Quantitation ofS-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

Abstract

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.

The role of sulfur chemistry in thermal generation of aroma

“…Its content is 2.8 mg/kg in tomatoes, 6 mg/kg in malt, 81 mg/kg in cabbage, 176 mg/kg in celery and 53-252 mg/kg in asparagus (Scherb et al, 2009). Its thermal degradation (Figure 9.13) produces dimethyl sulfide (50).…”

“…Consequently, the most important precursor for 16 in meat aroma appears to be thiamine, in combination with L-cysteine, which serves as a hydrogen sulfide source. Figure 9.13 Formation of dimethyl sulfide (50) from S-methylmethionine (49) (Scherb et al, 2009). Under acidic conditions, as in meat, the thiazole ring of thiamine (51) hydrolyses. The intermediate 52 loses formic acid and splits into the primary degradation products 53 and 5-hydroxy-3-mercapto-2-pentanone (54), as illustrated in the upper part of Figure 9.14.…”

The Effect of pH and Temperature on Cabbage Volatiles During Storage

“…Dimethyl sulfide is derived from the S‐methylated form of methionine, S‐methylmethionine (81 mg/kg in cabbage) (Casey and others ; Cerny ). S‐methylmethionine easily degrades into dimethyl sulfide during storage (Scherb and others ). An enzyme fraction from leaves of cabbage cleaves the S‐methylmethionine sulfonium salt to dimethyl sulfide and homoserine.…”

Effect of Enzyme Activity and Frozen Storage on Jalapeño Pepper Volatiles by Selected Ion Flow Tube—Mass Spectrometry

“…Dimethyl sulfide was 760% higher in blanched compared to unblanched peppers. This compound has a cooked, cabbage‐like aroma and it is considered to be a key volatile in the aroma of several cooked vegetables (Scherb and others 2009). Together with other volatile sulfur compounds, dimethyl sulfide has been reported to be responsible for the sulfurous off‐flavors of several heat‐processed foods (Vazquez‐Landaverde and others 2005, 2006; Lozano and others 2007).…”