Hydrocarbons, Esters

Fruits and vegetables (e. g., pineapple, apple, pear, peach, passion fruit, kiwi, celery, parsley) contain unsaturated C₁₁hydrocarbons which play a role as aroma substances. Of special interest are (E,Z)-1,3,5-undecatriene and (E,Z,Z)-1,3,5,8-undecatetraene, which with very low threshold concentrations have a balsamic, spicy, pine-like odor. It is assumed that the hydrocarbons are formed from unsaturated fatty acids by β-oxidation, lipoxygenase catalysis, oxidation of the radical to the carbonium ion and decarboxy-lation. The hypothetical reaction pathway from linoleic acid to (E,Z)-1,3,5-undecatrieneis shown in Formula 5.25.

R—CO—SCoA+ R^—OH

→ R—CO—O—R^+ CoASH (5.25)

Esters are significant aroma constituents of many fruits. They are synthetized only by intact cells:

(5.26)

Fig. 5.30. Biosynthesis ofγ- andδ-lactones from oleic and linoleic acid (according to Tressl et al., 1996) (1) R-γ-decalactone, (2) S-δ-dodecalactone, (3) R-δ-decalactone, (4)γ-decalactone, (5) R-(Z)-6-γ-dodecenelactone, (6) R-γ-nonalactone

Acyl-CoA originates from the β-oxidation of fatty acids and also occasionally from amino acid metabolism. Figure 5.30 shows an example of how ethyl (E,Z)-2,4-decadienoate, an important aroma constituent of pears, is synthesized from linoleic acid.

Table 5.30 gives information on the odor thresh-olds of some esters. Methyl branched esters, from the metabolism of leucine and isoleucine, were found to have very low values. The odor thresh-olds of the acetates are higher than those of the corresponding ethylesters.

When fruits are homogenized, such as in the pro-cessing of juice, the esters are rapidly hydrolyzed by the hydrolase enzymes present, and the fruit aroma flattens.

5.3.2.3 Lactones

Numerous lactones are found in food. Some of the representatives which belong to the typical aroma substances of butter, coconut oil, and vari-ous fruits are presented in Table 5.31.

Since the aroma of lactones is partly very pleas-ant, these substances are also of interest for com-mercial aromatization of food. In the homolo-gous series ofγ- andδ-lactones, the odor thresh-old decreases with increasing molecular weight (Table 5.32).

The biosynthesis of lactones was studied using the yeast Sporobolomyces odorus and it was shown that the results are valid for animal and plant foods. Labelling with deuterium indicates

5.3 Individual Aroma Compounds 381

Table 5.30. Odor thresholds of esters

Compound Odor

threshold (µg/kg, water) Methylpropionic acid methyl ester 7 2-Methylbutyric acid methyl ester 0.25 Methylpropionic acid ethyl ester 0.1 (S)-2-Methylbutyric acid ethyl ester 0.06

Butyric acid ethyl ester 0.1

Isobutyric acid ethyl ester 0.02 3-Methylbutyric acid ethyl ester 0.03

Caproic acid ethyl ester 5

Cyclohexanoic acid ethyl ester 0.001 (R)-3-Hydroxyhexanoic ethyl ester 270 Caprylic acid ethyl ester 0.1 (E,Z)-2,4-Decadienoic acid ethyl ester 100 trans-Cinnamic acid ethyl ester 0.06

Benzoic acid ethyl ester 60

Salicylic acid methyl ester 40

Butyl acetate 58

2-Methylbutyl acetate 5

3-Methylbutyl acetate 3

Pentyl acetate 38

Hexyl acetate 101

(Z)-3-Hexenyl acetate 7.8

Octyl acetate 12

2-Phenylethyl acetate 20

Table 5.31. Lactones in food

Name Structure Aroma quality Occurrence

4-Nonanolide Reminiscent Fat-containing food,

(γ-nonalactone) of coconut oil, fatty crispbread, peaches

4-Decanolide Fruity, peaches Fat-containing food,

(γ-decalactone) cf. Table 5.13

5-Decanolide Oily, peaches Fat-containing food,

(δ-decalactone) cf. Table 5.13

(Z)-6-Dodecen- Sweet Milk fat, peaches

γ-lactone

3-Methyl-4- Coconut-like Alcoholic

octanolide (whisky- beverages

or quercus lactone)

that the precursors oleic and linoleic acid are regio- and stereospecifically oxidized to hy-droxy acids (Fig. 5.30), which are shortened by β-oxidation and cyclized to lactones. The indi-vidual steps in the biosynthesis are represented in Fig. 5.31 using (R)-δ-decalactone, a key odorant of butter (cf. 10.3.4).

Linoleic acid is metabolized by cows with the for-mation of (Z)-6-dodecen-γ-lactone as a secondary product (Fig. 5.30). Its sweetish odor enhances the aroma of butter. On the other hand, it is un-desirable in meat.

Table 5.32. Odor thresholds of lactones Compound Odor threshold

The whisky or oak lactone is formed when alcoholic beverages are stored in oak barrels.

