Cyclic aroma compounds

Scheme 7.5 Formation of some aroma compounds after oxidative cleavage of a acyclic carotenoids (e.g., lycopene, phytofluene and phytoene) and b cyclic carotenoids (e.g. a-carotene and / -caro-tene)...

The major advantage of the seunpling technique developed, was that some trace chemicals could be trapped tind described for the first time as Black Truffle aroma constituents. In particular, some compounds, important flavor contributors, generally appearing in small concentrations, such as benzaldehyde, propanal, ethyl acetate, anisole or dimethyl disulfide - previously identified in Shiitake mushrooms (9) - could be characterized. This was also the case for three aromatic compounds, toluene, xylene and ethyl benzene, well known as raw vegetable constituents (1 ). In addition, two aliphatic esters, isopropyl and sec-butyl formates, and one cyclic sulfur compound (2-formyl thiophene) previously reported respectively in plums and apples (W) and in coffee and bread products (n) were identified. 

Fresh Shiitake mushrooms exhibit only a slight odor, but upon drying and/or crushing, a characteristic sulfurous aroma gradually develops. Lenthionine (1,2,3,5,6-pentathiepane, C2H4S5), a cyclic S-compound known to possess the characteristic aroma of Shiitake... 

A final group of important aroma compounds consists of the macro-cyclic lactones. Angelica root oil finds extensive use in flavor formulations for liquors such as vermouths, bitters, benedictines etc. 149). The musk-like odor of this essential oil is generally attributed to 15-penta-decanolide (67) (EXALTOLIDE ) (507). Recently three additional macrocyclic lactones were discovered in this oil, 13-tridecanolide (66), 17-heptadecanolide (68) and 12-methyl-13-tridecanolide (69) 625, 62 which, together, account for about 1% of the oil. 

Although the number of aroma compounds derived from acyclic carotenoids is much inferior to that of the mono- and bicyclic compounds, some of them can also be considered as breakdown products from genuine mono-, sesqui- and diterpenoids. The importance of the aliphatic isoprenoids (282) to (291) in the formation of total flavors of certain foodstuffs is not less than that of the cyclic compounds, the three methyl ketones (282), (287) and (290) which are related to the main tomato pigment lycopene were observed in tomato flavor (75). The hexahydro derivative (291) from coffee (595), jasmine oil (722) and green tea 438) is perceived as flowery and warm and can be considered as an oxidative biodegradation product of phytol and phyta-diene. 6-Methyl-3,5-heptadien-2-one (283), with a grassy and cinnamonlike aroma 438) [detected in tomato 668), the essential oil of Hama-metis leaves 383), Ceylon tea (722) and passion fruit (777)], and pseudo-ionone (288) [also isolated from passion fruit (777)] are believed to be formed from two different dehydrolycopenes. Compounds other than carotenoids, such as solanesol or squalene, can also be considered... 

In the natural world, carotenoid oxidation products are important mediators presenting different properties. Volatile carotenoid-derived compounds such as noriso-prenoids are well known for their aroma properties. Examples include the cyclic norisoprenoid P-ionone and the non-cyclic pseudoionone or Neral. Carotenoid oxidation products are also important bioactive mediators for plant development, the best-known example being abscisic acid. Apo-carotenoids act as visual and volatile signals to attract pollination and seed dispersal agents in the same way as carotenoids do, but they are also plant defense factors and signaling molecules for the regulation of plant architecture. 

The ring-opening of the cyclopropane nitrosourea 233 with silver trifiate followed by stereospecific [4 + 2] cycloaddition yields 234 [129]. (Scheme 93) Oxovanadium(V) compounds, VO(OR)X2, are revealed to be Lewis acids with one-electron oxidation capability. These properties permit versatile oxidative transformations of carbonyl and organosilicon compounds as exemplified by ring-opening oxygenation of cyclic ketones [130], dehydrogenative aroma-tization of 2-eyclohexen-l-ones [131], allylic oxidation of oc,/ -unsaturated carbonyl compounds [132], decarboxylative oxidation of a-amino acids [133], oxidative desilylation of silyl enol ethers [134], allylic silanes, and benzylic silanes [135]. 

Musty or potato-like flavor and aroma have been observed as a defect in milk (Hammer and Babel 1957) and Gruyere de Comte cheese (Dumont et al. 1975). This off-flavor results from the production of nitrogenous cyclic compounds by Pseudomonas taetrolens and P. perolens (Morgan 1976). Musty-flavored compounds produced by these organisms include 2,5-dimethylpyrazine and 2-methoxy-3-isopropyl-pyrazine. The Gruyere de Comte with potato off-flavor contained 3-methoxy-2-propyl pyridine, as well as alkyl pyrazine compounds (Dumont et al. 1975). Murray and Whitfield (1975) postulated that alkyl pyrazines are formed in vegetables by condensation of amino acids such as valine, isoleucine, and leucine with a 2-carbon compound. Details of the synthetic mechanism in pseudomonads are unknown. 

Many desirable meat flavor volatiles are synthesized by heating water-soluble precursors such as amino acids and carbohydrates. These latter constituents interact to form intermediates which are converted to meat flavor compounds by oxidation, decarboxylation, condensation and cyclization. 0-, N-, and S-heterocyclics including furans, furanones, pyrazines, thiophenes, thiazoles, thiazolines and cyclic polysulfides contribute significantly to the overall desirable aroma impression of meat. The Maillard reaction, including formation of Strecker aldehydes, hydrogen sulfide and ammonia, is important in the mechanism of formation of these compounds. 

