Sulfur Maillard reactions

Amino Acid Content. Amino acid content of field pea products is related to protein level, method of processing, and fraction (starch or protein). The protein fraction contains fewer acidic (glu, asp) amino acids than the starch fraction and more basic (lys, his, arg) amino acids than the starch fraction. Also, there are more aromatic (tyr, phe) amino acids, leu, iso, ser, val, and pro in the protein fraction than in the starch fraction (5). An amino acid profile of pea protein concentrate shows relatively high lysine content (7.77 g aa/16 g N) but low sulfur amino acids (methionine and cystine) (1.08-2.4 g aa/16 g N). Therefore, it is recommended that air classification or ultrafiltration be used because acid precipitation results in a whey fraction which contains high levels of sulfur amino acids (12,23). Also, drum drying sodium proteinates decreases lysine content due to the Maillard reaction (33). 

Many nitrogen- and sulfur-containing heterocycles have been identified in the aroma fractions of foods [214]. In roasted products (e.g., coffee) and heat-treated foods (e.g., baked bread or fried meat), these heterocycles are formed from reducing sugars and simple or sulfur-containing amino acids by means of Maillard reactions [215, 216]. Their odor threshold values are often extremely low and even minute amounts may significantly contribute to the aroma quality of many products [217, 218]. Therefore, N- and N,S-heterocyclic fragrance and flavor substances are produced in far smaller quantities than most of the products previously described. 

Cysteine can be obtained by hydrolysis from cysteine-rich proteins in hair or feathers or from petrochemical sources. Cysteine is an important raw material in Maillard reactions for the preparation of process flavours, but it can also serve as a source of ammonia and hydrogen sulfide for the preparation of flavour chemicals, such as the terpene sulfur compounds mentioned in Sect. 13.2.4 and furfuryl mercaptan mentioned in Sect. 13.4.2.4. 

Evers, et al. (34) identified several S-substituted furans having meaty aroma including 3-mercapto-2-methylfuran and 3-mer-capto-2,5-dimethylfuran from Maillard reaction mixtures. These compounds were readily oxidized to sulfides, some of which retained meaty odors. All furans having the sulfur atom bound to the 8-carbon had meaty aromas, whereas those with sulfur bound to the a-carbon had hydrogen sulfide-like odors. 

Maillard reaction products formed by interaction of reducing sugar and amino acids such as a-dicarbonyl compounds, aldehydes, hydrogen sulfide, and ammonia can react further to form derivatives that have been identified from meat or its components during heating. Important reviews of sulfur compounds that might be produced by these reactions have been published by Schutte (35) and... 

It is precisely to the production of meatlike flavors that the great majority of patents based on the Maillard reaction have been directed. Mos of them indicate cysteine or cystine as the essential sulfur-containing compound. Other patents claim alternative sources for sulfur, e.g., derivatives of mercaptoacetaldehyde (36), mercaptoalkylamines (37), S-acetylmercaptosuccinic acid (38), 2-thienvltetrasulfide (39), "a sulfide" (40), and hydrogen sulfide (heated with aqueous xylose without any amino acid) (41). 

The Maillard reaction has received much attention since the 1950 s as the source of flavor chemicals in cooked foods. Numerous compounds produced by this reaction have been reported in the last two decades. The major flavor chemicals are nitrogen- and sulfur-containing heterocyclic compounds. For example, nitrogen-containing pyrazines contribute a characteristic roasted or toasted flavor to cooked foods. Sulfur-containing thiophenes and thiazoles give a characteristic cooked meat flavor. A striking property of these compounds is their extremely low odor thresholds. 

Cystine and other sulfur-containing amino acids are recognized as important precursors of food flavors, especially meat flavors (3, 11-12). DMHF, a cyclic-a-dicarbonyl, possesses a sweet, caramel and fruity aroma (13). It is found in many food sources (14-17) and is used extensively in many flavor applications (18-19). DMHF can be formed from sugar via either sugar enolization (caramelization) by a Maillard reaction then cyclization (20). 

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. 

Heterocyclic aroma compounds found in meat primarily arise from interactions between mono- and dicarbonyl compounds, H2S and ammonia. The carbonyl compounds are derived from the Maillard reaction, including Strecker degradation of amino acids, oxidation of lipids and aldolization reactions. H2S is produced by thermal degradation of sulfur amino acids and ammonia by amino acid pyrolysis. 

