January 2009 Issue of Wines & Vines

Advantages of Plastic

Evolution of the plastic tank in the winery

by Richard Carey, Ph.D.

  • Numerous innovations in plastic tanks are now available, especially for small to mid-size wineries.
  • Plastic tanks save space, money and trees, compared to stainless steel tanks and wooden barrels.
  • Steel tanks allow potentially degrading air transfer to accommodate pressure changes, while plastic tanks simply flex with pressure.
  • Plastic offers a cheaper alternative to stainless, as well as energy savings from its lighter shipping weight.

For decades the wine industry has used concrete tanks, stainless steel tanks of all sizes, large and small wood tanks made of trees ranging from redwood to oak to chestnut, and a few glass-lined tanks. Fiberglass tanks have not been widely used in the United States because of concerns about oxygen transfer and the cost of labor for construction, but several Italian companies manufacture these types of tanks for use in Europe. Plastic tanks did not make any substantive penetration into the wine industry until the mid-1980s.

In the past few years, numerous innovations in plastic tanks became available to the wine industry, and especially for small to mid-size wineries, which can use tanks in 2hL (55 gallon) to 22hL (600 gallon) sizes. The focus of this article will be the advantages of using plastic tanks in the wine industry.

Early plastic tanks

Wine Tanks
This Flextank Pallet Tank is enclosed by a Schutz galvanized cage for structural support. As many as five units can be stacked on top of one another.

One of the earliest pioneers of plastic tanks for the American wine industry was Pasco Poly Inc., then in Washington state. Numerous winemakers in the Washington area asked Dave Rule, owner of Pasco Poly, to provide an inexpensive tank for wine storage, and this simple request started the introduction of plastic tanks into U.S. wine industry.

Various materials and construction methods were used to make the first plastic tanks, with variable results. Unfortunately, the early plastic tanks imparted a non-wine aroma to the wines stored in them for extended periods of time. These tanks had been used to store water for agricultural purposes. There had been no need to subject the tanks to the fine discriminations in aroma profiles of the water they stored for agricultural needs, but it was critical for the wine industry.

The main source of the off-aroma turned out to be the various plasticizers used in making the tanks. While this defect was corrected many years ago, it has taken a long time for winemaking personnel to understand that plastic wine tanks do not have to affect the aromas and tastes of the wine stored in them.

The tanks produced by Pasco Poly have been plastic variations of rather traditional wine tanks and have varied from about 20hL (500 gallons) to 160hL (4,300 gallons). Pasco Poly, now located in Idaho, has made several modifications on the traditional wine tank, including its "Killer Chiller" for cooling wine in a plastic tank and "Pump Under" tank for cap management.

Flextank--purpose-built plastic wine tanks

Flextank, an Australian company, began making oxygen-permeable wine tanks several years ago and now has multiple international locations, including an office in Athens, Ga. Where Pasco Poly concentrated on larger format tanks, Flextank has developed smaller tanks that range from the 22hL (600 gallon) stackable "Wine Cells" to 2hL (55 gallon) "Wine Tubes."

There are numerous reasons why these plastic tanks are attractive for the wine industry. First of all, they are designed for use in the wine industry; second, the tanks provide a maximum use of floor space; third, oxygen can permeate through the walls of Flextank plastic tanks because the polymers used in their construction allow for what Flextank calls "controlled permeation," or CP; fourth, the tanks are sealable; and fifth, these tanks provide significant cost savings.

Space saving

Wine tanks
A schematic drawing of 10 Wine Cells stacked in two layers of five each, which hold the equivalent of 100 wine barrels with a much smaller footprint.

One of the least expensive ways to increase a winery's capacity is to increase the height of its tanks. However, premium wines are not usually made in massively tall tanks, and tall narrow tanks are the most expensive. In addition, round tanks take up more floor space than a square tank of equal height. Filling the space with barrels is marginally better at space utilization, and barrels are both expensive and labor intensive. Barrels do have an advantage in that they can be stacked many layers high.

Flextank developed the "Pallet Tank" (shown on page 78) to solve space problems, control cost issues and manage tank issues without sacrificing wine quality. The most important fact about this tank and others in the Flextank family of small plastic tanks is that they function as a barrel in just about all ways except for their construction material.

If a winery has 15 feet or more of ceiling height, the equivalent of 20 barrels of wine can be stored in two Wine Cells and take up 29 x 82 inches of floor space. The drawing at left shows a stack of 5 x 2 Wine Cells that are equivalent to 100 barrels of wine. A four-high, two-barrel rack system would take about 155 square feet for 100 barrels, whereas the Wine Cell would take only about 85 square feet. The best use of space however, is the Pallet Tank, as a winery can get storage densities more than twice that of either the Wine Cell or barrel storage systems.

Tank permeation

Elsewhere in this issue, Tim Patterson discusses the redox potential of wine closure systems. A screwcap seals a bottle as effectively as the lid on a stainless steel wine tank. In contrast, one of the important aspects of barrels is the ability to maintain an oxidative environment for wine. Historically, barrels have been unique in their natural ability to meter oxygen in a controlled way that allows the gradual changes necessary for the development of the flavors and aromas desired in a finely aged wine. Now, the use of plastic tanks gives winemakers an even better container for more accurate control of a wine's development at a fraction of the cost of a barrel, when measured over the lifetime of the container. Oak amendments can then be used to add the flavors that are desired in fine wines.

