Sotolon, formation

Otlier mechanisms, which exclude 2-ketobutyric acid as a reactant have also been reported, such as Sotolon formation from chemical or enzymatic conversion of 4-hydroxyisoleucine (78,79) as a product of reaction from hexoses and pentoses in the presence of cysteine (20) and from the aldol... 

The presence of 2-ketobutyric acid in Port wine was investigated (P). Although the quantities found could reach 2 mg/L, these were always lower than 0.5 mg/L for wines older than 10 years. No correlation between the levels of keto-acid neither with age nor with the amount of furanone could be found. Conversely a very liigh correlation was observed with acetaldehyde (r=0.8906). Thus, 2-ketobutyric acid may contribute to Sotolon formation, but taking into accoimt the linear trend observed with time for the Sotolon levels, it seems unlikely that this keto-acid constitutes the only source of Sotolon in Port wine. 

The Sotolon formation was studied as a function of temperature, pH, SO2 and oxygen concentration. Temperature and to a lesser extent, dissolved oxygen were tlie most important parameters influencing the rate of Sotolon formation. A first order model was applied and the Arrhenius equation described well the temperature dependence of the reaction rate constant. Therefore, oxygen levels during aging have a major impact on age perception" in Colheita and Vintage Port wine. 

Blank, I., Lin, J., Fumeaux, R., Welti, D.H., and Fay, L.B. 1996. Formation of 3-hydroxy-4,5-di-methyl-2(5H)-furanone (sotolone) from 4-hy-droxy-L-isoleucine and 3-amino-4,5-dimethyl-3,4-dihydro-2(5)-furanone. J. Agric. Food Chem. 44 1851-1856. 

Pham, T. T., Guichard, E., Schilch, P., and Charpentier, C. (1995). Optimal conditions for the formation of sotolon from a-ketobutyric acid in the French "vin jaune". ]. Agric. Food Chem. 43, 2616-2619. 

Kobayashi (1989) reported the formation of Sotolon in wines by an aldol condensation of acetaldehyde and a-ketobutiric acid (derived from threonine) followed by lactonization (Fig. 7.11). During aging, ethanol is converted into acetaldehyde, thus allowing the formation of Sotolon (Silva Ferreira et ah, 2003). 

FIGURE 7.11 Formation of Sotolon (3-hydroxy-4,5-dimethyl-2(5H)-furanone) in wines by aldol condensation of acetaldehyde and 2-oxobutanoic acid followed by lactoni-zation (Kobayashi, 1989). 

Scheme 5.1 Proposed formation of 2-methyltetrahydro-3-furanone from I -deoxy-3 A-dike -tose212 and of sotolone and abhexonefrom 2-oxoacids214...

A. Kobayashi, Sotolon Identification, formation, and effects on flavor, in Flavor Chemistry Trends and Developments, R. Teranishi, R. G. Buttery, and F. Shahidi (eds),... 

Coffee (Arabica) /3-Damascenone, 2-furylmethanethiol, 3-sulfanyl-3-methylbutyl formate (126), homofuronol (127), furaneol, guaiacol, 4-vinylguaiacol, methional, sotolone, 2-ethyl-3,5-dimethylpyrazine, 2,3-diethyl-5-methylpyrazine, vanillin, 4-ethylguaiacol, homosotolone (128) 147... 

KOBAYASHI Sotolon Identification, Formation, Flavor Effect... 

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... 

Figure 7. Formation of Sotolon during an Amino-Carbonyl Reaction...

When Japanese rice wine (sake) is kept under unsuitable conditions, it develops an off-flavor with a burnt or soy-sauce (shoyu)-like odor. Takahashi al.(12) identified the main component as sotolon before our identification, and they claimed that the sotolon concentration was between 140-430 ppb in aged sake. This concentration is much higher than the threshold value of sotolon, and its aroma character would change from sugary to herbal or curry-like. The high sotolon content in aged sake could be a reason for the off-flavor defect. The formation of sotolon in aged... 

Table VIII. Formation of Sotolone from HIL as Affected by the Reaction Time Using Metliylglyoxal as Carbonyl Rcacinni ...

The purpose of the present study was to identify those volatile compounds which significantly contribute to the seasoning-like note of fenugreek using the approach of sensory directed chemical analysts. Gas chromatography in combination with olfactometry and mass spectrometry have been used as key steps of this approach (18,19). The formation of flavor impact compound(s) was studied in model systems using the quantification technique Isotope Dilution Assay (20, 21) The mechanistic study was based on a hypothetical pathway proposed for the formation of sotolone via thermally induced oxidative deamination of HIL (10). 

In the sample based on MIL and methylglyoxal, the ratio of Strccker aldehyde to sotolone was about 1 2 at pH 5 (Table VT). Consequently, the Slrecker degradation of HIL is a competitive reaction to the formation of sotolone. In contrast. only traces of Strecker aldehyde were detected In the sample containing the laciom-ADF, i.e about 50 times less than in the reaction with HIL 1 he formation orsololoiir from ADF is the favoured reaction, most likely due to the blocked carboxyl pmnp... 

The reaction of HIL and methylglyoxal at pH 5 performed in water yielded 2 8 mol% sotolone compared to 7.4 mol% when using the phosphate buffered system This suggests a catalytic effect of phosphate on the formation of sotoione from HIL. 

The presence of Sotolon in foodstuffs has been reported to be the result of an aldol condensation between 2-ketobutyric acid and pyruvic acid 10), similar to the mechanism proposed for the 5-ethyl-3-hydrox> -4-methyl-2(5H)-furanone formation in protein hydrolysates (77). The aldol condensation between acetaldehyde and 2-ketobutyric acid has also been suggested (77) to explain the presence of Sotolon in fortified type wines. 

Hence the aim of this work was to (/) evaluate the impact of Sotolon on the perceived age of Port wine (ii) determine which parameters (e.g. temperature, dissolved oxygen, pH and free SO2 content) have the greatest effect on Sotolon concentration, (Hi) and to establish mathematical models describing the rate of formation of Sotolon during storage, taking into account the critical parameters selected. 

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