Food Science Flavor Sensory Column: Professor Liu Yuan of Shanghai Jiao Tong University, et al.: Research hotspots on food umami: controversy and function

Studies have shown that umami substances can fully present their characteristic umami taste only when a certain amount of salt ions (Na+, Cl^- or K⁺, etc.
) surround the anion
.
In general, low concentrations of NaCl can enhance umami, but when NaCl concentrations are too high, umami will be masked
by saltiness.
The interaction of MSG with other basic tastes is related
to the concentration of both.
The latest research found that the addition of MSG did not affect the sweet, salty, sour and bitter taste of the solution in the simple aqueous solution system, but these four tastes had an inhibitory or masking effect
on the umami taste.
The current research is mostly carried out in model solutions, so umami perception and interaction need to be verified in a more realistic and diversified food system, from the level of receptor, signal transduction, brain response, behavior, physiological and psychological feedback
.
The umami perception pathway is shown in
Figure 2.

The taste of umami peptides is special

The controversy over the taste of umami peptides mainly stems from their different preparation methods, generally natural sources (food or enzymatic hydrolysates) umami peptides have obvious umami taste, while some umami peptides that are synthesized after determining the sequence have a more complex
taste.
Taking beef flavor peptide (BMP) as an example, Yamasaki et al.
first identified BMP in beef in 1978 and found that the umami taste of BMP was more prominent
than glutamic acid.
However, when the taste of the synthetic and isolated BMP was compared, the synthetic BMP was salty, sour and sweet, and had no umami taste
.
In the following nearly 20 years, different scholars proposed that the synthetic BMP has no sweetness, umami, no umami and other different results
.

The taste verification of umami peptides is mainly carried out by sensory analysis, and the combination of sensing evaluation, computational chemistry, molecular docking and other methods has gradually become a reliable method for comprehensively judging the taste characteristics of umami peptides (Figure 3).

The author's team constructed a series of umami biosensors based on receptors, cells and tissues, and realized umami determination based on the strength of receptor-ligand binding ability, and the results were positively correlated
with sensory evaluation results.
At the same time, our team used quantum chemistry, molecular docking and artificial intelligence to analyze the action law of umami receptor and umami peptide, systematically studied the taste pattern of currently known umami peptides from multiple angles, and predicted the active sites and umami receptor activity centers of the reported umami peptides, so as to establish a database
for the development of umami peptide evaluation system.
The comprehensive application of multiple technologies provides theoretical support
for umami peptide mining and analysis of freshness mechanism, including molecular, physiological and psychophysical.

Safety of umami ingredients

MSG and Chinese restaurant syndrome: There is a long-term excessive use of salt, monosodium glutamate and other condiments in some parts of China, which is particularly prominent
in some restaurants and takeaway foods.
The most concerned event about the safety of MSG was the Chinese restaurant syndrome (CRS),
first reported in 1968.
Kwok said he experienced numbness in his neck and arms and palpitations after eating Chinese food, and attributed these symptoms to the MSG
in the food.
In 1969, Schaumburg et al.
again proposed that MSG was a predisposing factor
for CRS.
However, a series of subsequent experimental studies and investigations showed that CRS symptoms were not associated
with MSG intake.
A large US questionnaire survey concluded that only 6 (0.
19%) of the 3 222 respondents had CRS related to eating Chinese food, but this part of the data was also questioned
because of the undesirable motives such as racial discrimination.

The safety of MSG: In fact, the "smell" of MSG can be partly attributed to the rapid development
of the food industry in the 60s of the 20th century.
During this period, MSG was widely added by food companies to the production of ingredients or foods with poor quality, rough production or poor flavor due to improper preservation, helping food companies to inferior or mask the characteristics
of inferior food.
Consumers have experienced adverse reactions after eating such foods, and have pointed the finger at MSG
added to food.

In 1969, Olney's research showed that a single subcutaneous injection of high concentrations of MSG into newborn mice caused neuronal necrosis in the brain, and other researchers successively injected or fed rats, dogs, and monkeys, and the detailed results are shown
in Table 1.

In addition to animal models, behavioral studies using human samples are also gradually being carried out
.
In healthy adults in China, the correlation between dietary intake of MSG and body mass index was investigated by 24-hour review method and questionnaire, and MSG intake was found to be positively correlated
with overweight.
Liu Qingzhi et al.
collected MSG intake and diabetes disease information in residents' diets through questionnaires, and found that diabetic patients' preference and intake of MSG were higher than those of non-diseased people, indicating that diabetes may be related to
excessive consumption of MSG.
However, such correlation results are influenced by behavioral research methods, sample size, and complexity, and the results are inconsistent and the mechanism has not been elucidated
.

