nasal aroma is related to the external environment, and the posterior nasal aroma is accompanied by oral processing behavior, reflecting the internal state of the oral cavity
.

There is a path dependence in aroma perception, due to the difference in the direction of the anterior and posterior nasal airflow and the influence of the oral environment, the olfactory receptors in the olfactory epithelium are activated to varying degrees to form perceptual differences, and this difference in olfactory perception is also called olfactory duality
.
For common and familiar aromas, there was no significant difference in aroma perception between the anterior and posterior nasal aromas, while matching unfamiliar or similar aromas to the test aroma differed greatly, and the posterior nasal sensibility of unfamiliar aromas was more sensitive, indicating a difference
in perception between the two pathways.
Compared with anterior nasal aroma perception, posterior nasal aroma perception path is longer and therefore slower
.

2.
Research progress of posterior nasal aroma analysis

The results of the emergence analysis of research topics are shown in Table 1, and the emergence of research topics represents the current development trend and research hotspots
.
According to the analysis of changes in research topics, the development trend in the field of posterior nasal aroma perception can be divided into two stages: the first stage, from 1995 to 2009, mainly focuses on the development of posterior nasal aroma analysis methods, such as in vivo and in vitro model experiments, offline analysis methods for posterior nasal aroma perception and the development of
online mass spectrometry instruments.
The second stage, from 2010 to present, mainly focuses on the influence of human sensory and physiological parameters on aroma perception, such as the cross-mode interaction between taste and posterior nasal smell, the release of aroma by saliva components during food oral processing, and the metabolism and perception evaluation
of aroma by nasal mucosa and oral mucosa.

3.
The main factors affecting the perception of nasal aroma in the posterior cavity

Basic properties of food

Dunkel et al.
reported that more than 10 000 aroma compounds were detected in more than 230 foods, but only 240 were
critical to the aroma profile of foods.
Aroma activity value (OAV) is the main method to measure the contribution of aroma to olfactory perception, usually OAV≥1, that is, the ratio of aroma concentration to the aroma threshold in the corresponding food system is greater than 1, it is considered to have an important contribution
to the aroma profile of food.
Different substituent positions, chiral differences, and enantiomer differences of aroma molecules all lead to differences
in aroma characteristics and intensity.
These structural differences lead to differences in the binding patterns of aroma to olfactory receptors, which ultimately form different activation signals
.
In addition, the boiling point and hydrophobic constant of aroma molecules affect their concentration distribution in the nasal cavity, which in turn leads to differences
in olfactory perception.
Due to the complexity of food oral processing, the food-saliva mixed system is affected by each component in the food matrix, and the partition coefficient is an important parameter to measure the relative concentration of aroma compounds released by the complex food matrix in the oral cavity, and the partition coefficient of each aroma molecule is constant
at a fixed temperature and in the solution system.

Oral processing parameters

As the first step in food digestion, oral processing is essential
for the nutritional intake and sensory enjoyment of food.
When food is ingested into the mouth, it is broken down into small particles by the synergistic action of the teeth, tongue, cheeks and salivary glands, eventually forming a soft, lubricated and viscoelastic swallowing food mass
.
Due to the complex and dynamic changes in the oral environment, aroma compounds are actively released from the food mass into the mouth and transmitted to the posterior nasal cavity
by respiratory airflow.
Therefore, the number of teeth, bite force, saliva composition and secretion, oral mucosa and other factors play an important role
in the perception of posterior nasal aroma.

Based on the dietary experience, chewing is easily affected
by the texture and composition of the food.
Chewing efficiency depends on bite force and food breakdown patterns, and is significantly related to
an individual's gender, age, ethnicity, and lifestyle habits.
In general, the bite force of men is greater than that of women, for example, the incisor bite force (168 N) of ethnic groups with higher carnivorous tendencies (168 N) is significantly higher than that of Han (146 N).

