Synthetic Aroma Compounds

Some synthetic flavorings which do not occur in food materials are compiled in Table 5.42. Except for ethyl vanillin, they are of little importance in the aromatization of foods. 

Sweet like vanilla (2 to 4-times stronger than vanillin) 

Fruity, pineapple-like Floral, jasmin and lilies 

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Undec-lO-enal is a widely employed synthetic aroma compound with a citrus, waxy, fatty odor accompanied by a green, soapy nuance. It can be produced by hydroformylation of 1,9-decadiene (Scheme 6.25). The required substrate is accessible, for example, by olefin disproportionation of cyclooctene with ethylene [100]. The diene was reacted with syngas in the presence of a heterogenized Rh catalyst in a flow reactor to give undec-lO-enal and 2-methyldec-9-enal in 80% overall yield and a slight dominance of the linear aldehyde. The isomers could be separated by distillation. The smell of the branched aldehyde is very similar to that of... 

Most of the studies on Botrytis-specific aroma compounds have investigated the aroma composition of wines (Table 6.7). Only a few have focused on what occurs in the grape. On synthetic media, the main odorous compounds produced by B. cinerea are aromatic aldehydes (benzaldehyde, phenylacetaldehyde), and furfural (Kikuchi et al., 1983). Sarrazin et al. (2007a) confirmed the higher concentration of phenylacetaldehyde in noble-rotted grapes. 

Other than studies on the role of proteins released by yeast during autolysis (mannoproteins) on wine aroma, little work has been reported on interactions of other proteins with aroma compounds. One study investigating such interactions was published by Druaux et al. (1995). They used synthetic wines and bovine serum albumin (BSA) as a model protein. This protein was found to bind 5-decalactone and there was greater binding when in water than in a model wine environment (pH 3.5 and 10% ethanol). To our knowledge this is the only study focused on elucidating the effect of proteins (others than mannoproteins) on the aroma release in wine or model wine. 

Interactions between mannoproteins from yeast cell walls and aroma compounds have been studied by Langourieux and Crouzet (1997). They performed the experiments with crude mannoproteins extracts and observed no effect on the activity coefficient of isoamyl acetate, and a slight decrease on the activity coefficients of ethyl hexanoate and limonene. However, when they purified the mannoproteins or when they used a model glycopeptide, they did not observe any effect on limonene volatility. If the synthetic peptide was heat treated (50 °C), they observed a slight reduction on the activity coefficient of limonene. This was explained by an increase in the hydrophobicity of the glycopeptide after the thermal treatment. 

Chalier et al. (2007), using mannoprotein at levels usually found in wines (150 mg/L), compared the effect of a whole mannoprotein extract (isolated from a synthetic medium subjected to alcoholic fermentation) to that of well characterized different mannoproteins fractions. From the four wine aroma compounds studied (isoamyl acetate, hexanol, ethyl hexanoate and /3-ionone), all except isoamyl acetate showed a decrease in volatility (up to 80%) when mannoproteins were present (Fig. 8F.3). They suggested that both the glycosidic and the peptidic parts of these macromolecules may be responsible for the interaction. They also found that the interactions of the whole mannoprotein extract Vs. mannoprotein fractions were different, suggesting that the conformational and compositional structure of these... 

Table 5.3 Calibration parameters of some aroma compounds in a synthetic wine solution enriched by automated HS-SPME with a 1 cm CAR-PDMS-DVB fiber and analysed by GC-MS. S.D. standard deviation.

The onset of the industrial production of essential oils can be dated back to the first half of the 19 century. After the importance of single aroma chemicals was recognised in the middle of the century, efforts were started to isolate such compounds from corresponding natural resources for the first time. This was soon followed by the synthesis of aroma chemicals. In this context, the most important pioneers of synthetic aroma chemicals have to be mentioned, such as methyl salicylate [1843], cinnamon aldehyde [1856], benzyl aldehyde [1863] and vanillin [1872], as they constitute the precursors of a rapidly growing number of synthetically produced (nature-identical) aroma chemicals in the ensuing years. 

Natural fruit aromas are mixtures of certain organic compounds and esters. Synthetic aromas prepared in laboratories are simple mixtures of these same esters and organic compounds. They are used in perfumes, foods and drinks to give taste and pleasant smells. Ethyl acetate, for example, is a colorless liquid with an apple flavour it is known as apple ester and is used in perfumery as a fruit essence. Propyl acetate has the smell of pears, isopentyl acetate that of bananas and ethyl butyrate smells of pineapples. All are colorless liquids. Higher molar mass esters are odorless. 

