Thermally generated volatile compounds

Thermally Generated Volatile Compounds in Packaging Materials... 

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. 

An integrated GC/IR/MS instrument is a powerful tool for rapid identification of thermally generated aroma compounds. Fourier transform infrared spectroscopy (GC/IR) provides a complementary technique to mass spectrometry (MS) for the characterization of volatile flavor components in complex mixtures. Recent improvements in GC/IR instruments have made it possible to construct an integrated GC/IR/HS system in which the sensitivity of the two spectroscopic detectors is roughly equal. The combined system offers direct correlation of IR and MS chromatograms, functional group analysis, substantial time savings, and the potential for an expert systems approach to identification of flavor components. Performance of the technique is illustrated with applications to the analysis of volatile flavor components in charbroiled chicken. 

Many of the aforementioned techniques are not appropriate to direct mass-spectrometric analyses of intact high-MW and heat-labile compounds. For such samples, thermal degradation techniques (analytical pyrolysis) can be performed to generate more-volatile compounds of lower molecular weight that are amenable... 

Most of the studies on the thermal degradation of carotenoids analyzed the volatile fraction, as the identification of nonvolatile fractions was probably more complex to analyze. A study was published recently on the volatile compounds generated by the thermal degradation of carotenoids in... 

Certain volatile elements must be analyzed by special analytical procedures as irreproducible losses may occur during sample preparation and atomization. Arsenic, antimony, selenium, and tellurium are determined via the generation of their covalent hydrides by reaction with sodium borohydride. The resulting volatile hydrides are trapped in a liquid nitrogen trap and then passed into an electrically heated silica tube. This tube thermally decomposes these compounds into atoms that can be quantified by AAS. Mercury is determined via the cold-vapor... 

Tenax TA 60-80 mesh Good thermal stability and desorption. Traps little water. Generates few artifacts. Limited adsorption and breakthrough of very volatile compounds For general volatile trapping. Among the most used adsorbents... 

Adsorbent choice. The choice of adsorbent material depends on the volatile compounds in the food. Of the synthetic porous polymers, the most widely used and best overall adsorbent is Tenax TA (poly-2,6-diphenyl-p-phenylene oxide) 60 to 80 mesh. While Tenax does not show an adsorption capacity for all volatiles, especially very small polar compounds such as acetaldehyde, it has good thermal stability and desorption capabilities. It also traps little water and generates very few artifacts. Table G1.2.2 shows a few limitations and advantages of various adsorbents, all of which can be purchased from chromatography suppliers. If very small volatiles are the goal, various Carbosieves could be used, or traps containing several adsorbents in series. Traps with mixed adsorbents should be desorbed immediately, before transfer between phases occurs. 

In addition to simple model systems, more complex systems which are closer to actual foodstuffs have been used to investigate the formation of flavor chemicals in the Maillard reaction. Sixty-three volatile chemicals were isolated and identified from starch heated with glycine (4). When beef fat was used as a carbonyl compound precursor in a Maillard model system with glycine, 143 volatile chemicals were identified (6). These included fifteen n-alkanes, twelve n-alkenes, thirteen n-aldehydes, thirteen 2-ketones, twelve n-alcohols, and eleven n-alkylcyclohexanes. Recently, the effect of lipids and carbohydrates on the thermal generation of volatiles from commercial zein was studied (7). 

Volatile Compounds in Ginger Oil Generated by Thermal Treatment... 

From our aroma research on boiled small shrimps, almost one hundred volatile components were identified. Among them, more than forty components were determined as sulfur- and/or nitrogen-containing heterocyclic substances, together with various kinds of volatiles that are well known to be thermally generated such as hydrocarbons, carbonyl compounds, alcohols and phenols. The shrimp... 

Various kinds of heterocycles and two unsaturated methylketones were identified as characteristic components in the volatiles from cooked small shrimps. Without exception, they were all thermally generated compounds. Some volatile components from cooked small shrimps were in common with those of other animal protein foodstuffs like meat however, various types of compounds found in another foodstuffs were composed of the volatiles from specific shrimp species. Both the precursors and the formation pathways for the typical aroma compounds have already been elucidated, even though it is difficult to explain the different constituents of the volatile components among shrimp species. In future, it will be necessary to investigate the key factors which define the possible pathway to form characteristic volatiles in each foodstuff. 

J. M. Ames and A. Apriyantono, Effects of pH on the volatile compounds formed in a xylose-lysine model system, in Thermally Generated Flavors Maillard, Microwave, and Extrusion Processes, T. H. Pariimenl, M. J. Morello, and R. J. McGorrin (eds), American Chemical Society, Washington, DC, 1994, 228-239. 

Several reports are available In literature regarding thermal decomposition of poly(ethylene oxide) not in flash pyrolysis conditions. One such report indicates that at temperatures between 324° C and 363° C, the polymer generates 9.7% of volatile compounds (at 25° C), 3.9% monomer with smaller amounts of CO2, formaldehyde, ethanol, and saturated C1-C7 compounds [4]. Another report indicates that at temperatures between 225° C and 250° C, the polymer generates CO2, HCHO,... 

The most important precursors for lipid oxidation are unsaturated fats and fatty acids like oleic (18 1), linoleic (18 2), linolenic (18 3) and arachidonic acid (20 4). The more unsaturated ones are more prone to oxidation. Lipid peroxidation and the subsequent reactions generate a variety of volatile compounds, many of which are odour-active, especially the aldehydes. That is why lipid oxidation is also a major mechanism for thermal aroma generation and contributes in a great measure to the flavour of fat-containing food. Lipid oxidation also takes place under storage conditions and excessive peroxidation is responsible for negative aroma changes of food like rancidity, warmed-over flavour, cardboard odour and metallic off-notes. 

The major precursors in meat flavors are die water-soluble components such as carbohydrates, nucleotides, thiamine, peptides, amino acids, and the lipids, and Maillard reaction and lipid oxidation are the main reactions that convert these precursors in aroma volatiles. The thermal decomposition of amino acids and peptides, and the caramelization of sugars normally require temperatures over 150C for aroma generation. Such temperatures are higher than those normally encountered in meat cooking. During cooking of meat, thermal oxidation of lipids results in the formation of many volatile compounds. The oxidative breakdown of acyl lipids involve a free radical mechanism and the formation of... 

Chen J, Wang M, Ho CT (1998) Volatile compounds generated from the thermal degradation ofN-acetylglucosamine. J Agric Food Chem 46, 3207-3209. 

Table I. Volatile Compounds Generated from the Thermal Reaction of WGH, DWGH and AWGH with Glucose...

The effect of lipids in the Maillard reaction has been studied by many authors who cooked or roasted mixtures of amino acids and reducing sugars in various vegetable oils. The thermal oxidative degradation of lipids generates lower molecules, for instance aldehydes, that contribute to the formation of heterocyclic volatile compounds. 

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