Enzymatic formation aldehydes from fatty acids

Unsaturated and saturated fatty aldehydes such as (Z, Z, Z)-8, 11, 14-heptadecatrienal, (Z, Z)-8, 11-heptadecadienal, (Z)-8-heptadecenal, (Z, Z, Z)-7, 10, 13-hexadecatrienal, pentadecanal, ( , Z, Z)-2, 4, 7-decatrienal, ( , Z)-2, 6-nonadienal and ( )-2-nonenal have been identified in essential oils from edible seaweeds as characteristic major compounds. The enzymatic formations of the long-chain fatty aldehydes from fatty acids such as linolenic acid, linoleic acid, oleic acid and palmitic acid, respectively, have been demonstrated. Based on enzymatic formation of (2/ )-hydroxy-palmitic acid and 2-oxo-palmitic acid from palmitic acid during biogeneration of pentadecanal and on incubation experiments of synthetic (25)- or (2/ )-hydroxy-palmitic acid and 2-oxo-palmitic acid as substrates with crude enzyme solution of Viva pertusa, the biogeneration mechanism of long chain aldehydes via oxylipins is discussed. 

Galliard and Matthew (5) have reported the biogenesis of C15, C14, C13 and C12-saturated fatty aldehydes from palmitic acid in cucumber fruits. However, that of the unsaturated Cn-aldehydes such as (Z, Z, Z)-8,11,14-heptadecatrienal, (Z, Z)-8, 11-heptadecadienal and (Z)-8-heptadecenal had not been studied so far. Thus, the enzymatic formation of the long-chain aldehydes from unsaturated fatty acids in a green seaweed, U. pertusa, was explored. 

It should be stressed that it is a prerequisite of successful flavor precursor studies that the contribution of the odorant under investigation to a food flavor or off-flavor has been established. Sometimes the structure of a precursor can be assumed on the basis of structural elements in the odorant. In such cases, additions of the respective isotope-labelled precursor to the food system is commonly used to elucidate the precursor and to clarify reaction pathways governing the formation of the odorant. This method has been frequently applied, especially, in studies on the enzymatic generation of odor-active aldehydes (e.g., (Z)-3-hexenal in tea leaves) or alcohols (e.g., l-octen-3-oI in mushrooms) [cf. reviews in 84, 85] as well as lactones [86] from unsaturated fatty acids. 

Figure 9. Enzymatic formation of aldehydes from unsaturated fatty acids.

Enzymatic Formation of Long-Chain Fatty Aldehydes in a Green Seaweed U. pertusa. In preliminary experiments, they were found to increase during incubations of unsaturated fatty acids with homogenates of U. pertusa fronds. The acetone powder preparations from the homogenates were shown to retain this activity (20). Thus, enzyme solutions solubilized from the acetone powders with 0.1% Triton X-100 were used as a source of the enzyme activity. The products, (Z)-8-heptadecenal, (Z, Z)-8, 11-heptadecadienal and (Z, Z, Z)-8, 11, 14-heptadecatrienal and pentadecanal, were identified by GC and GC-MS analysis using authentic specimens... 

In addition to the enzymatic pathway of aroma formation, a thermal route also exists. At high temperatures, interactions of amino acids and sngars resnlt in the formation of various aldehydes. After thermal treatment, the tea becomes more tasty and pleasant, and has a better aroma. An essential source of secondary volatiles, formed during tea leaf processing, is oxidative. o-Quinone resulting from the oxidation of catechins can oxidize, besides amino acids and carotenes, unsaturated fatty acids as well. Linoleic and linolenic acids can be converted into hexenal and trans-hex-2-enal, respectively, and in addition, small amounts of other volatile compounds, especially hexanoic acid and trani-hex-2-enoic acid, can be formed from the same acids, respectively. Also the monoterpene alcohols, linalool and geraniol, play an important role in the formation of the aroma of black tea [38]. 

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