Fruit ester formation

Esters are widespread in fruits and especially those with a relatively low molecular weight usually impart a characteristic fruity note to many foods, e.g. fermented beverages [49]. From the industrial viewpoint, esterases and lipases play an important role in synthetic chemistry, especially for stereoselective ester formations and kinetic resolutions of racemic alcohols [78]. These enzymes are very often easily available as cheap bulk reagents and usually remain active in organic reaction media. Therefore they are the preferred biocatalysts for the production of natural flavour esters, e.g. from short-chain aliphatic and terpenyl alcohols [7, 8], but also to provide enantiopure novel flavour and fragrance compounds for analytical and sensory evaluation purposes [12]. Enantioselectivity is an impor-... 

Esters, RCOOR, are formed by the reaction of carboxylic acids, RCOOH, and alcohols, R OH, where R and R are hydrocarbon groups. Many esters are responsible for the odors of fruit and, thus, have important uses in the food and cosmetics industries. The first two steps in the mechanism of ester formation are... 

Formic add elliyl ester [ethyl formate] see fruit esters. 

It is well established that 18 2 and 18 3 are the precursors of and C, aldehydes and alcohols by exogenous application of 18 2 or 18 3 and radiolabelled precursors, e.g. [93]. As for the hexyl and hexenyl esters, it is not clear if hexyl or hexenyl moieties of esters solely originate from C, aldehydes or alcohols. Incubation of apple fruits with hexanal or hexanol resulted in an increase in the formation of hexyl esters, e.g. [94], but increased levels of these esters at the ripe stage of apple fruits were much larger than the endogenous levels of aldehydes and alcohols [97]. Interestingly, methyl jasmonate application stimulated ester formation of the pre-climacteric apples, but had no effect on the ester formation of the post-climacteric fruits [17] however, it inhibited the hexyl ester formation in apples stored in a controlled atmosphere [96]. Fan et al. [17] speculated that the inhibitory effect found in the last report was owing to the toxic level of methyl jasmonate used. 

When whole strawberries were cut into pieces and incubated with alcohols and acids, it has been shown that if they were cut into two or four pieces, the formation of the acetate esters increased, while cutting them into eight pieces dramatically depressed the formation of esters. Moreover, no ester formation was observed when strawberry was homogenized (Yamashita et al. 1975). Gilliver and Nursten (1976) showed that without adding banana slices, it was impossible to produce esters in aqueous solutions containing lower alcohols and either acetyl or butyryl CoA, or acetic or butyric anydride. They concluded that there is need for enzymes and/or cofactors from the fruit slice for ester biosynthesis. 

Esters are quite important to the flavor of both natural foods (e.g., fresh fruit) and fermented foods. Of the fermented foods, esters probably are most important to the flavor of some of the alcoholic beverages. The TNO-CIVO [85] compilation of volatiles in foods lists 94 esters that had been identified in beer. Most of the esters found in beer are formed via primary fermentation. They are produced intracellularly in yeast by enzymatic action [86]. Lipid metabolism by the yeast provides a large number of acids and alcohols that may undergo esterification to yield a variety of esters. While pure chemical reactions can lead to ester formation, this reaction is... 

All the radicals concerned are important in oxidation processes of aromatic molecules, and this paper aims to offer a starting point for the thermochemical dissection of such oxidation processes. It is also hoped that it may stimulate further investigations of radical thermochemistry, especially in the aromatic field. Areas for fruitful work on bond energies include the formate and carbonate esters, including phenyl formate and phenyl carbonate, and the bond strengths in formic acid itself and in benzaldehyde. 

The berry or the small fruits consist of strawberry, raspberry, blackberry, black currant, blueberry, cranberry and elderberry. The volatiles responsible for the flavour of small fruits are esters, alcohols, ketones, aldehydes, terpenoids, furanones and sulfur compounds (Table 7.3, Figs. 7.1-7.7). As fruit ripen, the concentration of aroma volatiles rapidly increases, closely following pigment formation [43]. 

Plant waxes are concentrated on leaves and leaf sheaths and on fruit skins, or in some exceptional cases in the seeds of plants. Most vegetable waxes contain predominantly wax esters plus a variety of other lipid materials, which affect the degree of saturation and other properties of the wax derived from different sources. Most use has been made of plant waxes in the cosmetic sector, but there is increasing interest in the use of plant-derived sterols as dietary supplements to reduce cholesterol formation. 

Methods for the capillary gas chromatographic separation of optical isomers of chiral compounds after formation of diastereoisomeric derivatives were developed. Analytical aspects of the GC-separation of diastereoisomeric esters and urethanes derived from chiral secondary alcohols, 2-, 3-, 4- and 5-hydroxy-acid esters, and the corresponding jf- and -lactones were investigated. The methods were used to follow the formation of optically active compounds during microbiological processes, such as reduction of keto-precursors and asymmetric hydrolysis of racemic acetates on a micro-scale. The enantiomeric composition of chiral aroma constituents in tropical fruits, such as passion fruit, mango and pineapple, was determined and possible pathways for their biosynthesis were formulated. 

It seems that the biogenesis of 3-acetoxyacidesters in pineapple is an enantio-selective process, comparable to the formation of esters of secondary alcohols in passion fruits. As the enzymic hydrolysis of ethyl 3-acetoxyhexanoate by Candida utilis leads to the (S)-configurated hydroxycompound (see Figure 4), only (S)-3-hydroxyacid esters are esterified to the corresponding 3-aceto-xycompounds in pineapple. 

In a series of labeling experiments, the biogenesis of esters and alcohols by fruit tissue slices was investigated (7y .) While (U—1 4c) -acetate and (U-1 c) -butyrate were incorporated into the corresponding esters by postclimacteric banana tissue, (U-14c)-octanoate was transformed in climacteric and postclimacteric banana tissues into caproic and butyric acid by B-oxidation and into heptanoic acid by ot-oxidation (9). In addition, 1-octa-nol, Z-4-hepten-2-ol, pentanol-2 and the corresponding esters were labeled. The biosynthesis of alcohols and esters is an analogous reaction to the formation of wax esters as outlined by Kolattukudy ( O). Octanoyl-CoA is reduced by an acyl-CoA reductase (NADH dependent) to octanal which is further transformed into 1-octanol by an... 

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