Flavour chemical

Rh/(5)-BINAP Isomerization of allylic amine /.-Menthol Aroma and flavour chemical 96-98... 

Various schemes have been proposed to explain the production of nitrogen-containing heterocyclic compounds, such as pyrrolidines and piperidines, from proline. Nitrogen heterocyclic compounds are often potent flavouring chemicals. 

R)- -Decalactone contributes much of the characteristic taste and aroma of peach and many other flavours. Chemically synthesised T -decalactone has been cheaply available for a long time, but the consumer demand for naturally flavoured food and beverages that arose in the mid 1980s created a strong demand for the (RJ-lf -decalactone isomer as a natural food flavour molecule. This definition of natural grade required its production by entirely enzyme-based steps. In turn this led to the development of a number of biotransformation processes to make natural f -decalactone. 

Because T -decalactone is a flavour chemical its quahty is assessed by its taste quahty rather than by its chemical purity, and is used in oidy very low concentrations to achieve the desired effects. 

Market acceptance was eased by the previous consumption of T -decalactone from fruit sources, and as a chemically synthesised flavour chemical. 

In this chapter chemical conversions of natural precursors resulting in flavour chemicals are discussed. The main groups of natural precursors are terpenes for all kinds of terpene derivatives, vanillin precursors like lignin and eugenol, sugars for Maillard-associated flavour chemicals, amino acids and molecules obtained by fermentation or available as residual streams of renewable resources. 

The main renewable resource for L-carvone is spearmint oil (Mentha spicata), which contains up to 75% of this flavour chemical. There also exists a synthetic process for the manufacturing of L-carvone, which is based on (-t)-limonene, which is available as a by-product of the citrus juice industry as a major component of orange peel oil (Scheme 13.4). The synthesis was developed in the nineteenth century and starts with the reaction of (-t)-limonene and nitrosyl chloride, which ensures the asymmetry of the ring. Treatment with base of the nitrosyl chloride adduct results in elimination of hydrogen chloride and rearrangement of the nitrosyl function to an oxime. Acid treatment of the oxime finally results in l-carvone. 

Examples of Flavour Chemicals Derived from Sugars 13.4.2.1... 

Cysteine can be obtained by hydrolysis from cysteine-rich proteins in hair or feathers or from petrochemical sources. Cysteine is an important raw material in Maillard reactions for the preparation of process flavours, but it can also serve as a source of ammonia and hydrogen sulfide for the preparation of flavour chemicals, such as the terpene sulfur compounds mentioned in Sect. 13.2.4 and furfuryl mercaptan mentioned in Sect. 13.4.2.4. 

Many products from the flavour industry are primary products from renewable resources or secondary products obtained by chemical conversions of the primary products. In general these secondary products are key flavour chemicals with a high added value. The cost diiference between a precursor, the primary product and the flavour chemical can easily amount to a factor 20-1,000, especially when it concerns a natural flavour chemical. A large part of this cost reflects, of course, the efficiency of the reaction, the labour involved and the cost of the other reagents. 

Although quite often these flavour chemicals can be prepared from petrochemical sources, renewable resources are preferred by the flavour industry, because access to these renewable resources is very good and already existed when these companies were started. In addition, chemicals from renewable resources are natural, so they can be used in natural flavours and offer the possibility to be used for the production of natural secondary products. 

Heliotropin (also known as piperonal, or 3,4-methylenedioxy-benzaldehyde and having the structural formula is an important aroma and flavour chemical and is also an intermediate in syntheses of other aroma chemicals and some pharmaceutical chemicals. 

A typical application of this process is the protection of instable, highly volatile and high value-added flavour chemicals. Molecular inclusion can, for example, be used to achieve a long-lasting taste effect in chewing gum. The relatively high price of p-cyclodextrin, however, does not allow a broad use in this context. 

The previously described manufacturing processes for flavour chemicals and flavour extracts primarily regard the physical and physico-chemical isolation and purification of naturally occurring flavour chemicals derived from plant and animal tissue. The huge area of organic chemical synthesis of nature-identical and synthetical flavour chemicals is not within the scope of this book. 

Table 2.10 shows that the isolation and purification of naturally occurring flavour chemicals and extracts from animal and plant raw materials is most important for the preparation of natural flavours. About 75% of the commercially used flavours come from such natural sources. Physico-chemical reactions of typical flavour precursors may also lead to natural flavouring substances when mild conditions ( kitchen technology ) are applied. In addition, natural flavour chemicals may be prepared by biotechnological processes. This chapter outlines the most important biotechnical manufacturing techniques. 

