Dead-End Microfiltration Properties of Slurry of Bakers' Yeast Disrupted by Bead Milling

Dead-end microfiltration experiments were carried out at a constant pressure of 98 kPa by using slurries of bakers' yeast disrupted by bead milling under a variety of disruption conditions of frequency and milling time, and the results were compared with those obtained with untreated yeast slurry. The effect of the pore size of microfiltration membranes on the flux decline behaviors was examined by using membranes with nominal pore sizes of 0.1, 0.2, and 0.45 mu m. The flux decline behaviors of untreated slurry were similar to each other, irrespective of the pore size of the membranes used. However, in microfiltration of slurries treated by bead milling, the flux decline for the 0.1-mu m membrane was quite noticeable compared to the cases using 0.2- and 0.45-mu m membranes because the cell debris rejected by the 0.1-mu m membrane contributed to a pronounced increase in the specific filtration resistance of the filter cake formed on the membrane surface. Such flux decline became marked with the increase in the frequency and milling time in bead milling. The typical flux decline behaviors were described by a blocking filtration mechanism followed by a cake filtration mechanism, judging from the characteristic filtration curves for the blocking filtration laws, although cake filtration started from the beginning of filtration under the severe disruption condition. The average specific filtration resistance of filter cake was closely correlated to the product of the cube of frequency and the milling time, which was a measure of the cell disruption energy. In order to clarify the major factor influencing the significant membrane fouling, slurry of yeast disrupted by bead milling was separated into two portions of supernatant and sediment by centrifugal sedimentation under various centrifugation conditions, and the experimental data of microfiltration of supernatant and sediment slurry were compared with those obtained with the original slurry of disrupted yeast cells. As a result, it was demonstrated that the cell debris with the size larger than 0.047 mu m caused a significant flux decline observed in microfiltration using the 0.1-mu m membrane.