25º Congresso Brasileiro de Microbiologia

25º Congresso Brasileiro de Microbiologia

ResumoID:1708-1

Área: Fermentação e Biotecnologia ( Divisão J )

EFFECTS OF ACETIC ACID ON THE YEAST CELLS DURING GROWTH ON SOLID AND LIQUID MEDIA

Karen Fernanda Oliveira (IQ-UNESP); Maria Olivia Campos Masiero (IQ-UNESP); Vinicius Paschoalini Silva (IQ-UNESP); Cecilia Laluce (IQ-UNESP)

Resumo

Acetic acid is a minor by-product of fermentation by Saccharomyces cerevisiae; but inhibitory amounts of this acid can be excreted by acetic bacteria during ethanol production. Acetic acid bacteria give rise to more toxic contaminants to the yeast cells than lactic acid bacteria. Acetic acid is also the major toxic component in sugar-cane bagasse hydrolysates. Inhibition by acetic acid would increase at pH values below its dissociation constant (pKa of 4.74). The non-dissociated form of the acetic acid diffuses across the cell membrane passively penetrating into the cells limiting growth, due to its high solubility in the phospholipid portion of the plasma membrane. Acetic acid causes yeast cell death showing cellular phenotypes similar to necrosis and apoptosis. High sugar concentrations also induce apoptosis. It was recently shown in Saccharomyces cerevisiae that depletion of glutathione or exposure to low external doses of H₂O₂trigger the cell into apoptosis, whereas depletion of reactive oxygen species (ROS) or hypoxia prevents apoptosis. In the present work, the toxic levels of acetic acid that inhibit the growth yeast cells were determined on solid YPD medium containing acetic acid at low pH. A synthetic liquid medium was also improved by adding glycine, zinc sulfate and vitamins in order to show differences in growth of the yeasts strains with the acetic acid additions. Yeast strains of our culture collection were assayed on solid medium showing that the growth in the presence of added acetic acid was dependent on the yeast strain, glucose concentration, added acetic acid and pH. When the glucose was sterilized with the components of the solid medium, inhibition of the growth was observed at increasing glucose concentrations. On the other hand, the increases in sugar concentration encouraged the yeast growth when glucose was sterilized by filtration prior to its addition to the medium. An initial pH of 3.5 was seen to be more selective to the assay of resistance of the yeast strains to acetic acid in solid medium at 30°C. In liquid medium, the final pH increased from 2.6 (w/o acetic acid) to 3.7 after 8h growth in the presence of acetic acid (33 mM to 165 mM) without drops in viability. This suggests that the buffering capacity of the acetic acid/acetate buffers (pH ranging from 3.7 to 5.6) was able to protect the yeast cells against losses in viability.

Palavras-chave: Saccharomyces cerevisiae, fermentation inhibition, acetic acid, growth inhibition, resistance to acetic acid

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