Fruit aroma substance

Aliphatic monoketones are of minor importance as fragrance and aroma substances. 2-Alkanones (C3-C15) have been found in the volatile fractions of many fruits and foodstuffs, but they do not contribute signiflcantly to their aroma. An exception are the odd-numbered methyl ketones Cy, C9, Cn which possess a characteristic nutty note they are used, e.g., in cheese flavor compositions. In... 

Other fruit components. Other fruit components that may be used in tile manufacture of non-carbonated beverages, particularly dilutables, include pectins and aroma substances obtained during the concentration of fruit juices. These components do not normally count towards the fruit content of products as they are usually classifted as types of permitted additives. 

Bruemmer, J. H. Aroma substances of citrus fruits and their biogenesis. In "Geruch-und Geschmackstoffe". F. Drawert, ed. Verlag Hans Carl, Nurnberg, Germany, 1975, p 167-176. 

C,oH,gO, Mr 154.25, colorless oils with camphor-like odor, bicyclic monoterpene epoxides. 1,4-C., bp. 173-174 °C 1,8-C.,bp. 176-177 °C. 1,4-C. occurs in the essential oil of Fructus Cubebae [a drug obtained from fully grown but unripe fruits of the Indonesian endemic plant Piper cubeba (Piperaceae)] and 1,8-C. occurs to 40-60% in eucalyptus oils (Eucalyptus globulus, Myrtaceae). C. are used as expectorant for bronchial catarrh and as aroma substances in the perfume industry. 

Occurrence m-C. in the essential oils of leaves and fruits of the black currant(/(i7tes nigrum, Saxiftaga-ceae). p-C. is widely distributed in the plant kingdom and is isolated from, e.g., turpentine, cypress oil, cinnamon, eucalyptus, thyme oil and many others. C. is used as an aroma substance in cosmetics. 

Apple flavor The sometimes marked differences between aromas of individual varieties of apples are mainly due to quantitative variations in the composition of apple flavor substances. Key components are ethyl (+)-2-methylbutanoate and other esters of 2-methylbutanoic acid, in addition to ethyl and hexyl bu-tanoates, hexyl acetate, (E)-2- and (2)-3-hexenyl acetates (see fruit esters) and j3-damascenone. ( )-2- Hexenal, ( )-2- hexen-l-ol, and hexanal (see alka-nals) play a special role in A. f. These are trace aroma substances in intact apples. When the fruit cells are destroyed, the concentration of the Cg units increase strongly due to enzymatic processes. They are the main aroma components of apple juice. Accordingly, the aromas of fresh apples and apple juice differ markedly. Apricot flavor The typical aroma is due to the combined effects of numerous components with flowery and fruity characters these include linalool, 1-ter-pinen-4-ol, a-terpineol (see p-menthenols), 2-phen-ylethanol, a- and )8- ionones, /5- damascenone, and (Z)-jasmone for the flowery part together with fruit esters and lactones, e. g., 4-octanolide, 4- and 5-deca-nolide, 4-dodecanolide (see alkanolides), hexyl acetate and hexyl butanoate for the fruity part, rounded off by benzaldehyde. 

Banana flavor Of the 350 flavor compounds identified to date in B. f. 3-methylbutyl acetate as well as 3-methylbutyl butanoate and 3-methylbutyl 3-methyl-butanoate (see fruit esters) are of particular sensory relevance. Fatty-fruity and exotic-fruity notes result mainly from aroma substances with the (Z)-4-config-uration, e.g., Z)-4-hepten-2-one (C7H12O, Mr 112.17, CAS [90605-45-1 ]) and (Z)-4-hepten-2-ol (C7H14O, Mr 114.19, CAS [34146-55-9]), as well as their acetates and butanoates. Eugenol, elemicin (see safrole), and 0-methyleugenol are responsible for the spicy aroma. Bilberry flavor The aroma of the European bilberry (Vaccinium myrtillus) is mainly due to ( )-2- hexenal, ethyl 2- and 3-methylbutanoates (see fruit esters) as well as ethyl 3-hydroxy-3-methylbutanoate (C7H,403, Mr 146.19, CAS [18267-36-2]). 

To handle dehydration of fruit juices, a technology called slush drying was proposed and tested with apple juice for the potential loss of volatile flavor and aroma substances (Chandrasekaran and King 1971 Lowe and King, 1974). The principle of this method stems from the dependence of the freezing point on the concentration of dissolved solids (Figure 20.1). It boils down to the fact that drying takes place from an ice-liquid mixture (slush) in which 20% to 70% of the water present in the fruit juice is frozen (the... 

Sediment stabilization in fruit juices, obtaining body in beverage powders Stabilization of powdery tiroma emulsions, encapsulation of aroma substances... 

To a certain extent, unwanted aroma substances are concealed by typical ones. Therefore, the threshold above which an off-flavor becomes noticeable can increase considerably in food compared to water as carrier, e. g., up to 0.2 pg/kg 2,4,6-trichloroanisole in dried fruits. 

At the low pH values prevalent in fruit, non-enzymatic reactions, especially reactions 4-7 shown in Table 5.6, can interfere with the isolation of aroma substances by the formation of artifacts. In the concentration of isolates from heated foods, particularly meat, it cannot be excluded that reactive substances, e. g., thiols, amines and aldehydes, get concentrated to such an extent that they condense to form heterocyclic aroma substances, among other compounds (Reaction 8, Table 5.6). 

