Chapter 7 



VOLATILE ALCOHOLS AND 



CARBONYL COMPOUNDS 



A large variety of volatile alcohols, aldehydes, ketones and esters is found in plants 

 though usually in very small amounts. These compounds, despite their low concentration, 

 are of great aesthetic and commercial interest because of the contribution they make to 

 the flavor and odor of foods, flowers, perfumes, etc. In terms of total amount terpenoids 

 are the most important flavor and odor constituents of plants; but because of their special 

 pathway of biosynthesis, they are treated separately in Chapter 8. The compounds dis- 

 cussed in the present chapter have the same types of functional groups as many of the 

 terpenoids, but their carbon chains show much less branching and are often completely 

 unbranched. As minor constituents of essential oils they add distinctive characteristics 

 to products which are predominantly terpenoid; as flavor and odor constituents of fruits 

 and flowers they may be much more significant. Their role in the plant may lie in their 

 attractiveness to insect pollinators and animal seed-disseminators. The compound 2- 

 hexenal ("leaf aldehyde") is largely responsible for the distinctive odor of crushed leaves. 

 It is also reported to act as an antibiotic, wound hormone, and seed germination stimu- 

 lant (1). 



All of the straight chain alcohols up to Cjo have been found in plants either free or 

 esterified. A few branched chain and unsaturated alcohols are also known. Aliphatic 

 aldehydes up to C12 have been found and ketones up to C13. Unlike the fatty acids there 

 seems to be no preference for even-carbon chains among these compounds. Branching, 

 if present, is usually confined to a single methyl group near the end of the chain. Sec- 

 ondary alcohols are rather common, but they usually have their hydroxyl groups at C-2, 

 and never farther removed from the end of a chain than C-3. The ketones are almost 

 without exception methyl ketones. The esters usually have lower fatty acids comprising 

 their acyl groups, but sometimes aromatic acids are present. Acids of the citric acid 



cycle, or closely related acids, are never found in esters possibly because coenzyme 



A derivatives are not involved in their metabolism. To illustrate the complexity of some 

 fruit flavors. Table 1 lists some flavor constituents which have been found in apples (2), 

 pineapples (3) and strawberries (4). Table 2 lists some other compounds of this type with 

 examples of their natural occurrence. 



ISOLATION 



The volatile components of fruits and flowers are present in such minute amounts 

 that tremendous quantities of starting material are necessary for the isolation of any 

 workable quantity of product. Because of their volatility these compounds are also diffi- 

 cult to isolate, and they are frequently converted en masse to non-volatile derivatives 

 which can then be fractionated. 



Three general methods are available for removing volatile components from plants: 

 distillation, solvent extraction, and aeration. Distillation (or steam distillation) at at- 

 mospheric pressure may bring about some decomposition. Distillation under reduced 

 pressure and lower temperatures may permit enzymatic degradations to proceed causing 



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