The Mechanism of Sunlight Flavor Formation in Milk with Special Reference to Methionine and Riboflavin

Abstract

Evidence is presented that beta-methylmercaptopropionaldehyde (methional) is of importance in the sunlight flavor defect of milk. In the estimation of 3 experienced taste observers, it imparted typical sunlight flavor to milk at a level of 0.1 ppm, and was detectable in milk at concns. as low as 0.05 ppm. A number of other compounds closely related to methional, including methyl mercaptan, beta-methylmercaptopropylamine, beta-methylmercaptopropyl alcohol and beta-methylmercaptopropionic acid, were evaluated. None appeared to be so specifically an agent of flavor as methional. Methional was demonstrated, by means of infrared spectral data, as a product of the sunlight catalyzed reaction between methionine and riboflavin. It was concluded that amino acid methionine is a specific source of the flavor. Negative flavor development was noted from a number of other amino acids and several derivatives of methionine. In milk, the proteins are indicated as the primary source of sunlight flavor. It is suggested that casein is most important as an origin, since it is present in greatest concn., contains the highest level of methionine, and is the principal component limiting light transmission. The role of riboflavin, as well as certain dyes and dicarbonyl compounds, in the photolysis of methionine is discussed.