To: Regional Wine Producers

From: Bruce Zoecklein, Head, Enology-Grape Chemistry Group, Virginia Tech

Subject: Sterile Filtration

Sterile bottling is not expensive in dollars, but it does require planning and concern by the winemaker and everyone working in the winery.

Sterile filtration is the first step. Procedures for this are widely available from every filter sheet and/or membrane producer. If membranes are used, they must be preceded by a filter system which performs 99.9% of the work. Additionally, membrane filters must be bubble tested, before, (possibly during) and after each day's run.

If the membrane fails the integrity or bubble test, everything produced between that time and the previous test is suspect.

The "bubble point" is that gas pressure at which the surface tension of water in the capillary pores of a saturated filter is overcome and gas is allowed to pass through the pores. It is directly dependent on pore diameter. The bubble point test is a final check and will determine if leaks are present anywhere in the filtering system. The bubble point test should be run immediately after the holder is assembled and while the filters are still wet. It also should be used before the daily run and directly afterward as a means of checking the integrity of the system.

1. Close all valves.
2. Connect one end of a short flexible piece of tubing to the downstream vent valve on the membrane holder and submerge the other end in a container of water just under the surface.
3. Attach one end of a flexible hose, capable of withstanding 50 psi pressure, to the upstream vent valve.
4. Open the top and bottom vent valves and slowly increase pressure. Initial bubbling from the submerged end of the hose is not necessarily significant because it may be the result of air displaced by pressure in the system. However, prolonged bubbling at any constant pressure below the bubble point indicates a leak in the filter system. Most membrane producers can supply you with the bubble point pressure for each type of cartridge and micron size. When cartridge filters are being tested, the pressure at which steady bubbling occurs will normally be somewhat less than the bubble point specified for the membrane pore size, owing to interfacial forces at the seal between the membrane and cartridge end caps. This small reduction in bubble point does not mean the tube is defective.

Sterile filtration may also be achieved with responsible sterile filter sheets. Throughout the run, maximum flow rates must be rigorously observed. If a filter requires sheets with internal holes, close attention should be paid to ensure that there is no bypassing of the sheet in the vicinity of the holes. The final filter, be it sheet or membrane, must be sterilized before each bottling run.

Finally, at the end of the run, winemakers should take out each sheet and look at the downstream side of the pad. Sheets can be weakened on installation or use.

A major source of ruptured filter sheets is back pressure shock caused by no upstream support and rapid closing of valves, shutting off of pumps, pulsation of pumps, etc. In any of these cases, chances are good that an experienced eye looking at the used sheet will see the characteristic flaw or a dark line.

Every square millimeter of material in contact with the wine, from before the final filter to the bottle, must be thoroughly sterilized. To simplify this, the entire system should be analyzed. All fittings which are not absolutely necessary, should be removed. Get rid of all the extra valves, which are not needed for the proper functioning of the wine transfer and filling operation. Quick disconnects should replace threaded fittings wherever possible. All lines should be made so that they will drain thoroughly and properly with no liquid pockets.

I prefer hot water as a sterilant. Chemicals, unless they work on thoroughly bright surfaces, can be shielded from the organisms. (Chemical sterilization and cleaning will be discussed in a subsequent issue.)

Sterilization should be done at the end of a run, to prevent organism buildup. The procedure should be repeated before the next run.

Under either system, hot water or chemical sterilization before the run, the equipment must be thoroughly cooled and/or rinsed with sterile water. If rinse water is without suspended solids or micro-organisms, it can be introduced immediately before the membrane filter, otherwise it should be introduced before the sheet filter.

The bleaching process during cork manufacture in Portugal or Spain makes corks sterile at that instant. Unfortunately, corks are subsequently subject to contamination in the winery atmosphere. Sterile-finished corks in sterile bags are a must where there is residual sugar. New glass can carry micro-organisms, but it is relatively rare. If any biological contamination is even suspected, bottles should be rinsed with a SO₂ solution before use.

During the bottling run, filler tubes and spacers should be sprayed with 70% ethyl alcohol prior to a bottling and hourly thereafter. Cork jaw locks should be sprayed hourly with 70% alcohol.

If a spillage occurs, it should be cleaned up immediately and the area hosed with a water/cleaner/sanitizer mixture.

Some wineries use corkers with heated jaws. Heated jaw corkers normally run at 170°F, which is adequate to inhibit organism growth after splash, but it should be remembered that such temperatures are inconsistent with standard paraffin lubricant on corks. With heated jaws, siliconed corks should be used.

I believe that the alcohol spray mentioned above is a reasonable alternative to heated jaws. All things listed above are tedious, time consuming and above all, they require planning and management follow-through. But they are not expensive. Certainly, they are far less costly than two months of retention and/or dumping, refiltering, rebottling or even the anxiety of waiting for the axe to fall.

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Dr. Bruce Zoecklein
Associate Professor and Enology Specialist
Head Enology-Grape Chemistry Group
Department of Food Science and Technology
Virginia Tech
Blacksburg VA 24061
Enology-Grape Chemistry Group Web address: www.vtwines.info or www.fst.vt.edu/zoecklein/index.html
Phone: (540) 231-5325
Fax: (540) 231-9293
Email: bzoeckle@vt.edu