Aroma compounds distillation/extraction

The content of aroma compounds is, in general, low, and compositions of these compounds are often complex. Therefore, at the dawn of analytical chemistry, aroma compounds were extracted from a huge mass of raw material. Fractionation was carried out by means of distillation, and various other classical procedures (e.g., crystallization, pH control in extraction, derivatization) were employed. Quite obviously, compounds revealed using these procedures were inevitably restricted to a set of major constituents, if any. Occasionally, before the 1950s, additional techniques like UV-IR spectroscopy and open-column chromatography were employed and were helpful to some extent. 

Volatile constituents of cupuacu were isolated by steam distillation-extraction of pulp or juice [2].The identification of volatile constituents was based on mass spectral analysis. The pleasant aroma compounds were mainly esters (Fig. 8.2). Targe amounts of ethyl butanoate and small amounts of ethyl acetate, butyl acetate, and butyl isobutanoate were described. 

More recently, several aroma compounds were isolated from cupuacu pulp by vacuum distillation, solid-phase extraction, and simultaneous steam distil-lation-extarction and were analysed by GC, GC-MS, and GG-O [8]. The olfaction of the extracts obtained by solid-phase extraction indicated linalool, a-ter-pineol, 2-phenylethanol, myrcene, and limonene as contributors of the pleasant floral flavour. In this study, the esters ethyl 2-methylbutanoate, ethyl hexanoate, and butyl butanoate were involved in the typical fruity characteristics. 

Aroma compounds in distilled spirits and liqueurs, their levels, odour attributes, and thresholds are most important for quality and authenticity. Using gas chromatography and mass spectrometry, especially the composition of volatile aroma compounds in distilled spirits has been widely investigated [4-8]. By direct injection of an alcoholic distillate it is possible to determine more than 50 components within levels between 0.1 and 1,000 mg L b special methods of extraction can be used to increase this number up to more than 1,000 volatile substances [6]. However, sensory analysis is still indispensable to describe and evaluate spirit drinks. 

In terms of specificity in isolation, one will also isolate food constituents that are not aroma compounds (e.g. pesticides, herbicides, PCBs, plasticisers, and some antioxidants). Since these compounds are typically present in foods at very low levels, they generally present few complications. The primary volatile that complicates the application of this methodology is water. In all cases, one obtains an aroma isolate that consists of volatiles in an aqueous solution . Thus, unless the amount of water is small and the subsequent analytical step is tolerant of some water, volatility-based techniques must include some water-removal process. This may be freeze-concentration, the addition of anhydrous salts, or solvent extraction. Distillation is often used to isolate aroma compounds from fat-containing foods. Since fat is not volatile (under isolation conditions), its presence does not prohibit the use of this methodology. 

Aroma compounds are present in minute levels in foods, often at the ppb level ( ig/liter). In order to analyze compounds at these levels, isolation and concentration techniques are needed. However, isolation of aroma compounds from a food matrix, which contains proteins, fats, and carbohydrates, is not always simple. For foods without fat, solvent extraction (unit gu) can be used. In foods containing fat, simultaneous distillation extraction (SDE see Basic Protocol 1) provides an excellent option. Concentration of headspace gases onto volatile traps allows sampling of the headspace in order to obtain sufficient material for identification of more volatile compounds. A separate protocol (see Basic Protocol 2) shows how volatile traps can be used and then desorbed thermally directly onto a GC column. For both protocols, the subsequent separation by GC and identification by appropriate detectors is described in unitgu. 

In particular, liquid-liquid extractions, wastewater treatments, gas absorption and stripping, membrane, and osmotic distillation, are the processes more studied. For example, the VOCs removal, the extraction of aroma compounds and metal ions, the concentration of aqueous solutions, the acid-gases removal, the bubble-free oxygenation/ozonation, have been successfully carried out by using membrane contactors [1, 2]. 

The flavor constituents of plain and roasted cashew nuts have not been previously reported in the literature. In the present study, aroma compounds have been isolated from plain, oven-roasted and oil-roasted cashew nuts by simultaneous distillation extraction and by steam distillation followed by selective extraction, after pH adjustment. Compound identification was carried out by GC and GC-MS analyses. Esters and lactones are present in plain cashews whereas roasted samples also contain pyrazines. 

Reaction of an aqueous solution of cystine with thiamin, glutamate, and ascorbic acid produces a complex mixture of compounds with an overall flavor resembling that of roasted meat. The reaction was carried out at 120 C for 0.5h at pH 5.0 in a closed system. The aroma compounds were isolated by means of the simultaneous steam distillation/solvent extraction method. The flavor concentrate was pre-separated by liquid chromatography on silica gel and subsequently analysed by GC and GC/MS. Unknown flavor components were... 

