Aroma enzymatic formation

The work reported by Josephson and Lindsay illustrates the broad nature of aroma biogenesis. In the past, most food scientists did not consider that the desirable aroma of fresh fish was enzymatically generated after harvest. In seafood, post harvest aroma chemical formation is generally associated with negative odors. 

The main components of tea are essential oil (0.5%) which is formed during fermentation, caffeine (1.8%-5.0%), and tannins (13%-18%). Enzymatic formation of black tea aroma follows biosynthetic pathways. The main precursors are amino acids and carotenoids (P-carotene, lutein, neoxanthin, and violaxanthin). Fermentation significantly reduces carotenoids and forms ionone and terpenoid carbonyls as a result of primary oxidations. During firing, secondary epoxida-tion takes place and forms epoxyionone, dihydroactinidiole, and trimethyl substituted cyclohexanones [35]. 

The principle components of tea comprise (a) an essential oil (about 0.5%) formed during fermentation (b) caffeine (1.8-5.0%) and (c) tannins (13-18%). The enzymatic formation of the black tea aroma has been investigated and is reviewed by Yamanishi [60,61] who established the involvement of the following biosynthetic pathways ... 

A more complex flavor development occurs in the production of chocolate. The chocolate beans are first fermented to develop fewer complex flavor precursors upon roasting, these give the chocolate aroma. The beans from unfermented cocoa do not develop the chocolate notes (84—88) (see Chocolate and cocoa). The flavor development process with vanilla beans also allows for the formation of flavor precursors. The green vanilla beans, which have Htfle aroma or flavor, are scalded, removed, and allowed to perspire, which lowers the moisture content and retards the enzymatic activity. This process results in the formation of the vanilla aroma and flavor, and the dark-colored beans that after drying are the product of commerce. 

Scheme 7.1 Enzymatic degradation of fatty acids by the j -oxidation cycle and formation of various types of aroma compounds in fruits and vegetables...

On the positive side, thermization and minimum pasteurization should not cause the formation of undesirable flavours and aromas and should, in fact, result in improved flavour by reducing bacterial growth and enzymatic activity, e.g. lipolysis. If accompanied by vacuum treatment (vacreation), pasteurization removes indigenous off-flavours, i.e. those arising from the cow s metabolism or from feed, thereby improving the organoleptic qualities of milk. 

Non-enzymatic browning reactions play a central role in the formation of food aroma and flavor, especially in heat-treated foods. The purpose of this work is to present sensory data, scattered in the literature, for volatile non-enzymatic browning reaction products and related compounds. The compilation has no pretensions to completeness and only a small part of the extensive patent literature has been covered. Anyhow, it is felt that a compilation of this kind, which has not been available hitherto, would be useful to workers in the field. 

A powerful sulphury aroma compound, 3-mercapto-2-methylpentan-l-ol (51), has recently been identified [13] in a complex process flavouring. It has a low odour threshold of 0.15 mg/L of water. The formation could be traced back to the onions present in the process flavouring and its formation is explained from propanal present in onions via aldol condensation, addition of hydrogen sulphide and enzymatic reduction (Fig. 3.31). 

The sensory quality of kiwifruit was greatly improved, resulting from the softening of the texture, decrease in acidity, and increase in soluble solid content due to the conversion of starch into sugars. In addition to these changes, there were enzymatic reactions which cause formation of more volatiles that improve the aroma of the fruit. 

The three essential steps for the formation of Ci3-norisoprenoid aroma compounds can therefore be considered to consist of (i) e initial dioxygenase cleavage, (ii) the subsequent enzymatic transformation(s) of the primary degradation product in natural tissues, and (iii) the final acid-catalyz conversion of a flavorless polyol into an aroma compound under processing conations. 

Enantioselectivity of Biogenetic Routes. The enantiomeric composition of natural aroma compounds is known to reflect the enantioselectivity of their biogenesis (22). Thus, by elaborating the chiral composition of Cp-norisoprenoids in fruits, we expected to obtain some information about step II m the formation of Ci3-notiso-prenoids, i.e. the enzymatic transformation(s) of the primary carotenoid degradation products into labile aroma precursors (cf. Fig. 3). 

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]. 

The main classes of Maillard reaction end products of practical interest inelude volatile heterocyclic molecules responsible for aromas of thermally processed foods colored oligomeric and polymeric molecules that determine non-enzymatic browning of thermally processed foods protein and lipid cross-linking and other advanced glycation end products in vivo, which are implicated in pathogenesis of comphcations in diabetes and aging. Here we only consider the contribution of D-finctosamines implicated in the formation of these end products. A number of reviews containing detailed considerations of the formation of volatiles, melanoidins, and... 

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