Volatile compound, Maillard reaction

Maillard reaction. We are sure, however, they do not represent artefacts, formed upon work up or during the analytical procedure. Several of the heterocycles have been described earlier as volatile constituents of mother s milk (Pellizari et al. 1982 cf. Figure 31.2 compounds 5, 6, 9, 13, 17, 19 among other unknown oxygen con-... 

The aroma volatiles produced in the Maillard reaction have been classified into three groups by Nursten [6], and this provides a convenient way of viewing the origin of the complex mixture of volatile compounds derived from the Maillard reaction in foods ... 

Many desirable meat flavor volatiles are synthesized by heating water-soluble precursors such as amino acids and carbohydrates. These latter constituents interact to form intermediates which are converted to meat flavor compounds by oxidation, decarboxylation, condensation and cyclization. 0-, N-, and S-heterocyclics including furans, furanones, pyrazines, thiophenes, thiazoles, thiazolines and cyclic polysulfides contribute significantly to the overall desirable aroma impression of meat. The Maillard reaction, including formation of Strecker aldehydes, hydrogen sulfide and ammonia, is important in the mechanism of formation of these compounds. 

Meat aroma is not the result of one chemical constituent but the sum of the sensory effects of many of these volatiles. Over 90% of the volume of volatile constituents from freshly roasted beef is from lipid, but approximately 40 percent of the volatiles from the aqueous fraction are thought to be heterocyclic compounds, many resulting from Maillard reaction products or their interactions with other ingredients. 

Even though many compounds discussed in the above presentation are thought to be important in meat flavor, a delicate blend of these compounds and other ingredients at the appropriate concentration is needed to synthesize acceptable flavor. In view of the possible instability of the flavor compounds themselves, precursors that supply the precise mixture of volatiles upon heating will be needed. Attempts have already been made to use this approach as judged by the numerous patented mixtures of precursors listed in the literature. More effort should be given to the quantitative aspects of meat flavor production and work must be continued on the qualitative aspects of the volatiles and the appropriate Maillard reaction precursors chosen. 

Sensory Properties of Volatile Maillard Reaction Products and Related Compounds... 

The volatile compounds formed by the Maillard reaction are only one group of flavor compounds in foods. Schutte (1) presents a brief summary of the major classes and their modes of formation from precursors. Some of them can be formed by different pathways. An example is the furans, which can be formed by non-enzymatic browning reaction but also by biotransformation. 

It becomes clear that analytical methods based on the evaluation of the end products of deteriorative reactions will not be satisfactory. Therefore in our own experiments amino acid analysis of Amadori compounds and gas chromatography of volatile Strecker aldehydes were applied to detect the onset of the Maillard reaction well before detrimental sensory changes occurred. 

Evershed, R. P., Bland, H. A., van Bergen, P. F., Carter, J. F., Horton, M. C., and Rowley-Conwy, R. P. (1997). Volatile compounds in archeological plant remains and the Maillard reaction during the decay of organic matter. Science 278,432-433. 

Even though the occurrence of the Maillard reaction was recognized at the beginning of this century (1), volatile flavor compounds were not isolated and... 

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). 

Pyrazines are the major volatile flavor chemicals produced in Maillard reactions. The discovery of this role of pyrazines was one of the most significant advances in flavor chemistry and two comprehensive reviews of pyrazines have appeared (25, 26). In the 1970 s, pyrazines were well-characterized as the compounds which directly contribute to roasted or smoky flavors. Some pyrazines possess an extremely low odor threshold (25, 29). For example, odor threshold of 2-isobutyl-3-methoxypyrazine in water is 0.002 ppb. 

The Maillard reaction commonly occurs in food products and during food processing. A typical or pure Maillard reaction is simply the reaction of a sugar and an amino acid. Strictly speaking, the sugar must be a reducing carbohydrate and the amino acid can be either free or bound, as a peptide or protein. The reaction generates not only volatile compounds, which provide odor, but also odorless nonvolatile compounds, some of which are colored. 

The sensory properties of nearly 450 volatile Maillard reaction products and related compounds have been compiled (45). The review includes quantitative aroma and flavor descriptions, as veil as sensory threshold values for different media, classified according to chemical structure. 

Most of the original patents referring to meat flavors utilizing Maillard technology vere claimed by Unilever (48-52 56,57). More recent patents are involved with the production of meat-like flavors. While a majority of patents are concerned vith cysteine, cystine, or methionine as the sulfur source, others claim alternatives such as mercaptoacetaldehyde, mercaptoalkamines, etc. Several patents (53,54), declare the contribution to meat-like flavors produced from thiamine in the Maillard reaction. Alternately, a technical report describes the volatile flavor compounds produced by the thermal degradation of thiamine alone (55). 

N, S and 0 heterocyclic compounds, along with noncyclic sulfur compounds and hydrocarbons, are predominant in "meaty" flavor volatiles. The mechanisms of heterocyclic formation by Maillard and pyrolysis reactions have been reviewed by Vemin and Parkanyi (57) and the Maillard reaction itself is a recurring subject of review (58). Since other speakers contributing to this volume will discuss these aspects of meat flavor, they will not be repeated in this presentation. 

