Since ancient times the delicious taste and aroma of foods, herbs, spices and essential oils have been inspiring human beings in different cultures, geographical locations and ages . Over thousands of years people have developed a wealth of recipes, techniques and technologies for food preparations, mainly driven by flavour, comprising aroma, taste, texture, viscosity, temperature as well as cooling, tingling and pungency . Starting from distillation and extraction in the world of ancient Greece and Rome the medieval era led to an extended use of herbs and spices. In the Renaissance the studies of Lavoiser, Davy, Dalton, Priestly, Scheele and others laid the foundation for modern chemistry . Finally, in the industrial age the curiosity of chemists revealed the chemical nature of numerous flavouring substances. The so-called great cycle of the chemical industry - identification - laboratory synthesis -large-scale synthesis and commercialisation - introduced important aroma chemicals like cinnamic aldehyde, benzaldehyde, methyl salicylate, coumarin, phenyl acetalde-hyde and vanillin between 1830 and 1890. Numerous character impact compounds were identified in different segments of our diet such as allyl disulphide in garlic and furfuryl mercaptan in coffee.
The development of modern gas chromatography as well as the combination with olfactometry (GC/O) and in particular the introduction of gas chromatography with mass spectrometry (GC/MS) at a routine level increased the productivity of the great cycle of the flavour industry. Supported by continuously improved nuclear magnetic resonance (NMR) techniques and specific preparative techniques, more than 15,000 chemical compounds with flavouring properties have been reported so far . The use of GC/MS instrumentation in combination with powerful computer systems enabled deep insight into the world of volatile flavour compounds (see 6.2.1). Just recently taste-active molecules as well as trigeminal active compounds have been receiving increased attention while correcting the picture on volatile flavour compounds in modern flavour chemistry .
The roots of modern flavour industry grew in local and regional markets in the 19th and early 20th century (see also chapter 1). In Germany Haarmann & Reimer was founded in 1874. Leon and Xavier Givaudan started their company in 1896 in Switzerland. Robert G. Fries and his brother George Fries began selling flavourings in the USA around 1913. In Japan Takasago was founded in 1922. As the business grew and the first restructuring of companies such as the conversion of Naef, Chiut et Cie. to Firmenich et Cie. took place, government authorities started to lay the foundation for the legislation on foodstuffs and in particular flavourings.
In Europe chemically defined substances with flavouring properties, which are obtained by chemical synthesis, are defined in the European Union Flavouring Directive 88/388/EEC , article 1 No. 2(b) (ii) and (iii) in two categories (see 7.1 and 7.4.2):
These flavouring substances are characterised by the fact that the flavouring substance obtained by chemical synthesis or isolated by chemical processes is chemically identical to a substance naturally present in material of vegetable or animal origin.
These flavouring substances are defined by the criterion that the flavouring substance is not chemically identical to a substance naturally present in materials of vegetable or animal origin.
The legislation of some European countries comprises positive lists for artificial flavouring substances, such as ethyl vanillin.
Since this approach is based on the scientific proof of natural occurrence, the International Organisation of the Flavour Industry (IOFI), Brussels (B), established the Working Group on Analytical Methods (WGMA), which examines published data with regard to the validity of chromatographic (e.g. GC retention time) and spectroscopic (e.g. mass spectra, infrared and NMR spectra) data. In this context reference data, artefact formation and the nature of the source material (i.e. food use) are also considered.
This approach is different from the legal regulations relating to use of flavourings in foods, which are described in § 101.22 of Title 21 of the Code of Federal Regulations in the USA. In the USA, the term 'artificial flavour' or 'artificial flavouring' means any substance with flavouring properties, which is not derived from a spice, fruit or fruit juice, vegetable or vegetable juice, edible yeast, herb, bark, bud, root, leaf or similar plant material, meat, fish, poultry, eggs, dairy products, or fermentation products thereof. This definition of artificial flavourings also comprises synthetic flavouring substances and adjuvants, which are listed in §§ 172.515(b) CFR as well as synthetic flavouring substances listed in §§ 182.60 CFR. In practice, this means that any flavouring using synthetic aroma chemicals, with and without natural occurrence, is labelled as artificial flavour (see 7.5.2: USA).
Nature-identical and artificial flavouring substances are produced in order to evoke specific sensorial effects. The potency of the individual compounds is usually described by a set of parameters comprising the odour or taste threshold in the corresponding matrix, i.e. water or oil phase, as well as the dynamics of perception as determined by Steven's law . For many compounds a shift of the specific sensorial properties is observed in different concentrations.
