## 2 Aroma Volatile Composition and Their Biological Characteristic of Major Fruits
### Volatile Compounds and Their Biological Characteristic of Major Fruits  
More than 300 volatile molecules have been reported in fresh apples [23]. The total number, identity and concentration of volatile compounds emitted by ripening apple fruit are cultivar specific [24]. The contribution of each compound to the specific aroma profile of each cultivar depends on the activity and substrate specificity of the relevant enzymes in the biosynthetic pathway, the substrate availability, the odour threshold above which the compound can be detected by smell, and the presence of other compounds [25]. Esters are the most abundant volatile compounds emitted by apple and, together with \(\alpha\)-farmesene, have been proposed for cultivar classification [26]. Ethyl 2-methyl butanoate, 2-methyl butyl acetate, and hexyl acetate contribute mostly to the characteristic aroma of \"Fuji\" apples, while ethyl butanoate and ethyl 2-methyl butanoate are the active odor compounds in \"Elbstar\" apples, and ethyl butanoate, acetaldehyde, 2-methyl butanol, and ethyl methyl propanoate in \"Cox Orange\" [27]. For the \"Pink Lady\" cultivar, hexyl acetate, hexyl 2-methyl butanoate, hexyl hexanoate, hexyl butanoate, 2-methylbutyl acetate and butyl acetate were prominent within the blend of volatiles produced by fruit throughout maturation [28]. Among fruit tissues, it has been shown that epidermal tissue produces a greater amount of volatiles than internal tissues [29]. This higher capacity for aroma production by the peel has been attributed to either the abundance of fatty acid substrates or the higher metabolic activity [30; 31]. More than 300 volatile compounds have been identified in pear fruit (_Pyrus pyrifolia_ Nak.) [32], methyl and hexyl esters of decadenoate are the character-imparting compounds of the European pear [32,33]. Other volatile esters, for example, hexyl acetate, 2-methylpropyl acetate, butyl actate, butyl butanoate, pentyl acetate, and ethyl hexanoate possess strong pear-like aromas [32]. Ethyl octanoate and ethyl-(_E_)-2-ocenoate contribute to sweet or fruity odors in pears, while a high concentration of 2,4-decadenoates in fruit flesh is accepted by consumers [34]. In addition, hexanal, 2-methylpropyl acetate, ethyl acetate, hexyl acetate, 3-methylbutyl-2-methyl butanoate, ethyl butanoate, and butanol are identified as impact volatiles in \"Conference\" pears [35]. Although the melon aroma has been extensively investigated and more than 240 volatile compounds have been identified in different varieties [36], the literature is lacking in quantitative data. Numerous compounds of different degrees of volatility, especially those containing a C\({}_{9}\) straight chain, are the major determinants of melon fruit quality perceived by consumers. These compounds are strongly dependent on the variety and physiological behavior of the fruit; in fact, fresh climacter melons such as cantaloupe have greater aroma intensity and a shorter shelf life than less climacter melons such as honeydew [37]. In climacteric aromatic melon varieties, volatile esters are prominent, together with sulphur-containing aroma compounds, sesquiterpenes, norisoprenes, short-chain alcohols, and aldehydes [38]. Volatile esters, mainly acetate derivatives such as ethyl 2-methyl propyl acetate and 2-methyl butyl acetate are dominant with 37% of the total volatile profile [39]. In addition, lower amounts of lactones, sulfur compounds [such as (methylthio) acetate, 2-(methylthio) ethyl acetate and 3-(methylthio) propyl acetate], short-chain alcohols and aldehydes compose the complex mixture of volatile compounds [40, 41, 42]. Non-aromatic varieties often have much lower levels of total volatiles, and lack the volatile esters [43]. Volatiles derived from amino acids are major contributors to melon aroma [44, 45]. Both aromatic and non-aromatic varieties possess amino acid derived volatiles. In the aromatic varieties these volatiles are mostly esterified and their levels are usually higher than in the non-aromatic varieties.