​Filtration Mechanisms

Filters mechanically block larger particles from passing through a filter medium, while allowing smaller particles called the filtrate to pass through.  Generally, the beverage industry uses three kinds of filters in sequence. At each step, the cost per particle removed increases, and the particle size becomes more precise. Thus, the goal of each filtration step is to remove as much particulate as possible to preserve the life of the next filter for as long as possible.
  • Depth filters are used to remove the bulk of the particulate being filtered out.
  • Pre-filters (a specialized depth filter) are used to protect the membrane filter from any larger particles that may have escaped the depth filter. It is also more precise at particulate removal than the previous depth filter.
  • Membrane filters are used to remove a precise particulate size.
  • Turbidity
  • Temperature and Electrical Charge
  • Effective Filtration Area and Pore Size
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Turbidity’s impact on filterability
Turbidity refers to a liquid’s cloudiness and uses a nephelometer to measure light’s ability to pass through the liquid in nephelometric turbidity units (NTU). This measurement can be used to determine how the suspended material may block filtration media. However, the relevancy of the measurement is limited, as filterability is also impacted by colloids.

Filterability is a fluid’s ability to pass through a filter without a drop in pressure that would indicate clogging. It is measured by the filterability index: The difference of time needed to filter 200 ml and 400 ml of wine at 2 bars through a membrane Ø 25 mm with a porosity of 0.65 μm.2 This is impacted by turbidity, as well as by colloids, which can cause low filterability even if turbidity is low. These are macromolecules that can drop out of solution and form web-like clusters by binding to other charged particles and molecules. The colloids common in beer and wine include polysaccharides, proteins, mannoproteins, pectin, and hemicellulose. Liquids to be filtered with high colloidal concentrations can be referred to as “colloidally challenging” due to their filterability issues.
​For more on the filterability index read this article by Blue H2O Filtration:  https://bhftechnologies.com.au/understanding-filterability-index/

Given the discrepancy between turbidity and filterability, Scott Labs recommends filterability tests, though the company acknowledges they are not always practical. If not used, they recommend [3]:
  • Allowing colloidal webs enough time to break up with help from gravity.
  • Adding a filterability enzyme or fining agent during finishing to accelerate the time process. Filterability enzymes derived from the grape fungus Aspergillus niger include pectinases which degrade pectins, glucanases which degrade glucans, cellulase which degrade cellulose, and hemicellulase which degrades hemicellulose4.
  • Clarification to reduce larger particles, since the more clarified and filterable the product, the more efficient (fewer passes needed for filtration).
  • Residual particles can form a layer on the filter called the “filter cake” or “gel layer” and restrict filtration in a process called binding. Back-flushing is occasionally an option for unclogging the filter, however, it is limited to some circumstances which will be covered below.
Temperature
In cold or “chill” filtration, low temperatures promote the coagulation or crystallization of some compounds. This is common in the alcoholic beverage industry to promote protein and crystalline salt stabilization.

Electrical charge
The filtration media influences adsorption. Adsorption can be a positive or negative attribute, because it can help to remove unwanted compounds, but may also remove the desired ones. It also can increase the likelihood of membrane fouling. Membrane fouling mechanisms will be discussed in a later section.
Effective Filtration Area (EFA)
The total usable area of the filtration media that is exposed to the flow of liquid. This is typically higher in depth filters than in cartridge filters.

Pore Sizes of Filters
Filters are typically classified by their effective pore size. This represents the size of the particulate in microns (μm) that a filter removes, with anything smaller able to pass through as filtrate (the fluid that is filtered). The classifications of pore sizes are:
  • Nominal pore size. This is the general size of a particle trapped. Some companies will specify the exact percentage of particulate removed of the nominal pore size. Nominal pore size also means the filter cannot be integrity tested, which is “the quality or state of the complete membrane in perfect condition” and used to track incremental changes in membrane damage [5].
  • Absolute pore size is when the pore size is constant. This can be integrity tested.
Sterile Filtration [6]
Filtration is commonly used in alcoholic beverage production to remove yeast and lactic acid bacteria that can cause undesired changes in the bottle. This is more prevalent in the wine industry, as distilled spirits are microbially stable. Lactic acid bacteria has a maximum alcohol tolerance of 10-21% ABV [7], and beer is often microbially stabilized via refrigeration through the supply chain or by pasteurization. For sterile filtration, the following pore sizes are typical:
  • French winemakers .65 and 1.0 μm
  • Other alcoholic beverage producers: 0.45 μm
  • Pharmaceutical and non-beverage industries: 0.1- 0.2 μm [8]
Picture
*For illustration purposes only

​Depth Filters

  • Filtration mechanism: A labyrinth of channels in the filter media that trap particles. Due to the high volume of filter media used in the depth filtration process, it provides ample surface area to remove a large quantity of particulate. This makes it ideal for primary filtration.
  • Pore size: Nominal pore size. Depth filters can have unloading, which is when particles larger than the nominal pore size migrate downstream due to a system shock like a temporary increase in pressure.

