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Membrane Filtration
Choosing the right system is a cost-effective way to clarify or remediate most wines
Membrane filtration has become a major factor in current winemaking production. Because of the breadth of the topic, this Product Focus about current uses of membranes for wine production will be broken into two parts. This first part reviews the major classifications of membrane usage and their efficacy in the wine-production process and includes a list of the companies that supply equipment. In the second part, we will provide information about the different membrane companies and their specific equipment for use in the wine-production process.
My first exposure to membrane technology took place in 1983 or 1984. Peter Meier from Millipore came to my winery in California to demonstrate the “greatest advent of high-tech equipment for the wine industry.” He brought a demo unit to show me visually what membranes can do. We ran a recently fermented wine through his new device and got a drip, drip, drip of pristinely clear, wonderful wine out the delivery side of the filter.
That event stimulated my continuing interest in this technology. Today membrane technology has evolved and now covers a wide range of processes, from primary fermentation to molecular sieving of good and bad molecules in the almost finished wine.
Membrane filtration is not quite at the magical level in wine production, but it is not that far off. Consider the following: The device can clarify a recently fermented, racked wine to 0.2µ in one step. After that, the same technology can remove volatile acids, and then even remove the ethanol to a point where the “wine” produced is alcohol-free. It is also possible to remove selected compounds based on molecular size and, after that, select molecules based on hydrophobicity versus hydrophilicity. One of the more bizarre aspects of membrane technology is its ability to select compounds based upon their chiral orientation (two compounds that differ based upon their mirror image).
The membranes can separate compounds over a wide range of molecular weights and charges. One can find companies such as VA Filtration, Pall and Koch Membranes that cover the whole gamut of membrane types, configurations and sizes of systems. There are also companies such as Oenodia (Stars electrodialysis/Bipolar), Romfil (microfiltration) or ATP Group (microfiltration) that specialize in one type of system.
The basic membrane filtration process
Membrane filtration is different from conventional diatomaceous earth (DE), pad or lenticular filters. The depth filtration model uses the depth of the filtration media. This model relies on the statistical chance that any molecule will pass through the filter’s tortuous media path. Membranes use a horizontal flow path known as tangential flow filtration (TFF) or crossflow filtration. With this fundamental change in the architecture of the filter media, many winemaking processes can be improved and made much more selective.
Membranes allow for a more precise pore size differentiation, so the different levels of product flow can provide the winemaker with choices, depending on the need for clarification or molecule segregation. The concept of TFF removes one of the biggest obstacles of depth filters—the entrapment of insoluble particles.
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. Interspersed are irregular particles such as DE that trap the wine’s insoluble particles. Once the spaces are “trapped” (or clogged), the filter must be discarded.
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Product flow in membrane filtration is tangential to the membrane surface, which has defined pore spaces and provides a smooth surface on which the product (wine) can move rapidly. A combination of the insoluble particles and the wine’s velocity continuously scrubs the surface. Like a river, there is mass flow in the main body, and the liquid that is actually touching the membrane surface has zero flow. This zero-flow region is known as the gel layer. The main product flow space (the retentate) is subjected to pressure by the system’s pumps, providing the tangential flow. The pressure and speed keeps the liquid and its particles moving, which allows the molecules that are smaller than the pores to pass through the gel layer to the permeate side of the membrane. Statistically, more of the smallest molecules pass through in the beginning of the filtration cycle.
This is one reason some people believe membrane filtration strips out flavor. Continuous tasting of the progress of a batch will show that as the batch progresses, more of the larger molecular weight particles will pass the gel layer to the permeate side, and the flavors of the wine improve. This means the higher the concentration of the batch that can get through the membrane, the more the filter material replicates the original wine. This is an important criterion to judge the effectiveness of a filter’s action. The holdup volume is comprised of the total volume in the membrane system and the maximum concentration of the product until the membranes are fouled to the point that the process must be stopped and the equipment cleaned.
Basic membrane systems
Membrane systems for the wine industry break down into the following classifications: microfiltration, ultrafiltration, nanofiltration, reverse osmosis and electrodialysis.