Important physicochemical parameters are volatility, vapour pressure and polarity, which play an important role in the flavour release of solid and paste-like food as well as for beverages . The link between sensorial properties and the molecular structure has always been of highest interest for organic chemists in the past 150 years. Functional groups, stereochemistry, molecular size and heteroatoms have been varied in numerous isomers in order to understand the corresponding sensory profile and related effects. The use of molecular references like (E,E)-2,4-decadienal for the descriptor 'fatty' or (E)-2-hexenal for 'green' has become a standard approach to link sensory with the flavour chemistry of flavourings and finished food products. Analytical techniques like GC/O or the use of mouth model systems  are following the same approach. The aroma value concept  (see 6.2.4) impressively shows that in many foodstuffs only a few flavour compounds play an important role in the authentic flavour profile. The ratio of the concentration of a flavour compound in food and the retronasal odour threshold or taste threshold determine the odour activity value (OAV) or taste activity value (TAV) (see 6.2.4). Together with omission experiments these tools are essential for unmasking the blueprint of a flavour extract. Recently "LC Taste®", a new method for combining instrumental analysis and sensorial analysis, has been developed. Based on a separation via high-temperature liquid chromatography (HTLC) and online tasting of aroma and taste compounds in aqueous matrix, flavour researchers and developers have the possibility for a differentiated retronasal, taste-oriented evaluation of mixtures .
Industrial applications of nature-identical and artificial flavourings have to be based on two pillars: a clear understanding of flavour-relevant molecules and at the same time the use of technically feasible, high-purity, high-yield syntheses. In addition, performance criteria like pH, oxidation and chemical stability as well as toxicological safety have to be considered.
Flavour-active compounds cover an enormous dynamic range regarding perceivable concentration, starting with extremely powerful molecules like 1-menthen-8-thiol in the low ppt (ng/kg) range and going up to the few percent level for compounds like acetic acid. Together with the huge variety of chemical compound classes this fact generates additional challenges for the toxicological evaluation of flavouring substances.
In Europe, the EU Commission started to establish an inventory for flavouring substances covering chemically synthesised or chemically isolated flavouring substances chemically identical to flavouring substances naturally present in foodstuffs or in herbs and spices as well as vegetable and animal raw materials normally considered as foods. In addition, chemically synthesised or chemically isolated flavouring substances from other sources and not yet found in nature are also considered. By end of 2004 the list consisted of approx. 2800 compounds as a first inventory comprising natural, nature-identical and artificial substances, based on information received from the flavour industry. All listed compounds are subject to a detailed safety evaluation (see 7.4.2). In 2000, the EU Scientific Committee on Food (SCF), now recognised as the European Food Safety Authority (EFSA) (FSA panel), accepted the evaluation procedure of the World Health Organisation Joint Expert Committee on Food Additives (JECFA) and reviewed the list of flavouring substances already approved by JECFA. These compounds are part of the list of evaluated flavour compounds by the Panel of Expert Scientists (FEXPAN) of the Flavour and Extract Manufacturers' Association (FEMA) in the USA (see 7.5.2: USA). So basically two international expert committees are currently evaluating flavouring substances for their risks: JECFA and EFSA. They perform the 'risk assessment', whilst the responsibility for 'risk management' lies with the Codex Committee on Food Additives and Contaminants (CCFAC) and the EU Commission and its administration (DG Sanco).
For the FEMA approach as well as for the evaluation of flavouring substances of the European register, normal and maximum use levels of aroma-active compounds in different food applications play an important role. The EU approach comprises 16 food categories which cover a broad range of food applications, starting with category 1 (dairy products) and ending with category 16 covering composite foods. Basically, two different methodologies exist, namely MSDI (Maximised Survey-Derived Daily Intake) and TAMDI (Theoretical Added Maximum Daily Intake). The MSDI approach is based on the annual production of substances as stated by the producers divided by the total consumers. The TAMDI system requires a calculation of flavour addition to food and consumption of flavoured foodstuffs per day. For the EFSA concept, the first evaluation is conducted based on the MSDI method following a decision tree, which is published together with the so-called FGE statements . A second interactive step based on a modified TAMDI method helps to identify compounds with a higher use level than the calculated safety level.
All entries of the European inventories are classified in 34 chemical groups, which represent compounds of consistent chemical, metabolic and biological behaviour. In the case of compounds that are easily degraded to smaller fragments, the most toxic fragment is given higher priority in assigning the appropriate chemical group.
The wealth of powerful aroma chemicals is finally classified based on safety considerations. In order to give more insight, prominent representatives are explained in detail.
Important compounds of Group 1 are straight-chain primary aldehydes like hexanal (Flavis # 05.008, FEMA 2557) and octanal (Flavis # 05.009, FEMA 2797) with tallowy and fatty notes as well as widely used acids like acetic acid (Flavis # 08.002, FEMA 2006) and the powerful butyric acid (Flavis # 08.005, FEMA 2221). In the same group ubiquitous fruity esters like ethyl butanoate (Flavis # 09.039, FEMA 2427; annual consumption in the USA approx. 10 tons in 1991 ) and the applelike methyl 2-methyl butyrate (Flavis # 09.483, FEMA 2719) are registered.