Due to the different rates (frequencies) of chewing food, the rate of release of aromas in the food matrix is also different
.

Saliva (pH 6.
0~8.
0) is a complex biological liquid, the main components of which are water (99%), salt and protein
.
Saliva has a variety of functions
such as lubrication, cushioning, antibacterial activity, tooth protection, and flavor perception.
Changes in saliva secretion and basic components due to food chewing stimulation alter the release
of food aromas in the mouth.

Physiological parameters of the nasal cavity

Nasal airflow is the main way to transmit aroma molecules to olfactory epithelial cells, and when people are stimulated by aroma molecules in the external environment, nose pumping is a common behavior
to enhance aroma perception.
Changes in nasal airflow affect the concentration of aroma received by olfactory epithelial cells and affect the processing
of olfactory perception signals by the central nervous system of olfactory cells.
In the resting state, about 10% of the inhaled air reaches the olfactory epithelial area, and the nostrils can draw ambient gases (more than 300 mL/s per nostril) in a turbulent pattern for a short time, so that more aroma molecules are deposited on the olfactory mucosa to promote aroma perception
.
One of the commonly used research methods is to establish a mathematical model
of aroma transmission according to the transmission process of aroma molecules in the oral cavity and the interfacial mass transfer osmosis theory.
Due to the high and stable presence of acetone in the respiratory airflow, the signal of acetone is often used to correct the raw data to establish the effect
of different airflow rates on the perception of posterior nasal aroma.
However, there are differences between the numerical model and the actual situation, and it is necessary to better reveal the perception principle
of aroma in combination with the actual breathing airflow value.

Oral processing movement is a complex dynamic process, and the transport of aroma compounds to the posterior nasal cavity is a series of alternating dynamic and static events, so how to monitor the airflow change process in real time is the key to
interpreting the post-nasal aroma transport.
Due to the complexity of nasal anatomy and the high frequency of airflow, computational fluid dynamics (CFD) techniques allow for more precise study of detailed changes in
nasal airflow.
Computed tomography (CT) imaging techniques and magnetic resonance imaging combined with CFD analysis can provide valuable 3D reconstruction of the nasal cavity, which helps to accurately understand the structure of the nose and the delivery of nasal airflow during breathing, and is an effective way to
study the correlation between different anatomical regions and olfactory performance 。 Ni Rui et al.
used CT, CFD and 3D printing technology to reconstruct the three-dimensional model of the oral-nasal-throat-trachea (Figure 2A), and simulated the flow field in the respiratory airway, and found that during normal inhalation and exhalation, an air curtain will be formed outside the virtual cavity connecting the oropharynx and the back of the mouth to prevent the aroma components from being transported to the main stream of the lungs during exhalation (Figure 2B box area), and promote the transmission of aroma molecules in the mouth to the main stream of the nasal cavity during exhalation.
Promotes posterior nasal aroma perception
.
This study is the first to demonstrate that human oropharyngeal geometry can efficiently deliver odorous substances to olfactory receptors
in the nasal cavity.

4.
Posterior nasal aroma analysis method

Classification of aroma detection methods

Aroma is one of the main driving forces to promote food flavor perception, and it is of great significance
to clarify the composition and change of aroma in the posterior nasal cavity to improve the aroma perception theory.
Posterior nasal aroma detection is the analysis of nasal exhaled air, which occurs in a short time and has a low aroma concentration, so the researchers proposed an enrichment concentration method and designed a highly sensitive online analysis instrument
.
According to the sampling type, it is divided into offline, semi-offline, and online analysis modes, where the offline analysis mode uses the Tenax adsorption method to collect nasal exhaled gas, followed by gas chromatography-tandem mass spectrometry (GC-MS), which is mainly based on the capture and enrichment capacity of Tenax material for exhaled gas and the chromatographic separation and qualitative quantitative analysis capabilities
of GC-MS.
The semi-offline mode is gas chromatography-ion mobility spectroscopy (GC-IMS), which directly draws the exhaled gas from the nasal cavity and then separates
it by gas chromatography and ion mobility spectroscopy, respectively.
Because this method can directly analyze nasal exhaled air, but the sample analysis time is longer (<30 minutes), it is called semi-offline analysis mode<b14>.
In-line analysis modes include PTR-MS, selective ion flow tube mass spectrometry (SIFT-MS), and atmospheric pressure chemical ionization mass spectrometry (APCI-MS).

