Synthesis and Sensory Characterization of Novel Umami-Tasting Glutamate Glycoconjugates ERSAN BEKSAN, † PETER SCHIEBERLE, †,§ FABIEN ROBERT, ‡ IMRE BLANK, ‡,§ LAURENT BERNARD FAY, ‡ HEDWIG SCHLICHTHERLE-CERNY, ‡ AND THOMAS HOFMANN* ,†,§,# Deutsche Forschungsanstalt fu ¨r Lebensmittelchemie, Lichtenbergstrasse 4, 85748 Garching, Germany, and Nestec Ltd., Nestle ´ Research Center, Vers-chez-les-Blanc, 1000 Lausanne 26, Switzerland Two glycoconjugates of glutamic acid, namely, the N-glycoside dipotassium N-(D-glucos-1-yl)-L- glutamate (1) and the corresponding Amadori compound N-(1-deoxy-D-fructos-1-yl)-L-glutamic acid (2), have been synthesized in yields of 35 and 52%, respectively, using new Maillard-mimetic approaches, and their chemical structures have unequivocally been elucidated by 1D- and 2D-NMR and MS experiments. Systematic sensory studies revealed that both glycoconjugates exhibit pronounced umami-like taste with recognition taste thresholds of 1-2 mmol/L, close to that of monosodium glutamate (MSG). Contrary to an aqueous solution of MSG, 1 does not show the sweetish and slightly soapy by-note, but evokes an intense umami taste. Aqueous solutions of 2 were described by the descriptors umami, seasoning, and bouillon-like. Added to a bouillon base, which did not contain any taste enhancers, both glycoconjugates imparted a distinct umami character similar to the control sample containing the same amount of MSG on a molar basis. To the best of our knowledge, these types of glycoconjugates in general and, in particular, N-glucosyl glutamate and N-deoxyfructosyl glutamate have never been reported as taste active compounds having umami-like properties. Therefore, 1 and 2 represent a new class of umami-type taste compounds showing properties similar to the umami reference compound MSG. Systematic 13 C NMR measurements revealed that 1 was fairly stable in aqueous solutions under alkaline conditions (pH 8-10) as well as in dry form. However, it rapidly hydrolyzes in neutral and acidic solutions, giving rise to glucose and glutamate. In contrast, glycoconjugate 2 was observed to be rather stable in aqueous solution as well as in the presence of human saliva. KEYWORDS: Umami; taste; N-glucoside; Maillard reaction; Amadori product; glutamate; MSG; N-(D- glucos-1-yl)-L-glutamate; N-(1-deoxy-D-fructos-1-yl)-L-glutamic acid; saliva INTRODUCTION The Japanese word “umami” means “delicious” and is used as a synonym for the characteristic sensory properties of monosodium glutamate, abbreviated as MSG (1). Very recent molecular-biological investigations led to the identification of a truncated form of the metabotropic glutamate receptor mGluR4, which is present in the brain, from rat taste buds and suggested this protein as a taste receptor for L-glutamate (2). The identification of this novel G-protein coupled receptor in taste tissue provided evidence for the existence of glutamate- like taste as a basic taste quality. This was further strengthened by the recent finding that two taste receptor proteins, namely, T1R1 and T1R3, are able to form a universal heterodimeric sensor for L-glutamate (3, 4). Besides glutamic acid, also adenosine-5′-monophosphate (5′- AMP), inosine-5′-monophosphate (5′-IMP), and guanosine-5′- monophosphate (5′-GMP) are well-known to show umami-like sensory characteristics (5). These purine-5′-nucleotides occur in many savory foods such as meat, fish, seafood, and mushrooms and are able to enhance the flavor and mouthfeel including the impression of creaminess and viscosity of savory dishes. Therefore, these are widely used as ingredients and taste enhancers in culinary and snack products. In addition to their distinct sensory quality, another peculiar effect of these compounds is their mutual taste synergism. The synergistic effects between MSG and purine-5′-nucleotides and analogues of both groups were observed more than 35 years ago (6, 7), reporting an exponential increase in the glutamate- like taste intensity of an aqueous solution of MSG when 5′- IMP was added. According to these results the taste activity of * Corresponding author [telephone (49) 251-83-33-391; fax (49) 251- 83-33-396; e-mail thomas.hofmann@uni-muenster.de]. † Deutsche Forschungsanstalt fu ¨r Lebensmittelchemie ‡ Nestec Ltd. § These authors contributed equally to this work. # Current address: Institut fu ¨ r Lebensmittelchemie, Universita ¨t Mu ¨nster, Corrensstr. 45, D-48149 Mu ¨nster, Germany. 5428 J. Agric. Food Chem. 2003, 51, 5428-5436 10.1021/jf0344441 CCC: $25.00 © 2003 American Chemical Society Published on Web 07/19/2003