Factors affecting levels of ferulic acid, ethyl ferulate and taste-active pyroglutamyl peptides in sake

Abstract

Factors affecting ferulic acid, ethyl ferulate and taste-active pyroglutamyl (pGlu) peptides levels in sake were analyzed using small-scale sake brewing tests on eighteen rice samples with differing cultivar variety, cropped year and area, and polishing rate. Ferulic acid concentration in sake was highly positively correlated with its content in rice (r = 0.782**, double asterisk indicates 1% significance level), feruloylesterase (FE) activity (r = 0.804**) and feruloylated saccharide forming activity (FSFA) (r = 0.619**) in the rice koji. The results suggested that ferulic acid in rice induced FE activity and FSFA, and these two enzymes accelerated the formation of ferulic acid in sake mash. The concentration of bitter-tasting peptides in sake was highly positively correlated with crude protein content in rice (r = 0.786**), and negatively correlated with acid carboxypeptidase (ACP) activity to (pGlu)LFGPNVNPWH (r = -0.612**), fermentation length (r = -0.820**), and pyroglutamyl leucine ((pGlu)L) concentration in sake (r = -0.502*; 5% significance level). The observation suggested that bitter-tasting peptides are initially formed in sake mash in accordance with protein content in rice, and are then hydrolyzed to smaller peptides, such as (pGlu)L. An ACP specific to the hydrolysis of bitter-tasting peptides was suggested by the observation that ACP activity to (pGlu)LFGPNVNPWH was significantly correlated (-0.612**) to their formation whereas an ACP to Cbz-Glu-Tyr was not (r = 0.220). It was suggested that (pGlu) oligo-peptide ethyl esters were formed during the decomposition of bitter-tasting peptides to which the ACP to (pGlu)LFGPNVNPWH might contribute.

Keywords: Ethyl ferulate; Ferulic acid; Pyroglutamyl peptide; Rice; Sake.