GRAPE JUICE FERMENTATION - EFFECTS OF INITIAL NITROGEN CONCENTRATION ON CULTURE PARAMETERS

After the work we published previously (Bezenger et al., 1986), we are studying the evolution of the nitrogen content of the cells in an isothermal reaction (35.degree. C) with various initial ammonium concentrations in the fermentation medium, which simulates a white grape juice. Yeast strain, culture medium and analytical techniques are similar to those described previously. Kjeldahl''s method is used for nitrogen measurement. Nitrogen balance between cells and medium is satisfactory (fig. 1). With each initial nitrogen concentration, eight reactors are inoculated. Cell growth, CO2 production and ethanol production are measured. In order to known the cellular nitrogen evolution during the reaction, we stop fermentation at different times corresponding to various reaction progresses. In this way we confirm the positive effect of nitrogen on cell growth (tabl. 1). Figure 2 shows that the nitrogen content of the cells remains about 10% (dry weight) until exhaustion of the nitrogen in the medium. Then the nitrogen content decreases to values between 3.5 and 8% depending on the initial nitrogen concentration in the medium. After exhaustion of the nitrogen in the medium, growth continues (fig. 3). This indicates that cells "offer" their nitrogen to allow new growth. Biomass quantity has always a priority over nitrogen quantity (fig. 4). The initial nitrogen concentration does not affect growth kinetics: maximal specific growth rate is constant in all essays (tabl. 2). Nevertheless with higher nitrogen amounts, specific growth rate is maintained longer at maximal values and its decrease is slower (fig. 5). On the other hand, in a poor medium, growth rate and nitrogen decrease simultaneously (fig. 6a). This phenomenon is not observed with a rich medium (fig. 6b). This may indicate growth limitation by nitrogen. Alcohol tolerance appears to be increased by nitrogen in the initial medium (fig. 7). This parameter affects also CO2 and ethanol production kinetics: its increase gives a higher maximal CO2 production rate which will decrease later and more slowly (fig. 8). The decrease of the specific rate of CO2 production and the decrease of the nitrogen content are simultaneous in all media (fig. 9) which may indicate here too a production limitation by nitrogen. Alcohol tolerance is also improved by high nitrogen amounts (fig. 10). Figure 11 illustrates the combined effects of the ethanol produced and the decrease of the cell nitrogen on the rate of CO2 production. We can use Ludeking and Piret''s model to describe our fermentation after the maximal growth rate was reached and before growth stopped (an example is given figure 12). A relationship has been found between Ludeking and Piret''s coefficient and our deceleration factor (see Bezenger et al. 1986 for definition) for CO2 production at the end of the reaction (fig. 13). Table 4 shows the importance of the intial nitrogen concentration in the medium on the completion of the reaction. So these results show very positive effects of an increase in initial nitrogen concentration in the medium on cellular growth and on CO2 and ethanol productions.