UNDERSTANDING AROMA COMPOUNDS
When food is consumed, the interaction of taste, odor and textural feeling provides an overall sensation which is best defined by the English word “flavor”.
Flavor results from compounds that are divided into two broad classes: Those responsible for taste and those responsible for odors, the latter often designated as aroma substances. However, there are compounds which provide both sensations. Compounds responsible for taste are generally nonvolatile at room temperature. Therefore, they interact only with taste receptors located in the taste buds of the tongue. The four important basic taste perceptions are provided by: sour, sweet, bitter and salty compounds. Glutamate stimulates the fifth basic taste.
Aroma substances are volatile compounds which are perceived by the odor receptor sites of the smell organ, i. e. the olfactory tissue of the nasal cavity. They reach the receptors when drawn in through the nose (orthonasal detection) and via the throat after being released by chewing (retronasal detection).
The concept of aroma substances, like the concept of taste substances, should be used loosely, since a compound might contribute to the typical odor or taste of one food, while in another food it might cause a faulty odor or taste, or both, resulting in an off-flavor.
The amount of volatile substances present in food is extremely low (ca. 10–15 mg/kg). In general, however, they comprise a large number of components. Especially foods made by thermal processes, alone (e. g., coffee) or in combination with a fermentation process (e. g., bread, beer, cocoa, or tea), contain more than 800 volatile compounds. A great variety of compounds is often present in fruits and vegetables as well. All the known volatile compounds are classified according to the food and the class of compounds.
Of all the volatile compounds, only a limited number are important for aroma. Compounds that are considered as aroma substances are primarily those which are present in food in concentrations higher than the odor and/or taste thresholds. Compounds with concentrations lower than the odor and/or taste thresholds also contribute to aroma when mixtures of them exceed these thresholds.
Among the aroma substances, special attention is paid to those compounds that provide the characteristic aroma of the food and are, consequently, called key odorants.
The lowest concentration of a compound that is just enough for the recognition of its odor is called the odor threshold (recognition threshold). The detection threshold is lower, i. e., the concentration at which the compound is detectable but the aroma quality still cannot be unambiguously established. The threshold values are frequently determined by smelling (orthonasal value) and by tasting the sample (retronasal value).
The threshold concentrations (values) for aroma compounds are dependent on their vapor pressure, which is affected by both temperature and medium. Interactions with other odor-producing substances can result in a strong increase in the odor thresholds.
As already indicated, compounds with high “aroma values” may contribute to the aroma of foods. The “aroma value” Ax of a compound is calculated according to the definition: Ax = cx / ax (cx: concentration of compound X in the food, ax: odor threshold of compound X in the food).
The evaluation of volatile compounds on the basis of the aroma value provides only a rough pattern at first. The dependence of the odor intensity on the concentration must also be taken into account.
In addition, additive effects that are difficult to assess must also be considered. Examinations of mixtures have provided preliminary information. They show that although the intensities of compounds with a similar aroma note add up, the intensity of the mixture is usually lower than the sum of the individual intensities.
For substances which clearly differ in their aroma note, however, the odor profile of a mixture is composed of the odor profiles of the components added together, only when the odor intensities are approximately equal. If the concentration ratio is such that the odor intensity of one component predominates, this component then largely or completely determines the odor profile. Examples are (E)-2-hexenal and (E)-2-decenal which have clearly different odor profiles. If the ratio of the odor intensities is approximately one, the odor notes of both aldehydes can be recognized in the odor profile of the mixture. But if the dominating odor intensity is that of the decenal or of the hexenal that particular note determines the odor profile of the mixture.
The aroma profiles of foods containing the same aroma substances can be completely dissimilar owing to quantitative differences. For example, changes in the recipe or in the production process which cause alterations in the concentrations of the aroma substances can interfere with the balance in such a way that an aroma profile with unusual characteristics is obtained.
An off-flavor can arise through foreign aroma substances, that are normally not present in a food, loss of key odorants, or changes in the concentration ratio of individual aroma substances.
More resources:
For the full article click Aroma_Compounds
An extensive online database of aroma compounds is available here
Flavornet is a compilation of aroma compounds found in human odor space, click here
Oxford Journals article on aroma compounds, click here