Cucumber 2-nonenal

The fruit of the cucumber plant Cucumis sativus) is mainly eaten raw or as pickle. Approximately 30 volatile compounds have been detected in the volatile fraction of cucumber, with aliphatic alcohols and carbonyl compounds being most abundant [35]. Fresh cucumber flavour develops as a result of enzymatic degradation of linoleic and linolenic acid rapidly after the tissue is disrupted (Scheme 7.2), by which ( ,Z)-2,6-nonadienal and ( )-2-nonenal mainly are formed [184]. ( ,Z)-2,6-Nonadienal is the main flavour volatile of cucumber fruit, with ( )-2-nonenal as the second most important compound (Table 7.7) [185, 186]. 

Many materials used for food and beverage packaging have characteristic odors or sensory active compounds (Torri et ah, 2008). The intensity and description of the odor may be affected by the number and type of volatile compounds that are released under environmental conditions at the time of evaluation. Chemical composition of the material and polymer morphology may play a role in the sensory characterization. Sensory descriptors do not define a specific chemical compound but may be related to different compounds, a blend of compounds, and even a limited concentration range of a compound or class of compounds. For example, frans-2-nonenal in water changes in sensory (taste) description from "plastic (0.2 gg/1) to "woody" (0.4-2.0 p.g/1), "fatty" (8-40 pg/1), and "cucumber" (1000 gg/1) (Piringer and Ruter, 2000). Such terms are descriptive of the sensation and perception by human response to the chemical stimuli (Table 2.1). 

Hexenal (leaf aldehyde) is a constituent responsible for the smell of green leafs, ( )-2-octenal a main component of the aroma of raw potatoes ( )-2-nonenal is the organoleptic main constituent of the smell of cucumbers and is found in carot root oil, tomatoes, beef and raspberries 158). ( )-2-Decenal and ( )-2-dodecenal are components of some essential oils, ( )-2-tridecenal is responsible for the bug-like smell of coriander seed oil1S8). 

Figure 10-20 Lipoxygenase Catalyzed Formation of Aroma Compounds in Cucumber. Source Reprinted from Biochim. Biophys. Acta., Vol. 441, T. Galliard, D.R. Phillips, and J. Reynolds, The Formation of cw-3-nonenal, mwu-2-nonenal and Hexanol from Linoleic Acid Hydroperoxide Isomers by a Hydroperoxide Cleavage Enzyme System in Cucumber (Cucumis Sativus) Fruits, p. 184, Copyright 1976, with permission from Elsevier Science.

Galliard, T., et al. 1976. The formation of cw-3-none-nal, frarw-2-nonenal and hexanal from linoleic acid hydroperoxide isomers by a hydroperoxide cleavage enzyme system in cucumber (Cucumis sativus) fruits. Biochim. Biophys. Acta 441 181-192. 

When mushroom homogenates were incubated with surimi, enhanced plant-like aromas somewhat reminiscent of oysters were produced, and this treatment also resulted In the masking of some of the fish-like aromas of the surimi. Cucumber homogenates developed strong cucumber, cardboard-like aromas which appear to be contributed principally by 2-nonenal and 2,6-nonadienal. As a result, the cucumber homogenates caused undesirable and unbalanced aromas that did not suppress unpleasant fishiness. Watermelon fruit extracts behaved similarly, and also provided unbalanced sweet aromas to surimi. Tests to date have been limited to short-term incubations of crude enzyme preparations with surimi. Further exploration of more purified and controlled plant-based flavor-generating enzyme systems for the production of fresh seafood-like aromas, and especially those for the eight-carbon volatile aroma compounds, appear warrented. 

More recently, the total volatile composition of distilled cucumbers was investigated by a number of groups (2, ). The steam volatile con unds of muskmelons fCucumis melo) have also received attention (, , 6,) as have those of watermelons (Citrullus vulgaris) (7.) The most striking feature in the volatile con onents of all these fruits is the variety and magnitude of the nine-carbon compounds. These compounds include nonanol, nonanal, various nonenols, nonadienols, nonenals and nonadienals. 

The surprising feature of this fruit is the quantity of six-carbon alcohols and aldehydes. Taken as a group, these conpounds constitute over 70% of the total volatiles. We had fully expected to find a variety of nine-carbon compounds in our essence and we specifically looked for them since they are the major volatiles in other Cucurbitaceae. For exanple, nonanal, 2-nonenal, 6-nonenal and 2,6-nonadienal are some of the major components of cucumber ( ) and 6-nonenol is the major compound in frozen muskmelon ( ). None of these compounds were found if present, they are below the 1% level. 

The enzymic formation of aldehydes, ketones, alcohols, and oxoacids (from linoleic and linolenic acids) on disruption of plant tissues is an important biosynthetic pathway by which fruit and vegetable volatiles are formed. Some examples are (E)-2-hexenal ("leaf aldehyde") and ( )-3-hexenol ("leaf alcohol") in tea (E)-2-hexenal in apples (E,Z)-2,6-nonadienal ("violet Teaf aldehyde") and (E)-2-nonenal in cucumber ( Z)-5-nonenal in musk melon (Z,Z) -3,6-nonadienol in water melon, and 1-octen-3-ol ("mushroom alcohol") in certain edible mushrooms and Fungi. The enzyme system is highly substrate specific to a (Z,Z)-1,4-pentadiene system (like lipoxygenase) splitting the >C = C< double bond at the W - 6 and/or W - 9 position. Therefore linoleic-, linolenic-, and arachidonic acids are natural substrates. It seems to be a common principle in leaves, fruits, vegetables, and basidiomycetes. 

