Odour-active substances

Taste-active substances are usually water-soluble polar andnon-volatQe substances. Some taste-active substances may additionally be odour-active substances, but not necessarily. The complex (uniform) sensory perception of taste and smell is called flavour a strong pleasant smell, usually from food or drink is called aroma, and aromatic food or drink is that which has a pleasant smell. However, the term aromatic substance has a completely different meaning in organic chemistry. An aromatic substance, put simply, is a benzene derivative. The unwanted, altered, modified, unnatural or unpleasant odour, taste and flavour are called off-odour and off-taste or off-flavour, respectively. 

Saturated and unsaturated hydrocarbons with odd and even numbers of carbon atoms in the molecule (about C11-C35) are present as the primary substances in all vegetable oils and animal fats. Alkanes, alkenes, alkadienes and alkatrienes also arise as oxidation products of unsaturated fatty acids, catalysed by lipoxygenases or by autoxidation of fatty acids during food storage and processing. Only the lower hydrocarbons can play a role as odour-active substances. The main hydrocarbons resulting from oxidation of unsaturated fatty acids are ethane from Hnolenic acid, pentane and butane from Hnoleic acid and hexane and octane from oleic acid. The immediate precursors of hydrocarbons are the fatty acid hydroperoxides (Table 8.4). The unsaturated hydrocarbons are predominantly (Z)-isomers. Numerous other hydrocarbons, including ahcycHc hydrocarbons, appear as secondary hpid oxidation products. 

The most important diols occurring in alcoholic beverages and fermented dairy products are butane-2,3-diol and pentane-2,3-diol that are not, however, odour-active substances. These diols are produced as byproducts due to the activity of certain microorganisms, along with the corresponding acyloins (acetoin and 3-hydroxypentane-2-one) and sensory active a-diketones (biacetyl and pentane-2,3-dione). 

Other cinnamic acids also produce aldehydes, but they have no significance as odour-active substances. By analogy, coumaralde-hyde is produced from 4-coumaric acid, caffeoyl aldehyde from caffeic acid, conifer aldehyde (also known as ferulyl aldehyde) from ferulic acid, 5-hydroxyconifer aldehyde from 5-hydroxyferulic acid and sinapyl aldehyde from sinapic acid. These aldehydes can be reduced to the corresponding alcohols, with which they play a role as the building units of lignin biosynthesis. 

Hydroxycarboxyhc acids are mostly non-volatile and polar compounds that are not important as odour-active food components. Certain hydroxy acids are, however, the major substances influencing the sour taste of fruits, vegetables and other foods. Some of their reaction products are odour-active substances, especially lactones. 

Oxocarboxylic acids are major products of the metabolism of proteins, lipids and carbohydrates, and also arise as intermediates in fermentation processes, as products of other enz)miatic reactions, which are not linked with the metabolism of major nutrients, and also as products of non-enzymatic reactions. They usually occur in low concentrations in aU foods of animal and vegetable origin. Oxoacids are polar and non-volatQe compounds, but some of their reaction products are important odour-active substances in foods. An example is 5-ethyl-3-hydroxy-4-methyl-5H-furan-2-one (abhexon), which appears in acid protein hydrolysates from threonine via 2-oxobutyric acid. 

Numerous functional derivatives of carboxyhc and substituted carboxyhc acids have been found to be natural food components. In particular, important compounds are esters, lactones, anhydrides, amides and nitriles. The most important functional derivatives of carboxyhc acids that act as odour-active substances are esters and lactones. 

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