Perpendicular-Flow Filtration Absolute or Sterile

Sterile filtration requires the use of membranes of sufficient porosity that microorganisms are physically excluded. To remove bacteria, membranes with maximum pore sizes of 0.45 pm are recommended. Although attempts have been made to use membranes with pore sizes of 0.45pm (e.g., 0.2pm), flow rates are commonly too slow to be economically feasible. Where spoilage yeasts are believed to represent a threat, some winemakers have relied on membranes with larger pores (0.65—1.0pm). However, 

Membrane filtration utilizes cartridges that when properly cleaned and stored can be reused at a later date. For those cartridges that can be stored, it is common to thoroughly back-flush the membranes twice, first with cold water and then with warm water because the latter can bake-on macromolecules such as proteins. Membranes can be stored wet, most commonly in food-grade 95% v/v ethanol or a solution of citric (1200 mg/L) and SO2 (600mg/L). In all cases, the membrane manufacturer should be consulted for specific storage and back-flushing recommendations. 

To perform a bubble test, the membrane must be kept constantly wet. Gas, normally N2, is then connected to the inlet side of the filter and the outlet is connected to tubing that is placed into a vessel containing water. As gas is allowed to flow into the filter housing, the pressure is slowly increased using the gas tank regulator. The bubble point is that pressure where a stream of bubbles first appears on the outlet or filtrate side of the membrane. If the measured bubble point pressure is less than the manufacturer s specified value, the membrane has been compromised and should not be used. The bubble point should be recorded prior to start-up and at the end of botding as well as after any interruptions (i.e., upon return from worker break periods). 

Like bubble pointing, the diffusion test allows evaluation of the integrity of filter membranes. The membrane is first wetted and gas pressure on the upstream side of the filter is increased to that recommended by the 

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