3-Methyl-4-(3,4-dihydroxy-5-methoxybenzo)oc-tanoic acid is extracted from the wood. After elimination of the benzoic acid residue, this compound cyclizes to give the lactone. The odor thresholds of the two cis-oak lactones (3R, 4R and 3S, 4S) are about ten times lower than those of the trans diastereomers (3S, 4R and 3R, 4S).

5.3.2.4 Terpenes

The mono- and sesquiterpenes in fruits (cf. 18.1.2.6) and vegetables (cf. 17.1.2.6), herbs and spices (cf. 22.1.1.1) and wine (cf. 20.2.6.9) are presented in Table 5.33. These compounds stimulate a wide spectrum of aromas, mostly perceived as very pleasant (examples in Table 5.34). The odor thresholds of terpenes vary greatly (Table 5.34). Certain terpenes occur in flavoring plants in such large amounts that in spite of relatively high odor thresholds, they can act as character impact compounds, e. g., S(+)-α-phellandrene in dill.

Monoterpenes with hydroxy groups, such as linalool, geraniol and nerol, are present in

Table 5.33. Terpenes in food

fruit juice at least in part as glycosides.

Linalool-β-rutinoside (I) and linalool-6-0-α-L-arabinofuranosyl-β-D-glucopyranoside (II) have been found in wine grapes and in wine (cf.

20.2.6.9):

(5.27) Terpene glycosides hydrolyze, e. g., in the production of jams (cf. 18.1.2.6.11), either enzymatically (β-glucosidase) or due to the low pH of juices. The latter process is strongly accelerated by a heat treatment. Under these conditions, terpenes with two or three hydroxyl groups which are released undergo further reactions, forming hotrienol (IV) and nerolox-ide (V) from 3,7-dimethylocta-1,3-dien-3,7-diol (cf. Formula 5.28) in grape juice, or cis- and trans-furanlinalool oxides (VIa and VIb) from 3,7-dimethylocta-1-en-3,6,7-triol in grape juice and peach sap (cf. Formula 5.29).

5.3 Individual Aroma Compounds 383

Table 5.33. (Continued)

Table 5.33. (Continued)

5.3 Individual Aroma Compounds 385

Table 5.33. (Continued)

aCompounds IVa and IVb are also denoted as pyranlinalool and furanlinalool oxide, respectively.

bCorresponding aldehydes geranial (Va), neral (VIb) and citronellal (VIIa) also occur in food. Citral is a mixture of neral and geranial.

c(−)-3,7-Dimethyl-1,5,7-octatrien-3-ol (hotrienol) is found in grape, wine and tea aromas.

Table 5.34. Sensory properties of some terpenes Compound^a Aroma quality Odor threshold

(µg/kg, water)

Myrcene (I) Herbaceous, 14

metallic

Linalool (IV) Flowery 6

cis-Furanlinalool Sweet-woody 6000 oxide (IVb)

Geraniol (V) Rose-like 7.5

Geranial (Va) Citrus-like 32

Nerol (VI) 300

Citronellol (VII) Rose-like 10 cis-Rose oxide (VIIa) Geranium-like 0.1 R(+)-Limonene (IX) Citrus-like 200 R(–)-α-Phellandrene Terpene-like, 500

(XI) medicinal

S(–)-α-Phellandrene Dill-like, 200

(XI) herbaceous

α-Terpineol (XVII) Lilac-like, 330 peach-like

(R)-Carvone (XXI) 50

1,8-Cineol (XXIII) Spicy, 12 camphor-like

(all-E)-α-Sinensal Orange-like 0.05 (XXXIX)

(–)-β-Caryophyllene Spicy, dry 64 (XLIX)

(–)-Rotundone (L) Peppery 0.008

aThe numbering of the compounds refers to Table 5.33.

(5.28)

(5.29) Most terpenes contain one or more chiral centers.

Of several terpenes, the optically inactive form and the l- and d-form occur in different plants.

The enantiomers and diastereoisomers differ regularly in their odor characteristics. For exam-ple, menthol (XIV in Table 5.33) in the l-form

Fig. 5.31. Formation of R-δ-decalactone from linoleic acid (according to Tressl et al., 1996)

(1R, 3R, 4S) which occurs in peppermint oil, has a clean sweet, cooling and refreshing peppermint aroma, while in the d-form (1S, 3S, 4R) it has remarkable, disagreeable notes such as phenolic, medicated, camphor and musty. Carvone (XXI in Table 5.33) in the R(−)-form has a pepper-mint odor. In the S(+)-form it has an aroma similar to caraway. Other examples that show the influence of stereochemistry on the odor threshold of terpenes are 3a,4,5,7a-tetrahydro-3,6-dimethyl-2(3H)-benzofuranone (cf. 5.2.5) and 1-p-menthene-8-thiol (cf. 5.3.2.5).

5.3 Individual Aroma Compounds 387

Some terpenes are readily oxidized during food storage. Examples of aroma defects resulting from oxidation are provided in Table 5.5 and Section 22.1.1.1.