Terpenoids aroma compounds

Sugars, acids and aroma compounds contribute to the characteristic strawberry flavour [85]. Over 360 different volatile compounds have been identified in strawberry fruit [35]. Strawberry aroma is composed predominately of esters (25-90% of the total volatile mass in ripe strawberry fruit) with alcohols, ketones, lactones and aldehydes being present in smaller quantities [85]. Esters provide a fruity and floral characteristic to the aroma [35,86], but aldehydes and furanones also contribute to the strawberry aroma [85, 87]. Terpenoids and sulfur compounds may also have a significant impact on the characteristic strawberry fruit aroma although they normally only make up a small portion of the strawberry volatile compounds [88, 89]. Sulfur compounds, e.g. methanethiol. 

Terpenoids are synthesised by the condensation of a series of isoprene (2-meth-ylbuta-1,3-diene) units, followed by enzymatic cyclisation by a terpene cyclase, and subsequent modification such as hydroxylation, and are grouped on the basis of their carbon chain length. Monoterpenes and sesquiterpenes consisting of ten and 15 carbon atoms, respectively, are ranked among the most important aroma compounds. Despite their diversity, all terpenoids are synthesised... 

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). 

Loscos N, Hemandez-Orte P, Cacho J, Ferreira V (2007) Release and formation of varietal aroma compounds during alcoholic fermentation from nonfloral grape odorless flavor precursors fractions. J Agric Food Chem 55(16) 6674—6684. doi 10.1021/jfl)702343 Martin VI, Pitera DJ, Withers ST, Newman JD, Keasling JD (2003) Engineering a mevalonate pathway in Escherichia coli for production of terpenoids. Nat Biotechnol 21(7) 796-802. doi 10.1038/nbt833... 

The oils have a high terpene hydrocarbon content (>90%, mainly (+)-limonene), but their content of oxygen-containing compounds differs and affects their quality. Important for aroma are aldehydes, mainly decanal and citral, and aliphatic and terpenoid esters. The sesquiterpene aldehydes a-sinensal [17909-77-2] and/3-sinensal [6066-88-8], which contribute particularly to the special sweet orange aroma, also occur in other citrus oils, although in lower concentration [369-370a, 370d, 394,421, 430-438]. 

The berry or the small fruits consist of strawberry, raspberry, blackberry, black currant, blueberry, cranberry and elderberry. The volatiles responsible for the flavour of small fruits are esters, alcohols, ketones, aldehydes, terpenoids, furanones and sulfur compounds (Table 7.3, Figs. 7.1-7.7). As fruit ripen, the concentration of aroma volatiles rapidly increases, closely following pigment formation [43]. 

Approximately 230 volatile compounds have been identified in raspberry fruit [35]. The aroma of raspberries is composed of a mixture of ketones and aldehydes (27%) and terpenoids (30%), alcohols (23%), esters (13%) and furanones (5%). The raspberry ketone (Fig. 7.5) along with a-ionone and jS-ionone have been found to be the primary character-impact compounds in raspberries. Other compounds such as benzyl alcohol, (Z)-3-hexen-l-ol, acetic acid, linalool, geraniol, a-pinene, jS-pinene, a-phellandrene, jS-phellandrene and jS-caryophyllene contribute to the overall aroma of mature red raspberries [101-105]. The most important character-impact compounds of raspberries are summarised in Table 7.3. 

The root of carrot Daucus carota) is eaten raw or cooked. The characteristic aroma and flavour of carrots are mainly due to volatile compounds, although non-volatile polyacetylenes and isocoumarins contribute significantly to the bitterness of carrots [1,2]. More than 90 volatile compounds have been identified from carrots (Table 7.9) [207-215]. The carrot volatiles consist mainly of terpenoids in terms of numbers and amounts and include monoterpenes, sesquiterpenes and irregular terpenes. Monoterpenes and sesquiterpenes account... 

The powerful potentialities of SBSE followed by thermal desorption and GC-qMS methodology to characterize Madeira wine was also explored by Perestrelo et al. (2009). This methodology provided higher ability for profiling traces and ultratraces of compounds in Madeira wines, including esters (80.7-89.7%), higher alcohols (3.5-8.2%), Ci3 nor-isoprenoids (1.7-6.5%), carboxylic acids (1.6H-.2%), aldehydes (0.9-3.7%) pyrans (0.2-1.7%), lactones (0.3-2.7%), and mono (0.1-1.4%), and sesqui-terpenoids (0.1-0.8%). The authors reported that the concentration of some of them is above their odor threshold, and therefore can probably play a remarkable impact on the aroma complexity of the corresponding wines. 

Aroma-active molecules of natural origin are mainly formed via well-known biosynthetic pathways.17,27-29 The major class is the terpenoids followed by phenylpropanoid compounds (see Chapters 1.15, 1.16, and 1.24). Enzymatic and biosynthetic transformation and cleavage of fatty acid is another important source of aroma-active compounds (see Chapter 8.07). Transformation of amino acids and carbohydrates by fermentation is also... 

To illustrate the approach to the identification of unknown materials by using infrared spectroscopy, we will work through an example. Citronellal is the terpenoid responsible for the characteristic aroma of lemon oil, and is used in perfumes and also as a mosquito repellent. The infrared spectrum of citronellal is shown in Figure 4.5a. We can use this spectrum to determine the molecular structure of this compound. 

Terpenes and terpenoid compounds do not play a major role. Only linalool was detected by GC-0 (no. 8). Most of the terpenes identified by GC-MS were odorless at the concentration present in the aroma extract, i.e. a- and p-piiiene, sabinene, 3-carene, menthol, p-ierpitieol. cineol, anethol, p-terpinyl acetate, l-/i-... 

Some natural products containing a furan ring have an intense odour, e.g. 2-furylmethanethiol 27, a component of coffee aroma, rose furan 28, a component of rose oil, and menthofuran 29, which occurs in peppermint oil. The compounds 28 and 29 are reminiscent of terpenoid structures ... 

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. 

Lewinsohn, E., Schalechet, F.,Wilkinson, J., Matsni, K.,Tadmor,Y., Nam, K.H., Amar, O., Lastochkin, E., Larkov, O., Ravid, U., Hiatt, W. et al.. Enhanced levels of the aroma and flavor compound S-linalool by metabolic engineering of the terpenoid pathway in tomato fruits. Plant Physiol., 127, 1256, 2001. 

These compounds are also important constituents of the aroma of tomatoes, tea, strawberry, oUve oil, grape, apples, and pear. Moreover, plant essential oils, composed mainly by terpenoids, were extensively studied for their antimicrobial activity against many microorganisms including several pathogens. In particular, the activity of oils from Labiatae and citrus fruits were investigated. 

Another important class of volatile compounds originating from terpenoids are the cleavage products of carotenoids which have carbon skeletons ranging from C8 to Cl 8 (Fig. 2) [9, 75]. Many of these volatile carotenoid derivatives are key aroma constituents of finits and flowers due to their extremely low aroma thresholds. For example, P-damascenone, found in numerous fruits, has a floral/finity odor with an aroma threshold of a few parts per trillion, making it one of the most potent odorants known... 

Lewinsohn, E., etak (2001) Enhanced levels of the aroma and flavor compound S-linalool by metabolic engineering of the terpenoid pathway in tomato fruits. Plant Physiol. 127, 1256-1265... 

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