These three online analysis methods can directly collect nasal exhaled air, and can continuously monitor and analyze the changes of aroma type and content in exhaled air, reflecting the aroma composition and concentration
in the nasal cavity in real time.

Comparison of detection methods

Fast response, direct analysis, trace detection limit and high sensitivity are the main features of posterior nasal aroma analysis instruments, and scholars have developed analytical instruments
that can be applied to online detection of posterior nasal aroma 。 At present, 3 analytical instruments have been used in multiple fields, and each technique has its own specific field of focus according to its historical development and the difference in the focus angle of scientific questions: 1) SIFT-MS is mainly used for respiratory gas analysis for medical purposes, including the identification of respiratory volatiles in patients with diseases and dynamic monitoring of specific medical biomarkers, such as Španěl, etc.
quantification of volatile respiratory metabolites ammonia, acetone, hydrogen cyanide, alcohol, pentane, acetic acid, methane and sulfide through SIFT-MS and discussed its potential as a biomarker; 2) PTR-MS is mainly aimed at environmental science and various ecological fields, including meteorological monitoring, environmental gas pollutant monitoring, and the impact of plants on
ambient air quality.
However, with the rapid development of food flavor analysis and flavor perception in food oral processing, PTR-MS is also widely used in the field of food flavor, such as the metabolic changes of aroma components in the nasal mucosa and the changes in the aroma of bread oral processing.
3) APCI-MS is mainly for food and flavor related research, such as strawberry, mint, gum and tomato during oral processing of aroma release
.
4) GC-IMS is mainly used in the field of safety protection, such as rapid detection of explosives, detection of drugs and detection
of toxic and harmful gases.
Due to its high sensitivity and two-dimensional separation ability, it has been widely used
in many fields such as food traceability analysis, authenticity identification and flavor analysis.

At present, the most widely used posterior nasal aroma analysis instrument with the most detection compounds is PTR-MS
.
However, both online and semi-online analytical instruments have common problems
such as difficult isomer separation, difficult identification of identical fragment ions, and difficult capture of high boiling point components.
Therefore, a variety of extraction methods were used to analyze the composition of food aroma in combination with GC-MS, and then combined with online analysis instruments to compare and monitor the target aroma compounds perceived by the nasal cavity to analyze the nasal aroma perception
more comprehensively.
In addition, the development of a new mass spectrometry with a more sensitive and wider capture range is an effective solution for
comprehensive analysis of nasal aroma release after analysis.

5.
Aroma sensory evaluation

Sensory evaluation is an effective representation method to measure aroma perception, which can directly reflect consumers' evaluation of products, and is also the main method
to measure consumers' acceptance of products.
The aroma perception of food in oral processing is a dynamic process, and the aroma perception evaluation methods commonly used in food oral processing include time-intensity method (TI), dynamic quantitative descriptive analysis method (D-QDA), dynamic dominant attribute test method (TDS) and dynamic selection of appropriate vocabulary analysis method (TCATA).

