Copyright 1999
Chapter 16
WINE FILTRATION
Most red wines self-clarify in a few months, and they usually do not require special clarification treatments. Even when red wines are not brilliantly clear, the dark color obscures any slight cloudiness. White and blush wines are different. Here, any lack of clarity will be painfully evident, and most white or blush wines require one or more specific clarification steps.
Cloudy wine is difficult to sell, and sometimes getting a white or blush wine clear and "bottle bright" without using some kind of filtration is difficult. Consequently, practically all commercially produced white and blush wines are filtered before they are bottled.
Of course, home winemakers always prefer to produce brilliantly clear wine, but such clarity is seldom essential unless the home winemaker actively participates in homemade wine competitions.
FILTER TECHNOLOGY
In the past thirty years, great strides have been made in filter technology. Now, a variety of filters are available for removing microorganisms, and these filters can produce sterile wine when used properly. "Molecular sieve" filters can remove color and other large phenolic molecules from wine. Over the past few years, "reverse osmosis" filtration techniques have been developed specifically for the wine industry. These sophisticated ultra-filtration techniques can selectively remove acetic acid and other small molecules from wine.
Desirability
Sterile filtration equipment gives wineries a very useful production tool. Practically all of the yeast cells can be removed from wine by simple, inexpensive filtration. Off-dry and sweet wines can be bottled with little worry the residual sugar will start fermenting, and wine quality does not suffer significantly when the filtration is done properly.
In the past, wines containing excessive quantities of acetic acid were difficult to handle. Often, these wines were added to sound fermentations in small quantities, or acetic wines were sent to the "still house" to be converted into alcohol. Now, acetic acid can be effectively removed by ultra-filtration apparatus, and many other defective wines can be salvaged using modern filtration techniques. In addition, the new filter technologies have significantly reduced wine production costs, and the quality of inexpensive jug wines has improved significantly.
Even so, extreme levels of filtration are not necessary or desirable for normal wines. Many winemakers believe the less treatment a healthy wine receives, the better the quality of the finished product. These winemakers pursue a minimum filtration philosophy for the production of fine wines, but minimum filtration does not mean zero filtration. Producing brilliantly clear white or blush table wines without using some reasonable level of filtration is difficult and sometimes impossible.
FILTER ASSEMBLIES
Both "pad" type filter assemblies and "cartridge" type filters are used by small wineries. Normally, pads are made of depth type materials, but some manufacturers produce special adapters that allow membrane filter media to be used in their pad type filter assemblies. Cartridge type filter assemblies are used by commercial wineries and by many home winemakers. Both depth type cartridges and membrane type cartridges are made to fit standard cartridge housings. Each filter material and each filter type is most efficient when used for the intended purpose, so the manufacturer's recommendations should be followed.
Pad Filters
Pad type filter assemblies consist of a stack of ridged plastic or stainless steel frames held together by a powerful clamping mechanism. Each frame is separated by filter pads. The wine flows into half the frames, through the filter pads and then out of the other frames. Pad type filter assemblies are made of stainless steel and molded plastic. The clamping mechanisms are large, and the materials are expensive. In addition, the frames contain many ridges, so they are difficult to manufacture. Consequently, pad type filter assemblies are expensive. However, when large quantities of wine are filtered, the depth type filter pads are quite economical. Practically all commercial wineries use pad type filters.
Cartridge Filters
Cartridge type filters consist of filter materials made in the shape of a hollow cylinder. The cylindrical filter cartridge is housed in a plastic or stainless steel container. Wine flows into the housing and through the filter cartridge from the outside to the inside. After passing through the filter material, the wine flows out of the housing. Both depth type filter cartridges and membrane type filter cartridges are available. Cartridge type filter housings made of plastic are used extensively for domestic water filtration. These plastic housings are made in large quantities, and they are inexpensive. Filter systems can be built from inexpensive plastic housings simply, so home winemakers often use cartridge type filter assemblies.
FILTER MECHANISMS
Filtration can be accomplished by two different mechanisms. Unwanted particles are attracted and entangled in a porous filter material when the first mechanism is used. This type of filtration is an adsorption process, and electrostatic and adhesion forces are responsible for trapping and retaining the particles. The pathways and cavities in the filter material are much larger than the trapped particles. This kind of filtration is called "depth" filtration because the particles are trapped within the thickness of the filter material.
The second filter mechanism is a simple sieve process. Here, the liquid is forced through a membrane containing many small holes or pores. Pore size is large enough to allow desirable (smaller) particles to pass through the membrane, but the pores are small enough to block the larger, unwanted particles. Filter membranes are made of special plastic materials. The plastic is very thin, and the pore diameters can be made very small. Membrane filters are usually called "sterile" filters when the pores are small enough to remove wine microbes.
Depth Filters
Depth filters are used to remove grape fragments, tartrate crystals, yeast cells and general debris from wine. Depth filters are made by compressing and bonding fibrous substances into a mat or pad. In the past, asbestos fibers were used extensively, but modern depth filters are made from specially prepared cellulose. The cellulose fibers are specifically processed to give them a net residual electrical charge. The long path lengths created by the overlaid fibers trap the particles within the body of the pad. Smaller size particles are retained when the pads are made tighter and more dense. However, liquid flow through the pad becomes slower as the density of the pad increases. Rates of flow for typical commercial filter pads are shown in the Table below. The rates of flow are for water, and they are given in gallons per minute for a one square meter of pad area at a pressure of 10 pounds per square inch.
