Natural Chemistry in Human Olfaction: A Foodborne Perspective from Future Biotechnology
biocatalytic production of natural flavors that determine the chemical perception of food and beverages has been a daunting goal of academic and industrial research. Advances in chemical trace analysis and post-genomic advances at the chemo-biological interface have revealed that the odor quality of natural chemosensory entities is defined by odor-induced patterns of olfactory receptor activity. In a break from conventional wisdom, this review and big data analysis now show that there are only 3 to 40 signatures of truly critical odors for each food, with 230 extracted from 10000 or so food volatiles. This suggests that the stimulus space produced by food has co-evolved with our approximately 400 olfactory receptors and is roughly matched to the best natural agonists. This perspective provides insight into the natural chemical characteristics of odors, provides chemical odor coding for more than 220 food samples, and solves the industrial significance of producing a recombinant, which can completely reconstruct the natural odor characteristics for edible and daily fragrance, Fully immersive interactive virtual environment or human-like biological electronic nose.
1. Preface
For more than 8000 years, natural biochemical processes have been used in the production of food products, such as bread, beer, wine, cheese and yogurt, vinegar, soy sauce or fish sauce. Thus, flavor biotechnology was born by microbial fermentation or enzyme activity to produce new flavors and flavors. [1]
Although the metabolic properties of diverse microorganisms hold great potential for de novo flavor biosynthesis, the yields of valuable compounds found in nature are often too low for commercial application. In addition to some flavor compounds from primary metabolism, such as L-glutamic acid and citric acid, the diversity of metabolism often leads to a fairly wide range of closely related compounds, for example, a series of fusel alcohols from amino acid metabolism. [2]
in the last century, tremendous advances in organic synthesis have made the preparation of high-purity, naturally occurring odor molecules and chiral odors a cost-effective method [3]. For example, mint odor and racemic menthol have been industrially produced by Haarmann & Reimer in the mid -1960 s, while (-)-menthol with cooling effect (1R,2S,5R) structure needs to be separated by chiral resolution [4]. In the late 1980 s, a team led by Ryoji Noyori first developed the synthesis of myrcene-based (-)-menthol, which is known today as the "Takasago method", using the catalyst [{(S)-BINAP}2Ru]ClO4 to convert the asymmetric isomerization of diethylgeranamine to 3-(R)-citronellenamine, a new key reaction step. Ryoji Noyori won the 2001 Nobel Prize in Chemistry [5] using BINAP ruthenium catalyst. The synthesis of (-)-menthol is one of the best-selling flavor ingredients in the world today, reaching global demand of 25000 to 30000 metric tons per year. BASF developed a new process in 2012 to use a chiral rhodium catalyst for the asymmetric hydrogenation of (Z)-menthol [6]. In addition, organic chemistry has also successfully produced more influential non-natural homologs, for example, the currently widely used ethyl vanillin, which has a flavor intensity about 4 times stronger than its natural analog vanillin [7].
Despite the extraordinary success of industrial flavor production, alienated consumers are increasingly averse to non-natural chemicals added to food, cosmetics or household products, creating a growing demand for flavors with truly authentic natural flavor labels, as well as for flavor molecules of "organic" or biological origin [2]. In the past few decades, this has led to a serious shortage of several plant resources such as vanilla and mint, and promoted the application of "greener" chemistry and more "environmentally friendly" biotechnology manufacturing processes. The rapid development of synthetic biology to produce biomass flavors through plant cells, tissue culture or microbial processes involving bacteria, fungi, yeast and their enzymes. [2,8 - 10] Even insect-derived enzymes have recently been considered as undervalued treasures in industrial biotechnology. Biotechnological production of fine chemicals, such as organic acids, amino acids, nucleotides, vitamins and alcohols [12], biocatalytic regional and stereoselective transformations [13], rational protein design and computer-aided enzyme design, combined with directed evolution technology, to develop new biocatalysts [14,15], multi-step processes employing sequential biological or chemical catalytic transformations or engineered recombinant whole cells expressing multiple enzymes [16] and selective recovery of target molecules by efficient downstream processing have evolved rapidly over the past few years and are now a well-established discipline in the chemical industry [2,17]. Although volatile alcohols (such as fusel alcohols), odor-active organic acids and esters (such as 2-phenylethyl acetate), aldehydes (such as (Z)-3-hexenal and vanillin), ketones and 3-And 5-alkyl lactones have been produced industrially, but their chemical diversity makes odor molecules still a truly challenging target for biotechnology, in the food, feed, cosmetics and pharmaceutical sector has a wide range of applications. [2,9,10]
