Influence of Sulfur Amino Acids on the Volatile and Nonvolatile Components of Cooked Salmon (Salmo salar) LISA METHVEN,* MARIA TSOUKKA,MARIA JOSE ORUNA-CONCHA, JANE K. PARKER, AND DONALD S. MOTTRAM Department of Food Biosciences, The University of Reading, Whiteknights, Reading, RG6 6AP, United Kingdom Volatile and nonvolatile compounds, which could contribute to flavor, were analyzed in salmon. One hundred twenty-three volatile compounds were identified in the headspace of two different samples of cooked salmon, including lipid-derived volatiles, Maillard-derived volatiles, sulfur volatiles, Strecker aldehydes, nitrogen heterocyclic compounds, terpenes, and trimethylamine. Significant differences between samples were found for 104 of the volatiles. Although the levels of free cysteine and methionine were low in the salmon, sulfur volatiles were formed in the cooked fish, demonstrating that there were sufficient sulfur amino acids present for their formation. Notable differences in sulfur compounds between the samples suggested that small changes in sulfur amino acids could be responsible. When this hypothesis was tested, salmon heated with cysteine had increased levels of many thiophenes, thiazoles, alicyclic sulfides, and nitrogen heterocycles. With the addition of methionine, levels of dimethyl sulfides, two alicyclic sulfides, pyrazines, some unsaturated aldehydes, and alcohols and 2-furanmethanethiol increased. The largest difference found among the nonvolatile (low molecular weight water-soluble) compounds was in inosine monophosphate. KEYWORDS: Aroma volatiles; salmon flavor, cooked fish flavor; sulfur volatiles; tastants; cysteine; methionine; inosine monophosphate INTRODUCTION Fish flavor is derived from nonvolatile taste-active compounds and volatile compounds, which may stimulate the odor receptors. In a previous study of haddock, the nonvolatile low molecular weight components were found to be responsible for major flavor notes of the cooked haddock (1). The nonvolatile low molecular weight water-soluble components can also be the precursors of the volatile aroma compounds. The aroma of raw fish is the result of a complex mixture of volatile compounds, which result from many processes such as microbial enzymic breakdown and lipid degradation (enzymic and oxidative). In the case of cooked fish, the Maillard reaction and Maillard- lipid interactions are also important (2). Much of the literature has concentrated on lipid degradation and the effects of storage on fish flavor due to the highly unsaturated nature of fish polyunsaturated fatty acids (PUFAs) (2-5). Typical lipid- derived volatiles in fish are unsaturated C8 and C9 alcohols, aldehydes, and ketones, which give fresh plantlike aromas (4). Trimethylamine (TMA) is the most characteristic volatile in fresh saltwater fish (2) and is responsible for “old fishy, ammonia” odors (4). It is formed by the reduction of trimethy- lamine oxide (TMAO) by bacterial enzymes (3, 4). TMAO can also act as an oxidizing agent, for example, promoting the degradation of both cysteine and fish oils (6). Cooked fish are generally low in typical Maillard reaction flavor volatiles, such as pyrazines (6), in comparison to levels in most meats, probably due to lower levels of free sugars (7, 8). There are some aroma volatiles that are specific to certain fish. Canned tuna fish has a meatlike aroma thought to be due to 2-methyl-3-furanthiol (9). Salmon is high in carotenoid pigment, resulting from either crustacea in the diet in the case of wild fish or added carotenoids in the case of reared fish, which leads to the pink flesh and also to specific flavor volatiles with alkyl furanoid type structures (10). However, little is known regarding the contribu- tion of volatile sulfur compounds to fish flavor. The level of volatile sulfur compounds in fish is typically low in comparison to the lipid-derived compounds (2), but as many sulfur volatiles have very low odor thresholds, they can be important to the overall flavor perception (11). The sulfur volatiles are generally thought to result from reactions involving cysteine or methionine despite the very low levels of these free amino acids in fish (11, 12). Whereas methionine is an essential amino acid in the fish diet, cysteine is considered nonessential as it can be synthesized from the methionine (13). Methional and thiophen- ecarboxaldehyde have been identified as odor-active compounds of raw salmon (14). Methional has been found to be a potent odorant in boiled cod and boiled salmon, while other methion- ine-derived compounds such as methanethiol and the dimethyl sulfides were potent odorants in boiled cod alone (15). Dimethyl * To whom correspondence should be addressed. Fax: +44 118 931 0080. E-mail: l.methven@reading.ac.uk. J. Agric. Food Chem. 2007, 55, 1427-1436 1427 10.1021/jf0625611 CCC: $37.00 © 2007 American Chemical Society Published on Web 01/25/2007