VOLATILE ALCOHOLS AND CARBONYL COMPOUNDS 



113 



TABLE 1. VOLATILE FLAVOR CONSTITUENTS OF SOME FRUITS 



Apples 



Pineapples 



Strawberries 



ethyl acetate 

 ethyl propionate 

 ethyl butyrate 

 ethyl valerate 

 propyl acetate 

 propyl propionate 

 propyl valerate 

 acetic acid 

 formic acid 

 propionaldehyde 

 propyl butyrate 



methyl iso-caproate 



ethyl acetate 



ethyl iso-caproate 



methyl iso-valerate 



ethyl iso-valerate 



methyl valerate 



methyl caprylate 



ethyl acrylate 



acetaldehyde 



methyl p'-methylthiolpropionate 



methyl propyl ketone 



iso-amyl alcohol 



amyl alcohol 



ethanol 



1-propanol 



ethyl butyrate 



iso-butyl alcohol 



2-pentanol 



1-hexanol 



/r««s-2-hexenol-l 



ethyl iso-valerate 



iso-amyl acetate 



ethyl caproate 



1-hexyl acetate 



/ra«s-2-hexenyl-l acetate 



changes in constituents of the tissue. If oxidative reactions are a problem, distillation 

 can be carried out in a nitrogen atmosphere. Solvent extraction methods are usable in 

 special cases especially for the less polar compounds. Some of the lower molecular 

 weight volatile compounds are too soluble in water to be extracted efficiently by organic 

 solvents. In the aeration process only compounds evolved into the air are isolated by 

 passing a stream of air over the plant material for a long period of time and condensing 

 any entrained substances in cooled receivers; or the stream of air may be passed through 

 reagents which react with at least some of the compounds to produce non-volatile deriva- 

 tives. 



A mixture of volatile substances isolated by one of the above procedures maybe frac- 

 tionated by distillation or chromatography. Chromatographic separations can be made 

 on liquid partition columns or by gas chromatography. Alternatively, derivatives of dif- 

 ferent components of the mixture may be used to separate them. Thus aldehydes and 

 ketones form water-soluble bisulfite compounds from which other components may be re- 

 moved by solvent extraction. They may also be converted to solid 2, 4-dinitrophenylhy- 

 drazones, and remaining volatile compounds removed by distillation. Aldehydes form 

 water-insoluble derivatives with dimedone. Ketones do not react with this reagent. Al- 

 cohols can be converted into solid urethanes by reaction with isocyanates or into 3, 5- 

 dinitrobenzoates by reaction with 3, 5-dinitrobenzoyl chloride. Remaining volatile com- 

 pounds are then removed by distillation. Esters cannot be separated as such by chemical 

 means since derivatives formed represent either the alcohol or the acid portion of the 

 ester rather than the intact compound. After the various types of derivatives have been 

 prepared, they are usually separated by chromatographic procedures as described below 

 under "Characterization". In a few cases it is possible to recover the original compounds 

 from purified derivatives, but there is usually no reason to do this. 



