Pierre Strehaiano

Low frequency thermo-ultrasonication of Saccharomyces cerevisiae suspensions: Effect of temperature and of ultrasonic power

Abstract The combined effect of low frequency ultrasound (20 kHz) with temperature on the survival of a strain of Saccharomyces cerevisiae suspended in water was studied. The treatment temperatures tested were 45 °C, 50 °C and 55 °C; the actual ultrasonic powers tested were 50 W, 100 W and 180 W. Application of ultrasonic waves at a non-lethal temperature did not display a deactivating action; a higher effect of ultrasound at higher temperatures was observed, and the synergy between ultrasound and temperature was confirmed. These results proved that the ultrasonic waves do not destroy the yeasts cells; they damage them, thus increasing their sensitivity to heat. The existence of an optimal ultrasonic power for a maximal deactivating effect was shown, and was found to have a value of around 100 W (actual power).

An original kinetic model for the enzymatic hydrolysis of starch during mashing

Abstract This paper presents a kinetic model for the enzymatic degradation of the starch during mashing for beer production. Based on a new set of experiments, an original reaction scheme for the hydrolysis of starch by α- and β-amylases has been proposed. Here, the kinetics of the reactions involved in this scheme are described: starch gelatinisation, amylase activities and carbohydrate production. One originality of this model is the mathematical representation of the amylase activities temperature dependency. The model requires to know the initial composition of malt (starch concentration and amylase potential which are classically measured to qualify the raw material) and the operating conditions (temperature chart) to predict the evolutions of amylase activities, dextrins and fermentable carbohydrate concentrations during mashing. The model parameters have been estimated by fitting nine experiments performed on one malt variety. Five other experiments validate the model for different malt varieties.

Optimization of a culture medium for xylanase production by Bacillus sp. using statistical experimental designs

The concentrations of oat spelt xylan, casein hydrolysate and NH4Cl in the culture medium for production of xylanase from Bacillus sp. I-1018 were optimized by means of response surface methods. The path of steepest ascent was used to approach the optimal region of the medium composition. The optimum composition of the nutrient medium was then easily determined by using a central composite design and was found to be 3.16g/l of xylan, 1.94g/l casein hydrolysate, 0.8g/l of NH4Cl. The xylanase production was increased by 135% when the strain was grown in the optimized medium compared to initial medium.

Metabolic transition step from ethanol consumption to sugar/ethanol consumption by Saccharomyces cerevisiae.

The metabolic pathway shift between only ethanol consumption to both sugar/ethanol consumption was measured by on-line analysis of respiratory quotient of a Saccharomyces cerevisiae. The experiments were carried out in a fed-batch culture under aerobic conditions. During the transition phase, respiratory quotient (RQ) profile shows that sugar can be metabolized through the fermentative pathway even to values of RQ lower than 1.

Influence of pH, malic acid and glucose concentrations on malic acid consumption by Saccharomyces cerevisiae

Malic acid consumption by Saccharomyces cerevisiae was studied in a synthetic medium. The extent of malic acid degradation is affected by its initial concentration, the extent and the rate of deacidification increased with initial malate concentration up to 10 g/l. For malic acid consumption, an optimal pH range of 3–3.5 was found, confirming that non-dissociated organic acids enter S. cerevisiae cells by simple diffusion. A full factorial design has been employed to describe a statistical model of the effect of sugar and malic acid on the quantity of malate degraded (g/l) by a given amount of biomass (g/l). The results indicated that the initial malic acid concentration is very important for the ratio of malate consumption to quantity of biomass. The yeast was found to be most efficient at higher levels of malate.

Production of xylanases by Bacillus polymyxa using lignocellulosic wastes

Production of xylanolytic enzymes with no detectable cellulase activity was investigated using two strains of Bacillus polymyxa. The optimum pH and temperature for activities of the two xylanases ranged from 6.0 to 7.0 and from 45 to 50°C, respectively. The highest titres of xylanase, up to 24 nKat ml^-1 were produced within 36 and 42 h, respectively in shake flask cultures at 30°C. Enzyme production showed a cell growth associated profile. One of two strains, B. polymyxa CECT 153 was chosen for further detailed study. Numerous carbohydrates were examined for their ability to induce xylanase. It was found that xylan or xylan containing substrates, such as wheat straw, induced maximum and comparable levels of xylanase, while pure cellulose (avicel, a-cellulose) and the easily metabolisable sugars (glucose, sucrose) did not improve xylanase synthesis. Low levels of constitutive enzyme were produced as evidence from the culture medium without carbon source addition. Among various nitrogen sources tested, yeast extract was optimal for the production of xylanase.

Effects of inoculum level on kinetics of alcoholic fermentation

SummaryWork on alcoholic fermentation has often been performed with different conditions of inoculation, and here we describe the influence of the inoculation level on yields and growth rate. Taken with previous results we suggest that there may be a growth inhibitor accumulated in fermentation broth which could be introduced with the inoculum in the fresh culture medium.

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

SummaryThe 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.

Oxygen effect on batch cultures of Leuconostoc mesenteroides: relationship between oxygen uptake, growth and end-products.

Growth and lactose metabolism of a Leuconostoc mesenteroides strain were studied in batch cultures at pH 6.5 and 30° C in 101 modified MRS medium sparged with different gases: nitrogen, air and pure oxygen. In all cases, growth occurred, but in aerobiosis there was oxygen consumption, leading to an improvement of growth yield Yx/s and specific growth rate compared to anaerobiosis. Whatever the extent of aerobic growth, oxygen uptake and biomass production increased with the oxygen transfer rate so that the oxygen growth yield, Yx/o2, remained at a constant value of 11 g dry weight of biomass/mol oxygen consumed. Pure oxygen had a positive effect on Leuconostoc growth. Oxygen transfer was limiting under air, but pure oxygen provided bacteria with sufficient dissolved oxygen and leuconostocs were able to consume large amounts of oxygen. Acetate production increased progressively with oxygen consumption so that the total molar concentration of acetate plus ethanol remained constant. Maximal Yx/s was obtained with a 120 l/h flow rate of pure oxygen: the switch from ethanol to acetate was almost complete. In this case, a 46.8 g/mol Yx/s and a 0.69 h−1 maximal growth rate could be reached.

Malate utilization bySchizosaccharomyces pombe

SummaryMalic acid degradation bySchizosaccharomyces pombe was studied in synthetic and natural media. Using different concentrations from 1 to 29 g/l, malic acid was degraded. Initial deacidification rate increased with initial malate concentration. During the stationnary phase,S. pombe was able to decompose malate added to the medium before or after sugar exhaustion.