Chemistry of aged garlic: Diversity of constituents in aged garlic extract and their production mechanisms via the combination of chemical and enzymatic reactions

Abstract

Raw garlic contains characteristic compounds, such as S-alk(en)ylcysteine sulfoxides, γ-glutamyl-S-alk(en)-ylcysteines and polysaccharides. These compounds undergo various transformation processes during the aging process. Among these compounds, the change of sulfur-containing molecules is diverse and time-dependent. Previously, by means of the liquid chromatography (LC)/LC-mass spectrometry (MS) method, a number of unidentified peaks corresponding to candidates of sulfur-containing molecules were detected in the chromatogram of aged garlic extract (AGE), and identified using MS and nuclear magnetic resonance (NMR). The production mechanisms of these compounds were then examined by model reactions and laboratory experiments mimicking the aging process. Three γ-glutamyl tripeptides [γ-glutamyl-γ-glutamyl-S-methylcysteine, γ-glutamyl-γ-gluta-myl-S-allylcysteine (GGSAC), γ-glutamyl-γ-glutamyl-S-1-propenylcysteine], γ-glutamyl-S-allylmercaptocysteine (GSAMC) and cis-S-1-propenylcysteine (cis-S1PC) were isolated and identified. GGSAC was produced from GSAC through the enzymatic reaction catalyzed by γ-glutamyltranspeptidase (GGT), and two other tripeptides could be produced in similar reactions. GSAMC was produced by the reaction between γ-glutamyl dipeptides and allicin. Furthermore, GSAMC was a precursor compound of S-allyl-mercaptocysteine (SAMC), and thus it was produced from GSAMC by GGT. cis-S1PC was produced from trans-S1PC by the isomerization reaction. A number of other compounds were also identified, including Maillard reaction products; however, their production mechanisms have not been elucidated. In this review, we present the changes in characteristic constituents in raw garlic and garlic extract during the aging process and discuss their production mechanisms involving the various chemical and enzymatic reactions.

Keywords: Maillard reaction products; aged garlic extract; aging process; enzymatic reaction; sulfur-containing compounds; γ-glutamyl transpeptidase.

Figure 1.

Chemical structures of compounds found in garlic and aged garlic extract. Chemical structures of compounds in (A) raw garlic and (B-D) aged garlic extract. (B) Hydrophobic compounds, (C) S-alk(en)ylcysteine and γ-glutamylpeptide derivatives, and (D) phenolic compounds and saponins. Chemical structures of compounds found in garlic and aged garlic extract. (E) Chemical structures of compounds in Maillard reaction-relating compounds.

Figure 1.

Chemical structures of compounds found in garlic and aged garlic extract. Chemical structures of compounds in (A) raw garlic and (B-D) aged garlic extract. (B) Hydrophobic compounds, (C) S-alk(en)ylcysteine and γ-glutamylpeptide derivatives, and (D) phenolic compounds and saponins. Chemical structures of compounds found in garlic and aged garlic extract. (E) Chemical structures of compounds in Maillard reaction-relating compounds.

Figure 1.

Chemical structures of compounds found in garlic and aged garlic extract. Chemical structures of compounds in (A) raw garlic and (B-D) aged garlic extract. (B) Hydrophobic compounds, (C) S-alk(en)ylcysteine and γ-glutamylpeptide derivatives, and (D) phenolic compounds and saponins. Chemical structures of compounds found in garlic and aged garlic extract. (E) Chemical structures of compounds in Maillard reaction-relating compounds.

Figure 2.

Constituents in raw garlic and production pathways of characteristic compounds in aged garlic extract. (A) Constituents in raw garlic. (B) Production mechanism of cis-S−1-propenylcysteine by isomerization. (C) Production mechanism of S-alk(en)ylcyteine by enzymatic reaction and compounds produced. (D) Production mechanism of γ-glutamyl tripeptides by enzymatic reaction and compounds produced. (E) Hydrolysis of polysaccharide, fructan. (F) Production mechanism of Amadori compounds by the Maillard reaction and compounds produced. (G) Production mechanism of S-allylmercaptocysteine. (H) Hydrophobic compounds derived from alliin and their transformed compounds. Combination of block arrows and compound names indicate the providing of raw materials to the reaction pathways involved. Please refer to the corresponding names spelled out and the structures shown in Fig. 1 for abbreviated compound names in this figure. GGT, γ-glutamyl transpeptidase.