Browsing by Author "Loureiro-Dias, M. C."
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- Effects of salts on Debaryomyces hansenii and Saccharomyces cerevisiae under stress conditionsPublication . Almagro, A.; Prista, C.; Castro, S.; Quintas, Célia; Madeira Lopes, A.; Ramos, J.; Loureiro-Dias, M. C.The effect of Na+ and K+ on growth and thermal death of Debaryomyces hansenii and Saccharomyces cerevisiae were compared under stress conditions as those commonly found in food environments. At the supraoptimal temperature of 34 degrees C both cations at concentrations of 0.5 M stimulated growth of D. hansenii, while K+ had no effect and Na+ inhibited growth of S. cerevisiae. At 8 degrees C, close to the minimum temperature for growth in both species, both cations inhibited both yeasts, this effect being more pronounced with Na+ in S. cerevisiae. At extreme pH values (7.8 and 3.5) both cations at concentrations of 0.25 M stimulated B. hansenii while Na+ inhibited S. cerevisiae. K+ inhibited this yeast at pH 3.5. Thermal inactivation rates, measured at 38 degrees C in D. hansenii and at 48 degrees C in S. cerevisiae, decreased in the presence of both cations. This protective effect could be observed in a wider range of concentrations in D. hansenii. These results call the attention to the fact that not all yeasts have the same behaviour on what concerns synergy or antagonism of salt together with other stress factors and should be taken into consideration in the establishment of food preservation procedures. (C) 2000 Elsevier Science B.V. All rights reserved.
- Extrusion of benzoic acid in Saccharomyces cerevisiae by an energy-dependent mechanismPublication . Henriques, M.; Quintas, Célia; Loureiro-Dias, M. C.When grown in the presence of benzoic acid, Saccharomyces cerevisiae was able to extrude [C-14]benzoic acid when a pulse of glucose was given to preloaded cells. While octanoic, sorbic, hexanoic, salicylic, butyric and propionic acids were also inducers, ethanol and acetic acid were not. The mechanism of extrusion required energy and prior growth in the presence of the inducers. Diethylstilbestrol, an inhibitor of ATPases, prevented benzoic acid extrusion. Propionic acid was not actively extruded in cells adapted to either benzoic or propionic acid, behaving as an appropriate probe to measure intracellular ph. Even though the extrusion mechanism was active, benzoic acid entered the cells by a simple diffusion mechanism.
- Problems in the use of benzoic acid for estimating the internal pH of yeastsPublication . Henriques, M.; Quintas, Célia; Loureiro-Dias, M. C.Some yeasts have the peculiar ability to grow in the presence of weak acids at rather low pH. These conditions are predominant in preserved foods and beverages such as fruit concentrates, juices, wine, where these yeasts are responsible for spoilage. The main preservatives currently utilized by food industries are sorbic, propionic, benzoic acids and SO2. It is usually assumed that weak acids diffuse through the plasma membrane in the undissociated form. In the cytoplasm, where the pH is higher, dissociation occurs resulting in accumulation of the lipid-insoluble anion and internal acidification. This is probably a very general mechanism of preventing microbial growth in foods.
- The effect of ethanol on the plasma membrane permeability of spoilage yeastsPublication . Quintas, Célia; Lima-Costa, Maria Emília; Loureiro-Dias, M. C.The effect of ethanol on the passive proton influx and on leakage of compounds absorbing at 260 nm, as representatives of intracellular content, was studied on food spoilage yeasts such as Saccharomyces cerevisiae, Zygosaccharomyces bailii, Pichia sp. and Debaryomyces hansenii. For volume fraction below 10 %, the effect of ethanol on the proton influx was in general weak, but above a certain treshold of ethanol high values were observed. In Z. bailii ethanol had no effect up to 20 % (volume fraction). Previous growth of the cells in the presence of benzoic acid or ethanol did not affect the influx of protons in the presence of ethanol. Leakage of compounds absorbing at 260 nm was not observed at 25 degrees C and occurred at 30 degrees C only after a rather long incubation in high concentrations of ethanol, which induced cell death. This suggests that in the yeasts this process does not control the leakage of compounds in the presence of ethanol.