Heterocyclic compounds are dominant among the aroma compounds produced in the Maillard reaction, and sulfur-containing heterocyclics have been shown to be particularly important in meat-like flavors. In a recent review, MacLeod (6) listed 78 compounds which have been reported in the literature as possessing meaty aromas seven are aliphatic sulfur compounds, the other 71 are heterocyclic of which 65 contain sulfur. The Strecker degradation of cysteine by dicarbonyls is an extremely important route for the formation of many heterocyclic sulfur compounds hydrogen sulfide and mercaptoacetaldehyde are formed by the decarboxylation and deamination of cysteine and provide reactive intermediates for interaction with other Maillard products. 

As with the Maillard reaction, odorous compounds are formed, water and carbon dioxide are liberated, the pH drops during the reaction, colour formation is markedly increased by increasing the pH, oxygen has only a slight enhancing effect on colour production, and the reaction is inhibited by sulfur dioxide. 

Although sulfite occupies most of the stage in terms of modifying the Maillard reaction, other sulfur compounds have an effect too for example, the use of N-acetylcysteine to reduce burnt off-flavours in canned liver sausage272 (see Chapter 5). 

In Chapter 13, six main ways of inhibiting the Maillard reaction in foods received attention refrigeration, the use of sulfur dioxide, lowering of pH, adjusting aw, and removing one or other of the reactants. Four of these are clearly not appropriate for in vivo systems, which normally cannot be refrigerated, have their pH or aw adjusted, or be exposed to sulfur dioxide. The reactants (and intermediate products derived from them) therefore become the main targets. 

This chapter deals with the Maillard reaction, the oxidation of sulfur-containing amino acids, isopeptide bonds, alkaline treatments, the interaction of proteins with polyphenols, and the oxidation and heat treatment of tryptophan. 

Beside the conversion of endogenic sulfur compounds the addition of S-compounds like sulfite, as an antimicrobial agent, antioxidant and enzyme inhibitor (75) or like thiamine (vitamin Bl), as a nutrient for yeasts, are allowed in the EEC (within defined maximum values). Furthermore chemical reactions like sulfite addition to aldehydes, Maillard reaction or Strecker degradation play an important role with regard to the sulfur chemistry of wines. 

Apart from sotolon, the other compounds in Fig. 5 can be explained as the products of a Maillard reaction, and their carbon skeletons simply originate from the active Amadori intermediate in other words, they still preserve the straight carbon chain structure of monosaccharides. In spite of being a simple Cg lactone, sotolon has a branched carbon skeleton, which implies another formation process in the Maillard reaction. Sulser e al.(6) reported that ethyl sotolon (ll) was prepared from threonine with sulfuric acid, and that 2-oxobutyric acid, a degradation product of threonine, was a better starting material to obtain II. This final reaction is a Claisen type of condensation, which would proceed more smoothly under alkaline conditions. As we(lO) obtained II from 2-oxobutyric acid (see figure 6) with a high yield in the presence of potassium carbonate in ethanol, a mixed condensation of 2-oxobutyric and 2-oxo-propanoic (pyruvic) acids was attempted under the same conditions, and a mixture of sotolon (22% yield) and II were obtained however, the... 

The Maillard reaction plays an important role in flavor development, especially in meat and savory flavor (Buckholz, 1988). Products of the Maillard reaction are aldehydes, acids, sulfur compounds (e.g., hydrogen sulfide and methanethiol), nitrogen compounds (e.g., ammonia and amines), and heterocyclic compounds such as furans, pyrazines, pyrroles, pyridines, imidazoles, oxazoles, thiazoles, thiophenes, di- and trithiolanes, di- and trithianes, and furanthiols (Martins et al., 2001). Higher temperature results in production of more heterocyclic compounds, among which many have a roasty, toasty, or caramel-like aroma. 

Although they are not methods of preparative value in a chemical sense, cysteine, cystine, N-acetylcysteine, 4-thiazolidinecarboxylic acid and cysteine methyl ester when heated in soybean oil at 200 °C produce many sulfur-containing heterocycles among which are found vanishingly small amounts of various 1,2,4-trithianes <76MI 620-01). The reaction is termed the Maillard reaction and is widely employed in laboratories in the flavor industry. 

According to Rizzi (2000) furfuryl mercaptan (1.128) and related compounds are readily formed by reactions of furfuryl alcohols, formed in Maillard reaction, and sulfur-containing amino acids at acid pH. 

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