Microfiltration is the most common production process in the wine industry, since this is the step where general clarification occurs. Particle sizes for microfiltration range from above molecular weight 100,000 to 0.45µ. The pore size most often used in the wine industry for microfiltration systems is 0.2µ, which has been identified as the sweet spot for clarity/product flow/minimum fouling. The 0.2µ pore size is large enough to allow virtually all molecules that are important for wine flavor, structure and enjoyment of wine to pass through. This type of filtration applies to the vast majority of all wines that are filtered. The only real exceptions are bigger, full-bodied reds, which have other options for preservation at bottling to provide safe packaging.
Ultrafiltration comprises the next step down in molecul
ar separation and includes the molecular range from 1,000 molecular weight (mw) to 100,000 mw. Depending on the design of the system, a membrane can be supplied that accomplishes tasks including decolorization of various wines, concentration of phenolic compounds, astringency adjustment and oak extract concentration. However, there is not one ultrafilter membrane that will do all of these tasks.
Nanofiltration filters at the molecular size range from 100 mw to 1,000 mw, which is one of the more difficult filtration ranges because of potential impact on the wine. Anyone who decides to use this range of tangential-flow filtration should be aware of the molecular species that are moving from retentate to permeate side of the membrane and the potential impact that movement will have on the wine. Think carefully about the molecules in wine that are above 100 and below 1,000 mw. All organic acids, esters, terpenes and many small molecular weight phenolic compounds fall into this category.
Nanofiltration should be used as a molecular sieve to remove flaws that small molecular weight compounds cause in wines. If further processing of the permeate stream can remove those compounds and then return the permeate back to the wine, that is a good use of nanofiltration. Use nanofiltration cautiously, because it can irreversibly remove the heart of a wine.
Reverse osmosis (RO) is a process using membranes with a molecular size range less than 100 mw to 150 mw. Within this process range there are two designations: tight and loose RO. In the wine industry, the practical use of RO is for removal of some small molecular weight spoilage compounds such as volatile acid compounds (acetic acid and ethyl acetate). This process focuses on tight RO (less than 100 mw). It is also used for Brett compounds such as 4-ethylguaiacol and 4-ethylphenol. Other smoke taint-type compounds can also be removed (loose RO less than 150 mw). The other common compound targeted for removal by RO is ethanol for alcohol adjustment.
The key to these processes is closed-loop processing for TTB regulatory efficacy. To keep a wine’s identity intact, RO needs to split the permeate stream and run that stream through either resins or a separate stream to a distillation column and then send that alcohol-free water back to the wine.
Unlike nanofiltration, RO does not remove the heart of a wine when it is used as a vehicle to remove VA, Brett or smoke taint. Be sure your RO is used in accordance with TTB regulations so it does not get classified as a distilling unit. Any permanent separation of the permeate stream from the loop will force the classification of it as a still and require operation under a Distilled Spirts Permit.
(Editor’s note: For a more complete discussion of RO, see the column “Selecting a Machine for Reverse Osmosis” in the January 2017 issue of Wines & Vines.)
Electrodialysis
This type of membrane separation uses a specific membrane type driven by electrical current and pore size. In this type of separation, the molecules are driven through a membrane that is either positively or negatively charged. The flow is still tangential, but it is not under high pressure. The driving force is electricity.
The primary use of electrodialysis is to produce cold-stable wines. The membranes are set up in a specific order of a cathode side, neutral product and an anode side as a three-part sandwich made into a membrane stack. When the surface is charged, the cathode attracts the negative component of a salt, and the anode attracts the positive component of a salt. As one component passes through the membrane on one side, a matching component passes on the other, which keeps the neutral product at an electrically neutral condition. The particles exchanged are the respective H+ and OH-.
When a special membrane type and configuration are used (bipolar membrane), the ionic species that pass through the membrane, from the wine’s perspective, are potassium to the anode and tartrate to the cathode. This changes some membrane parameters, and the system can be used to adjust the pH of a wine.
Summary
Many companies supply these types of equipment. It would be in your business’s best interest to first clearly define what type of filtration is needed to improve the quality of your wine and what fits into your budget. Familiarize yourself with the operation of the equipment and be sure to understand what is retained on one side of the membrane and what passes through.
Most manufacturers provide some means for testing their equipment, and don’t hesitate to take the equipment for a test drive. Tangential-flow filtration is a powerful tool; be sure you can harness that power to improve your wines.
Richard Carey, Ph.D., is a wine consultant in Lancaster, Pa., and owner of Tamanend Wine Inc. He wrote a software program to help small wineries keep track of their wine production records and results of laboratory analyses.
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