FILTER NUMBER |
FLOW RATE |
| 2 | 30 |
| 3 | 26 |
| 4 | 20 |
| 5 | 16 |
| 6 | 14 |
| 7 | 11 |
| EK | 5 |
| EKS | 3 |
Depth filter pads are manufactured in several sizes and in bulk. Eight by eight, sixteen by sixteen and thirty-six by thirty-six-inch square pads are common sizes. Sixteen inch pads are the most popular size for small winery use. Pad thickness depends on porosity, but most pads are about 3/16 inches. Depth type filter cartridges are made from a variety of materials including cellulose and spun plastic fibers. Cartridges are made in several sizes and with different end patterns. Double open-ended, 10-inch cartridges are the most popular size. Single open-ended cartridges are equipped with Buna "N" gaskets, and cartridges with gaskets are preferred for wine filtration.
Washing depth filters before filtering wine is a standard practice. Washing removes any contaminates, bad odors or off tastes that may be present. The standard washing procedure is to assemble the filter system and circulate a 1% citric acid in water solution through the system. Then the system is drained, and clean water is circulated. After a short rinse, the water coming out of the filter is tasted, and if the water has a paper taste, washing is continued.
Porosity
Depth filters can be purchased with porosities ranging from 50 microns to about 0.2 microns. Two different scales are used to rate the porosity of depth filters, and a large difference in particle retention ability exists between the two scales. "Nominal" filters are rated using a 50 percent criterion. For example, a 1-micron nominal filter will pass about half the 1-micron particles present. "Absolute" filters use a more stringent standard. Absolute filters use a 10 out of a million criterion. A 1-micron absolute filter only passes ten 1-micron particles out of a million. The other 999,990 1-micron particles are retained by the filter.
Obviously, a 1-micron nominal filter is far more porous than a 1-micron absolute filter. Filter pads are usually rated using the absolute scale. Depth type cartridge filters are rated using the nominal scale unless the rating is specified as absolute.
Membrane Filters
Membrane filters work in a different way. The filter material is a thin, flexible plastic membrane. A special manufacturing technique is used to produce the plastic, and the plastic sheet contains an enormous number of very small holes. A membrane filter acts just like a sieve. Particles larger than the hole size are mechanically blocked at the surface of the membrane, but smaller particles pass right through the holes.
Filter membranes are made from several plastic materials including polypropylene, cellophane and polyester. Since the membrane is very thin, additional mechanical support must be provided, and filter membranes are placed on top of a backing structure to provide greater strength. Membrane filter cartridges are more expensive than depth filter cartridges because of more complicated construction methods and more costly materials.
The porosity of membrane filters is rated using the absolute scale, and porosities are designated by the equivalent size of the holes. For wine industry use, the three most popular membrane filter porosities use 0.65, 0.45 and 0.2 micron hole diameters. Because of the simple sieve action, membrane filters are easily plugged or blocked. To reduce plugging, all wine going into a tight membrane filter must be filtered with a 1-micron absolute depth filter first. Membrane filters can be washed and reverse flushed to remove some of the blocking particles, and these filters can be reused several times.
Membrane filters are used to sterilize wine just before bottling. A membrane filter with a 0.45-micron absolute rating will remove all wine yeast and bacteria from the wine. Membrane filters are commonly used in all commercial wineries, and 0.45-micron membrane filters are often mounted on the bottling line just ahead of the bottle filler.
Filters for Home Winemakers
A filter assembly suitable for the home winemaker can be easily made from standard, 10-inch housings. These filter housings are used to improve drinking water quality. They are made of plastic, and they can be purchased for about $25. Two plastic tubing fittings ($2) are needed to connect plastic tubing to the inlet and outlet ports in the housing. This type of filter assembly can be used with any small transfer pump that can deliver a pressure of at least 10 pounds per square inch. Alternatively, a gas transfer system can be used to move the wine through the filter.
One-micron (nominal) depth type cartridges cost about $4, and they are suitable for rough filtration of all types of wine. Either 0.5-micron (nominal) depth cartridges, costing about $7, or 0.2-micron (nominal) depth cartridges costing $20 can be used for polish filtration. A 0.2-micron nominal cartridge is preferred for final filtration of white and blush wines. Cartridge life can be extended by carefully cleaning up the wine by fining and racking before it is filtered.
This type of cartridge filter assembly can produce excellent wine clarity, and it gives the small producer a very practical filtration system. Best of all, the filter can be easily assembled from standard parts for a few dollars.
SUMMARY
Getting white and blush wines sparkling bright is difficult without using some kind of filtration, but excessive filtration can strip desirable aromas, flavors and color. Home winemakers often use cartridge type filters, and these filters can produce excellent wine clarity.
Commercial wineries use sterile filtration techniques when they bottle wines containing residual sugar. The filter removes all of the yeast cells and prevents fermentation from occurring after the wine is bottled.
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