Biosynthesis of flavor compounds

Pheromone identification is still difficult because the structure of unique compounds present in small amounts in mixtures of similar molecules has to be elucidated. This topic will be discussed in detail by Ando as well as by others, showing nicely the recent progress in analytical techniques. The following chapter by R. Jurenka deals with insect pheromone biosynthesis with special emphasis on lepidopteran pheromones and also covers genetic aspects. The subsequent chapter by C. Keeling et al. describes the hymenopteran semio-chemicals (bees and ants), describing pheromones and allelochemicals. The hymenoptera add a certain flavor to the scene, because now the complexity of social insects with their many interactions comes into play, as well as the multi-level (multi-trophic) signals used by parasitoids. 

The biosynthesis of geosmin is of interest because an enhanced understanding of the pathways and enzymes involved may support the development of effective off-flavor control strategies. Increased biochemical information could also enhance the use of biochemical systems to produce large amounts of geosmin for the flavor and fragrance industries, which may be interested in this compound to provide a desirable earthy note to certain products (88). Currently, difficulties associated with the synthesis of the three chiral carbons renders the chemical synthesis of geosmin extremely difficult (88). 

Grbin, P.R., Herderich, M., Markides, A., Lee, T, Henschke, P.A. (2007). The role of lysine amino nitrogen in the biosynthesis of mousy off-flavor compounds by Dekkera anomala. J. Agric. Food Chem., 55, 10872-10879. 

Biosynthesis of Chiral Flavor and Aroma Compounds in Plants and Microorganisms... 

Research over the last few years has revealed that the flavor compounds are not end products of monoterpene biosynthesis in the grape. Oxidative pathways, leading to flavorless polyhydroxylated forms of the monoterpenes, are active in Vitis vinifera. Also, the... 

L-Phenylalanine,which is derived via the shikimic acid pathway,is an important precursor for aromatic aroma components. This amino acid can be transformed into phe-nylpyruvate by transamination and by subsequent decarboxylation to 2-phenylacetyl-CoA in an analogous reaction as discussed for leucine and valine. 2-Phenylacetyl-CoA is converted into esters of a variety of alcohols or reduced to 2-phenylethanol and transformed into 2-phenyl-ethyl esters. The end products of phenylalanine catabolism are fumaric acid and acetoacetate which are further metabolized by the TCA-cycle. Phenylalanine ammonia lyase converts the amino acid into cinnamic acid, the key intermediate of phenylpropanoid metabolism. By a series of enzymes (cinnamate-4-hydroxylase, p-coumarate 3-hydroxylase, catechol O-methyltransferase and ferulate 5-hydroxylase) cinnamic acid is transformed into p-couma-ric-, caffeic-, ferulic-, 5-hydroxyferulic- and sinapic acids,which act as precursors for flavor components and are important intermediates in the biosynthesis of fla-vonoides, lignins, etc. Reduction of cinnamic acids to aldehydes and alcohols by cinnamoyl-CoA NADPH-oxido-reductase and cinnamoyl-alcohol-dehydrogenase form important flavor compounds such as cinnamic aldehyde, cin-namyl alcohol and esters. Further reduction of cinnamyl alcohols lead to propenyl- and allylphenols such as... 

Low Molecular Weight Carbonyl Compounds. In the dairy field, a major product made this way is starter distillate. The main component is diaceyl which is a very important aroma compound responsible for the characteristic buttery flavor of fermented dairy products such as sour cream or buttermilk. The dairy industry relies upon fermentation by lactic streptococci for the production of diacetyl in cultured products. Starter distillate is a natural product rich in diacetyl which is produced by distilling such lactic cultures. The key intermediate in the biosynthesis of diacetyl is aL-acetolactic acid which is decarboxylated to form diacetyl (Figure 3). The starting material of the biosynthetic pathway is citrate which is a natural component of milk. 

Oxidation of Phenolic Compounds. Phenolic compounds are widespread throughout the plant kingdom and are prevalent in fruits where they are important contributors to color and flavor (46). Phenolic compounds, particularly flavonoids and derivatives of chlorogenic acid, play a crucial role in the development of a number of postharvest disorders through their oxidation to brown compounds that discolor many fruits and vegetables and substantially reduce their quality. A number of enzymes catalyze the biosynthesis or oxidation of phenolic compounds, among them phenylalanine ammonia lyase (PAL), tyrosine ammonia lyase (TAL), cinnamic acid-4-hydroxylase (CA4H), polyphenol oxidase (PPO), and catechol oxidase (CAO). The chemistry... 

C,H,60S, Mr 160.27 discovered and synthesized in 1976 as the typical flavor compound of the yellow passion fruit (Passiflora edulis) (see fruit flavors). Racemic cis-Zira/is-mixture 1 10, oil with an exotic fruity odor, bp. 85 -86 °C (1.6 kPa). Synthesis and sensory evaluation of the four stereoisomers demonstrated that the (-F)-(2S,4/f)-form was the compound with the typical passion fruit character, olfactory threshold in water 2 jmb, [alo" -f56.1 (CCI4). According to chiral analysis, however, the passion fruit contains the (4S)-epimer pair with an untypical, sulfurous, herby-green or, respectively flowery odor (-)-(2R,4S), [a]g -56.1 , and (-)-(2S,4S), (a]g -117.6°. For biosynthesis, see Ut.. ... 

Raab, T., et al. (2006) FaQR, required for the biosynthesis of the strawberry flavor compound 4-hydroxy-2,5-dimethyl-3(2H)-furanone, encodes an enone oxidoreductase. Plant Cell 18,1023-1037... 

Unwanted carbonyl compounds, which can produce an off-flavor in stored beer, are bound by sulfite derived from yeast metabolism. Yeast reduces the sulfate present in the wort to sulfite and sulfide, which is then consumed in the biosynthesis of sulfur-containing amino acids. If the growth of yeast comes to a standstill, excess sulfite is eliminated, increasing the stability of the beer to oxidative processes. 

Benzaldehyde is considered to be the character-impact compoimd for cherry aroma (77), and is the key component in imitation cherry flavor compositions (18), The biosynthesis of this character-impact compound has been attributed to the acid or enzymic hydrolysis of the glucoside amygdalin in cherry pips. The importance of benzaldehyde in the aroma of cherry is emphasised by Schmid and Grosch (19), On sniffing stepwise diluted flavor extracts from cherries, seven compounds were revealed to have the highest aroma values - these compounds were benzaldehyde, linalool, hexanal, /ran5-2-hexenal, phenylacetaldehyde, (Z, )-2,6-nonadienal and eugenol. 

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