The chart above shows the oxygen transfer rates of various tanks, procedures and wine closures. It is important to note the range of oxygen transfer in old vs. new barrels. The average is virtually the rate of the Pallet Tank (1,100L) or the Wine Cell (2,300L) tank. The current smaller tanks appear to allow oxygen to transfer at a rate closer to a new barrel, which must be taken into consideration when storing wine in tanks smaller than 1,000L.

The larger the specific lot of wine, the more cost effective the labor invested in that wine will be. There is a fixed amount of time that it takes to hook up a source vessel to a destination vessel. The greater the number and the smaller the unit of transfer, the greater the cost allocated to that lot of wine. In the case of Flextank, these plastic "barrels" are about 1,000L. In comparison to oak barrels at about 200L, the setup time is reduced by 80%, which lowers the cost of a Flextank-aged wine in comparison with oak barrels.

Other than the oxygen permeation into wine, barrels are used to impart the flavor of the oak after toasting. It is generally assumed that the oak flavor achieved from a barrel is limited to the 8 to 12mm on the inside of the oak stave (the average stave is 25 to 30mm thick). That would indicate that the flavor contribution to the wine from the oak comes from only 25% to 33% of the oak in a barrel. The use of plastic tanks designed for maturation of the wine combined with oak amendments is a "green" solution: Virtually 100% of the oak is used in a controlled manner to give the flavor desired by the winemaker.

Tank sealability

Plastic tanks have another feature that makes them more useful than variable capacity tanks. One of the problems of variable capacity tanks is the seal at the top. Leaks frequently occur, and the seal deflates, exposing the wine to air and microbiological contamination. The wine may degrade and ultimately reduce the profits for the winery. This also happens with standard winery stainless steel tanks because the pressure vacuum release valve at the top allows air to go in and out based upon daily wine volume changes in air pressure inside the tank. It is common knowledge that winery tanks have about as much relative strength as a beer can with respect to the wine pressures in the tank, and so they must have a method to relieve even the slightest differential pressure.

Liquids expand and contract in volume directly with temperature. For example, water has a thermal coefficient of expansion of 0.00021ml/1°C at 20°C. This number equals the change in volume of water either one degree more or one degree less than 20°C. If a 1,000L tank were completely sealed from the outside, and the cellar air warmed the wine inside from 17°C to 18°C, the wine inside the tank would change its volume by 210ml. The larger the temperature change, the larger the volume change, resulting in a bulge and possible rupture of a wine tank--if it were completely sealed.

However, no standard wine tank is completely sealed, and stainless steel wine tanks are designed to let that volume of air ebb and flow. This air reacts with the wine at the top of the tank and reduces the effectiveness of the free SO2 in the headspace. The exchange of SO2 with air may allow surface film yeasts that have come into the tank along with the air to grow and prosper in the headspace.

The Flextank, like a barrel, is sealed. The same temperature variations occur, but the plastic is flexible and allows for changes in volume without danger of rupture under normal temperature fluctuations in a winery environment. In this type of tank, the wine is microbiologically maintained in the same condition as when it was placed in the tank. Larger plastic tanks, however, are effectively inert to oxygen transfer due to thick walls needed for structural integrity.

Cost savings

Anyone purchasing stainless steel equipment for the wine industry is well aware that the cost of stainless steel has gone through the roof. Plastic offers a cheaper alternative as well as an energy savings, in that it is lighter in weight from a shipping standpoint.

In addition, there are significant savings that occur with the use of a plastic tank instead of oak barrels. Using the Pallet Tank as an example, there is up to a 90% savings on cooperage costs by choosing the Flextank alternative. A barrel imparts flavor for three to five years, and a Flextank claims a useful life of at least 20 years. In the real world, the useful life numbers would most likely be lower for barrels and higher for the Flextank, as long as it does not suffer from abuse, such as "forklift puncture disease."

Other cost comparison studies show that a Flextank Pallet Tank barrel is about 12% of the cost of an equivalent volume in conventional oak barrels. The savings are due to the cost for the actual wood used in wine production and the labor saved in handling the amount of wine.

Future Articles

Subsequent articles will discuss the newest innovations in small plastic tanks, including pallet rack tanks, wine skins and a new 300L (80 gallon) Dexter tank system, as well as innovations in large format plastic tanks.

Manway and specific fittings

One of the important factors in Flextank plastic tanks is that they are designed for the wine industry. They have a 17-inch manway in the top of the tank. The lid has a screwed bung that can seal the tank or can be made to fit a fermentation lock. The manway can be sealed by a clamp fitting, and the valve flange fitting at the bottom can be outfitted to match any winery hose configuration.
Wine Tanks
The tanks can have a standard sample valve installed in the front recess to protect it. A patent has been applied for on a newly designed, simple sample valve that can be placed in the tank recess as well. To take a sample, the valve head can be pushed in for as long as necessary. The flow stops when the valve head is released. A safety lock prevents accidental release as well as a lock open so that inert gas can be bubbled into the tank.


Richard Carey, Ph.D., is president of Vitis Wine Center and winemaker for Tamanend Winery in Lancaster, Pa. He has written numerous articles on new technologies for the grape and wine industry as well as a series of articles on laboratory analyses in Wine East magazine. Please send your comments and questions about this article, as well as ideas for the basis of future articles, to Carey at rcarey@vitisresearch.com.

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