Aroma biosynthesis

Similarly, in potato Solanum tuberosum), silencing LOX-Hl caused a severe decrease in the amount of volatiles produced by the leaves and in the intensity of their aroma, while the depletion of HPL increased the content of C5 (2-pente-nal, pentanal, l-penten-3-ol and ds-2-pentenol) volatiles [27]. These examples clearly demonstrate that the fatty acid metabolism involved in aroma biosynthesis is not as simple as initially supposed. 

Lavid, N., Schwab, W., Kafkas, E., Koch-Dean, M., Bar, E., Larkov, O., Ravid, U., and Lewinsohn, E. 2002. Aroma biosynthesis in strawberry 5-adenosylmethio-nine Euraneol 0-methyltransferase activity in ripening fruits. J. Agric. Food Chem. 50 4025-30. 

In this chapter, we review the biosynthetic pathways responsible for the formation of floral and fhiit volatiles, the regulation and localization of scent and aroma biosynthesis, and the recent advances in biotechnological manipulation of floral fragrances and fruit aromas. 

There are also enzymes present that participate in the formation of many of the several hundred volatile compounds found in tea aroma. The important enzyme systems responsible for the biosynthesis for the methylxanthines have already been mentioned. 

Ctoteau R, Biosynthesis of cychc monoterpenes, in PatlimentTH, Croteau R (eds.). Biogeneration of Aromas, ACS Symposium series 317, Washington DC, USApp. 13 156, 1986. 

Schottler M, Boland W (1996) Biosynthesis of dodecano-4-lactone in ripening fruits Crmal role of an epoxide-hydrolase in enantioselective generation of aroma components of the nectarine ( Prunus persica var. nucipersica ) and the strawberry (Fragaria ananassa). Helv Chim Acta 79 1488... 

While most of the studies carried out to date have focused on the evolution of the primary compounds, such as the sugar and acid components, little data exist for the other quality characteristics, and in particular, for aroma. However, most of these studies have been conducted on the aromatic varieties. For instance, some experimental studies conducted on different aromatic cultivars under different conditions mainly reported the evolution of terpene compounds during the dehydration process. Accordingly, it has been showed that after the grape harvest, activation or inhibition of the metabolism involved in the biosynthesis of the aroma compounds is strictly dependent on the grape dehydration technique. 

Methods for the capillary gas chromatographic separation of optical isomers of chiral compounds after formation of diastereoisomeric derivatives were developed. Analytical aspects of the GC-separation of diastereoisomeric esters and urethanes derived from chiral secondary alcohols, 2-, 3-, 4- and 5-hydroxy-acid esters, and the corresponding jf- and -lactones were investigated. The methods were used to follow the formation of optically active compounds during microbiological processes, such as reduction of keto-precursors and asymmetric hydrolysis of racemic acetates on a micro-scale. The enantiomeric composition of chiral aroma constituents in tropical fruits, such as passion fruit, mango and pineapple, was determined and possible pathways for their biosynthesis were formulated. 

Different studies have focused on the biosynthesis of aroma compounds during MLF and the concomitant organoleptic consequences (Laurent et al. 1994). Maicas et al. (1999) demonstrated that MLF noticeably changes major and minor volatile... 

Carotenoids are regarded as part of the aroma potential of grape, as they are the biogenetic precursors of C13-norisoprenoids, a chemical family with many powerful odorants, which are mainly found as glycoconjugates in grape (Baumes et al. 2002 Enzell 1985 Mathieu et al. 2005 Winterhalter 1993). Sunshine favors the biosynthesis of carotenoids in the berry, from the first stage of fruit formation... 

The smell and taste of plants rely on aroma and fragrance compounds, many of which (besides fhe terpenoids) are derived from phenylpropanoid metabolism. In food and cosmetic industry, such fragrance and aroma compounds play an important economical role. Simple phenolic fragrance compounds are, e.g., eugenol, isoeugenol or (methyl)chavicol (Fig. 4.2), the biosynthesis of which has been clarified recently more complex compounds are phenolic esters. Evolutionary aspects of the bios)mthesis of flavours and scents have been reviewed by Gang (2005). 

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

The knowledge about the stereochemical properties of enzymes catalyzing the biosynthesis of chiral volatiles is not only interesting from a strictly scientific standpoint of view it is also an essential basis for future improvement of natural flavor and aroma by ge-netical engineering of plants and microorganisms. 

Flavor formation in fruit products has also extensively been reviewed (10), A distinction can be made between the primary aroma components, which are biosynthesized by the whole fruit and secondary aroma compounds (e.g. hexanal, 2-hexenal), formed after disruption of the cells during processing or chewing (11). The Importance of the peel for aroma formation has also been stressed by several authors (12). An extensive literature on the respiration climacteric (13), the role of ethylene (14) and the enzymes and substrates required for biosynthesis is available (15). 

The enantiomeric composition of chiral fruit aroma components such as alcohols, 3-hydroxyacid esters and lactones was determined and possible pathways for their biosynthesis were presented. 

Many aroma compounds in fruits and plant materials are derived from lipid metabolism. Fatty acid biosynthesis and degradation and their connections with glycolysis, gluconeogenesis, TCA cycle, glyoxylate cycle and terpene metabolism have been described by Lynen (2) and Stumpf ( ). During fatty acid biosynthesis in the cytoplasm acetyl-CoA is transformed into malonyl-CoA. The de novo synthesis of palmitic acid by palmitoyl-ACP synthetase involves the sequential addition of C2-units by a series of reactions which have been well characterized. Palmitoyl-ACP is transformed into stearoyl-ACP and oleoyl-CoA in chloroplasts and plastides. During B-oxi-dation in mitochondria and microsomes the fatty acids are bound to CoASH. The B-oxidation pathway shows a similar reaction sequence compared to that of de novo synthesis. B-Oxidation and de novo synthesis possess differences in activation, coenzymes, enzymes and the intermediates (SM+)-3-hydroxyacyl-S-CoA (B-oxidation) and (R)-(-)-3-hydroxyacyl-ACP (de novo synthesis). The key enzyme for de novo synthesis (acetyl-CoA carboxylase) is inhibited by palmitoyl-S-CoA and plays an important role in fatty acid metabolism. 

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... 

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