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. 

In heated foods the main reactions by which flavors are formed are the Maillard reaction and the thermal degradation of lipids. These reactions follow complex pathways and produce reactive intermediates, both volatile and non-volatile. It has been demonstrated that lipids, in particular structural phospholipids, are essential for the characteristic flavor development in cooked meat and that the interaction of lipids with products of the Maillard reaction is an important route to flavor. When model systems containing amino acids and ribose were heated in aqueous buffer, the addition of phospholipids had a significant effect on the aroma and on the volatile products. In addition a number of heterocyclic compounds derived from lipid - Maillard interactions were found. The extent of the interaction depends on the lipid structure, with phospholipids reacting much more readily than triglycerides. 

The volatiles from cooked meat contain large numbers of aliphatic compounds including aldehydes, alcohols, ketones, hydrocarbons and acids. These are derived from lipids by thermal degradation and oxidation (J7) and many may contribute to desirable flavor. In addition, the aldehydes, unsaturated alcohols and ketones produced in these reactions, as well as the parent unsaturated fatty acids, are reactive species and under cooking conditions could be expected to interact with intermediates of the Maillard reaction to produce other flavor compounds. 

Effect of Phospholipids on Reaction Volatiles. As would be expected, the inclusion of phospholipids in the reaction mixtures produced many volatiles derived from lipid degradation these included hydrocarbons, alkylfurans, saturated and unsaturated alcohols, aldehydes and ketones. However, two other important observations were made. First, the concentrations of most of the hetero- cyclics, formed by the amino acid + ribose Maillard reaction, were reduced. For most of the major volatiles this reduction was of the order of 40 - 50%, but in the case of thiophenethiol and methyl- furanthiol the reduction was over 65%. This appears to support the findings that in meat and coconut, lipids exert a quenching effect on the amount of heterocyclic compounds formed in Maillard reactions during heat treatment (11,12). Second, and perhaps more important, the addition of phospholipid to the reaction mixtures resulted in the production of large amounts of compounds derived from the interaction of the lipid or its degradation products with Maillard reaction intermediates. 

Volatile compounds generated by model systems of zeln, corn amylopectin and corn oil extruded at barrel temepratures of 120°C and 165°C were analyzed by GC and GC/MS. The largest quantities of lipid oxidation products were detected in systems containing all three components. In each system, the quantity of 2,4-deca-dienal was low relative to the quantities of hexanal, heptanal and benzaldehyde. Identification of the Maillard reaction products, 2-methyl-3(or 6)-pentyl-pyrazine, 2-methyl-3(or 6)-hexylpyrazine and 2,5-di-methyl-3-pentylpyrazine, suggested that lipid-derived aldehydes might be involved in the formation of substituted pyrazines. 4-Methylthiazole was identified as a major decomposition product of thiamin when corn meal containing 0.5% thiamin was extruded at a final temperature of 180°C. 

Of the different types of lipids in foods, the phospholipids, being more unsaturated, are particularly important in relation to aroma formation in meat.151 The aroma of cooked meat was not affected by the prior extraction of triglycerides with hexane, but the use of a more polar solvent (chloroform-methanol), which extracts all lipids, including phospholipids, resulted, after cooking, in the replacement of the meaty aroma by a roast or biscuit-like one. This was reflected in the volatiles, the dominant aliphatic aldehydes and alcohols being replaced by alkylpyrazines. This implies that the participation of the lipids in the Maillard reactions inhibited the formation of heterocyclic compounds. 

Flavour is a complex sensation, made up principally of smell and taste, but touch and hearing contribute as well. The human senses of smell and taste differ in sensitivity, between each other and depending on the nature of the component eliciting the sensation. Substances may have no impact at all (such as oxygen or carbon monoxide) or exhibit very low thresholds (such as 2 X 10 14g mL 1 water for the odour of bis-2-methyl-3-furyl disulfide).204 In general, odour thresholds are much lower than taste thresholds and so flavour tends to be dominated by odorous components, by substances able to reach the olfactory epithelium high up in the nose, that is, substances with at least some volatility. Hence, the emphasis on the volatile compounds derived from the Maillard reaction. 

The formation of volatile compounds by the Maillard reaction has been most recently reviewed by Mottram152 and Tressl and Rewicki.30... 

C.-T. Ho and J. Chen, Generation of volatile compounds from Maillard reaction of serine, threonine, and glutamine with monosaccharides, in Flavor Chemistry Thirty Years of Progress, R. Teranishi, E. L. Wick, and I. Homstein (eds), Kluwer/Plenum, New York, 1999, 327-333. 

K. Yanagimoto, K. G. Lee, H. Ochi, and T. Shibamoto, Antioxidant activity of heterocyclic compounds found in coffee volatiles produced by Maillard reaction,./. Agric. Food Chem., 2002, 50, 5480-5484. 

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