Experiments I to III were carried out under comparable conditions (250 g cucumbers, phosphatebuffer pH 6.8, 40 s homogenization, addition of 100 mg precursors, volatiles were enriched by extraction / or distillation-extraction and determined by GC-MS). It can be seen that linolenic acid is transformed into hexanal, ( )-3-nonenal, pentylfuran, and (E)-2-nonenal in the 40 s trial I and I (I = extraction I = distillation-extraction). During 120 s homogenization the labile intermediate (Z)-3-nonenal decreased from 0.23 mg to 0.02 mg and all carbonyls are reduced to some extent to the corresponding alcohols. 

The purified dihydroxy acid was a labile component and decomposed during heating into carbonyls and oxoacids. Figure 8 presents some results of the thermal fragmentation at pH 4 - 5 in a Likens-Nickerson distillation. It can be seen that we characterized constituents similar to those in the linoleate experiment of cucumber homogenates. 50 mg precursor (9,10-dihydroxy- ( Z) -1, 2-octadecenoic acid) were decomposed into 1.95 mg ( )-3-nonenal, 0.25 mg ( )-2-nonenal, 0.51 mg 2-pentylfuran, and C -oxo- and Cg-dicarboxylic acids. In our opinion this is the most important reaction in oxidized beer (producing cardboard flavor). In an analogous reaction linolenic acid was trans-... 

Occurrence E)-2-A. Cg-C,3 in citrus oils, especially bitter orange, Cg also in guava and ginger aromas, C, in bread, cucumber, carrot (see vegetable flavors) and rice flavor, c,o in coriander oil, butter, chicken and guava aroma, C,2 in coriander oil, peanut and meat flavor. (Z)-4-Heptenal is found, among others, in "butter, seafood and tea flavor, (Z)-3- and (Z)-6-nonenal in cucumber, melon and fish aroma, and (Z)-4-decenal in calamus oil and Citrus junos oil. ... 

Cucumbers The impact compound is ( ,Z)-2,6- nona-dienal, together with ( )- and (Z)-2-nonenal (see alkenals), hexanal (see alkanals), and (Z)-l,5-octadien-3-one (see tea flavor). 

As shown in Fig. 3, the initial products from the 9- and 13-hydroperoxides of linoleic acid are the volatile aldehydes c/j-3-nonenal and hexanal and the corresponding C9 and C12 0x0 acid fragments. Analagous volatile products from linolenic acid are cw-3,c/s-6-nonadienal and cw-3-hexenal. However, in most plants, an isomerase enzyme converts the cis-3-enals to the trans-2 isomers (see Fig. 3). Such an enzyme cis-Z, trans-l-emA isomerase has been partially purified from cucumber fruits (Phillips et al., 1979). 

Because of their structural diversity, the C-9 compounds occupy first place among the aliphatic alcohols and carbonyl compounds. Cucumber 164, 296, 619) and various melon fruits 293, 294, 297, 298) seem to be particularly rich in derivatives having this number of carbon atoms. In addition to nonanal and the corresponding alcohol, their unsaturated analogs (16) to (25) were also found in these Cucurbitaceae fruits. ( ,Z)-2,6-Nonadienal (23) and ( )-2-nonenal (17) constitute the organoleptic principle for cucumber flavor Cucumis sativus L.) (164, 296). Although it is present in a concentration of only 0.9 ppb (about 45 times its threshold value) (295), (Z)-6-nonenal (21) is the essential... 

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). 

Cucumber, pear 9(S)-hydroperoxy-10-trans, 12-cis-octadecadienoic acid (9-LOOH) (Z)-3-nonenal + 9-oxo-nonanoicadd... 

The following aldehydes play an important role in cucumber aroma (E,Z)-2,6-nonadienal and (E)-2-nonenal. Linoleic and linolenic acids, as shown in Fig. 3.31, are the precursors for these and other aldehydes (Z)-3-hexenal, (E)-2-hexenal, (E)-2-nonenal. 

Cucumber fruits were studied by using the same analytical approach as they contain only five key volatiles and their concentration in cucumber tissue seems to be less variable than in tomato (see, for example. Ref. 12). Table 4 shows the El and API correlations for the five compounds each one could be attributed to a single ion mass on the API, and calibration with authentic standards allowed conversion of the ion signal into concentration units (parts per billion by volume). The amount of the Cg volatiles present in the macerated tissue was estimated by microwaving a sample of cucumber to inactivate the enzyme system that produces Cg and Cg volatile compounds. Inactivation was confirmed by APIMS analysis of the headspace above the treated samples. Microwaved samples were macerated after spiking them with known amounts of the Cg volatiles, then measuring volatile compound release in the blender apparatus. The values obtained from the spiked standards were then compared with the release traces from cucumber samples and the amounts of nonenal and nona-2,4-dienal estimated as 5 and 8mg/kg fresh tissue, respectively. These values compare well with the... 

The importance of Cg aldehydes to the character impact of cucumber flavor was recently confirmed by calculating their odor unit values (ratio of concentration to odor threshold). ( ,Z)-2,6-Nonadienal and (Z)-2-nonenal were determined to be the principal odorants of cucumbers (32). 

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