Isolation of aroma compounds

Engel, W., Bahr, W., Schieberle, P. (1999) Solvent assisted flavour evaporation—a new and versatile technique for the careful and direct isolation of aroma compounds from complex food matrices. Eur. Food Res. Technof 209 237-241. 

It should be no surprise that the methods used to produce most flavourings from plant sources are based on similar principles as those used in the isolation of aroma compounds from foods. However, economics and scale play major roles in dictating methods. Additionally, the physical characteristics of the plant material, and concentrations and properties of flavouring materials also... 

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. 

The availability of and improvement in membranes has rekindled some interest in dialysis in aroma research. Benkler and Reineccius (19, 20) initially published studies on the use of Nafion (Dupont) membranes for the separation of fat from flavor isolates. This would permit solvent extraction to be used in the isolation of aroma compounds from fat containing foods. Chang and Reineccius (21) later used a continuous tubular counter current flow system to accomplish this fat/aroma separation more efficiently. These membranes can be obtained commercially and have been improved in terms of membrane thickness and purity. While the aroma isolate obtained using this membrane may not perfectly reproduce the aroma being studied, this is an alternate technique for aroma isolation. 

Isolation of aroma compounds from wheat and rye samples prior to determination using isotope dilution methods. 

FIGU RE 3.7 Solvent assisted flavor evaporation system for the isolation of aroma compounds from solvent extracts. (From Werkhoff, R, S. Brennecke, W. Bretschneider, H.J. Bertram, Flavor, Fragrance, and Odor Analysis, R. Marsili, Ed., Marcel Dekker, New York, 2002, p. 139. With permission.)... 

Many factors affect the volatile composition of fruit and vegetables, e.g. genetics, maturity, growing conditions and postharvest handling. Furthermore, preparation of the fruits and vegetables for consumption and the method for isolation of volatile compounds may change the volatile profile and key aroma compounds compared to non-processed fruits and vegetables. 

Solvent extraction is an excellent choice for aroma-compound isolation from foods when applicable. Unfortunately, many foods contain some lipid material, which limits the use of this technique since the lipid components would be extracted along with the aroma compounds. Alcohol-containing foods also present a problem in that the choice solvents (e.g. dichloromethane and diethyl ether) would both extract alcohol from the product, so one obtains a dilute solution of recovered volatiles in ethanol. Ethanol is problematic since it has a high boiling point (relative to the isolated aroma compounds), and in concentration for analysis, a significant proportion of aroma compounds would be lost with the ethanol. As one would expect, the recovery of aroma compounds by solvent extraction is dependent upon the solvent being used, the extraction technique (batch or continuous), and the time and temperature of extraction. 

A similar apparatus has been used for recovery of aroma compounds from cacao during processing [34]. In this process, water and acetic acid are removed from the aroma-laden gas stream by the initial traps and then the gas is passed through traps of the same design as those described by Cams and Tuot [29]. The aroma isolate so provided is suggested to be useful for the flavouring of soluble cocoa beverages, cake mixes, and confectionery products. 

If one is considering the recovery of aroma compounds from waste gas streams, one should investigate the pollution-control literature. There are a large number of patents and scientific articles that deal with this issue. The techniques used are generally aimed at the removal of trace volatiles in air streams and are potentially suited to aroma recovery. The primary consideration is whether the techniques yield an isolate safe for human consumption. 

The world of aroma compounds is becoming more and more complex. In the early days people used aromatic products like fruit juices or fruit juice concentrates which were relatively weak and still close to the related foodstulf. Later, with more knowledge of separation techniques, infusions, extracts, oleoresins and absolutes ranging from weak to strong impact were used to impart aroma. Essential oils such as spice oils already had a very strong impact. Modern analytical technologies allowed the evaluation of the chemical compositions of extracts and essential oils, so that isolates either as powerful mixtures or even as single compounds could be obtained. 

The analysis of aroma compounds starts with the isolation of the volatile fraction from the food. Techniques used in the preparation of flavor extracts from foods have recently been reviewed [7-9], The most important task in the choice of the isolation procedure is to test whether the flavor of the extract is identical or at least similar to the flavor of the food itself. This has to be checked by a sensory evaluation of the food extract (e.g., after dilution with an appropriate medium like water or oil) before a time consuming chemical analysis is performed. 

Faraday <1800 1825 Isolation of natural compounds with various aromas Isolation of benzene (illuminating gas condensate)... 