our progress in the molecular basis of olfaction, it is necessary to more effectively guide post-genomic flavor production technology to target molecules selected by natural evolution to create truly authentic olfactory perceptions of various foods and beverages. However, this requires new knowledge of how our sense of smell can solve the world of food odors at the molecular level. Seven transmembrane helix receptors (called odorant receptors (OR)) coupled 400 multiple rhodopsin-like G proteins work at the interface between the volatile molar chemical world and sensory perception in the brain, converting external chemical stimuli into internal information that can be processed by neural circuits [18]. Assuming that these receptors have evolved to handle this task, analysis of their coding strategies is expected to yield valuable insights into how to efficiently encode chemical information in nature [19]. To meet this need, there is a need to define the chemical odour space and decode a comprehensive population of sensorially active key molecules that reflect sensory phenotypes and trigger specific food taste characteristics, known as the "sensory metabolome (sensometabolome). [20, 21]
Despite the great technological advances in molecular sensory science in recent years to elucidate the sensory metabolomics of food at the molecular level ("sensoromic Sensomics"),[1,20,21] there is still a need for new bodies of knowledge to address many of the puzzles, particularly at the chemical-biological interface of olfaction. [22]
In other words, our understanding of odor coding relies heavily on the knowledge of the interaction of key odors with their best cognate receptor proteins, as well as the deciphering of combinatorial codes, in which the characteristics of odors are encoded by the specific subset of receptors they activate. [22]
to clarify the human receptor coding of a single odor related to organisms, and even more importantly, to clarify the receptor coding of the natural chemical sensory mixture that encodes the olfactory image that our brain perceives when eating, will be an important milestone. Efficient and cost-competitive biotechnology reconstructs truly authentic odor signatures-all of which are constructed from key odors produced by the same organism.
2. Analytical coverage of the food chemical odor space needs to be comprehensive and quantitative
search for odor molecules in our daily diet began with the introduction of gas chromatography (GC) in the early 60 s. At the time, research was conducted on the assumption that a whole set of volatiles occurring in food, body odors, or environmental odors contributed to the specific odors of chemosensory entities. However, although about 8000 volatiles have been identified so far in 2013, and a total of 10000 volatiles have been predicted to occur in foods, early experiments to reconstruct the aroma of foods with identified volatiles, such as olive oil [26] and orange juice [27] have not been successful. The main reasons for the difference in sensory perception between the flavor of authentic food and the aroma of simulated food are the lack of high-impact trace odors that cannot be detected by analytical equipment-gas chromatography flame ionization detector, and inaccurate quantitative data. Thus, dose/olfactory vigor considerations have increasingly led researchers to question whether all of the suggested 10000 volatiles in food contribute to a specific smell of food?[28-30] This prompted a paradigm shift in the search for key smells of food and the introduction of the "sensoromics" approach. Applying the idea of coupling gas chromatography to sniffing devices [31], gas chromatography sniffing (GC-olfactometry, GC-o) is a method that uses the human nose as the most sensitive and selective biodetector. It has been widely used in experimental entomology to locate active odors in a large number of inactive volatiles in chromatographic isolates through "sniffing" detection, it is used to detect volatiles perceived by the antennal olfactory organs of insects. [28,30,32 - 34] Techniques based on repeated GC-O analysis of serial dilution of aroma fractions, such as CHARM analysis [35,36] or AEDA aroma extract dilution analysis [28,30], enable comprehensive detection of odor-active molecules and ranking of their sensory effects according to their relative thresholds in the air.
this olfactory vigor-directed strategy has greatly helped us to focus our laborious identification experiments on the most odor-active molecules in food. However, because the olfactory screening of key odor molecules by gas chromatography-sniffing is based on their thresholds in air, rather than in the respective food matrix, researchers began to study the contribution of individual odors to the aroma of a given food based on "odor units" or "odor activity values" (OAV), the "odor unit" or "odor activity value" (OAV) is defined as the ratio of the concentration of one odor in a food to its odor threshold in an appropriate matrix. [29,30,37,38] However, the huge chemical complexity of volatile components and the huge differences in concentration, volatility and chemical stability of key odors challenge their precise quantification [28,30].
made a breakthrough by using stable isotope (13C, 2H) labeled bimolecular key odors as the most suitable internal standard for high-resolution gas chromatography/mass spectrometry analysis. [21,28,34, 39-42] Considering the identification of analytes during extraction, sample cleaning and chromatography, this so-called stable isotope dilution analysis (SIDA) allows robust quantitative analysis of key food odors, The required accuracy is less than 10%.