Heterocyclic compounds contribute significantly to the overall aroma impression of meat. They include 0-, N-, and S-hetero-cyclic structures. Meat flavor heterocyclics include furans,... 

Lactones are cyclic compounds formed through the intramolecular esterification of a hydroxy fatty acid. 7-Lactones and 8-lactones, with fivesided and six-sided rings, respectively have been found in cheese (Jolly and Kosikowski, 1975 Wong et al., 1975 Collins et al., 2004). The origin of the precursor hydroxy fatty acids has been ascribed to a 8-oxidation system in the mammary gland of ruminants (see Fox et al., 2000), the reduction of keto acids (Wong et al., 1975) and/or the action of lipoxygenases and other enzymes present in members of the rumen microflora (Dufosse et al., 1994). Lactones have low flavor thresholds and while their aromas are not specifically cheese-like (their aromas have been described variously as peach, apricot and coconut ), they may contribute to the overall flavor of cheese (see Collins et al., 2004). 

Lactones are cyclic compounds with relatively high boiling points and an ester functional group. They have the characteristic ester notes fruity, oily, and sweet. y-Undecalactone, with a peach-hke aroma, has a boiling point of 297°C. 

The main purpose of using hops in beer is to add bitterness to the final product. A series of compounds referred to as Ci and B-acids are responsible for this taste (1 ). A secondary, ill defined flavor (flavor referring to smell and taste) is also imparted to beer by brewing with "aroma hops." Not all hop varieties are considered "aroma hops," and there is evidence that the flavors Imparted to beer by different aroma hops are different ( ). There has been considerable controversy in recent years as to the nature and source of this flavor. Researchers have credited ter-pene alcohols (2, ), humulene oxidation products (, ), multi-cyclic terpenoid ethers (6) and carotenoids (.6) as being in part responsible for this flavor. 

Two excellent papers on YE aroma have arisen recently from Werkhoff s group at Haarmann and Reimer in Germany (13,14). Fifty sulfur compounds were reported in the first study (13) while 115 such components were described in the second paper (14). Aliphatic compounds, sulfur-substituted furans, thiophenes, thiazoles, 3-thiazolines, cyclic polysulfides, perhydro-l,3,5-dithiazines, a perhydro-1,3,5-oxathiazine, a perhydro-l,3,5-thiadiazine and aliphatic and heterocyclic dithiohemiacetals were all mentioned, and the structures of most of the newly identified compounds were confirmed by synthesis. These sulfur compounds elicit a wide variety of aromas but are often savory, e.g., meaty, vegetable-like, and YEs are a valuable source of potent components which provide meaty notes. 

The English language definition of aromatic is "of or having an aroma smelling sweet or spicy fragrant or pungent." Historically, aromatic compounds were identified as such by their characteristic odor. The modern chemical definition of aromatic has to do with the cyclic delocalized bonding in a molecule rather than its odor. 

The flavor of flying oils and fried products is mainly due to the volatile compounds which influence the aroma. Nonvolatile products may also change the taste and the mouth-feel. Billek et al. (Billek et aL, 1978) reported that RBD soybean oil contains about 1.2% oxidized triacylglycerols. The nonvolatile oxidized compounds include cyclic carbon-to-carbon linked dimers, noncyclic hydroxyl dimers, carbon-to-carbon or carbon-to-oxygen... 

In addition to the 5-lactones of structure (41), other lactonic aroma components with a saturated ring are known, the structure of which, however, is somewhat different from that of the n-series represented by (41). These are 6-hydroxy-5-hexanolide (42), identified in tomato (557), and three cyclic compounds carrying an unsaturated chain in position 5, viz. (Z)-7-decen-5-olide (jasmine lactone) (44) 712), identified in the flavor of black tea (50) together with methyl jasmonate (779). (Z)-9-dodecen-5-olide (45) and (Z)-9-tetradecen-5-olide (46), the latter two lactones having been isolated from butter flavor (50, 725). On the other hand, lactone (43) was isolated from milk where it is present in a quantity of 10 pg/kg (0.001 ppb) (247) this quantity, however, is probably much lower than the threshold value. 

Aromaticity has been deeply rooted in chemical literature, even before the structural and bonding principles have been clearly established [1-6]. It has been one of the most ubiquitous concepts and its origin dates back to the isolation of benzene by Faraday in 1825 illuminating the gas from whale oil [1]. Historically, the molecule benzene is closely associated with aromaticity. The first known use of the word aromatic as a chemical term is by Hofmann in 1855 [2]. However, even before that in 1833, Mitscherlich distilled benzene from benzoic acid and lime [3]. The cyclo-hexatriene structure for benzene was first proposed by Kekule in 1865 [4] while the cyclic nature of benzene was finally confirmed by the crystallographer Kathleen Lonsdale [5]. The term aromaticity owes its name to the pleasant aroma that some members of this class have and later on it was denoting exceptional stability that this family of compounds exhibit. An explanation for the exceptional stability of benzene is attributed to Sir Robert Robinson, who coined the term aromatic sextet... 

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