Food volatiles, isolation methods

The isolation of volatile flavor compounds from foods represents a major problem in einalytical studies of food flavor. Headspace methods are especially attractive since they are rapid, simple and measure what is typically presented to the nose. 

This chapter discusses some more recent variations of methods for isolation of volatiles fiom food and plant materials. For particular problems there are advantages to each of the three main types of isolation methods, direct extraction, steam distillation and dynamic headspace. Direct solvent extraction is the only method which is reasonably efficient in isolating components of both high and low water solubility. Because food and plant volatiles are usually water soluble at their ppm concentrations their isolation by steam distillation does not fit the theory s required non-miscible conditions and this may be better considered a type of dynamic headspace isolation. By atkpting ideas fiom a recently published direct solvent extraction metiiod, which used excess sodiiun fate to bind all water in aqueous foods, the authors discovered an effective dynamic headspace meAod for isolating Furaneol and other water soluble volatiles. 

A consideration of volatility as a means of aroma isolation requires an appreciation for the factors that influence the amount (or proportion) of an aroma compound in the gaseous phase vs. in the food at equilibrium (static headspace isolation methods) and nonequiUbrium conditions (dynamic headspace isolation methods). In both cases, our methodology requires that the aroma compound partitions into the gas phase for isolation. Considering equilibrium conditions first, the amount of an aroma compound in the gaseous phase is defined by the gas food partition coefficient (kg,). This can be most simply expressed as ... 

The primary interference with this basis of isolation is the water present in a sample. In most foods, the aroma components seldom make up more than 300 ppm (0.03%) of the product. Yet, the moisture content of a food, even a dry food, is generally above 2% and thus isolation methods based solely on volatility will produce a dilute solution of aroma componnds in water. The high boihng point of water precludes a simple concentration and analysis, i.e., the aroma compounds would be lost during concentration since they are present in low concentrations and are often more volatile than water. Thus, most dynamic headspace methods of aroma isolation involve some additional method to remove water from the isolate. 

It is relatively difficult to provide an exact quantitative analysis of compounds that are unstable and occur only in traces in foods. Serious errors are possible, particularly when the volatile flavor compounds are present in extremely small amounts, are unstable, and/or are reactive, and the workup conditions selected often result in considerable losses. It is known that quantitative results obtained for flavor compounds in foods can be significantly affected by the isolation method and the structure of the analyte. Quantitative analysis can readily give erroneous results, particularly when the internal standards that are used differ from the analytes in terms of their chemical and physical properties, as is usually the case. Although the method of quantifying volatile compounds with the aid of internal standards has been in use for quite some time, the literature repeatedly... 

The flavor compounds that are labeled with stable isotopes (isotopomers) differ only slightly from the analyte in terms of mass, and their physical and chemical properties—e.g., volatility, reactivity, distribution coefficient and chromatographic behavior—are the same as those of the unlabeled flavor compounds, with the exception of minor and negligible isotope effects. They are added to foods as internal standards as early as possible, namely before the first extraction, so that they undergo virtually the same losses as the flavor compounds to be studied during the isolation method and enrichment steps that are employed. For this reason, labeled compounds satisfy nearly all of the requirements for an ideal internal standard and can also tolerate workup methods with very low recovery percentages, provided that the detection sensitivity is not too low. 

Major emphasis in studies of N-nitroso compounds in foods has been placed upon volatile nitrosamines, in part because these compounds are relatively easy to isolate from complex matrices by virtue of their volatility. Procedures utilizing atmospheric pressure or vacuum distillation have been used by most investigators, with variations of the method of Fine e al. (2) being among the most popular. This procedure employs vacuum distillation of a mineral oil suspension of the sample with optional addition of water to improve nitrosamine recovery from low moisture content samples (6) The usual approach to prevention of nitrosamine formation during analysis involves adding sulfamic acid or ascorbate to destroy residual nitrite at an early stage of sample preparation. 

Concentration Methods. The GCMS analysis of an environmental sample starts with the isolation of the organic compounds from the matrix (air, water, food, etc.) into a form suitable for introduction into the GCMS instrument, typically a solution in a volatile solvent. This concentration step includes essentially three major methods vapor stripping, solvent extraction, and lipophilic adsorption. We have recently reviewed the detailed operation of these methods (Ij, (See also Bellar, Budde and Eichel-berger, this volume) but their general features will be outlined here. 