The production and modification of flavour chemicals from precursors by... 

A flavouring substance is a defined chemical component or substance with flavouring properties (Tab. 3.3). Various synonyms are in use, e.g. flavour substance, flavour chemical, flavour(ing) component, flavour(ing) compound, flavouring agent, aroma compound, aroma chemical, and others. Since flavour includes both taste and odour, a flavouring substance may be a substance that causes either taste or odour impressions, or both. 

Regardless of this problem biotechnically produced flavour extracts are very attractive. Under properly defined conditions biotechnical reactions allow a significant increase of yield or strength of the flavours compared to purely natural flavours. It is also practised to run biotechnical reactions in such a way that the purity of the produced flavours is increased and undesired side products are suppressed. In comparison to the complicated chemical syntheses of pure isomers of flavours chemicals, the biotechnically manufactured substances are generally homogeneously composed by nature . 

These advantages have a positive influence on the economy of biotechnical manufacturing processes. In many cases expensive starting materials can be replaced by cheaper and simpler substrates of the biochemical reaction which results in a favourable cost price for the flavour extract. Compared to commodities which meanwhile include numerous biotechnical products (e.g. technical enzymes), the relatively high prices obtainable for flavour chemicals justify the relatively complicated techniques necessary for biotechnical processes. Production of flavour chemicals is, therefore, an interesting further application of biotechnology in line with e. g. the generation of pharmaceutical products. 

In chapter 3.2.1.1.2 biotechnical syntheses leading to pure flavour chemicals are already demonstrated. The metabolic reaction mechanisms for most of these cases are well-known and properly defined. The detailed knowledge available about their biochemistry today even allows the specific modification especially with genetic engineering methodology. The generation of complex mixtures of flavours by biotechnical reactions, however, is not yet very well understood. The application of such processes on an empirical basis, however, is practised successfully and will be described in the following. 

As shown above flavour substances may be mobilized by destruction of cells and framing structures of plants and by extraction of flavour materials dissolved in the plant juices. In addition, flavour precursors may also be liberated from covalent bonds [18a], The majority of these bonds are glycosidic links of flavour chemicals to molecular or particulate plant components. The enzymes used for cleavage of these bonds (p-glycosidases) tend to form equilibria of bound and free forms. 

The characteristic taste of edible mushrooms is mainly derived from the flavour chemicals 3-octanol and l-octen-3-ol. In order to fermentatively generate mushroom flavours it is possible to cultivate higher mushrooms in a submerged fermenter and propagate them by the formation of mycelia. In the fermentation broth, l-octen-3-ol is detectable after some time. In the mushroom this substance is formed by a lipoxygenase using unsaturated fatty acids as a substrate [21 ]. 

Plant cells show an extensive repertoire of chemical reaction mechanisms epoxi-dation, reduction, oxidation, hydroxylation, isomerisation. It is self-evident that plant cell cultures synthesize as enantioselectively as their mother organisms. Besides the well-known flavour extracts and single substances, also presently unknown naturally flavour chemicals and mixtures of these are in principle obtainable. Therefore the rapid progress in investigating this area is not surprising [26],... 

Although the main objective of present publications is the synthesis of pure flavour chemicals by plant cells there is a number of experiments with plant cell cultures for the manufacturing of complex flavour extracts. In these cases it is intended to produce flavour extracts using cell cultures of taste-delivering plants similar to the extracts rendered by extraction from the natural mother plant. 

The same holds for products from animal cell cultures where even less results are available. With animal cell cultures especially enzymes are accessible which may be used for biotransformations of flavour precursors. Compared to the progress in the production of pharmaceutical substances from animal cell cultures it is only a question of time when processes are developed which allow the economical production of flavour chemicals not yet accessible by other routes. Quick progress is expected, among others, for sweeteners, bitter substances, essential oils, fruity flavours and vegetable flavours from cell cultures. 

S. Arctander, Perfume and Flavour Chemicals, 2 volumes, S. Arctander, 1969. 

Extraction - making a cup of coffee involves extraction of the flavour chemicals and caffeine from the insoluble vegetable matter using hot water and is an example of liquid-solid extraction. 

Calo A, Stefano RD, Costacurta A, Calo G (1991) Caratterizzazione di Cabernet Franc e Carmenere (Vitis sp.) e chiarimenti sulla loro coltura in Italia. Riv Vitic Enol 44 3-25 Cronin DA (1982) Techniques of analysis of flavours chemical methods including sample preparation. In Morton ID, MacLeod AJ (eds) Food flavours. Part A. Introduction. Elsevier, Amsterdam, pp 15-48... 

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