Grapefruit flavour

Nootkatone is an important constituent from grapefruit flavour. It is synthesised by oxidation of valencene, which is obtained and isolated from orange peel oil where it occurs at a maximum level of 0.4% [9]. 

Ingredients Carbonated water. Concentrated fruit juices (pineapple, grapefruit) (5% when reconstituted). Citric acid. Acidity regulator (sodium citrate). Artificial sweeteners (aspartame, saccharin). Flavourings. Preservative (sodium benzoate). Antioxidants (ascorbic acid, ascorbyl palmitate). Colour (lutein)... 

Table 5.9 Ingredient formulae for flavourings used in the notional pineapple and grapefruit crush (litres)...

What about taste Take the grapefruit. The main flavour comes from another sulfur compound and human beings can detect 2 x 10-5 parts per billion of this compound. This is an almost unimaginably small amount equal to 1CT4 mg per tonne or a drop, not in a bucket, but in a good-sized lake. Why evolution should have left us abnormally sensitive to grapefruit, we leave you to imagine. 

In the 1970s it was supposed that the characteristic sharp fruity scent and flavour of grapefruit came mainly if not entirely from a simple bicyclic enone called nootkatone. There was quite a rush to synthesize this compound in various laboratories and a remarkable feature of many successful syntheses was the use of fragmentation reactions. We shall describe parts of three syntheses involving the fragmentation of a six-, a four-, and a three-membered ring. 

The synthesis of nootkatone occupied many chemists for some years and has given us some excellent examples of fragmentation reactions. However, the synthetic samples of nootkatone failed to deliver the intense grapefruit taste and smell of the material from grapefruits. The reason is simply that nootkatone is not the flavour principle of grapefruit The samples of nootkatone isolated from grapefruit contained minute traces of the true flavour principle—-a simple thiol. Humans can detect 2 x 10-5 p.p.b. (yes, parts per billion) of this compound, so even the tiniest trace is veiy powerful. At least the syntheses allowed chemists to correct an error. 

Grapefruit oils are mainly used in flavourings. In perfumery, they are only employed as modifiers for other citrus types (bergamot, bitter orange, lemon etc.). 

Buettner, A., Schieberle, P. (2001) Evaluation of key aroma compounds in hand-squeezed grapefruit juice (citrus paradisi Macfayden) by quantitation and flavour reconstitution experiments. J. Agric. Food Chem. 42, 1358-1363... 

Yoshikoshi s synthesis15 of nootkatone (then supposed to be the flavouring principle of grapefruit) uses an optically active enone 52 prepared from P-pinene 48 by ozonolysis to (+)-nopinone 49 and a chemo- and regioselective aldol condensation using the silyl enol ether 50. Though the aldol reaction produces a mixture of diastereoisomers of 51, all dehydrate to the same enone E-52. 

Interestingly, the detection threshold of different flavour compounds in water also varies considerably with a grapefruit compound being an astonishing low value of 2 parts in 10 ". So the human sensory system has evolved a highly specialised sensitivity to, amongst other things, citrus compounds. 

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