An interesting comparative study of aroma compounds in aged brandies and aqueous alcoholic wood extracts involving US-assisted extraction was carried with a view to identify the components of brandy aroma already present in grapes and wines, those formed in the distillation step and those coming from the oak wood [6]. After three extraction cycles with 30, 10 and 10 ml of dichloromethane in an ultrasonic bath, the volume of the overall extract was reduced to 100-200 pi in a rotary evaporator and the individual components quantified by GC-FID. The relatively high standard deviations obtained in some cases (0.1-18.4%) can be ascribed mainly to both irreproducibility in the US energy provided by the bath and the low final extract volumes. 

The volatiles of fresh pineapple (Ananas comosus [L] Merr.) crown, pulp and intact fmit were studied by capillary gas chromatography and capillary gas chromatography-mass spectrometry. The fnjit was sampled using dynamic headspace sampling and vacuum steam distillation-extraction. Analyses showed that the crown contains Cg aldehydes and alcohols while the pulp and intact fruit are characterized by a diverse assortment of esters, h rocarbons, alcohols and carbonyl compounds. Odor unit values, calculated from odor threshold and concentration data, indicate that the following compounds are important contributors to fresh pineapple aroma 2,5-dimethyl-4-hydroxy-3(2H)-furanone, methyl 2-methybutanoate, ethyl 2-methylbutanoate, ethyl acetate, ethyl hexanoate, ethyl butanoate, ethyl 2-methylpropanoate, methyl hexanoate and methyl butanoate. 

SPE extraction of wine aroma can also be performed by using a C18 300-mg cartridge previously activated with 2mL methanol and 2mL distilled water. Ten millilitres of wine is diluted with 30 mL of water and 2-octanol (internal standard) is added to it. The solution is passed through the cartridge, stationary phase is rinsed with 3 mL water and then dried with flushing air. Free aroma compounds are recovered with 3 mL of dichloromethane and the solution is concentrated under nitrogen flow to about 100 p,L. Glycoside compounds are recovered with 3mL of methanol (Di Stefano, 1991). 

Simultaneous Steam Distillation/Extraction An elegant apparatus was described by Nickerson and Likens ( 5) for the simultaneous steam distillation and extraction (SDE) of volatile components. This device has become one of the mainstays in the flavor field. In this apparatus, both the aqueous sample and water-immiscible solvent are simultaneously distilled. The steam which contains the aroma chemicals and the organic solvent are condensed together, and the aroma compounds are transferred from the aqueous phase to the organic phase. Typical solvents used are diethyl ether, pentane or a mixture thereof normal extraction times are one to two hours. 

A micro version of the distillation extraction apparatus has been described by Godefroot t al, (2J). This apparatus uses heavier than water solvents, e.g., methylene chloride or carbon disulfide as the extractant. Because only one milliliter of solvent is used, no further concentration of solvent is required. The authors found 15 minutes distillation/extraction time sufficient for recovery of nonpolar compounds, e.g., mono and sesquiterpenes, while one hour was required for oxygenated and higher boiling compounds. This apparatus was evaluated by Nunez and Bemelmans (28) for low levels of aroma compounds in water. They reported that results were satisfactory for volatile levels greater than 1 ppm. 

The dichloromethane solution is concentrated to 2-3 mL by distillation using a 40-cm length Vigreux column, and finally to 200 pL under a nitrogen flow prior to GC/MS analysis. The GC/MS profile of free aroma compounds of a Muscat grape skin extract is shown in Fig. 4.6. 

A more direct way of understanding the composition of the essential oils obtained by use of different extraction methods is to consider the solubility of compounds. Solubility reflects the extent to which a substance dissolves in a particular mixture, e.g. an organic solvent or water. Solubilization is the last step of the extraction process after desorption from the matrix surface and diffusion through the solvent boundary layer to the solvent, which is simply water for aqueous distillation of aroma compounds. 

Extraction recovery can be limited by one step or several steps. Microwaves cause more damage and destroy the essential oil cells in leaves or seeds more rapidly and effectively than conventional extraction methods, thus the desorption step, which can often be the limiting factor, is of minor importance in SEME. Solubility is rarely the limiting factor in solvent extraction if the solvent is well chosen. In the distillation of essential oils the solvent is always water, however, and aroma compounds can be totally different in structure and chemical characteristics, especially in their solubility. 

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