Filter Beds

  • Loose filter media
  • ​Frame with a screen or cloth
  • ​Filter sheets or pads
  • ​Filter modules
  • Pre-filters
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Loose filter media (no filter bed)
This only works if the filtration media functions through adsorption. For example, activated carbon.
  • Construction: None. In this process, the fine particulate is mixed into the liquid being filtered to increase the surface area contact with the filtration media in a short period of time. It is then filtered out.
  • Benefits: A low-cost form factor, since it is bulk material. It can also allow for more time in contact with the filtration medium than other form factors.
  • Drawbacks: More labor intensive because it can be messy, and removal can require significantly more processing than if used in other forms. These labor costs can make the overall cost of using powdered filters not economical.
Frame with a screen or cloth
  • Construction: A rectangular frame filled with the filtration media. A screen or cloth is used on either side of the frame to hold the media in place.
  • Benefit: This form factor only requires the purchase of the bulk material.
  • Drawback: Can be messy and labor intensive to work with.
Filter sheets or pads
  • Construction: 4–5 mm (0.16–0.2 in) thich cellulose sheets which can be impregnated with another filtration media.
  • Benefit: Easy to use, as they are pre-formed.​
  • Drawbacks: Sheets can quickly become full of particulate and no longer able to properly filter (binded), depending on the turbidity of the liquid.
Filter modules
  • Construction: Modules are layers of filtration medium which act as stacked filter beds.
  • Benefits: Modules are completely enclosed in a housing, thereby preventing atmosphere exposure. They also contain significant amounts of filtration media in a small footprint, allowing for high volumes of filtration.
  • ​Drawbacks: High cost, although this can be offset by labor savings, depending on the volume being filtered.
Pre-filters
A type of depth filter used to remove any particulate that escaped the primary depth filter. This protects the more expensive membrane filters.
  • Construction: Melt-blown polypropylene filter in a cylindrical housing is common. Membrane pre-filters are also an option.
  • Benefits: Has a better defined filter bed, which lessens the ability for particulate matter to pass through.
  • Challenges: Cannot hold as much particulate as a primary depth filter.

Depth Filter Media

In a depth filter, filter media, sometimes called filter aids, are the materials that do the filtering.
  • (Wood) Cellulose
  • ​Diatomaceous Earth
  • Perlite
  • Polypropylene
  • Activated Carbon
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(Wood) Cellulose [9]

Composed of: Plant fibers, which are manufactured using a wetlaid process similar to paper making. Coffee filters, for example, are made of cellulose.

Mechanism: Functions by trapping contaminants and other particles within its matrix of fibers.

​Porosity is defined by: The tightness of the matrix.

Benefits [10] 
  • Can be made from renewable fiber sources and is biodegradable.
  • High strength and durability.
  • Low impact on color and flavor, and it does not release ions of calcium, iron or other haze or crystal-causing metals.
  • Chemical and temperature stability.

Drawbacks
  • The cellulose fibers have inconsistent diameters, which create unpredictable pore sizes and low porosity.
  • Prone to biofilm/pathogen build-up if used for extended durations.

Common Filtration Form Factors
  • Frame and screen filters.
  • Filter sheets made of cellulose impregnated with diatomaceous earth.
  • Lenticular filter cartridges.
Diatomaceous Earth (Kieselguhr/“Earth Filtration”)

Composed of: Diatoms, a microscopic, unicellular algae with cell walls constructed of silicon dioxide (silica).

Porosity is determined by: The coarseness of the diatomaceous earth as it influences absorbency, as measured in Darcy. One Darcy is equal to 1 ml/s. For a commercially available example of different loose diatomaceous earth particle sizes. See Eaton chart.