Chemical Group 2 comprises cocoa flavour compounds like 3-methyl butanal (Flavis
# 05.006, FEMA 2692) and the banana-like 3-methyl butyl acetate (Flavis # 09.024, FEMA 2055).
In chemical Group 3 important terpene alcohols such as the rose-like geraniol (Flavis
# 02.012, FEMA 2507) and aldehydes such as the citrus-like compound citral (Flavis
# 05.020, FEMA 2303) are found. The two isomers of citral, namely neral (Flavis # 05.170, FEMA 2303) and geranial (Flavis # 05.188, FEMA 2303) are listed in the same group. The mixture and the two individual isomers are listed under the same FEMA number 2303.
Group 4 lists compounds with leaf-like notes such as (Z)-3-hexen-1-ol (Flavis # 02.056, FEMA 2563) and (Z)-3-hexenal (Flavis # 05.075, FEMA 2561) as well as esters such as citronellyl acetate (Flavis # 09.012, FEMA 2311) with rose-like, floral profile.
A typical representative for mushroom-like notes, 1-octen-3-one (Flavis # 02.023, FEMA 2805), is found in Group 5 together with the fatty, fruity tasting 2-undecanone
(Flavis # 07.016, FEMA 3093) and numerous other 2-alkanones, which are responsible for characteristic cheesy notes.
Group 6 shows additional terpene alcohols like linalool (Flavis # 02.013, FEMA 3045) with flowery profile and a-terpineol (Flavis # 02.014, FEMA 3045) with sweet, fruity aroma. In addition, numerous esters are listed like a-terpinyl acetate (Flavis # 09.065, FEMA 3047), which is characterised by a strong herbaceous odour.
In Group 7 the a- and p-isomers of santalyl acetate (Flavis # 09.034, FEMA 3007) are mentioned. These compounds have a characteristic sandalwood-like odour and a bitter-sweet taste. Additional compounds in this group are perilla aldehyde (Flavis # 05.117, FEMA 3557) with fatty, spicy notes and safranal (Flavis # 05.104, FEMA 3389) with a characteristic saffron-like odour and taste.
One of the most important compounds in Group 8 is menthol (Flavis # 02.015, FEMA 2665; world usage 11,800 tons in 1998 ), well known for its minty aroma accompanied by a strong cooling effect. Furthermore, Group 8 comprises also a-ionone (Flavis # 07.007, FEMA 2594) and p-ionone (Flavis # 07.008, FEMA 2595) with a strong violet-like odour and carvone (Flavis # 07.012, FEMA 2249), which occurs in two different forms. L-carvone exhibits the odour of spearmint, while 5-carvone shows a strong caraway note. Another important compound is nootkatone (Flavis # 07.089, FEMA 3166), which also occurs in two different forms. (+)-Nootka-tone has a highly appreciated grapefruit-like profile and (-)-nootkatone is characterised by a terpeny note. p-Damascenone (Flavis # 07.108, FEMA 3420), a very powerful, honey-like, fruity, sweet compound, is also a member of this group together with l-menthyl lactate (Flavis # 09.551, FEMA 3748), a typical cooling compound.
In Group 9 various organic acids like lactic acid (Flavis # 08.004, FEMA 2611), pyruvic acid (Flavis # 08.019, FEMA 2970) and succinic acid (Flavis # 08.024, registered in Food Chemical Codex) together with various diesters are found. Beside some diols, numerous lactones such as y-decalactone (Flavis # 10.017, FEMA 2360) with peach-like notes and S-decalactone (Flavis # 10.007, FEMA 2361) with a sweet coconut-like profile are represented.
Group 10 is a fairly small group covering secondary aliphatic saturated or unsaturated alcohols, ketones, ketals and esters. Important members are the typical dairy compounds diacetyl (Flavis # 07.052, FEMA 2370) and acetoin (Flavis # 07.051, FEMA 2370).
Group 11 is the home for mint lactone (Flavis # 10.036, FEMA 3764) and various phthalides, which are well known from the flavour chemistry of celery.
Maltol (Flavis # 07.014, FEMA 2656) and ethyl maltol (Flavis # 07.047) are key compounds in Group 12.
Group 13 sets the frame for the complex world of furanones. Important fruity and caramelic representatives are Furaneol® (Flavis #13.010, FEMA 3174) and the corresponding methoxy derivative mesifuran (Flavis # 13.089, FEMA 3664). In addition the linalool oxides and angelica lactone are in this group.
Group 14 lists among other compounds various furyl disulphides and in particular the coffee compound furfuryl mercaptan (Flavis # 13.026, FEMA 2493).
