Estimation of methionine in food grains
Methionine is one of the essential, sulphur-containing amino acids. Although it is present in many food proteins, methionine is the limiting amino acid in most of the grain legumes. Methionine estimation is a routine in nutritional screening programmes of the grain legumes.
The protein in the grain is first hydrolyzed under mild acidic condition. The liberated methionine gives an yellow color with nitroprisside solution under alkaline condition and turns red on acidification. Glycine is added to the reaction mixture in order to inhibit color formation with other amino acids.
» 2N Hydrochloric Acid
» 10N NaOH (40%)
» 10% NaOH
» 10% Sodium Nitroprusside
» 3% Glycine
» Orthophosphoric Acid (Sp. gr. 1.75)
» Standard Methionine: Dissolve 100mg of DL-Methionine in 4mL of 20% HCl and dilute with water to 100mL.
Draw a standard curve and calculate the methionine content from the graph.
Methionine content in the sample = (Methionine content from the graph x 4) mg per g
Methionine is usually expressed as percentage of protein or g per 16g N.
1. Horn, M J, Jones, D B and Blum, A E (1946) J Biol Chem 166 313.
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