Selective distillation column 40° proof (20%)

Aldehyde and ester draw 185° proof (92.5%)

Dilution H,0

Selective distillation column 40° proof (20%)

Figure 5. Aldehyde (heads) concentrating column.

In 1962 the U.S. Treasury Department increased the maximum allowable entry proof (storage in barrels) for straight whisky from 110 to 125° (55 to 62.5%). The industry engaged in a study in December 1985 to determine the flavor development, chemical composition, and evaporation loss rates of distillates at six different proofs from 109 to 155° (54.5 to 77.5%) over a planned 8-yr period, and the study was continued an additional 4 yr. Evaporation losses averaged approximately 3.0% a year over the 12-year period.

In addition to chemical analysis, they applied regressive analysis for most congeners listed in Table 4. Regression analysis illustrates the functional relationship between congeners. Except for fusel oil, there was an appreciable continuing development of congeners through 12 yr. Esters, the most important flavor constituent in whisky, developed linearly over this time period. Later studies, using radioactive carbon for direct monitoring, determined the congeners derived from ethanol during whisky maturation. Esters (ethyl acetate) are formed in a system that is affected by changes in concentration of acetic acid, ethanol, and water, ie, a loss of water by evaporation; an increase in acetic acid results in an increase of ethyl acetate.

The maturing progress of a type of bourbon as measured by the principal ingredients is illustrated in Tables 4 and 5. It is evident that the congeners amount to only about 0.5-0.75% of the total weight. Yet it is this small fraction that determines the quality of the final product. Thus matured whiskies vary widely in taste and aroma because of the wide variation in congener concentration. For example, esters may vary from 8.0 to 18.0 g/100 L, and aldehydes from 0.7 to 7.6 g/100 L. Nevertheless, there is no correlation between chemical analysis and quality, ie, taste and aroma. Only the consumer can detect the fine variations and thus evaluate the quality of whiskies.

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