The role of malic acid in the metabolism of Schizosaccharomyces pombe: substrate consumption and cell growth

Summary The effect of initial concentrations of malate varying from 0 to 28.6 g/l was studied. The acid was found to be inhibitory for growth of Schizosaccharomyces pombe but not for its deacidification activity. Malate was never integrated into biomass but partly transformed into ethanol if the aeration rate was weak (oxygen limitation). In the absence of glucose, resting cells of S. pombe were able to degrade malic acid if their concentration was sufficient, but their viability gradually decreased. However, for 0.15 g/l of growing cells (inoculum) 6 g/l of glucose was necessary to consume 8 g/l of malate. When the medium did not contain sugar no growth was observed despite the partial consumption of malate, showing that the acid was neither a carbon source nor an energy source. Offprint requests to: P. Strehaiano     

Effects of acetic acid on growth and fermentative activity of Saccharomyces cerevisiae

The effect of acetic acid on the growth and the fermentative activity of S. cerevisiae was analysed comparatively with the pH. This study showed that the pH does not affect these two activities. On the contrary, the acetic acid has an inhibition effect. This effect was modelised by the relation of Levenspiel. Finally, it was shown that the quantities of acetic acid produced by Brettanomyces were not sufficient to explain the inhibition of Saccharomyces.     

<Emphasis Type="Italic">Brettanomyces bruxellensis</Emphasis>: effect of oxygen on growth and acetic acid production

The influence of the oxygen supply on the growth, acetic acid and ethanol production by Brettanomyces bruxellensis in a glucose medium was investigated with different air flow rates in the range 0-300 l h(-1 ) x (0-0.5 vvm). This study shows that growth of this yeast is stimulated by moderate aeration. The optimal oxygen supply for cellular synthesis was an oxygen transfer rate (OTR) of 43 mg O(2) l(-1) x h(-1). In this case, there was an air flow rate of 60 l h(-1) (0.1 vvm). Above this value, the maximum biomass concentration decreased. Ethanol and acetic acid production was also dependent on the level of aeration: the higher the oxygen supply, the greater the acetic acid production and the lower the ethanol production. At the highest aeration rates, we observed a strong inhibition of the ethanol yield. Over 180 l h(-1) x (0.3 vvm, OTR =105 mg O(2) l(-1) x h(-1)), glucose consumption was inhibited and a high concentration of acetic acid (6.0 g x l(-1)) was produced. The ratio of "ethanol + acetic acid" produced per mole of consumed glucose using carbon balance calculations was analyzed. It was shown that this ratio remained constant in all cases. This makes it possible to establish a stoichiometric equation between oxygen supply and metabolite production.     

Oxygen effect on lactose catabolism by a Leuconostoc mesenteroides strain: modeling of general O2-dependent stoichiometry

Lactose metabolism of a Leuconostoc mesenteroides strain was studied in batch cultures at a pH of 6.5 and 30 degrees C in 10 L of a modified MRS (De Man, Rogosa, Sharp) broth. The end products of this heterolactic bacterium were D-lactate, acetate, ethanol, and carbon dioxide. To test the effect of oxygen on their synthesis, the medium was sparged with different gases: nitrogen, air, and pure oxygen. When oxygen was available, oxygen uptake occurred, which caused a modification in acetate and ethanol production but not in lactate or carbon dioxide production; acetate plus ethanol together were produced in constant amounts, which were independent of the level of aeration. The influence of oxygen on end-product formation could be summed up by the general equation: lactose + x O(2) --> 2 D-lactate + (x + 0.1) acetate + (2 - x) ethanol + 2 CO(2). Maximal oxygen uptake (x = 2) was reached under a 120 L/h flow rate of pure oxygen. In addition, this equation provided useful information on the possible pathway of galactose catabolism by a heterofermentative microorganism. (c) 1996 John Wiley & Sons, Inc.     

Mixed culture of killer and sensitive Saccharomyces cerevisiae strains in batch and continuous fermentations

This paper presents a kinetic study of the dynamics of the population of two Saccharomyces cerevisiae strains (designated K1 and 522D) in mixed culture. These two strains are commonly used in wine making. The K1 strain (killer yeast) secretes a glycoprotein (killer toxin) which causes the death of the 522D strain (sensitive yeast). Initially, the mixed cultures were realized in batch fermentations. Initial concentrations of killer yeast were 5 and 10% of the total population. The influence of the killer strain on the sensitive cultures was measured in comparison with a reference fermentation. The reference fermentation was inoculated only with the sensitive strain. Results show that an initial concentration of 10% of killer strain affects the microbial population balance and the rate of ethanol production. However the fermentation was only slightly disturbed when the proportion of killer to sensitive yeast at the beginning of mixed culture was 5%. To achieve total displacement by the killer yeast at low concentrations, the mixed cultures were carried out in a continuous system. The results obtained in continuous fermentations with the same strains have shown that a level of contamination as low as 0.8% of killer strain was sufficient to completely displace the original sensitive population after 150 h incubation.     

Study of xylitol formation from xylose under oxygen limiting conditions

The fermentation of D-xylose byCandida parapsilosis was studied in continuous cultures. From the results obtained, xylitol formation seems to be directly coupled to growth of biomass, and strongly influenced by oxygen consumption.     

Influence of medium composition, pH and temperature on the growth and viability of Lactobacillus acidophilus

The Lactobacillus acidophilus growth was investigated to find the optimal concentrations of 2 nitrogen sources (thought to be in excess) and 2 organic acids. A Plackctt and Burman experimental design was used allowing identification of the more important factors with very few experiments. In the studied range, all the factors had a linear effect and a great influence on the final viability. 20 g/L of each nitrogen sources, 3 g sodium-citrate/L and 5 g sodium acetate/L are necessary. pH, temperature and glucose had poor influence.     

Xylitol production in repeated fed batch cultivation

The ability of Candida parapsilosis to produce xylitol was tested using successive substrate supplies, and the importance of the amount of viable cells in enhancing the conversion rate was demonstrated. The suitability of this yeast for the production of xylitol was investigated in repeated fed-batch cultivation, using pure xylose or mixtures of xylose and glucose. The use of this process increased productivity by about 40% compared with simple batch cultivation without loss of yield of product on substrate. The presence of glucose in the culture medium seemed to stimulate the specific growth rate, but had no influence over other fermentative parameters.     

Nutritional requirements of Brettanomyces bruxellensis: growth and physiology in batch and chemostat cultures

The nutritional requirements of Brettanomyces bruxellensis have been investigated. Batch culture and chemostat pulse techniques were used to identify growth-limiting nutrients. The study included determination of the essential components of the culture medium and quantification of the effects of the components. Among the components tested, ammonium sulfate and yeast extract had a significant effect on glucose consumption, growth, and ethanol production. However, if the ammonium sulfate concentration is above 2 g/L, an inhibitory effect on B. bruxellensis growth is observed. The yeast extract appears to be the most important and significant component for growth. The maximum amount of synthesized biomass is proportional to the concentration of yeast extract added to the culture broth (in the tested range). Magnesium and phosphate ions are probably not essential for B. bruxellensis. These ions appear to be supplied in sufficient amounts by the yeast extract in the culture medium. Brettanomyces bruxellensis appears to have very low nutritional requirements for growth.