Grapefruit odour

These often have very powerful odours (Boelens and Gemert, 1994) for example, 2-isobutyl-3-methoxypyrazine from galbanum oil has a powerful green note and l-para-menthene-8-thiol, a very powerful natural material, has a strong grapefruit odour at concentrations in the p.p.b. range. 

The key components of fresh grapefruit juices with a typical grapefruit odour are both isomers of p-mentha-l-en-8-thiol (8-188). The (- -)-(Ji)-enantiomer is present in minute concentrations (less than 1 (xg/kg), but has a very low odour threshold concentration. The (-)-(S)-p-mentha-l-ene-8-thiolhas aweak and non-specific smell. Of the other sulfur compounds, 4-mercapto-4-methylpentan-2-one (8-125) is significant, and also occurs in blackcurrants, some hop cultivars, aromatic wines and basil. A relatively high content of sesquiterpenoids is also typical. The smell and bitter taste of grapefruits arise from (-l-)-nootkatone and (-1-)-8,9-didehydronootkatone. Important odour-active compounds are numerous cyclic ethers, which are likewise found in other essential oils. For example, the essential oil contains about 13% of hnalool oxides that arise from linalool via 5,6-epoxide, and another important epoxide is ( )-4,5-epoxydec-2-enal. The fresh odour of juices is mainly influenced by aliphatic aldehydes, such as acetaldehyde. 

The especially fresh, juicy grapefruit note is due to the presence of R(-i-)-para-menthene-8-thiol which can be detected as a trace constituent in the ppb range of grapefruit oils ]46]. Other important potent odour-active volatiles of grapefmit juice are l-penten-3-one, l-hepten-3-one, hex-anal, 4-mercapto-4-methylpentan-2-one and 4,5-epoxy-(E)-2-decenal ]47]. 

CIC The bitter taste originates from naringin. Nootkatone imparts the characteristic fresh woody odour. Acetaldehyde and ethyl butyrate improve the juicy note and 1-p-menthane-8-thiol is responsible for the typical exotic grapefruit character. 

The concentrations of the two sulphur compounds, MMP and MTO, in grapefmit juices were 0.8 and 0.01 pg/kg, respectively [71 ]. Omission experiments indicate that the grapefruit-like odour was lacking when MMP was absent. MTO, which occurs in even lower concentration in oranges, contributes to the aroma but its impact is not so typical when compared to that of MMP. 

The characteristic aroma of blackcurrants (varieties oiRibes nigrum, Grossulariaceae) is mainly a consequence of aliphatic and aUcycUc thiols. The bearer of the characteristic cat odour is 4-methoxy-2-methyl-2-thiol, called blackcurrant mercaptan (8-124), which occurs also in olive oil and green tea. Other important components are 4-mercapto-4-methylpentan-2-one called cat ketone (8-125) and (lS,4R)-p-menthan-8-thiol-2-one (see Section 8.2.13.1.1). Cat ketone is also found in grapefruits, some hop cultivars, basil and some aromatic wines. The leaves contain about 0.7% essential oil, whose main component isp-cymol, the sulfur compounds of which contribute to the strong smell, mainlyp-menthan-8-thiol-2-one. 

For example, the pleasant smell of a-terpineol (8-19) resembles lilac flowers, but the substitution of oxygen for sulfur turns this into a very aggressive smell of p-menthene-8-thiol (8-188), which only at very low concentrations (below 1 (xg/kg) recalls the scent of grapefruits for which (R)-p-mentha-l-en-8-thiol is a key component. The individual enantiomers of a-terpineol and 1-p-menthene-8-thiol also have different odours. (-F)-(R)-a-terpineol (8-19) has a typical heavy smell of lilac flowers, (-)-(S)-a-terpineol (8-19) has smell of coniferous trees and tar, the natural isomer (-F)-(R)-l-p-menthene-8-thiol (8-188) has the aroma of grapefruit, but the (S)-enantiomer (8-188) has a weak and non-specific flavour. 

The only isomer of nootkatone that resembles the odour of grapefruits is (-l-)-nootkatone (8-52), while the remaining isomers have a more woody and spicy smeU. 

On the other hand, perfumes, and also the pure fragrance chemicals that compose the perfume, can also be classified according to the note they provide, i.e. according to the fragrance type. So, one can find different types, like floral, which reminds one of scents similar to jasmine, rose, heliotrope, etc citrus, which are aromas reminiscent of lemon, orange, lime, grapefruit, etc. fruity, based on non-citrus fruity odours like peach. 

A. Buettner and P. Schieberle, Stable isotope dilution assays for the quantification of odour-active thiols in hand-squeezed grapefruit juices (Citrus Paradisi Mac-Fayden), Frontiers of Flavour Science (P. Schieberle and K. H. Engel, eds.), WB-Druck GmbH, Rieden am Forggensee, 2000, p. 132. 

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