In Group 15 phenyl acetaldehyde (Flavis # 05.030, FEMA 2874) with honey-like, flowery notes has to be mentioned.
Group 16 contains 1,8-cineol or eucalyptol (Flavis # 03.001, FEMA 2465), a compound with a characteristic camphoraceous odour and a fresh cooling taste.
Groups 17 and 18 contain among other compounds isoeugenol (Flavis # 04.004, FEMA 2468) and eugenol (Flavis # 04.003, FEMA 2467), respectively.
Group 19 is dedicated to a few amides like N-ethyl-p-menthane-3-carboxamide (Flavis # 16.013, FEMA 3455).
Group 20 is a nice collection of wonderful sulphur compounds. Disulphides such as diallyl disulphide (Flavis # 12.008, FEMA 2028) with onion- and garlic-like notes are followed by extremely powerful mercaptans like the grapefruit compound p-1-men-thene-thiol (Flavis # 12.085, FEMA 3700). In addition, thiols with tropical sulphury notes complete the picture of interesting molecules.
Group 21 comprises phenyl derivates like raspberry ketone (Flavis # 07.055, FEMA 2588), which is a compound with sweet, fruity, raspberry-like notes.
In Group 22 we find cinnamic aldehyde (Flavis # 05.014, FEMA 2286) and cinnamic acid (Flavis # 08.022, FEMA 2288) as well as the corresponding esters.
Vanillin (Flavis # 05.018, FEMA 3107; world usage approx. 12,000 tons in 1990 ) and benzaldehyde (Flavis # 05.013, FEMA 2127; world usage approx. 13,000 tons in 1994 ) are major entries in Group 23.
Group 24 represents the world of pyrazines. The sensory profile of the listed compounds ranges from toast to coffee and cocoa notes reminiscent of the wonderful aroma of many culinary pleasures.
In Group 25 various phenols like thymol (Flavis # 04.006, FEMA 3066) and salicylates are listed. In addition naringin (Flavis # 16.058, FEMA 2769) is also found on the list.
Group 26 contains a series of aromatic ethers and Group 27 shows numerous anthranilates.
Pyridine, pyrrols and quinoline derivatives are found in Group 28. Thiazole, thi-ophenes, thiazolines and thienyl derivatives are summarised in Group 29. Among miscellaneous compounds Group 30 lists hydrogen sulphide and ammonia. In Group 31 we find aliphatic and aromatic hydrocarbons like limonene (Flavis # 01.001, FEMA 2633).
Group 32 shows various epoxides like p-caryophyllene epoxide (Flavis # 16.043).
In Group 33 we find amines like trimethyl amine (Flavis # 11.009, FEMA 3241) with a prominent fishy note.
Finally, Group 34 is dedicated to a collection of amino acids which are important for mouthfeel and taste in the world of savoury, sweet and beverage products.
After the completion of the safety evaluation of the above mentioned flavour compounds, the current register will be transferred into the European positive list of flavouring substances, comprising natural, nature-identical and artificial molecules. The status will not be indicated in the foreseen list. As a consequence, the currently existing positive lists for artificial compounds on the level of country-specific legislation will then be obsolete.
The future use of nature-identical and artificial flavour compounds will be based on a broad set of criteria: sensory properties, safety and use levels and finally performance parameters like stability in application and flavour release.
For the synthesis of flavour-active compounds numerous methodologies have been developed . In many cases natural products served as starting materials such as eugenol from clove oil for the synthesis of vanillin. In case of complex stereochemistry natural materials are still welcome for the synthesis of valuable flavour compounds such as nootkatone, which is obtained by oxidation from valencene.
Certainly economical considerations are an important starting point for the development of a successful aroma chemical. The sensorial profile, impact and the absence of off notes is often underestimated. It is part of the knowledge and the professional skills of a flavourist to understand the relevance of effects like aging, isomerisation and oxidation. Over the years numerous strategies for the synthesis of fairly simple aroma chemicals, like straight-chain esters, and of complex structures, like the different isomers of rose oxide (2S, 4R rose oxide and 2R, 4R rose oxide), have been developed.
A special discipline is represented by the world of sulphur-bearing compounds. In general all representatives of this class are very powerful chemicals, which often leads to their being handled in smaller amounts.
The sensorial quality of sulphur-bearing aroma chemicals is an important criterion, because of the reactivity of, for example, free thiols. For the synthesis of prominent representatives like 8-mercapto-3-menthanone or 1-menthen-8-thiol different synthetic methods have been published.
More than in the past, sensorial delights for millions of consumers will be based on the close interaction of multiple disciplines including flavourists, chemists, toxicolo-gists, technologists and chefs. In this context nature-identical and artificial substances will continue to play a major role in modern flavourings.
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