Alcohols and Other Volatile Compounds

Besides water, ethanol (ethyl alcohol) is the most plentiful compound in wine. A wine s strength is expressed in terms of alcohol content, or the percentage of alcohol by volume. As ethanol has a density of 0.79, a wine with an alcohol content of 10% vol contains 79 g/1 of ethanol by weight. The alcoholic strength of wine is generally 100 g/1 (12.6% vol), although it may exceptionally be as high as 136 g/1 (e.g. an alcohol content of 16% vol). 

Due to the low density of ethanol, dry wines, containing negligible amounts of sugar, have densities below that of water (1.00), ranging from 

91 to 0.94. This value decreases as the alcohol content increases. 

Ethanol in wine is mainly produced by the alcoholic fermentation of sugar in must. However, grape cells are also capable of forming small quantities, mainly under anaerobic conditions (carbonic maceration see Volume 1, Section 12.9.3). The appearance of traces of ethanol in grapes results from alcohol dehydrogenase activity, which acts as a marker for ripeness. 

As approximately 18 g/1 of sugar is required to produce 1% vol of ethanol during alcoholic fermentation, grape must has to contain 180, 226 and 288 g/1 of sugar to produce wines with 10, 

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Essential oils (Section 26 7) Pleasant smelling oils of plants consisting of mixtures of terpenes esters alcohols and other volatile organic substances Ester (Sections 4 1 and 20 1) Compound of the type... 

Gas Chromatography of Alcohols This procedure will identify and quantify the common alcohols in blood and mine. The column used is 0,3% Carbowax 20M on Carbopak C, The system is the same as System GI for Solvents and Other Volatile Compounds (p. 199) except that for this purpose it is operated isothermally at 120°. identification of alcohols... 

Oxidation of cumene to cumene hydroperoxide is usually achieved in three to four oxidizers in series, where the fractional conversion is about the same for each reactor. Fresh cumene and recycled cumene are fed to the first reactor. Air is bubbled in at the bottom of the reactor and leaves at the top of each reactor. The oxidizers are operated at low to moderate pressure. Due to the exothermic nature of the oxidation reaction, heat is generated and must be removed by external cooling. A portion of cumene reacts to form dimethylbenzyl alcohol and acetophenone. Methanol is formed in the acetophenone reaction and is further oxidized to formaldehyde and formic acid. A small amount of water is also formed by the various reactions. The selectivity of the oxidation reaction is a function of oxidation conditions temperature, conversion level, residence time, and oxygen partial pressure. Typical commercial yield of cumene hydroperoxide is about 95 mol % in the oxidizers. The reaction effluent is stripped off unreacted cumene which is then recycled as feedstock. Spent air from the oxidizers is treated to recover 99.99% of the cumene and other volatile organic compounds. 

The edible portion of broccoli Brassica oleracea var. italica) is the inflorescence, and it is normally eaten cooked, with the main meal. Over 40 volatile compounds have been identified from raw or cooked broccoli. The most influential aroma compounds found in broccoli are sulfides, isothiocyanates, aliphatic aldehydes, alcohols and aromatic compounds [35, 166-169]. Broccoli is mainly characterised by sulfurous aroma compounds, which are formed from gluco-sinolates and amino acid precursors (Sects. 7.2.2, 7.2.3) [170-173]. The strong off-odours produced by broccoli have mainly been associated with volatile sulfur compounds, such as methanethiol, hydrogen sulfide, dimethyl disulfide and trimethyl disulfide [169,171, 174, 175]. Other volatile compounds that also have been reported as important to broccoli aroma and odour are dimethyl sulfide, hexanal, (Z)-3-hexen-l-ol, nonanal, ethanol, methyl thiocyanate, butyl isothiocyanate, 2-methylbutyl isothiocyanate and 3-isopropyl-2-methoxypyrazine... 

Monitoring the purification of the first pheromone (Ca. hemipterus) by wind-tunnel bioassay was complicated by a synergistic relationship with food odors. It was difficult to demonstrate the activity of the pheromone once it became chromatographically separated from the food scent that was also present in the volatile collections (Bartelt et al., 1990a). Consequently, food scent, or a synthetic version of this, was routinely added to each bioassay treatment, and the food scent itself became the control when pheromone activity was being evaluated. A variety of esters, alcohols, and other compounds were found to syner-gize the pheromone of Ca. hemipterus (Dowd and Bartelt, 1991). Synergistic effects were seen with all the species. 

Spices impart aroma, colour and taste to food preparations and sometimes mask undesirable odours. Volatile oils give the aroma, and oleoresins impart the taste. Aroma compounds play a significant role in the production of flavourants, which are used in the food industry to flavour, improve and increase the appeal of their products. They are classified by functional groups, e.g. alcohols, aldehydes, amines, esters, ethers, ketones, terpenes, thiols and other miscellaneous compounds. In spices, the volatile oils constitute these components (Zachariah, 1995 Menon, 2000). 

Other aliphatics. In addition to the compounds described above, plants generate a variety of hydrocarbons and other aliphatic compounds ranging from low molecular weight volatiles to high molecular weight alcohols, acids, ketones and esters found in the waxy external cuticle of leaves and fruit. 

