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.

2-Substituted-5-methyl-3-oxazolines, a novel class of aroma precursors that are able to release the respective Strecker aldehydes by hydrolysis, were identified. Hydrolysis can take place after the addition of water or with human saliva during mastication, respectively. 2-Isobutyl-, 2-sec-isobutyl-, 2-isopropyl, and 2-benzyl-5-methyl-3-oxazolines were synthesized and structurally identified by means of gas chromatography-mass spectrometry (GC-MS) in the electron impact mode and in the chemical ionization mode as well as by one- and two-dimensional NMR experiments. With these compounds at hand, a variety of stability experiments were performed using headspace-GC-MS or proton transfer reaction-MS techniques on the basis of stable isotope dilution assays, proving the ability to release the respective Strecker aldehydes was dependent on the pH value as well as on the hydrolysis time. After the addition of water at 37 °C, for example, >70 mol % of 3-methylbutanal or >40 mol % of phenylacetaldehyde was liberated from a solution of 2-isobutyl-5-methyl-3-oxazoline or 2-benzyl-5-methyl-3-oxazoline, respectively, after 5 min. Furthermore, the presence of 2-isobutyl-5-methyl-3-oxazoline in dark chocolate containing 70% cocoa was proven by GC-MS.

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