Effect of ionic environment on the crossflow microfiltration behaviour of yeast suspensions

Comparison of Polypropylene and Ceramic Microfiltration Membranes Applied for Separation of 1,3-PD Fermentation Broths and Saccharomyces cerevisiae Yeast Suspensions

In recent years, microfiltration (MF) has gained great interest as an excellent technique for clarification of biological suspensions. This paper addresses a direct comparison of efficiency, performance and susceptibility to cleaning of the ceramic and polymeric MF membranes applied for purification of 1,3-propanediol (1,3-PD) fermentation broths and suspensions of yeast Saccharomyces cerevisiae. For this purpose, ceramic, titanium dioxide (TiO₂) based membranes and polypropylene (PP) membranes were used. It has been found that both TiO₂ and PP membranes provide sterile permeate during filtration of 1,3-PD broths. However, the ceramic membrane, due to the smaller pore diameter, allowed obtaining a better quality permeate. All the membranes used were highly susceptible to fouling with the components of the clarified broths and yeast suspensions. The significant impact of the feed flow velocity and fermentation broth composition on the relative permeate flux has been demonstrated. Suitable cleaning agents with selected concentration and duration of action effectively cleaned the ceramic membrane. In turn, the use of aggressive cleaning solutions led to degradation of the PP membranes matrix. Findings of this study add to a growing body of literature on the use of ceramic and polypropylene MF membranes for the clarification of biological suspensions.

Behavior of yeast cells in aqueous suspension affected by pulsed electric field

This work discusses pulsed electric fields (PEF) induced effects in treatment of aqueous suspensions of concentrated yeast cells (S. cerevisiae). The PEF treatment was done using pulses of near-rectangular shape, electric field strength was within E=2-5 kV/cm and the total time of treatment was t(PEF)=10(-4)-0.1 s. The concentration of aqueous yeast suspensions was in the interval of C(Y)=0-22 (wt%), where 1% concentration corresponds to the cellular density of 2x10(8) cells/mL. Triton X-100 was used for studying non-ionic surfactant additive effects. The electric current peak value I was measured during each pulse application, and from these data the electrical conductivity sigma was estimated. The PEF-induced damage results in increase of sigma with t(PEF) increasing and attains its saturation level sigma approximately sigma(max) at long time of PEF treatment. The value of sigma(max) reflects the efficiency of damage. The reduced efficiency of damage at suspension volume concentration higher than phi(Y) approximately 32 vol% is explained by the percolation phenomenon in the randomly packed suspension of near-spherical cells. The higher cytoplasmic ions leakage was observed in presence of surfactant. Experiments were carried out in the static and continuous flow treatment chambers in order to reveal the effects of mixing in PEF-treatment efficiency. A noticeable aggregation of the yeast cells was observed in the static flow chamber during the PEF treatment, while aggregation was not so pronounced in the continuous flow chamber. The nature of the enhanced aggregation under the PEF treatment was revealed by the zeta-potential measurements: these data demonstrate different zeta-potential signs for alive and dead cells. The effect of the electric field strength on the PEF-induced extraction of the intracellular components of S. cerevisiae is discussed.