Benefits
  • Inexpensive.
  • Has a negative charge, which means higher colloidal retention. This can also be a bad thing, because it can lead to color stripping and retention of desirable aroma compounds.

Drawbacks
  • Dust hazard, especially as crystalline silica can cause silicosis, a type of lung disease. This makes working with and disposing of this filtration media difficult.
  • Can impart iron into the filtrate.
  • Can be color stripping to wine.

Perlite [11]
Composed of: volcanic glass, which is pulverized, then rapidly heated to about 900 °C (1,700 °F). This causes entrapped water molecules in the rock to turn to steam, expanding the particles like popcorn and softening the glass. The jagged, interlocking structures create billions of microscopic channels between the filter aid particles to produce optimum flow rates and clarification abilities.

Benefits
  • Lightweight, with 30-50% less bulk density relative to diatomaceous earth.
  • Inert, therefore it imparts no taste minerals or odor to liquids being filtered.
  • Virtually insoluble in mineral and organic acids at all temperatures.
  • Spent filter aid cakes from wineries, breweries or other food-related industries may be added to animal feed and has been approved for this application in the United States by the Association of American Feed Control Officials (AAFCO) and European Union.

Drawbacks
  • Generally contain no, or very little crystalline silica (whereas diatomaceous earth does), however it still can be a dust hazard.
  • It does not have a precise particulate size.
  • Not a renewable material.​

Polypropylene
Composed of: Polypropylene which is melt blown, a process which uses a stream of high-velocity air to blow a molten thermoplastic resin from an extruder die tip onto a conveyor, or what is called a take-up screen. 12
Porosity is determined by: The tightness between the matrix.

Benefits
  • The matrix can be tighter than cellulose, due to a smaller diameter fiber.
  • More resistant to bacteria than cellulose.
  • Easy to work with compared to other depth filters, due to its pre-formed structure and defined pore size.
  • Has a low adsorption of polyphenols because of its low polarity.13
  • Less prone to adsorption effects, resulting in higher measurable flux rates and longer service life of the respective filtration modules in wine clarification.14

Drawbacks
  • Can be costly relative to diatomaceous earth or pearlite, if filtering large amounts of particulates.
  • Cannot remove as much particulate matter relative to the other types of depth filters.
Activated Carbon [15]

Composed of
  • A large surface area, consisting of macropores (>25 nm), mesopores (1 nm < D < 25 nm), and micropores (<1 nm).
  • The pores work by adsorbing many of the volatile byproducts of fermentation, including higher alcohols, cyclic compounds, and ethers.16
  • Not all activated carbon filtration is the same, since the type, amount, size, and the duration of exposure of the liquid to activated carbon all impact its efficacy.
  • Porosity is defined by: Typically not specified.

​Activated carbon removes:
  • “Organic admixtures from the ethanol solution and ensures the catalysis of a number of chemical reactions (oxidation, esterification, isomerisation, hydration, etc.)”17
  • Adsorbs a wide range of polar compounds, especially phenols and their derivatives.
  • Effective in removing off-odors, especially mercaptan off-odors.

Benefits
  • Non-specific as to what it adsorbs. This can be good or bad, depending on the goals of its usage.

Challenges
  • Acetaldehyde, which can be described as having a tart flavor reminiscent of green apples, is not easily filtered by activated charcoal.
  • Excessive filtration can cause excessive reduction in desirable color and/or aromas.

Form Factors
  • Powdered activated carbon (loose).
  • Filter sheets of cellulose impregnated with powdered activated carbon.
  • Lenticular filtration module of cellulose impregnated with powdered activated carbon.
  • Charcoal filtration and the Lincoln County process:
    • Large pieces of charcoal are used to filter out the liquid. This means less surface area, therefore is a more gentle activated carbon process than other activated carbon techniques.
    • Charcoal Production Process:
    • Charcoal is produced by burning wood until it is charred into coals. The coals may then be reduced to the desired particle size. In the case of Tennessee Whiskey, the Lincoln County Process specifically requires the charcoal to be made from “maple,” according to the State of Tennessee Public Charger 341 House Bill 1084/Senate Bill 1195.18
    • The liquid being filtered is passed through charcoal. The duration and methodology of filtration is dependent on the producer.
    • The liquid is then filtered to remove any charcoal particulate.
    • The specifics of the Lincoln county process as it applies to Tennessee whiskey will be discussed in a future issue of Hawaii Beverage Guide. More can be found on the topic at: Kerley, Trenton, "Characterization of Changes in Whiskey Odorants by the Lincoln County Process. " Master's Thesis, University of Tennessee, 2019. https://trace.tennessee.edu/utk_gradthes/5636

