Abstract: In this study, fifteen combination treatments were designed using five yeast assimilable nitrogen (YAN) concentrations (150, 240, 330, 420, and 500 mg/L) and three reducing sugar concentrations (170, 200, and 230 g/L) to study the effects of different concentrations of assimilable nitrogen and reducing sugar in synthetic grape must on the fermentation characteristics of Saccharomyces cerevisiae. For this purpose, yeast growth, sugar consumption rate and nitrogen consumption were measured. The results showed that 150 mg/L of YAN nitrogen in the synthetic grape must was too low to support yeast growth and simultaneously restricted the consumption rate of reducing sugar by yeast. The rate of alcoholic fermentation was increased by increasing the initial concentration of reducing sugar to 200 g/L. Yeast could grow normally in the synthetic grape must with initial assimilable nitrogen concentrations of higher than 240 mg/L. In this case, initial reducing sugar and assimilable nitrogen concentration had no significant effect on yeast growth, and reducing sugar concentration had the most direct effect on the fermentation characteristics of S. cerevisiae strains, determining the fermentation time. In the synthetic grape must with low initial reducing sugar concentration (170 g/L), the yeast growth rate increased with increasing the initial assimilable nitrogen concentration, while at high initial reducing sugar concentration (200–230 g/L), the yeast growth rate was not affected by the initial assimilable nitrogen concentration. An initial assimilable nitrogen concentration of higher than 330 mg/L was not completely consumed, and the remaining amount increased with the increase in the initial concentration of assimilable nitrogen, providing enough assimilable nitrogen for yeast to grow. Moreover, yeast could decrease assimilable nitrogen consumption with increasing the initial reducing sugar concentration.

Key words: synthetic grape must, yeast assimilable nitrogen, reducing sugar, Saccharomyces cerevisiae, fermentation characteristics