Aldehydes organoleptic properties

A rise in free fatty acids may also cause changes in the flavour components. Increased P-oxidation of the free fatty acids leads to P-keto acids which are decarboxylated to methyl ketones. There are also increases in unsaturated aldehydes, especially 2-enals and 2,4-dienals, probably formed by oxidation of unsaturated fatty acids. This marked increase in carbonyls and unsaturated aldehydes may cause a significant alteration in the flavour, as these compounds have considerable organoleptic properties. 

Composition Genuine essential oils consist exclusively of volatile components with boiling points mainly between 150 and 300 °C. They contain predominantly hydrocarbons or monofunctional compounds such as aldehydes, alcohols, esters, ethers, and ketones. Parent compounds are mono- and sesquiterpenes, phenylpropane derivatives, and longer-chain aliphatic compounds. Accordingly, essential oils are relative non-polar mixtures, i.e., they are soluble in most organic solvents. Often the organoleptic properties are not determined by the main components but by minor and trace compounds such as, e.g., 1,3,5-undecatrienes and pyrazines in galbanum oil. In many of the commercially important oils, the number of identified components exceeds 100. Very many of the constituents are chiral, frequently one isomer predominates or is exclusively present, e. g., (- )-menthol in peppermint oils or (-)-linalyl acetate in lavender oil. 

The hydroformylation in ethanol solutions gives, depending on the reaction temperature, either almost exclusively the aldehydes or a mixture of aldehydes and acetals with different organoleptic properties [93]. 

Oxidation products of Hpids (hydroperoxides, free alkoxyl and peroxyl radicals, epoxides and aldehydes) react with a number of food constituents during the processing and storage of food. These reactions often lead to a reduction in the nutritional value of foods (such as reactions with proteins and vitamins) and a deterioration of their organoleptic properties (e.g. reactions with flavour active substances). 

Table 8.9 Organoleptic properties of aldehydes arising from amino acids and fatty acids.

The organoleptic properties of some aldehydes produced by oxidation of fatty acids and organoleptic properties of the so-called Strecker aldehydes are hsted in Table 8.9 together with their precursors. Aldehydes produced from fatty acids are often carriers of a rancid odour and taste. They are formed even from minor unsaturated fatty adds. In beef and mutton tallow and butter, for example, small amounts of (llZ,15Z)-octadeca-ll,15-dienoic acid occur, autoxidation of which yields (Z)-hept-4-enal (8-39),... 

With improved analytical methods there has been a large increase in the number of compounds identified in wines. Volatile compounds from different families (such as alcohols, esters, aldehydes, terpenes, etc.) play an important role in the organoleptic characteristics of wines. This wide variety of compwimds with different chemical properties and with different concentrations makes the flavour pa-ofile of wines very complex. Therefore, it is necessary to standardise the terminology so as to facilitate the knowledge of the aromatic profile of wines. 

A careful organoleptic evaluation of the a,p-unsaturated aldehyde (17) which was detected in various food products such as carrot root oil (72), tomato (74), beef (342) and cranberry (17) has shown that this aldehyde has some remarkable flavor properties. Above the threshold concentration of about 0.1 ppb (72), for example at 0.4 to 2 ppb in water, ( )-2-nonenal (17) possesses a woody character (470). Above 8 ppb the sensory impression turns into a fatty one which becomes unpleasant above 30 ppb. Finally, an aqueous solution of 1,000 ppb of aldehyde (17) has a strong taste of cucumber. The fresh-brew woody note of roasted and ground coffee as well as the woody effect in bell peppers is due to the presence of this compound (470). Addition of (jE)-2-nonenal (17) to cranberry juice at a level of 1 ppb causes a considerable reduction in the normal astringent character of the juice (470) without any change of the original odor impression. An antagonistic effect of (17) is observed on the flavor of (Z)-3-hexenal (5) (375). Above 2 flavor units (E) 2 nonenal (17) causes the stale flavor of spoiled beers (381). 

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