。 The TI method requires the evaluator to score the overall aroma intensity or one or two aroma characteristics on the timeline, which has the advantages of simple and fast operation; The D-QDA method, which requires evaluators to simultaneously evaluate the strength of multiple sensory profiles during food tasting, is suitable for samples with few aroma properties, and reduces accuracy
for excessive aroma profiles.
Therefore, on this basis, TDS and TCATA evaluation methods are developed, TDS method requires evaluators to screen out a dominant sensory attribute, fast and targeted, more conducive to product comparison and product development, suitable for consumers
without sensory evaluation experience.
The TCATA method is derived from the selection of appropriate vocabulary analysis method, which requires evaluators to screen the set descriptive words in the form of questionnaires, and can select multiple sensory characteristics and other psychological states such as related consumer emotions, which can analyze consumers' preference information
on samples from more dimensions.

knot

Post-nasal aroma perception is affected by many factors, among which genetic variation and dietary culture differences determine consumers' olfactory cognition, which is also an important basis for
food production enterprises to adjust regional food flavor preferences.
In addition, the physical and chemical properties of the food itself and the oral processing behavior determine the aroma release law during the oral processing of food, and the respiratory airflow and the physiological structure of the nasal cavity play an important role in the transportation of aroma compounds, and determine the final type and concentration
of aroma compounds perceived through the nasal cavity 。 Posterior nasal aroma perception analysis methods mainly include instrumental analysis and dynamic sensory evaluation, in which the combination of traditional mass spectrometry and online mass spectrometry analysis methods can achieve a wider range (low boiling point and high boiling point) of aroma component capture, and more complete analysis of post-nasal perceived aroma components.
Due to the complexity and variability of oral processing, a variety of dynamic sensory evaluation methods can be used to analyze consumers' perceptions and preferences
from multiple dimensions.
In the future, it is necessary to conduct in-depth research on the law of in situ aroma release and perception of the mouth, analyze the mechanism of action of saliva components on aroma release, clarify the release, retention and metabolism mechanism of oral mucosa on aroma, and explore the transmission effect of nasal airflow on aroma, so as to achieve targeted and controlled release of flavor in the oral processing process of food, and design delicious food
that meets consumer needs.

Expert profiles

Professor Zhang Yuyu, doctoral supervisor, School of Light Industry Science and Technology, Beijing Technology and Business University, National Natural Science Foundation of China - Outstanding Youth Science Fund (2021), standing director of Food Additives Branch of China Society of Food Science and Technology, member of the Youth Working Committee of China Animal Products Processing Research Association, member of Poultry Products Sub-technical Committee of National Meat, Poultry and Egg Products Standardization Technical Committee, editorial board member
of "Fine Chemicals", "Food Research and Development", and eFood journals.
He has been committed to the theoretical and applied research
of oral flavor perception and food-derived substances in the field of food flavor chemistry.
Based on the salty fitting effect of typical umami substances, the scientific basic research and product technology innovation of salt reduction and sodium control and fresh salty fusion are implemented, and the research is carried out around the interaction perception law of food flavor substances in the process of oral processing, the salty aroma fusion effect mechanism of key aromatic substances in food, and the mechanism of multiple flavor enhancement, which provides new ideas and methods
for the development of low-sodium health foods.
He was supported by the National Natural Science Foundation of China - Outstanding Youth Fund and selected as the Outstanding Young Scientist Program
of Beijing Universities.

So far, he has presided over 12 projects such as the "13th Five-Year Plan" National Key R&D Program, the National Natural Science Foundation of China, and enterprise cooperation projects, published 1 academic work, authorized 7 national invention patents, 2 patented technology transformations, and published more than 160 academic papers, including 1 ESI hot paper and 1 highly cited paper
.
He won 1 special prize of the Science and Technology Award of the China Chamber of Commerce, and was a member of
the "Food Additive Teaching and Research Teacher Team" of the Huang Danian-style teacher team of national colleges and universities.

This article "Research Progress on Influencing Factors and Analysis Methods of Postnasal Aroma Perception" is derived from Food Science, Vol.
43, No.
19, pp.
17-27, 2022, authors: Pu Dandan, Shaan Yimeng, Shi Yige, Zhang Lili, Zhou Xuewei, Zhang Yuyu
.
DOI:10.
7506/spkx1002-6630-20220524-295
。 Click to view information about
the article.

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