Citrus fruits, smell

It may seem strange to classify a type of bond as a functional group, but you will see later that C=C double bonds impart reactivity to an organic molecule just as functional groups consisting of, say, oxygen or nitrogen atoms do. Some of the compounds produced by plants and used by perfumers are alkenes (see Chapter 1). For example, pinene has a smell evocative of pine forests, while limonene smells of citrus fruits. 

Conifer wood, balm trees, citrus fruits, coriander, eucalyptus, lavender, lemon grass, lilies, carnation, caraway, peppermint species, roses, rosemary, sage, thyme, violet and many other plants or parts of those (roots, rhizomes, stems, leaves, blossoms, fruits, seed) are well known to smell pleasantly, to taste spicy, or to exhibit specific pharmacological activities. Terpenes predominantly shape these properties. In order to enrich terpenes, the plants are carved, e.g. for the production of incense or myrrh from balm trees usually, however, terpenes are extracted or steam distilled, e.g. for the recovery of the precious oil of the blossoms of specific fragrant roses. These extracts and steam distillates, known as ethereal or essential oils ("essence absolue") are used to create fine perfumes, to refine the flavor and the aroma of food and drinks and to produce medicines of plant origin (phytopharmaca). 

Hydroxycinnamic acids (HCAs), comprising p-coumaric, ferulic, caffeic and sinapic, and their bound forms, are found in citrus fruit parts (d5). During processing and storage of citrus juices, vinyl phenols are produced from the free HCA by acid-catalyzed decarboxylation. The decarboxylation of all HCAs would potentially produce p-vinyl phenol (from p-coumaric acid), p-vinyl guaiacol (ferulic acid), p-vinyl catechol (caffeic acid) and 3,5-dimethyl-4-hydroxystyrene (sinapic acid). P-vinyl phenol and p-vinyl guaiacol have been identified in processed citrus juices, but p-vinyl catechol and 3,5-dimethyl-4-hydroxy styrene have yet to be reported. Vinyl phenols are unpleasant smelling compoimds with very low perception thresholds their presence adversely affects acceptability of citrus juice products. 

This is extracted from the peel of the fruit of Citrus limomm by pressure, either manual or mechanical. 11 is a pale yellow, sometimes slightly greenish liquid with the smell of fresh lemons in time it resinifies, decolorises and acquires a special resinous odour. It consists mostly (about 90%) of limonene and contains also small proportions of other terpenes, citral (which determines its value, as it contributes largely to the aroma) and other aldehydes and traces of esters. 

Figure 15 Partial odor profiles obtained using Dravnieks s [81] system of descriptors. Descriptors (some edited for brevity) appear along the x axis. The y axis represents applicability of descriptors to three odors. Gray bars represent a profile for ethyl butyrate ( tutti-fruity ). Black bars represent a profile for ethyl propionate, another, homologous ester. Profiles for the two similar-smeUing esters both include many fruit-related descriptors (particularly Fruity, not Citrus ). They also have Fragrant and Aromatic in common. White bars represent a profile for eucalyptol, an odorant that smells quite different from the two esters. As the profiles of the esters do the profile for eucalyptol contains the descriptors Fragrant and Aromatic. However, the profile differs from those of the esters in the lack of fruit-related descriptors and the addition of Minty, Camphor, and Eucalyptus. ...

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