For isolated cases, the first indication of the presence of a volatile substance may be the presence of an unknown peak when a sample is analyzed for ethanol and other volatile alcohols (see the previous section). In these instances, method development focuses on the compound encountered and frequently involves modification of the ethanol analysis procedure. Numerous procedures for single VOCs or classes of VOCs have been developed and summarized in review articles. HS-GC remains the most frequently used... 

Later, Tressl et al. (1976) also proceeded to the thermic degradation (2 h, 200 JC) of ferulic acid (H.87) and identified the same phenols as Fiddler et al., plus 4-isopropylguaiacol and vanillin alcohol (4-hydroxy-3-methoxybenzenemethanol) which have not been found in coffee. For isoeugenol (H.38), the formula is written as the (E)-( trans -) isomer, but nothing was specified in the text. Tressl et al. (1976) also published the results of thermal decomposition of cinnamic, p-coumaric (H.84) and sinapic (H.90) acids. Many of the simple phenols (and other aromatic compounds) formed have also been identified in roasted coffee volatiles. A thermic fragmentation of quinic acid (E.62) has shown that simple acids, phenols and polyphenols originate from this precursor (Tressl et al., 1978a). 

Methyl alcohol is obtained by first neutralizing the crude acid with milk of lime, which forms calcium acetate with the acetic acid present, and then submitting the liquid to fractional distillation. A second distillation from quicklime yields an alcohol of 99 per cent. The.alcohol so obtained contains acetone and other substances in small quantities, from which it can not be separated by distillation, and which impart to it an unpleasant odor. Pure methyl alcohol can be prepared from the commercial wood-spirit in a number of ways. In one of these the impure alcohol is treated with anhydrous calcium chloride, which forms with the alcohol a crystalline compound having the formula CaCl2.4CH30H. This substance is first heated to 100°, a temperature at which the acetone and other volatile impurities are vaporized then, after treatment with water, the alcohol is distilled off. 

Tenax is generally useful for compounds of the same or less volatility than benzene. Chromosorb 106 is useful for compoimds of medium volatility and Porapak N is capable of collecting alcohols and other highly polar compounds of medium volatility. Carbosieve B is best suited for very volatile compounds. Although these sorbents are suitable for mixtures of compounds, mixtures with a wide range of volatility may require two or three sorbents connected in series. Tenax is... 

In addition to the enzymatic pathway of aroma formation, a thermal route also exists. At high temperatures, interactions of amino acids and sngars resnlt in the formation of various aldehydes. After thermal treatment, the tea becomes more tasty and pleasant, and has a better aroma. An essential source of secondary volatiles, formed during tea leaf processing, is oxidative. o-Quinone resulting from the oxidation of catechins can oxidize, besides amino acids and carotenes, unsaturated fatty acids as well. Linoleic and linolenic acids can be converted into hexenal and trans-hex-2-enal, respectively, and in addition, small amounts of other volatile compounds, especially hexanoic acid and trani-hex-2-enoic acid, can be formed from the same acids, respectively. Also the monoterpene alcohols, linalool and geraniol, play an important role in the formation of the aroma of black tea [38]. 

Studies of other alcoholic beverages like wine have used dynamic head-space to quantify and identify volatile compounds (Leino. 1993, Mestres. 1998). More than 45 volatile compounds have been identified, some of which had very low concentrations. Head space analysis is a good method for separating, quantifying, and identifying compounds and for determining the authenticity of the wines being tested (Etievant. 1986, Savchuk. 2001). 

Heat treatment of eggs by cooking and frying creates a variety of other volatile compounds, more from egg yolk than from egg white. The main volatile components are aldehydes, alcohols, free fatty acids, esters and other compounds. Those occurring in the highest quantities are (2Z,4Z)- and (2Z,4E)-deca-2,4-dienals, (Z)-oct-2-enal, nonanal, hexanal, phenylacetaldehyde and hexyl butyrate. Egg white odour resembles (2E,4Z,7Z)-2,4,7-tridecatrienal, which arises as a product of arachidonic acid autoxidation (see Section 3.8.1.12.1). Its odour threshold was estimated to be 0.07ng/1 air. Sulfane and ammonia are also important protein degradation products. 

Pineapple Ananas comosus, Bromeliaceae) aroma consists of about 200 alcohols, esters, lactones, aldehydes, ketones, monoterpenes, sesquiterpenes and other volatiles. About 80% of the total volatile substances are esters. The main components in the green fruit are ethyl acetate, ethyl 3-(methylthio)propionate (8-189) with a distinctive pineapple aroma and ethyl 3-(acetoxy)hexanoate (8-190). The ripe fruit contains, as the main esters, ethyl acetate, (2J ,3i )-butane-2,3-diol diacetate (8-191) and ketone 3-hydroxy-butan-2-one. An important compound for the typical character of pineapple aroma, as in strawberry aroma, is 2,5-dimethyl-4-hydroxy-2//-furan-3-one (furaneol), present as a glycoside, and 2,5-dimethyl-4-methoxy-2H-furan-3-one. 

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