​Types of Depth Filters

  • Plate-and-Frame (sheet) Presses/ Filter Presses
  • "Mash Filters"
  • Candle Filter
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Plate-and-Frame (sheet) Presses/ Filter Presses [19]
One of the oldest styles of filters, this method consists of:

Process:
  • Pressure is used to pump the liquid through the filter bed.
  • After operation, filtration is temporarily halted to allow removal of the accumulated filter aid and retained material.

Benefits:
  • Commonly in operation, as it is one of the oldest methods of filtration still in use.
  • Can be low-cost to own.
  • Is generally reliable, and the filter sheets are relatively low-cost and easy to find.

Drawbacks:
  • Some of the liquid being filtered inevitably leaks out, which is a flaw of ALL filter presses.
  • Exposure of the filter media and product to contaminants in the environment.
  • High maintenance costs, as plates can become damaged and are not easy to clean. Also, lots of parts are susceptible to degradation.
  • High labor cost, due to the time required to load, unload, and clean the press.
  • Filters are typically one time usage.
  • Large footprint.
  • Sheet filters have less surface area than other powder filters, making them suitable for secondary or polishing filters.
Scott Labs has a great video on operating a sheet filter:
www.youtube.com/watch?v=A8veNLKB90o&t=5s
Mash Filter
Plate-and-frame filters can also be used with a very rough filter size to separate the wort from the spent grist (solids) in place of a lauter tun vessel.20 For a great video on mash filters, watch: Advantages of Mash Press Filtration by Brewmation Brewing Equipment at: www.youtube.com/watch?v=J7nS-5keyb8

Benefits:
  • Can handle grist of up to 100% adjunct because barley malt husks are not required to act as a filter medium, as is the case with a lauter tun.
  • Can handle very fine hammer mill crushed grist, which increases extraction.
  • Quickly filters large volumes of wort, allowing for increased production.

Drawbacks:
  • Compared to a lautering vessel, a mash filter can be difficult to use if a brewery employs a wide variety of mash sizes, because of its relatively small range of optimal loading.
Candle Filter
(a type of DE/Kieselgühr Filter)
Named after the filter element’s long, thin, cylindrical shape, early candle filters were developed by Wilhelm Berkefeld of Germany in the 1890s for water purification. Usage began in the brewing industry in the 1950s and 1960s. [21]

Filtration Medium: Diatomaceous earth is most common. Perlite and/or activated carbon can also be used.

Filter Bed: The candle itself is a porous metal or ceramic tube, which is coated by the filter medium but does not perform any filtration.

Filter Frame
  • A cylindrical pressure vessel contains internal “candles,” which are typically top-mounted, but can also be bottom-mounted.
  • An attached mixing vessel where the diatomaceous earth is added.

Process: [22]
  • The filter operator adds diatomaceous earth to the dosing tank, where it is combined with water. Once mixed, it is pumped through the filter and onto the candles to form a precoat.
  • Once the precoat is formed, the slurry (liquid being filtered) is injected into the vessel from the bottom so that the air in the vessel becomes compressed. The increasing pressure forces filtrate first through the filter media on the candles, and then through the center of the candle before it exits through a mounting plate, or manifold, holding the candles and out of the filter housing. The solids collect on the filter media and form a filter cake.

Benefits:
  • Vertical rather than horizontal screen filter.
  • Can remove high concentrations of particulate

Usage
Predominantly used for clarification of beer that has been fermented and lagered, although other applications, such as sterile filtration or wort filtration, are possible.
  • Pressure Leaf Filtration
  • Rotary Vacuum Drum
  • Lenticular Filters
  • Bag Filters
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Pressure Leaf Filtration
(a type of DE/Kieselgühr Filter) [23]
Filtration Medium: Diatomaceous earth is most common. Perlite and/or activated carbon can also be used.
Filter Bed: Filter screens coated with the filter medium.
Filter Frame: A bell housing contains the metal screens.