The method frequently applied to determine ee values is the enantioselective gas chromatographic analysis of the aroma substance on a chiral phase, e. g., peralkylated cyclodextrins. This method was used, e. g., to test raspberry fruit juice concentrates for unauthorized aromatization with trans-a-ionone. The gas chromatograms of trans-a-ionone from two different samples are shown in Fig. 5.13. The low excesses of the R-enantiomer of ee = 8% (concentrate A) and ee = 24% (B) can probably be put down to the addition of synthetic trans-a-ionone racemate to the fruit juice concentrate because in the natural aroma (C), the ee value is 92.4%. 

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

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

Tertiary thiols (Table 5.35) are some of the most intensive aroma substances. They have a fruity odor at the very low concentrations in which they occur in foods. With increasing concentration, they smell of cat urine and are called catty odorants. Tertiary thiols have been detected in some fruits, olive oil, wine (Scheurebe) and roasted coffee (Table 5.35). They make important contributions to the aroma and are possibly formed by the addition of hydrogen sulfide to metabolites of isoprene metabolism. In beer. 

Essential (volatile) oils are obtained preferentially by steam distillation of plants (whole or parts) such as clove buds, nutmeg (mace), lemon, caraway, fennel, and cardamon fruits (cf. 22.1.1.1). After steam distillation, the essential oil is separated from the water layer, clarified and stored. The pressure and temperature used in the process are selected to incur the least possible loss of aroma substances by thermal decomposition, oxidation or hydrolysis. 

Aroma compounds contribute significantly to the importance of fruits in human nutrition. The aroma substances of selected fruits will be outlined below in more detail. The structures and synthesis pathways of common aroma substances are explained in Chapter 5. 

The aroma of fruits can change on heating due to the liberation of aroma substances from gly-cosidic precursors (cf. 5.3.2.4), oxidation, water addition, and cyclization of individual compounds (cf. 5.5.4). 

The aroma of the most important citrus fruit, the orange, has been analyzed in detail. The potent aroma substances identified in the freshly pressed juice of the variety Valencia late by dilution analyses are shown in Table 18.28. 

The following S-containing aroma substances have been isolated from the yellow fruit 3-methylthiohexane-l-ol which probably gives rise to 2-methyl-4-propyl-l,3-oxathianes (cis/trans isomers in the ratio of 10 1) (Xa, b Formula 18.39). Of the two cis isomers, only the 2R,4S-isomer (Xb), which has a sulfurous herb-like odor (threshold = 4 pg/kg water), has been found in the fruit. However, the aroma note more typical of passion fruit is exhibited by the 2S,4R-isomer (Xa). 

The formation of typical aromas takes place during the ripening of fruit. In bananas, for example, noticeable amounts of volatile compounds are formed only 24 h after the climacteric stage has passed. The aroma build-up is affected by external factors such as temperature and day/night variations. Bananas, with a day/night rhythm of 30 °C/20 °C, produce about 60% more volatiles than those kept at a constant temperature of 30 °C. The synthesis of aroma substances is discussed in section 5.3.2. 

In the capsular fruit of vanilla, incorrectly called vanilla bean, 170 volatile compounds have been identified. However, the only fact that is certain is that apart from the main aroma substance vanillin, which is released from the glucoside on fermentation of the fruits, and (R)(+)-trans-a-ionone, the p-hydroxybenzyl-methylether (XVII) contributes to the aroma since its concentration (115-187 mg/kg) greatly exceeds the odor threshold (0.1 mg/kg, water). A mixture of 99% of sugar and 1% of ground vanilla is sold as vanilla sugar and a mixture of 98% of sugar and 2% of vanillin is sold as vanillin sugar. 

The most important aroma substance in dill fruit is (S)-carvone, which smells of caraway. In fact, dill seeds were used as a substitute for caraway in the past. 

It rapidly became apparent that the reaction of ethyl pyrocarbonate in wine was more complex than indicated by the reaction shown above. Ethyl alcohol and carbon dioxide were certainly the main degradation by-products, but small quantities of ethyl carbonate were also formed, and its fruit aroma was perceptible above a certain threshold. Most importantly, ethyl pyrocarbonate is a highly reactive molecnle and combines with certain substances in wine (organic acids, polyphenols, and nitrogen-based componnds) to produce urethanes, e.g. ethyl carbamate, which is toxic and carcinogenic. Quantities never exceeded 2-4 mg/1, significantly below the ofQcial threshold of 30 mg/1 in Canada. However, this risk was sufficient for the product to be completely abandoned. 

The aroma substances that comprise flavors are found in nature as complex mixtures of volatile compounds. A vast majority of volatile chemicals that have been isolated from natural flavor extracts do not provide aroma contributions that are reminiscent of the flavor substance. For instance, n-hexanal is a component of natural apple flavor (1) however, when smelled in isolation, its odor is reminiscent of green, painty, rancid oil. Similarly, ethyl butyrate has a nondescript fruity aroma although it is found in strawberries, raspberries, and pears, it does not uniquely describe the aroma quality of any of these individual fruits. It has long been the goal of flavor chemists to elucidate the identity of pure aroma chemicals that have the distinct character impact of the natural fruit, vegetable, meat, cheese, or spice that they were derived from. Often, these are referred to as character impact compounds (2). 

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