Patricia Taillandier

Isolation of Bacillus spp. from Thai fermented soybean (Thua‐nao): screening for aflatoxin B1 and ochratoxin A detoxification

Aims:  To study the interaction between Bacillus spp. and contaminating Aspergillus flavus isolated strains from Thai fermented soybean in order to limit aflatoxin production. To study the detoxification of aflatoxin B1 (AFB1) and ochratoxin A (OTA) by Bacillus spp. in order to find an efficient strain to remove these toxins.

Impact of the co-culture of Saccharomyces cerevisiae-Oenococcus oeni on malolactic fermentation and partial characterization of a yeast-derived inhibitory peptidic fraction.

The present study was aimed to evaluate the impact of the co-culture on the output of malolactic fermentation and to further investigate the reasons of the antagonism exerted by yeasts towards bacteria during sequential cultures. The Saccharomyces cerevisiae D strain/Oenococcus oeni X strain combination was tested by applying both sequential culture and co-culture strategies. This pair was chosen amongst others because the malolactic fermentation was particularly difficult to realize during the sequential culture. During this traditional procedure, malolactic fermentation started when alcoholic fermentation was achieved. For the co-culture, both fermentations were conducted together by inoculating yeasts and bacteria into a membrane bioreactor at the same time. Results obtained during the sequential culture and compared to a bacterial control medium, showed that the inhibition exerted by S. cerevisiae D strain in term of decrease of the malic acid consumption rate was mainly due to ethanol (75%) and to a peptidic fraction (25%) having an MW between 5 and 10 kDa. 0.4 g l(-1) of L-malic acid was consumed in this case while 3.7 g l(-1) was consumed when the co-culture was applied. In addition, there was no risk of increased volatile acidity during the co-culture. Therefore, the co-culture strategy was considered effective for malolactic fermentation with the yeast/bacteria pair studied.

Quantitative study of interactions between Saccharomyces cerevisiae and Oenococcus oeni strains

This study examines the interactions that occur between Saccharomyces cerevisiae and Oenococcus oeni strains during the process of winemaking. Various yeast/bacteria pairs were studied by applying a sequential fermentation strategy which simulated the natural winemaking process. First, four yeast strains were tested in the presence of one bacterial strain leading to the inhibition of the bacterial component. The extent of inhibition varied widely from one pair to another and closely depended on the specific yeast strain chosen. Inhibition was correlated to weak bacterial growth rather than a reduction in the bacterial malolactic activity. Three of the four yeast strains were then grown with another bacteria strain. Contrary to the first results, this led to the bacterial stimulation, thus highlighting the importance of the bacteria strain. The biochemical profile of the four yeast fermented media exhibited slight variations in ethanol, SO2 and fatty acids produced as well as assimilable consumed nitrogen. These parameters were not the only factors responsible for the malolactic fermentation inhibition observed with the first bacteria strain. The stimulation of the second has not been reported before in such conditions and remains unexplained.

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.

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.

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.

Influence of Oxygen on Alcoholic Fermentation by a Wine Strain of Torulaspora delbrueckii: Kinetics and Carbon Mass Balance

Torulaspora delbrueckii metabolism was assessed in a synthetic culture medium similar to grape must under various conditions: no aeration and three different oxygen feeds, in order to determine the effect of oxygen on metabolism. Carbon and nitrogen mass balances were calculated to quantify metabolic fluxes. The effect of oxygen was to decrease the flux of carbon going into the fermentation pathway in favor of growth. In the absence of aeration, higher amounts of glycerol were produced, probably to maintain the redox balance. The oxygen requirement of this strain was high, since even for the highest air supply oxygen became limiting after 24 h. Nevertheless, this strain developed well in the absence of oxygen and consumed 220 g/L of sugars (glucose/fructose) in 166 h at 20 °C, giving a good ethanol yield (0.50 g/g).