Chapter 15

Beer Filtration

Membrane Filters

Traditional membrane filters, also referred to as cartridge filters, are the perpendicular flow membranes used for the sterile filtration of beers just prior to bottling. They are called membrane filters since they collect particles at the surface. Membrane filtration is different from depth filters. Membranes allow for a more precise pore size differentiation, so the different levels of product flow can provide the beermaker with choices, depending on the need for clarification. Depth filters rely on the fibrous nature of the path to trap these particles in a gradation of the weave fibers through the filter, starting from coarse to fine from entry to exit.

Membrane Materials

Membrane filter cartridges are constructed out of a wide range of synthetic materials, including cellulose acetate, cellulose nitrate (collodion), polyamide (nylon), polyvinylidene difluoride (PVDF), polytetrafluoroethylene (PTFE), polyethersulfone (PES), and polypropylene (PP).

Membrane Pore Sizes

Membranes are manufactured in a range of different pore sizes and an effective way of maximizing their life is to use them in sequence, the beer passing from larger pore size to smallest. The common sizes in membrane filters are 1.2, 1.0, 0.8, 0.65, and 0.45 µm. Because of their small pore-diameter, membrane filters tend to have a slower flow rate than depth filters. They are also more likely to plug, because most of the filtering occurs at the surface.

Working Pressure

In a membrane filter the flowing particles are blocked on the membrane surface by the size of the holes which specify the filter mesh characteristic. As beer flows into the cartridge, it leaves microscopic particles on its outer surface. During operation, the accumulation of particles on the surface increases the resistance and causes pressure to build up. Pressure gauges should be present upstream and downstream of all filter systems.

Regeneration of Membrane Filter Cartridgess

The membrane cartridge will last as long as it continues to let beer through, while also passing regular integrity testing. The point in which membranes will clog is dependent upon the rough and polish filtration of the beer, as well as the constituents of the beer (e.g., colloids and gums).

Water Regeneration

After filtering beer and while the membrane is still in the filter housing, the membrane is rinsed with cold water (to prevent denaturing and subsequent deposit of proteins), followed by a hot water flush at 40 degrees C (104°F) for 15 minutes, both in the direction of the original flow of beer. Water temperature is then increased to 90 degrees C (194°F) and the filter is sterilized for 20 minutes.

Chemical Regeneration

When a hot water washing cycle does not properly restore flow rate, chemical cleaning may be recommended. Typical chemical cleaners include tri-sodium phosphate (TSP), sodium hydroxide, citric acid solution, peroxyacetic acid (peracetic acid), and many other commercially available clean-in-place (CIP, Section 20.2) chemicals.

Integrity Testing of Membrane Filters

Integrity testing is considered essential in those processes that utilize a membrane filter for bioburden control to assure that the filter meets the removal criteria. Several tests have been developed for testing the integrity of filters. However, the test pressures/flows vary according to manufacturer and type of membrane used and are only valid if correlated to the retention of a specific microorganism, preferably a beer spoiler.

Bubble Point Test

Although effective, a membrane filter has limitations based on length of use, the beer matrix, and volume of beer. Therefore, it is critical to perform an integrity test before and after each bottling run to ensure filtration performance. Filter failure due to a rip, tear, and defect, or plugging will be detected by these tests. Arguably, the most common integrity test being performed is known as the bubble point test.

Pressure Hold Test

A pressure hold test is similar to a bubble point test in that a gas pressure is applied on the upstream side of a wetted filter element(s). Instead of raising the pressure slowly and waiting for the gas passage, a known pressure is applied upstream of the filter membranes, just below the bubble point of the filter.

Forward Flow Test

The forward flow test is similar to the pressure hold test, except in this case, the set pressure is continuously applied upstream of the wetted filter membrane and then the diffusional gas flow through the wetted pores across the membrane is measured using instrumentation.

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