In Figure 3, as in Figure 2, samples isolated from oats with 7.4% and 8.3% lipid content were different with regard to chemical composition. Since the oil itself may play several roles, for example as generator of aroma compounds as well as solvent for other volatile compounds, it is of interest to follow the aroma pattern in the abovementioned samples. The amount of heterocycles decreased in most cases when the initial oat lipid content increased. Compounds such as pyrazine derivatives (methyl, 2,5-dimethyl, 2,6-dimethyl, 2-ethyl-5-methyl, trimethyl, 2,5-methyl-3-ethyl), furfural, 5-... 

Techniques for Further Isolation and Identification of Aroma Compounds. 403... 

Lee, K.G. and Shibamoto, T., Antioxidant properties of aroma compounds isolated from soybeans and mung beans, J. Agric. Food Chem., 48, 4290, 2000. 

Although the number of aroma compounds derived from acyclic carotenoids is much inferior to that of the mono- and bicyclic compounds, some of them can also be considered as breakdown products from genuine mono-, sesqui- and diterpenoids. The importance of the aliphatic isoprenoids (282) to (291) in the formation of total flavors of certain foodstuffs is not less than that of the cyclic compounds, the three methyl ketones (282), (287) and (290) which are related to the main tomato pigment lycopene were observed in tomato flavor (75). The hexahydro derivative (291) from coffee (595), jasmine oil (722) and green tea 438) is perceived as flowery and warm and can be considered as an oxidative biodegradation product of phytol and phyta-diene. 6-Methyl-3,5-heptadien-2-one (283), with a grassy and cinnamonlike aroma 438) [detected in tomato 668), the essential oil of Hama-metis leaves 383), Ceylon tea (722) and passion fruit (777)], and pseudo-ionone (288) [also isolated from passion fruit (777)] are believed to be formed from two different dehydrolycopenes. Compounds other than carotenoids, such as solanesol or squalene, can also be considered... 

In general, three conventional methods were used for the extraction of bioactive compounds such as solvents, steam, and supercritical fluids. On a global level, water extraction is practised while making cofiee or tea. Basically, pretreated plant material is extracted with hot water which takes up the flavor, taste, and color of the components. After filtration, the extract is ready for consumption. In case of the isolation of certain bioactive compounds from plant material by means of liquid extraction, some technological problems needs to be resolved [3]. First the plant material has to be pretreated in order to obtain reasonable extraction yields. Another problem is the need for special solvents to be used in the extraction procedure [4]. More recently, attention has been focussed towards the isolation of specific compounds that can be used in the food industry. Of particular interest is the isolation of bioactive compounds, aromas, and fiiagrances from plants and fruits [5,6]. The sequential extractions of bioactives using nonpolar to polar solvents are depicted in Figure 7.1. Various polarity solvents are reported as follows (1) nonpolar solvents (hexane, heptanes, petroleum ether,... 

As was noted above, most aroma compounds tend to be lipophilic. Log P values for a range of aroma compounds found in foods are presented in Table 3.1. (Log P as defined here is the logarithm of the oikwater partition coefficient and relates to an octanoliwater systan. The values presented are calculated as opposed to experimentally determined.) If one considers the Log P values presented in Table 3.1, one notices that the vast majority of these aroma compounds will partition into the oil phase (i.e., the organic solvent in an extraction). Thus, solvent extraction of a food can be a very effective means of preparing an aroma isolate for study. The weaknesses of this method include the obvious factor that different aroma compounds have different partition coefficients and thus, will be extracted to varying extents in the extraction process. Certainly, the more water-soluble components such as diacetyl... 

A nonequilibrium isolation method (e.g., dynamic headspace) depends upon many of the same factors as the equilibrium method discussed above, and thus also poorly represents what is found in the food. However, additional considerations must be made for the nonequilibrium nature of the method. In nonequilibrium situations, the rate of release of aroma compounds from a food into a gas phase has to be considered. In this method, one may pass a gas stream through a food with the gas stream picking up aroma compounds under nonequilibrium conditions. The aroma compounds would be subsequently stripped from the gas stream to produce an aroma isolate. 

Some of the aroma isolation procedures discussed above produce dilute solutions of aroma compounds in an organic solvent (e.g., distillation and solvent extraction). 

Historically, modem aroma research began with the isolation and identification of aroma compounds in foods. It was thought that if we could identify all of the aroma compounds in foods, we would be able to reproduce the aroma of that food by formulating a flavor based on the analytical data. This did not prove to be the case. Researchers found that there were very large numbers of aroma compounds present in foods and not all could possibly be contributors to the aroma of a food. Thus, an era began where researchers attempted to determine which aroma compounds were traly needed to recreate the aroma of a food. It was postulated that somewhere between 20 and 30 compounds should be adequate to reproduce the aroma of a food. The question then was, which compounds were needed Several approaches were developed to meet this challenge. These methods will be briefly discussed. 

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