FILTER TYPES

Filtration can be accomplished by two different ways. Unwanted particles are attracted and entangled in a porous filter material in the first mechanism. 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.

Membrane filters employ a different filter mechanism. 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

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 denser. 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 Table 12. The rates of flow shown are for water. They are given in gallons per minute for one square meter of pad area and at a pressure of 10 pounds per square inch.

Depth filter pads are manufactured in several sizes and in bulk roles and sheets. 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 contamination, bad odors or off tastes that may be present. The standard washing procedure is to assemble the filter system and circulate a 1-percent 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. 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 filter retains the other 999,990 1-micron particles. 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, but depth type cartridge filters are often rated using the nominal scale.

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 Flow Number Rate 2 30 3 26 4 20 5 16 6 14 7 11 EK 5 EKS 3

Table 12. Typical flow rates through filter

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 are 0.65, 0.45 and 0.2-microns. 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, reverse flushed to remove some of the blocking particles and 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 or 0.2-micron (nominal) depth cartridges costing can be used for polish filtration. These cartridges cost about seven dollars and twenty dollars respectively. 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.

Chapter 17

BOTTLING

Light, white table wines and most blush wines are bottled a few months after primary fermentation. These wines are valued for their young fruity characteristics, and they do not benefit from aging, and light. Heavier-bodied white table wines, such as Chardonnay and Sauvignon Blanc, are usually given several months of bulk aging. These heavier white wines and light, fruity red wines are usually consumed when one or more years old. Heavier style red wines are usually given one to three years of bulk aging before they are bottled, and then the highest quality red wines are often several years old before they are consumed.