Aroma isolation

Reactions of thiols, amines, and aldehydes (cf 5.3.1.4) in the aroma concentrate 

Reduction of disulfides by reductones from the Maillard reaction 

At the low pH values prevalent in fruit, non-enzymatic reactions, especially reactions 4-7 shown in Table 5.6, can interfere with the isolation of aroma substances by the formation of artifacts. In the concentration of isolates from heated foods, particularly meat, it cannot be excluded that reactive substances, e. g., thiols, amines and aldehydes, get concentrated to such an extent that they condense to form heterocyclic aroma substances, among other compounds (Reaction 8, Table 5.6). 

In the isolation of aroma substances, foods which owe their aroma to the Maillard reaction should not be exposed to ten eratures of more than 50 °C. At higher tenqteratures, odorants are additionally formed, i. e., thiols in the reduction of disulfides by reductones. Fats and oils contain volatile and non-volatile hydroperoxides which fragment even at tenqteratures around 40 °C. 

An additional aspect of aroma isolation not to be neglected is the ability of the aroma substances to bind to the solid food matrix. Such binding ability differs for many aroma constituents (cf. 5.4). 

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Aroma isolation, 11 516-521 distillations for, 11 519 solvent extraction for, 11 518 Aroma perception, taste and, 11 522-523 Aroma therapy, 18 354 Aromatic-(poly)cycloaliphatic diphenols, interfacial condensation of, 23 723-724... 

Azeotropic and extractive distillation Distillation processes Extractive distillation(s) argon, 13 460 for aroma isolation, 11 519 atmospheric, 13 646 batch versus continuous, 3 780 of coal-tar naphthalene, 17 78-79 corrosion, 3 779-780 of crude oil, 12 401-402 13 593 debottlenecking, 13 521 in fatty acid neutralization, 22 740 favorable vapor-liquid equilibria, 3 778 feed composition, 3 778 general separation heuristics for, 22 316-317... 

Solvent exposure tests, on plastics, 79 583 Solvent extraction, 70 744-746, 781-782 advantage of, 70 746 for aroma isolation, 77 518 in food processing, 70 787 in hazardous waste management,... 

Despite its unsaturated nature, benzene with its sweet aroma, isolated by Michael Faraday in 1825 [1], demonstrates low chemical reactivity. This feature gave rise to the entire class of unsaturated organic substances called aromatic compounds. Thus, the aromaticity and low reactivity were connected from the very beginning. The aromaticity and reactivity in organic chemistry is thoroughly reviewed in the book by Matito et al. [2]. The concepts of aromaticity and antiaromaticity have been recendy extended into main group and transition metal clusters [3-10], The current chapter will discuss relationship among aromaticity, stability, and reactivity in clusters. 

Sampling and Analysis. A frozen slice of bread was cut in pieces and stacked in an enlarged sample flask of an aroma isolation apparatus according to MacLeod and Ames (74). Volatile compounds were trapped on Tenax TA and afterwards thermally desorbed and cold trap injected in a Carlo Erba GC 6000 vega equipped with a Supelcowax 10 capillary column (60 m x 0.25 mm i.d.) and a flame ionisation detector. Similar GC conditions were used for GC-MS identification of volatile compounds by dr. M.A. Posthumus (Dept. Organic Chemistry, VG MM7070F mass spectrometer at 70 eV El, 75). 

With this introduction, we will present an overview of some of the more commonly used methods for aroma isolation. The reader is encouraged to obtain a comprehensive review of the topic for more detail and is referred to [1-3]. 

Absorption methods (sorptive extraction) have become the method of choice for many researchers. They offer advantages of being rapid, solventless, automated, and reasonably sensitive and broad in isolation properties. However, they provide an aroma isolate that reflects the biases resulting from compound volatility and affinity for the absorbent matrix. 

One of the few properties all aroma compounds have in common is they must be volatile if they are not volatile, they cannot make a contribution to olfaction. With this said, there is a very broad range in volatility across aroma-active compounds so one obtains a disproportionately large proportion of very volatile compounds and lesser amounts of low-volatility compounds in all aroma isolates obtained based on this property. 

