Methional

Methional

Names
IUPAC name 3-Methylsulfanylpropanal
Other names 3-(Methylmercapto)propionaldehyde; 3-(Methylthio)propionaldehyde; 3-(Methylthio)propanal
Identifiers
CAS Number	3268-49-3
3D model (Jmol)	Interactive image
ChEBI	CHEBI:49017
ChemSpider	17597
ECHA InfoCard	100.019.893
EC Number	221-882-5
PubChem	18635
InChI InChI=1S/C4H8OS/c1-6-4-2-3-5/h3H,2,4H2,1H3 Key: CLUWOWRTHNNBBU-UHFFFAOYSA-N
SMILES CSCCC=O
Properties
Chemical formula	C4H8OS
Molar mass	104.17 g·mol−1
Appearance	Colorless liquid
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Methional is an organic compound with the formula CH₃SCH₂CH₂CHO. It is a colorless liquid that is a degradation product of methionine. It is a notable flavor in potato-based snacks, namely potato chips, one of the most popular foods containing methional.^[1] Traces of the compound can also be found in black tea and green tea based products. Methional contains both aldehyde and thioether functional groups. It is readily soluble in alcohol solvents, including propylene glycol and dipropylene glycol.

Occurrence

In nature, methional is a thermally-induced volatile flavor compound. For instance, the heat-initiated Maillard reaction of reducing sugars and amino acids forms the initial basis of methional's composition. The formation of methional stems from the interaction of α-dicarbonyl compounds (intermediate products in the Maillard reaction) with methionine (Met) by the Strecker degradation reaction:^[1]

CH₃SCH₂CH₂(NH₂)CHCO₂H + O → CH₃SCH₂CH₂(HN=)CCO₂H + H₂O

CH₃SCH₂CH₂(HN=)CCO₂H + H₂O → CH₃SCH₂CH₂CHO + NH₃ + CO₂

Methional easily degrades into methanethiol, which then oxidizes into dimethyl disulfide. Dimethyl disulfide is partly responsible for the "reactive sulfur" that causes the overall taste of potatoes. Furthermore, the methionine resulting from the Strecker degradation reaction produces alkyl pyrazines, which contribute to the flavors in roasted, toasted, or thermally processed foods. Due to the ease of its decomposition, a large portion of methional is lost during potato processing.

Similarly, in the presence of flavin mononucleotide (FMN) and light, methionine is nonenzymatically oxidized into methional, ammonia, and carbon dioxide.^[2]

CH₃SCH₂CH₂(NH₂)CHCO₂H → CH₃SCH₂CH₂CHO + NH₃ + CO₂

Preparation

In the laboratory, methional is prepared by the Stephen aldehyde synthesis. Accordingly, 3-methylmercatopropionitrile is reduced to the iminium salt with stannous chloride, which is subsequently hydrolyzed as represented by these idealized reactions<!idealized because the SnCl4 gets wrecked in the final step, BTW, first reaction is even questionable>^[3]

CH₃S(CH₂)₂CN + SnCl₂ + 2 HCl → [CH₃S(CH₂)₂CHNH₂Cl]⁺ [SnCl₃]⁻

[CH₃S(CH₂)₂CHNH₂Cl]⁺ [SnCl₃]⁻ + H₂O → CH₃S(CH₂)₂CHO + NH₃ + SnCl₄

Another methional synthesis involves the reaction of methanethiol and acrolein.^[3]

CH₃SH + CH₂=CHCHO → CH₃SCH₂CH₂CHO

Biological routes

Because of their high cost, methional or its precursor methionine are not added during potato processing.^[1] In order to intensify flavoring of heat-processed potato foods, biotechnological approaches are used to increase methionine levels, and thus methional levels, in potato foods.

Biologically, the enzyme aminotransferase acts on methionine to remove the amine and form an α-keto-γ-methylthiobutyric acid. As catalyzed by α-keto acid decarboxylase, this ketomethylthiobutyric acid converts to methional.^[4]

CH₃S(CH₂)₂CH(NH₂)CO₂H + O → CH₃S(CH₂)₂COCO₂H + NH₃

CH₃S(CH₂)₂COCO₂H → CH₃S(CH₂)₂CHO + CO₂

References

1. 1 2 3 Faith C. Belanger, Rong Di & Daphna Havkin-Frenkel (2009) Increasing the Methional Content in Potato through Biotechnology, Biotechnology in Flavor Production, 185-188, doi:10.1002/9781444302493.ch9
2. ↑ S. F. Yang, H. S. Ku & H. K. Pratt (1967) Photochemical Production of Ethylene from Methionine and Its Analogues in the Presence of Flavin Mononucleotide, The Journal of Biological Chemistry, 242, 5274-5280.
3. 1 2 Charles Hurd & Leon Gershbein (1947) Reactions of Mercaptans with Acrylic and Methacrylic Derivatives, Chemical Laboratory of Northwestern University, 69, 2328-2335
4. ↑ María Del Carmen Martínez-Cuesta , Carmen Peláez & Teresa Requena (2013) Methionine Metabolism: Major Pathways and Enzymes Involved and Strategies for Control and Diversification of Volatile Sulfur Compounds in Cheese, Critical Reviews in Food Science and Nutrition, 53:4, 366-385. doi:10.1080/10408398.2010.536918