Process:
  • Diatomaceous earth is added to the dosing tank, where it is combined with water. Once mixed, it is pumped through the filter and onto the filter leaves to form a precoat.
  • Once the precoat is formed, the slurry (liquid being filtered) is injected into the vessel from the bottom so that the air in the vessel becomes compressed. The increasing pressure forces filtrate first through the filter media on the candles and then through the center of the leaves, before it exits through a mounting plate or manifold holding the leaves and out of the filter housing. The solids collect on the filter media and form a filter cake.

Benefit
  • Has the highest depth, adsorption, and surface filtration capabilities.
  • High flow rate and high capability of processing wines with higher levels of solids.
  • Filtration media is relatively inexpensive.
  • A layer of powdered carbon can be added to simultaneously remove flavor and color during the depth filtration pass.

Drawbacks:
  • Cannot be used for sterile filtration.
  • Ideal for medium to larger wineries because of:
    • Relatively high cost of the equipment.
    • Relatively high volume of wine loss during operation.
    • Requires a highly skilled operator to produce a consistent filtrate quality each time.
  • Due to DE and other filter aids being powdered, inhalation is a concern which can pose a health risk.
  • Maintenance can be expensive, as the equipment has built-in feed and recirculation pumps that must be explosion-proof for safe operation.
Picture
Plate Filter Press Image provided by Della Tofolla
Picture
Filter Sheet Images provided by Scott Labs
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Filter Press Images provided by Scott Labs
​Rotary Vacuum Drum
(a type of DE/Kieselgühr Filter)
Filter Medium: Typically diatomaceous earth.
Filter Bed: A hollow rotating drum, precoated with filter media.
Fiter Frame: A perforated, hollow rotating drum, with a basin underneath and an attached dosing tank.

Process [24]
  • The filter operator adds diatomaceous earth to the dosing tank where it is combined with water. Once mixed, it is pumped through the filter and onto the drum to form a precoat.
  • The bottom portion of the drum is immersed in the liquid being filtered. The wine is drawn through the filter bed into the drum and out through the outflow. Once exhausted, the filter aid and particulate matter is shaved off the drum as it rotates.
  • Video of a rotary vacuum drum: www.youtube.com/watch?v=V-1oufa3a6w

Benefits:
The scraping of the drum allows for higher volumes of solids to be removed. This makes rotary vacuum drum filters useful for the recovery of product from settled lees that may otherwise be discarded.

Drawbacks:
  • High labor costs for operation.
  • High potential for oxidation. Aeration is difficult to limit, since the drum is partially raised out of the liquid being filtered during rotation.
Picture
Rotary Vacuum Drum Filter Image provided by Della Tofolla
Lenticular Filters
(Stack Filter/ Disk Filter)25
The name comes from the convex shape of the lentil bean; lenticular filters are a modern and efficient form factor for filtration.
Filtration Medium: Layers of cellulose with the option of diatomaceous earth, activated carbon, and resin formed into a filter module.
Filter Bed: One or more cylindrical filter modules stacked on each other.
Filter Frame: A cylindrical-shaped housing with a center post that is both the filtrate intake and what the filter module sits on.

Process:26
•Fluid is pumped into the lenticular filter housing through the inlet. The liquid being filtered passes through the filter module and into the center post before exiting through the outlet.
•For a great video on lenticular filtration usage: Scott Laboratories Lenticular Filter Setup and Usage: www.youtube.com/watch?v=AldbLDW1EtY
•For technical insight into the operation of commercial lenticular filtration products, read: scottlab.com/lenticular-filtration-operating-instructions-2

Benefits:
•Lenticular filters have a smaller footprint relative to screen or plate and frame filters, because the layers of filtration media in each module are more efficient than filter sheets. This makes them portable.
•Back flushing is enabled by the space around the filter module and allows for the regeneration of the filtration media. This can be contrasted with sheet and frame presses where media is generally one-time use.
•Can be stored between usages, enabling reuse.26
•Higher yield on final product, as it should not drip and does not retain as much residual product as sheet presses. Nitrogen or carbon dioxide can be used to expel residual liquid, depending on what is being filtered.
•Can come in multiple grades/sizes.

Drawbacks
•Filter media is more expensive, but the cost can be offset by reuse and regeneration of the media, the higher yield, and the ease of operation as a function of time savings.
•Plastics are used in the construction of the material, making them not as environmentally friendly.
Picture
Lenticular Filter Housing and Module Images provided by Scott Labs
Bag Filters

​Filtration Medium: Cellulose or polypropylene.

Filter bed: Essentially a sheet filter in the shape of a bag.