The authors observed that an exhaustive list of all the chemicals present in coffee flavor had not yet been compiled, but they believed they had identified the components that are present at the higher ratio of weight, and those which principally control the odor note. Most of the substances identified were well-known compounds present in other roasted products as well, for instance in caramel sugar, cocoa, baked bread and—partially—even in wood tar. However, some of the chemicals detected were new and, obviously, characteristic of roasted coffee. Traces of methyl mercaptan, which was already known at that time and which smells even worse, were also detected in coffee aroma. Commenting on this observation, Reichstein and Staudinger note that it is generally known that many popular raw materials and synthetic perfume compounds owe their characteristic note, which is extremely pleasant to the olfactory sense, to their content of small quantities in additives which carry a rather unpleasant odor in themselves but prove very attractive in thinned solutions and in admixture with other oils. The authors tried to reconstitute coffee aroma, and only by combining over 40 of the substances extracted from coffee... 

Aroma chemicals, fragrance chemicals. Name for natural or synthetic organic compounds used, on account of the usually pleasant odors, in the manufacture of perfumes and perfumed products. The fIavor compounds are distinguished from aroma chemicals by their occurrence and use in foods The term odorant chemical applies generally to conmounds that can be detected using the sense of smell. The term fragrance chemicals is used, except for aroma chemicals, also for odor complexes, e.g., the absolutes and essential oils used in the perfume indust. For the physiology of the sense of smell, see Zif. . 

Researchers, using the analytical techniques available, are now able to separate and identify most of the compounds responsible for the aroma of foods [1]. From this vast list of aroma compounds (>7,000), a more limited subset of compounds, i.e., those that have significant flavoring properties, has been selected for manufacture by synthetic means. To be of any value to the flavorist or flavor manufacturer, a... 

The purity requirement imposed on synthetic aroma substances is very high. The purification steps usually used are not only needed to meet the stringent legal requirements (i. e. beyond any doubt safe and harmless to health), but also to remove undesirable contaminating aroma compounds. For example, menthol has a phenolic off-flavor note even in the presence of only 0.01% thymol as an impurity. This is not surprising since the odor threshold value of thymol is lower than that of 1 (—)-menthol by a factor of 450. 

The original identifications of character aroma compounds were from isolates of spice oils and herbs. Many of these early discoveries paralleled developments in synthetic organic chemistry (9). The first identifications and syntheses of character flavor molecules include benzaldehyde (cherry), vanillin (vanilla), methyl-salicylate (wintergreen), and cinnamaldehyde (cinnamon). A listing of character impact compounds found in herb and spice flavors is presented in Table 1. 

Two important character-impact compounds of strawberry flavor are the ftiranones 2,5-dimethyl-4-hydroxy-2//-furan-3-one (FuraneoF ) and 2,5-dimethyl-4-methoxy-2/f-furan-3-one (mesifuran) (24). However, at various concentrations, FuraneoF can simulate other flavors, e.g., pineapple (11) or Muscadine grape (25) at low levels, and caramel at high levels. Mesifuran exhibits a sherry-like aroma and is a contributor to sherry and French white wine aroma. Other important character-impact compounds of strawberry flavor are methyl cin-namate and ethyl 3-methyl-3-phenylglycidate, a synthetic aroma chemical (4,26). Representative chemical structures for fruit flavor impact compounds are shown in Fig. 2. 

For estimating the contribution of volatile compounds to bread aroma Rothe and coworkers (S) defined "aroma value" as the ratio of the concentration of some volatile compounds to the taste threshold value of the aroma. This concept was further developed by Weurman and coworkers (9) by introducing "odor value", in which aroma solutions were replaced by synthetic mixtures of volatile compounds in water. These mixtures showed the complexity of the volatile fractions of wheat bread, because none of them resembled the aroma of bread. Recently two variations of GC-sniffing were presented (10-11), in which the aroma extract is stepwise diluted with a solvent until no odor is perceived for each volatile compound separately in the GC effluent. The dilution factors obtained indicate the potency of a compound as a contributor to the total aroma. 

Nature-identical aroma substances are, with very few exceptions, the only synthetic compounds used in flavors besides natural products. The primary functions of the olfactory and taste receptors, as well as their evolutionary development, may explain why artificial flavor substances are far less important. The majority of compounds used in fragrances are those identified as components of natural products, e.g., constituents of essential oils or resins. The fragrance characteristics of artificial compounds nearly always mimic those of natural products. 

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