A contemporary of the method just described is the use of an absorbent (e.g. C-18) bonded onto granular or disk-type supports (solid-phase extraction [5]). The granular material is used in cartridge form (typically less than 5 ml), while disk forms are placed in a funnel/holder such as shown in Fig. 18.1b. A liquid (e.g. water, milk, or juice) would be passed through the cartridge (or filter disk), the analytes absorbed in the stationary matrix, the absorbent washed with water, and then the analytes of interest eluted from the absorbent with an organic solvent. This method has found limited use in the isolation of volatiles from foods but continues to find significant application in the analytical field overall [6]. 

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]. 

In retrospect, there are no totally new techniques for the isolation of thermally generated aroma compounds. The developments we have seen in recent years have been modifications of techniques which have existed for several years. As in the past, each method has its own unique strengths and weaknesses. The choice of method is determined by the food product to be analyzed, the volatiles of interest and the analytical methods to be appl ied. 

Two oat varieties were studied with respect to their oil content. The composition of these SCCO2 extracted oils, with regard to fatty acids, free fatty acids, phosphorus and thermal stability has previously been reported (Fors and Eriksson, submitted for publication 1988). Volatile compounds were isolated from the oat oils by molecular vacuum distillation. The fractions obtained were transferred to aqueous alkali and extracted by CH2CI2. The adjustment in pH was made to remove fatty acids which could otherwise interfere with the later work. Moreover, it is well established that many heterocycles are important flavor compounds in heated food items. These compounds are normally isolated in the basic fraction. The isolates were analysed by chemical and sensory methods. 

The flavor industry has introduced, over the years, methods of developing meat flavors by processing appropriate precursors under carefully controlled reaction conditions. As a result, meat flavors having a remarkably genuine meat character in the beef, chicken and pork tonalities are available for the food industry. It has repeatedly been stated that the Maillard reaction is particularly important for the formation of meat flavors. However, of the 600 volatile compounds isolated from natural beef aroma, only 12% of them find their origin in sugar/amino acid interactions and of these 70% are pyrazine derivatives. 

Numerous techniques have been developed to isolate and concentrate aroma compounds derived from lipids or other components, from other constituents than dairy foods. The most widely used methods are based on volatility and/or solubility. 

AEDA and SHA-0 are suitable tools for recognition of odor active compounds (13, 14), but the methods are afflicted with some simplifications no corrections were made for the losses of odorants during isolation procedure. By AEDA the complete amounts of the odorants present in the solvent extracts are volatilized during GC-0 and therefore ranked according to their odor thresholds in air, but the contribution of an odorant to the overall flavor in a food is strongly affected by its odor threshold in the food... 

Shimoda, M. and Shibamoto, T. (1990) Isolation and identification of headspace volatiles from brewed coffee with an on-column GC-MS method,/. Agric. Food Chem., 38(3), 802-804. 

Extraction of Essential Oils from Plants. Essential oils are aromatic substances widely used in the perfume industry, the pharmaceutical sector, and the food and human nutrition field. They are mixtures of more than 200 compounds that can be grouped basically into two fractions a volatile fraction, which constitutes 90-95% of the whole oil, and a nonvolatile residue, which constitutes the remaining 5-10%. The isolation, concentration, and purification of essential oils have been important processes for many years, as a consequence of the widespread use of these compounds. The common methods used are mainly based on solvent extraction and steam distillation. SFE has been used for the extraction of essential oils from plants, in an attempt to avoid the drawbacks linked to conventional techniques (57). Such is the case with the extraction of flavor and fragrance compounds, such as those from rose (58), rosemary (59), peppermint (60), eucalyptus (61), and guajava (62). The on-line coupling of the extraction and separation ietermi-nation steps (by SFE-GC-FID) has been proposed successfully for the analysis of herbs (63) and for vetiver essential oil (64). 

BUTTERY UNG Methods for Isolating Food and Plant Volatiles... 

An advantage of the direct extraction method (3-6) is that volatiles of both low and high water solubility are isolated in one operation. Other commonly used methods such as steam distillation or dynamic headspace are not effective in isolating highly water soluble compounds such as Furaneol or maltol fiom mostly aqueous food and plant materials whereas these compounds can often be isolated efficiently by direct solvent extraction. 

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