Esters in Foods
Most of the hydrocarbons covered so far in this module aren't the first molecules that come to mind, in terms of edibility. The alkane and unsaturated hydrocarbon groups are commonly found in petroleum, and most alcohols oxidize to produce aldehydes, which are fatal when ingested.
However, ethanol, which we know combusts to produce CO2, 2O, and heat, is used as a fuel and yet is the key ingredient in alcoholic beverages. It is not so much the ethanol that provides the flavor in such drinks, despite what an increasing proof (percent composition of ethanol) may suggest, but the family of esters. The process of yeast fermentation produces alcohols, which react with carboxylic acids in a condensation reaction to make esters.
The most common ester in wine is ethyl acetate, whose condensation reaction is shown above. This makes sense, considering ethanol is the only alcohol present in large quantities. One of the reasons why connoisseurs value aged fine wines is because over time, the concentration of esters increases through the aforementioned natural condensation reactions.
Dilution of beverages obviously decreases their flavor, but this is not only because the solution is less concentration. By adding a large amount of water to a solution with esters, the condensation reaction can be undone, splitting the ester into its component alcohol and carboxylic acid. This process is called hydrolysis.
Although the ester functional group has a polar carbonyl, it contains no hydrogen atoms suitable for hydrogen bonding. Therefore esters have low boiling points relative to most molecules of similar size. In many cases, even though its molecules are almost twice as large as those of the constituent alcohol and acid, an ester is found to have a lower boiling point than either. Ethyl acetate, for example, boils at 77.1°C, lower than ethanol (78.5°C) or acetic acid (117.9°C).
While they are important flavoring agents in wines and other such drinks, esters occur naturally in many fruits and enhance their flavors. A short table is given below.
|N-amyl acetate||Pears, bananas|
|Methyl salicylate||Oil of wintergreen|
|Linalyl acetate||Lavender, sage|
It is interesting to note that while esters are fragrant and responsible for many flavors, the alcohols and carboxylic acids that comprise them, as aforementioned, are toxic to the body. For example, methyl butrate gives apples their familiar taste, but methanol is poisonous, and butyric acid has an acrid smell that gives rancid butter its odor - another example of how effective a chemical reaction can be in changing chemical properties!