Thermal generation of aroma compounds

Thermal Generation of Aroma Compounds from Tea and Tea Constituents... 

Additional investigations are required to more fully understand the thermal generation of aroma compounds from tea. 

Reduction of desirable meat aroma remains as a serious impediment to addition of soy protein. When highly purified soy protein is added to ground patties, thermally generated meat aroma intensity is decreased. Adsorption of flavor compounds onto vegetable protein is a primary mechanism for this aroma loss (5-7). 

Meat flavor is due to a great number of volatiles from different chemical classes. However, most of the odorants described as meaty aroma contain sulfur. The two most important reactions which generate meaty aroma compounds are the reactions between sulfur containing amino acids and reducing sugars (Maillard reaction) and the thermal degradation of thiamin [35], Sulfur-containing furans are the basic chemicals responsible for the aroma of thermally treated meat. 

The thermal generation of flavor is a very essential process for the "taste" of many different foodstuffs, e.g. cocoa, coffee, bread, meat. The resulting aromas are formed through non-enzymatic reactions mainly with carbohydrates, lipids, amino acids (proteins), and vitamins under the influence of heat. Thiamin (vitamin B ) and the amino acids, cysteine and methionine, belong to those food constituents which act as flavor precursors in thermal reactions. The role of thiamin as a potent flavor precursor is related to its chemical structure which consists of a thiazole as well as a pyrimidine moiety. The thermal degradation of this heterocyclic constituent leads to very reactive intermediates which are able to react directly to highly odoriferous flavor compounds or with degradation products of amino acids or carbohydrates. 

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. 

Gas Chromatography—Matrix Isolation Infrared Spectroscopy—Mass Spectrometry for Analysis of Thermally Generated Aroma Compounds... 

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. 

It is demonstrated that a great many flavor compounds are formed in both model systems. On the other hand, phenylalanine formed by aldol condensations some special aroma products. Furthermore, the generation of thermal aroma compounds depend on the pH, the sugar/amino acid ratio and the temperature. 

We have carried out some model reactions on the formation of thermal aromas in order to test the conditions for the analysis of such aromas and to study the mechanisms of their formation and their dependence on concentration and temperature. Last but not least we were interested to get an overview about the compounds which can be formed by generation of thermal aromas. 

The thermal reaction of cystine and DMHF is important for the generation of meat flavors. The products of this reaction, their flavor compounds, aroma profiles and yields, however, vary according to the reaction parameters. This study focused on determining the effect of the reaction medium, duration, water content, temperature, pH and oxygen on the products of this reaction. 

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. 

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. 

Phenolic aroma compounds can be generated by the thermal radical degradation of phenolic acids such as ferulic acid (52), which is a constituent of many vegetable raw materials [76]. Fig. 3.32 shows the formation scheme for vinylguaiacol (53), vanilline (54) and guaiacol (55) from 52. 

Many aroma compounds generated biologically are unstable. Examples of this are mercaptans which can be oxidized to sulfides, and terpenes which can be thermally degraded. 

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... 

The Maillard reaction between reducing carbohydrates and amines is among the most important flavor generating reactions in thermally processed foods (5). Thus, it might be expected that in foods treated with HHP, but at low temperatures, some of the typical aroma compounds might not be formed. Only two studies about the influence of HHP on the formation of volatiles in Maillard model systems are currently available (6, 7). Bristow and Isaacs (d) reported that at 100°C, the formation of volatiles from xylose/lysine was generally suppressed when HHP was applied. Hill et al. (7) confirmed this observation for a glucose/lysine system. However, it has to be pointed out that the samples analyzed were not reacted in a buffered system and, also, the reaction time of the pressure-treated and untreated sample were not identical. 

Many biochemical reactions can be induced by temperature increase in foods Maillard reactions, vitamin degradation, fat oxidation, denaturation of thermally unstable proteins (resulting in variation of solubility or of the germinating power of grains, for example), enzyme reactions (which can either be promoted or inhibited), and so on. Some of these biochemical reactions generate components suitable, for example, for their sensory properties (flavor development) others may be more or less undesirable for nutritional or potential toxicity reasons (vitamin losses, changes in color, taste or aroma, formation of toxic compounds). All the reactions are linked to the simultaneous evolution of product composition, temperature and water content (or chemical potential, or water activity), these factors varying diflferently from one point to another, from the center to the surface of the products. 

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