Quantitation of S-methylmethionine in raw vegetables and green malt by a stable isotope dilution assay using LC-MS/MS: comparison with dimethyl sulfide formation after heat treatment.
The potent odorant dimethyl sulfide (1), showing a low odor threshold of 0.12 microg/L in water, is known to contribute to the aromas of various foods. Its cabbage-like odor plays an important role, particularly, in cooked vegetables, such as cabbage, celery, or asparagus. On the other hand, in fruit juices or beer, 1 may generate off-flavors. S-Methylmethionine (2) has previously been characterized as precursor of 1 during thermal processing, and several methods for its quantitation have been proposed. Using deuterium-labeled 2 as the internal standard, a stable isotope dilution assay (SIDA) using LC-MS/MS was developed for the fast quantitation of 2 in vegetables and malt. Application of the method to different foods revealed amounts between 2.8 mg (fresh tomatoes) and 176 mg (celery) of 2 per kilogram. To correlate the amount of 1 formed upon processing with the amounts of 2 present in the raw material, 1 was quantified before and after a thermal treatment of the same raw materials by a SIDA. Concentrations between 1.1 mg/kg (fresh tomatoes) and 26 mg/kg (celery) were determined in the processed samples. The quantitation of 2 during steeping, germination, and malting of barley, and a correlation of the data with the amounts of 1 formed after thermal treatment of the malt, resulted in yields between 24 and 27 mol % calculated on the basis of the amounts of 2. The results suggested that the extent of the formation of 1 can be predicted, for example, in plant materials, from the amount of 2 present in the raw foods.
Scherb
Julia
J
Lehrstuhl fuer Lebensmittelchemie, Technische Universitaet Muenchen, Garching, Germany.
Kreissl
Johanna
J
Haupt
Sonja
S
Schieberle
Peter
P
D002851:Chromatography, High Pressure Liquid / Q000379:methods*; D003903:Deuterium; D004121:Dimethyl Sulfoxide / Q000032:analysis*; D004355:Drug Stability; D002523:Edible Grain / Q000737:chemistry*; D001467:Hordeum; D006358:Hot Temperature; D007201:Indicator Dilution Techniques; D053719:Tandem Mass Spectrometry / Q000379:methods*; D014675:Vegetables / Q000737:chemistry*; D014814:Vitamin U / Q000032:analysis*
D014814:Vitamin U; D003903:Deuterium; D004121:Dimethyl Sulfoxide