MSG is a daily condiment, and the changes in intestinal flora during digestion are also gradually attracting attention
.
Liu Shujun et al.
used MSG as the substrate for in vitro fermentation of intestinal microorganisms, and according to the surge in butyric acid production in the fermentation broth and the increase in the abundance of Escherichia and Bacteroides, it is speculated that these two genera play a potential role
in the conversion of MSG to γ-aminobutyric acid.
In addition, an appropriate amount of MSG can maintain the structural integrity of the intestine, alleviate the rise of intestinal inflammation levels, maintain the homeostasis of the intestinal flora, and increase the relative abundance of probiotics
.

MSG exists in the form of glutamic acid and Na⁺ after entering the body, both of which are common components in foods
.
Mice are more susceptible to MSG than primates due to differences in the blood-brain barrier, and differences between mouse model results and human outcomes need to be taken into account
.
The data show that the daily intake of MSG in Asia is about 1.
2~3.
0 g/d, while that of other industrialized countries is generally 0.
3~1.
0 g/d
.
Modern epidemiological studies have shown that long-term moderate intake of MSG has no significant toxicity or other adverse effects
.

Safety assessment of MSG by international agencies: As shown in Table 2, international agencies and food safety administrations of various countries have widely affirmed
the safety of MSG.
In 2017, EFSA reassessed the acceptable daily intake of MSG and determined the allowable daily intake (ADI) for MSG to be 30 mg/kg mb
.
Based on this data, MSG is generally safe
in the daily diet.

3.
The function of umami regulates the flavor

Umami plays a key role
in the palatability and acceptability of food.
Palatability may determine whether a particular food is consumed, intake, absorbed and digested, and more
.
Umami solutions themselves have limited palatability, but adding them to food can significantly improve taste, reduce odor, and enhance palatability
.
In addition, the umami synergy between MSG, IMP and GMP can also increase the palatability of the product; The development of complex umami agents based on natural sources has also improved the diversity of taste characteristics and taste of products
.

Stimulates appetite and regulates satiety

Just as sweetness represents energy and bitterness warns of potential toxicity, umami represents nutrition
to some extent.
Umami is a signal of protein intake that helps the body obtain nutrients such as essential amino
acids.
As a result, loss of taste function or altered sensitivity may lead to loss of appetite, decreased dietary intake, and decreased
body mass.
Umami helps stimulate appetite, induce saliva secretion, trigger gastric acid and insulin secretion, and increase food intake
.

The elderly are prone to malnutrition and anorexia due to the deterioration of oral processing, taste perception and digestion
.
Studies have shown that adding 0.
6% MSG to customized foods can promote food intake
.
Therefore, special dietary products for the elderly or patients with dysphagia can increase the palatability of food, improve salivation and swallowing, and promote eating
by adding umami substances.

Taste receptors are widely found in body tissues including the mouth, such as respiratory tract, urinary tract, intestine, brain, lung pancreas and heart, and play the functions
of antibacterial immunity, promoting digestion and nutrition sensing.
The specific mechanism by which umami can increase satiety may be that during protein digestion, the release of gastrointestinal hormones (satiety hormones) such as neuropeptide Y, cholecystokinin and gastrin-like polypeptide-1 can affect protein-induced satiety, which in turn regulates eating behavior and digestion
.
Satety is produced by activating subdiaphragmatic vagus neurons, and the secretion of cholecystokinin can reduce food intake, which is triggered from the upper end of the intestine in response to food intake, preventing continuous eating and producing satiety
.
However, the mechanisms of how this physiological effect arises and exerts remain to be studied, and more work is needed to uncover the role
of taste receptors in tissues outside the mouth.

conclusion

A good life must be delicious, and human beings have never stopped searching for
umami.
In this paper, the existing controversial hotspots and functions of umami are discussed, and the safety questions about the umami peptide and umami representative substance MSG are discussed, and the function of
umami is summarized.
Umami peptides are important components of umami ingredients and play a multi-role role
.
Comprehensive sensory evaluation, biomimetic sensing, computational chemistry and other methods can realize the effective analysis
of the taste characteristics of umami peptides.

It is generally safe
to consume umami agents such as MSG in the daily diet.
Umami plays an active role
in regulating taste and eating behavior and feedback.
Due to the complexity of the food system and the diversity of potential flavor substances, it is still necessary to continuously explore new umami substances and study the physical and chemical properties and sensory properties
.
For commercial umami agents, it is necessary to continue to build and promote a green manufacturing system to help promote the seasoning industry
.
In terms of exploring the mechanism of umami perception, the structure of umami receptors and their binding and dissociation modes with ligands, the transmission of downstream cascade signals, and the physiological and psychological feedback of umami perception need to be explored
urgently.
On this basis, a diversified umami evaluation system is constructed, and the health functional effects
of umami (substance) are further explored.
With the continuous deepening of research on the basics and applications of umami, umami has great potential
in the development of healthy-flavor-oriented future foods.

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