Food volatiles, heterocyclic compounds

Mottram, D.S. 1985. The effect of cooking conditions on the formation of volatile heterocyclic compounds in pork. J. Sci. Food Agric. 36 377-382. 

F. B. Whitfield, D. S. Mottram, S. Brock, D. J. Puckey, and L. J. Salter, Effect of phospholipid on the formation of volatile heterocyclic compounds in heated aqueous solutions of amino acids and ribose, J. Sci. Food Agric., 1988, 42, 261-272. 

Velisek J., Davidek J., Cuhrova J. and Kubelka V. (1976) Volatile heterocyclic compounds in the reaction of glyoxal with glycine. J. Agric. Food Chem. 24, 3-7. 

Complex mixtures are produced by non-enzymatic browning reactions between thermally oxidized lipids and amines, amino acids and proteins (see Chapter 11.B.4). Interactions between aldehydes, epoxides, hydroxy ketones, and dicarbonyls with proteins cause browning that has been related with losses of lysine, histidine, and methionine. Schiff base formation results in polymerization to form brown macromolecules. Interactions between epoxyalkenals formed at elevated temperatures and reactive groups of proteins produce protein pyrroles polymers and volatile heterocyclic compounds. Much of the published research in this complex chemical area was based on model systems. More stmctural information is needed however with real foods subjected to frying conditions. 

Since proline already contains a pyrrolidine ring it provides a potential source of nitrogen heterocyclics in the MaiUard reaction, and a number of proline-con-taining model systems have been examined. Tressl et al. [32] identified more than 120 proline-specific compounds in the reaction of proline or hydroxypro-line with various sugars. These include pyrrolines, pyrroles, pyridines, indolines, pyrrolizines and azepines, but relatively few of the compounds have been identified among food volatiles. 

Heterocyclic compounds as meat volatiles have been reviewed recently by Ohloff and Flament (23) and by Shibamoto (24) More comprehensive coverage of these constituents as food flavorants is currently being published (Vernin, 25). 

More than 10,000 compounds have been identified as volatiles of foods. Of these, heterocyclic compounds are an important class, be cause of their exceptional sensory properties (1). Heterocyclic compounds contain one or more heteroatoms (0, S and/or N) in rings or fused ring systems. 

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. 

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. 

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. 

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. 

Heterocyclic compounds have been Identified as important volatile components of many foods. The odor strength and complexity of these compounds makes them desirable as flavoring Ingredients. 

For years researchers have investigated the sulfur compounds present in various foods. Cooked foods typically contain numerous sulfur compounds, especially heterocyclic compounds like thiazoles, thiophenes, thiazolines, etc. In 1986, Sha-hidi et al. (7) reported that 144 sulfur compounds had been identified in beef. Other heated food systems like bread, potato products, nuts, popcorn, and coffee also contain many sulfur compounds. Aliphatic thiols have been found in fruits, vegetables, dairy products etc., as well as in heated foods. No discussion of the occurrence of sulfur compounds in foods would be complete without mention of their major role in the various allium species. Indeed, more than half of the volatile compounds reported in garlic, onion, leek, and chive contain sulfur (2). Comprehensive reviews of the literature concerning the role of thiazoles, thiophenes, and thiols in food flavor through 1975 can be found in Maga s series of review articles (3-5). 

Additional minor volatile compounds are found in both frying oils and in fried foods, including cis,trans- and trans,trans-2,4-decaderived from oxidized linoleate and 2,4-heptadienal derived from linolenate. The isomers of 2,4-decadienal impart a desirable fried food flavor in fried potatoes when present in small amounts, but excessive amounts of this aldehyde would be expected to cause undesirable rancid flavors. Furfural compounds may be derived from interactions between food sugars and proteins. Minor amounts of sulfur compounds and nitrogen-containing heterocyclic compounds (methyl pyrazine and 2,5-dimethylpyrazine in potatoes) may originate from Maillard reactions (Chapter 11). 

The heterocyclic compounds containing O, N, or S, typically have very potent and often characteristic odors [22-24], They are essential to the flavor of some plants and are key volatiles in defining the flavor of many thermally processed foods. Examples of members of this class of aroma compounds have been presented in Chapter 5. 

Ammonia, volatile amines imines and amides and, in particular, heterocyclic compounds containing nitrogen, have certain importance as flavour-active substances in non-acidic foods. 

Amino acids may also undergo thermal degradation, which is almost always coupled with some other food components, particular sugars. The major types of volatile compounds formed from amino-sugar interactions include Strecker degradation aldehydes, alkyl pyrazines, alkyl thiazolines and thiazoles and other heterocycles [35, 36]. As the subject has mainly relevance for baked and roasted vegetable food products, this subject will not be discussed in further detail. 

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