Simultaneous Distillation Extraction

Steam distillation finds application in the analyses of volatiles from beverages and high-water-content foods, although it is less applicable to fats and oils. It has the disadvantage that large quantities of aqueous distillate require further extraction with a solvent, to separate the volatiles from the water. Concentration of the extract is then necessary. The formation of artifacts may also be a problem.

One of the most widely used techniques in aroma analysis combines steam distillation with solvent extraction in a Likens—Nickerson apparatus (Figure 14.1), which was first reported in 1964 for the extraction of hop oil [32]. The extracting solvent is immiscible with and less dense than water. Upon heating, volatile compounds in the steam are transferred to the solvent and both liquids condense. The glassware is constructed so that both solvent and water are returned to their starting vessels. After an extraction time of 1—12 h, the extract is collected and dried, either using anhydrous sodium sulfate, or by freezing and decanting the solvent from the ice. The extract is then concentrated before analysis to a volume of approximately 0.1 mL; a low-boiling extracting solvent is therefore desirable, so that it can be removed without substantial losses of compounds of interest. In addition, the solvent should be of high purity, so that impurities do not become major chromatographic peaks, when the extract is concentrated. Appropriate solvents, which have been widely used, are pentane, diethyl ether, or a combination of the two. Solvents denser than water, for example, dichloromethane, could be used in a modified apparatus.

Likens And Nickerson Apparatus

Heating mantle

Figure 14.1 Likens-Nickerson apparatus for simultaneous distillation-extraction.

Heating mantle

Figure 14.1 Likens-Nickerson apparatus for simultaneous distillation-extraction.

Simultaneous distillation-extraction (SDE) has been widely used for the analysis of cooked meat volatiles. In SDE, the sample is boiled for 1—2 h and hence precooking may not be necessary; the meat is usually minced, to maximize the surface area for extraction. MacLeod and Coppock used SDE, with 2-methylbutane and a range of extraction times, to examine the aroma of cooked beef [22,33]. Mussinan et al. [34] used SDE with diethyl ether to isolate 33 pyrazines from pressure-cooked beef, whereas Ohnishi and Shibamoto [35] used SDE with dichloromethane to identify 112 compounds in heated beef fat. Raes et al. [36] also used SDE with dichloromethane and showed that they could distinguish four breeds of cattle by the volatile compounds from their grilled steaks. Ramarathnam et al. [37] using pentane as extracting solvent, showed that the aroma extracts of cured beef, pork, and chicken contained far lower levels of lipid oxidation products than their uncured equivalents. Madruga et al. [38] studied the effects of castration and age of slaughter on goat meat aroma, extracting with pentane:ether (9:1) for 2 h and identifying 108 volatile compounds.

Although SDE has been used for many years, it has several advantages compared to other commonly used extraction techniques, such as those involving headspace collection on a polymer, followed by heat desorption into a GC injection port. Efficient stripping of volatiles from foods allows quantitative recoveries to be achieved for many compounds [39]. The aroma extract is obtained in a solvent; therefore, many injections can be performed from one extraction. Hence, one sample could provide material for GC, GC-MS, and quantitative GC-olfactometry (GC-O) techniques, such as CharmAnalysis™ (Datu Inc., Geneva, New York) and AEDA [40]. Fraction-ation of the extract can be carried out, resulting in increased separation of the components in the extract, facilitating the identification of minor components of the extract. Werkhoff et al. [41] used medium-pressure liquid chromatography to separate and identify numerous sulfur compounds in cooked beef, pork, and chicken. Gasser and Grosch [2] washed an SDE extract of cooked beef with acidic and basic buffer solutions to obtain three fractions. The neutral fraction was fractionated further, using silica gel chromatography. The same authors used similar methodology [3] to identify the key odorants of cooked chicken.

As with all aroma extraction techniques, SDE has drawbacks. When the extract is concentrated, by distilling off the solvent, low-boiling volatile compounds can be lost. These compounds include some present at high levels in headspace extracts of cooked meat, such as 2-butanone, 2-pentanone, 2- and 3-methylbutanal, diacetyl, 1-propanol, and 1-penten-3-ol. Artifacts can be formed as a result of the high temperatures used. Mottram and Puckey [42] found 2-methyl-3-nitro-2-butanol and 2-methyl-3-nitro-2-butyl nitrate in an SDE extract of bacon. These compounds were formed from the reaction between 2-methyl-2-butene, a solvent impurity, and breakdown products from nitrite, the curing agent. In addition, volatiles can be generated when samples are overcooked during extraction, for example, enhanced lipid oxidation [43].

If SDE is carried out under reduced pressure, thermal degradation of labile components can be diminished. By maintaining the system under a static vacuum, sample loss is reduced and higher boiling solvents, such as heptane and octane, can be used. In fact, extraction at room temperature is possible. An excellent discussion of the SDE technique is available in Ref. 39.

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