Methionine methional

Other examples are glycine — formaldehyde, alanine — acetaldehyde, valine — isobutyraldehyde, phenylalanine — phenylacetaldehyde, and methionine — methional (106). Products such as dried skim milk, dried eggs, and dehydrated vegetables and fmits are particularly susceptible to deteriorative flavor changes ascribed to this reaction (Table 10). 

Methionine Methional, methane-thiol, 2-propenal Vegetable-hke aromas... 

Strecker aldehydes are a frequent source of off-flavors in fermented products. Development of off-flavors in oxidized white wines typically marks the end of shelf life. Methional (3-methylthiopropionaldehyde) was identified as producing a cooked vegetables off-flavor character in a young white wine that had undergone spontaneous oxidation (101). Methional levels increased in wines spiked with methionol or methionine, suggesting its formation via direct peroxidation or Strecker degradation of methionine. Methional was recently demon-... 

Methionine can be obtained from enzymatic protein hydrolysates or from petrochemical sources. To a lesser extent than cysteine, it is a raw material in Maillard reactions for the preparation of process flavours and it can also be utilised as a precursor for the chemical preparation of the sulfide methional, which is an important flavour constituent for potato, malt, seafood and many other flavours. Methional can be reduced to methionol, which can be esterified with organic acids to, for instance, methionyl acetate and methionyl butyrate, which are useful compounds for pineapple and other fruit flavours (Scheme 13.16). 

Methional, formed by the degradation of the amino acid methionine, has been reported (Patton 1954 Velander and Patton 1955) to be the principal contributor to the activated flavor. Samuelsson (1962) reported, in studies of dio- and tripeptides containing methionine, that irradiation did not result in any hydrolysis of the peptides, and the... 

The methyl-branch is introduced at the C-3 position of tautomer 11 of the keton 9 by an electrophilic.attack with active methione (S-methyl-5 -adenosyl methionine). Thus the 2,6-dideoxy-4-keto-3-C-methyl-D-ery//iro glycoside 12 results which after reduction furnishes, e.g., D-mycarose (cf. also Ref. [5]). 

Maillard products which were identified in methionine/ reducing sugar model experiments, result predominantly from the Strecker aldehyde (methional) and methylmer-captan, respectively. Figure 10 summarizes compounds... 

Sulfur Compounds of Beef Flavor. Methional, which results from the degradation of methionine, is an important contributor to flavor in meat. Thiolanes, formed during the cooking of beef, have peculiar oniony flavors that also augment the quality of the meaty flavor. Thiophenes and thiofurans are also important to meaty flavors. Sulfides, such as methyl sulfide, are oxidized to methyl sulfoxide and methyl sulfone. Condensation reactions of Maillard browning products also result in thiazoles such as benzothiazole, an important component of meat flavor. 

Light-induced oxidation of proteins has been shown to lead to off-flavors and destruction of essential amino acids in milk. Patton (1954) demonstrated that sunlight attacks methionine and converts it into methional (( -methylmercaptopropionaldehyde), which can cause a typical sunlight off-flavor at a level of 0.1 ppm. It was later demonstrated by Finley and Shipe (1971) that the source of the light-induced off-flavor in milk resides in a low-density lipoprotein fraction. 

Methionine sulfoxide formation may occur without noticeable changes in physical or immunochemical properties of the protein. Thus reduction of sulfoxide to thioether often completely restores the lost protein function. Many cells, including human polymoprhonuclear neutrophilic leukocytes, contain enzyme methionine sulfoxide reductase, which is able to convert methionine sulfoxide to the reduced methione form in a variety of proteins (B25, F8). Methionine reacting with a strong oxidant effects methionine sulfone production, which in vivo is not reduced back to methionine. 

C. Lumiflavin and lumichrome edso catalyze oxidation of lipids (to Upid peroxides) md methionine (to methional), resulting in the development of an unpleasant flavor - the so-called sunlight flavor. 

Methanethiol results among others from the pyrolysis of methionine (Merritt et al., 1970). Like short-chain aliphatic thiols, it is also produced in model reactions involving cysteine and sugars, and also by heat treatment of methional (Q.22). 

Methional results from a Strecker degradation of methionine. It is at the origin of formation of methanethiol (Q.2), dimethyl disulfide (Q.14) and acrolein (C.16). 

Homogenization markedly reduces the propensity to oxidative rancidity, perhaps due to redistribution of the susceptible lipids and pro-oxidants of the MFGM (however, the propensity to hydrolytic rancidity and sunlight oxidized flavour (due to the production of methional from methionine in protein) is increased). 

Thus, if the amino acid (Figure 8.23) is alanine (R3 = -CH3), widely represented in must and wine, the corresponding aldehyde is ethanal. If the amino acid is methionine (R4 = CH3-S-CH2-CH2-), which is certainly only present in small quantities but is reputed to be highly reactive with carbonylated compounds, then methional, or -methyl-S-propanal, is produced. This compound is thermally unstable and evolves rapidly, via a Retro-Michael reaction, into acrolein and methanethiol (Figure 8.28). These smell of cooked cauliflower, wet dog, etc. In wine, part of the methional returns to methionol via catalyzed reduction by alcohol dehydrogenase with NADH. 

Investigation of the reaction mechanism (Maujean and Seguin, 1983) showed that sunlight flavor was mainly due to the oxidative photodegradation of methionine (Figure 8.30). It was observed that methional was the primary product of oxidative photolysis of the amino acid. 

The Strecker degradation involves the oxidative deamination and decarboxylation of a a-amino acid in the presence of a dicarbonyl compound. The products formed from this reaction are an aldehyde containing one less carbon atom than the original amino acid and an a-aminoketone (Table 9.2). The Strecker degradation of methionine and cystein is a source of sulfur-containing intermediates (e.g hydrogen sulfide and 2-methylthiopropanal = methional) [48]. 

---