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. 

Fig. 18.2 Aroma-isolation techniques based on distillation. Left simultaneous distillation/extrac-tion right high-vacuum distillation with cryotrapping. (Reprinted with permission from [15]. Copyright 1998 American Chemical Society)...

It is very common to combine methods in obtaining aroma isolates. The simultaneous distillation/extraction method previously described is an example. Another popular combination method initially involves the solvent extraction of volatiles from a food and then high-vacuum distillation of the solvent/aroma extract to provide a fat-free aroma isolate. This technique is broadly used today to provide high-quality aroma extracts for numerous purposes. The apparatus used in solvent removal has been improved upon to reduce analysis time and efficiency the modified method is termed solvent-assisted flavour extraction (SAFE) [16]. 

The efficiency of the SCC is illustrated in comparison with a traditional distillation-based essence recovery unit in Table 18.2. One can see that the aroma volatiles are preferentially stripped from the infeed in the SCC compared with the situation in a single-stage evaporator. Thus, highly concentrated aroma isolates can be produced. Flavortech [22] noted that essences of 1,500-fold maybe produced from juices if a double pass is used (the infeed is the flrst pass and the acquired essence the second pass). 

While the first two examples of using adsorption methods to produce aroma isolates were from gas streams. Tan et al. [28] applied adsorption methods to the isolation of flavouring extracts from mushroom blanching water. Unfortunately, only an abstract was available of this work so it lacks detail. It appears that they evaluated the use of two different resins (not described) and ethanol, pentane, hexane, and other solvents for desorption. They claim to have had good success in obtaining a useful aroma isolate. 

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. 

For aroma extracts, the blank sample is a mixture of the solvents used in the extraction, and are concentrated in the same way as the aroma isolate. Some volatiles in aroma extracts may derive from trace impurities of the solvents. For headspace techniques, a blank run is also recommended to check impurities coming from the tubings and/or adsorbents used. 

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

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. 

Sensory Validation of Sampling and GC Techniques. The sensory evaluation was carried out by a panel of three judges (employees of Pebeyre Ltd.). For this study, an external odor port was attached to the gas vent of the D.C.I. system. After the thermal desorption of the volatiles from the trap, the rotary valve was positioned so that the unresolved aroma isolate went to our sniffing port. The response was mesured as similarity or dissimilarity to canned black truffle aroma. 

Aroma isolates of processed and unprocessed truffles which were assessed at our odor port were described as typical of the respective aromas. 

The headspace technique developed in the present study to isolate volatiles from canned black truffles performed satisfactorily. The aroma isolate obtained was described as typical, and 36 compounds were identified for the first time as canned clack truffle constituents. The formation of the major part of them could be correlated to the thermal treatment applied. 

The aroma isolation method and heat desorption techniques were therefore validated. 

The headspace sampling technique developed in the present study to collect volatiles from cold stored Black Truffles performed adequately. Indeed, the aroma Isolate obtained was described as typical, and 11 minor compounds could be described for the first time as Black Truffle aroma constituents. Moreover, these results allowed the formulation of the first Nature-Identical Black Truffle aromatizer. 

Willemsen JHA, Dijkink BH, and Togtema A. Organophilic pervaporation for aroma isolation—industrial and commercial prospects. Memb. Tech. 2004 2 5-10. 

A number of years ago, Weurman (2 ) reviewed techniques of aroma isolation more recently, a number of additional reviews on aroma research have appeared (3-11). 

Fruit samples. Fruits were picked from 3 year old bushes grown on a commercial fruit farm, and were used immediately for aroma isolation. Stems used for labelling experiments were obtained from the same bushes. 

Comparison of cultivars. The analysis of the monoterpene fraction of the aroma isolated from the three cultivars is given in table I. The analysis was carried out using both 0V-101 and Carbowax 20M... 

Willemsen, J.H.A., Dijkink, B.H., and Togtema, A., Organophilic pervaporation for aroma isolation-industrial and coimnercial prospects, Membr. TechnoL, February, 5-10, 2004. 

Relative percentage abundance of total aroma isolate... 

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