Filter Frame: None though a filter bag housing can mitigate oxygen exposure.

Process: The filter bag is placed over the end of a hose or spout. The bag captures the solids, and functions similarly to the lint bag at the end of a washing machine hose.

​Surface Filtration/Membrane Filters

Surface filtration occurs within the first few millimeters (at the surface) of the thin film material (the membrane). There, pores of a specified size allow the filtrate to pass, while rejecting the residue larger than the pore size. For this reason, membrane filters are more precise at filtering out a specific particle size than depth filters; however, they are best used for filtering out small quantities of material.
Membrane terminology 
Semi-permeable membrane: This just means that particles smaller than the membrane can pass through, while particles larger than the membrane are rejected.

Pore size: If the pores are of a defined size (for instance, up to 5 μm), filtration is said to be “absolute” to the pore size. They are also integrity-testable to confirm functionality. This is done before and after each filtration run by finding the bubble point, the pressure at which a gas can overcome the surface tension of water in the capillary pores of a saturated filter and pass through the pores. [27]

​Reverse Osmosis: Liquid is pushed by a pump through a .001 μm membrane. In beverage filtration, only water and ethanol can pass through while everything else including dissolved minerals are rejected. This can be done using a cross flow filtration system or cartridge filtration system. Technically: “Osmosis is the spontaneous net movement or diffusion of solvent molecules through a selectively permeable membrane from a region of high water potential to a region of low water potential, in the direction that tends to equalize the solute concentrations on the two sides.”28 In reverse osmosis, as the pore size is so small, the natural flow of water would go from the filtered side to the unfiltered side due to osmosis. By applying pressure, the wine, beer, ​or tap water is pushed through the membrane thereby reversing the osmosis.

Filtration Mechanisms

  • Membrane Symmetry
  • Filtration Flow
  • Molecule Shape
  • Membrane Polarity and Membrane Fouling
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Membrane symmetry
The pore size throughout the depth of the membrane influences its structural integrity. Membranes can either be:
  • Asymmetric when the pore size gets smaller as it gets further from the membrane surface.
  • Symmetric when the pore size is constant throughout the depth. This provides greater structural stability.

​Filtration flow across the filter can happen in two directions which create similar end products, but allow for different volumes of liquid to be filtered. These are:

Perpendicular flow, or dead-end filtration, is where liquid is pumped directly into the filter.
  • Benefit: Lower cost of equipment.
  • Challenges: More prone to membrane fouling.
​
Cross-flow filtration, also known as tangential flow filtration, is where the liquid flows parallel to the filter surface. This creates a turbulent stream across the surface of the membrane to prevent particles from settling.
  • Benefits:
    • Minimizes clogging and maximizes efficiency.
    • Requires little energy to work.
    • The quality of the filtration is constant over time, because the fouling is reduced.
  • Challenges:
    • High cost for the system. This can be offset by the labor savings of high volume production.
    • For more on cross flow filtration:

Wine Filtration: Cross Flow Technology by Gusmer Wine www.youtube.com/watch?v=cCdupYOBIuo

For great insight into the differences between cross-flow systems read:
www.pall.com/content/dam/pall/food-beverage/literature-library/nongated/How_to_choose_a_wine_crossflow_microfiltration_system.pdf
The shape of the molecule being contained. This is important, as linear molecules have a lower rejection rate than proteins of the same molecular weight, because they may be able to snake through the membrane pores, whereas tightly wound globular coils of protein cannot deform to pass through the membrane pores [29].
Polarity of the membrane material
  • The distinction between hydrophobic (like PTFE and polypropylene) and hydrophilic (like ceramic, nylon and polyethersulfone) membranes are critical, because hydrophilic membranes have significantly increased binding of colloidal material. In particular, Vernhet et al (2002) found [30]:
  • Polyphenol amounts, nature of the deposited molecules and reversibility of the deposit were largely influenced by membrane polarity.
  • Polysaccharides were not influenced by membrane polarity. 
Membrane Fouling
  • ​Membrane fouling has been studied as a result of the impact of membrane’s polarity on clogging. As the plugging of the membrane reduces volume throughput, it has the compounding problem of potentially increasing the exposure of the liquid to oxygen and heat, as well as increasing the pressure on the membrane leading to earlier degradation. Although membrane fouling can be mitigated by back-flushing (running filtrate in a counter flow at high pressure to unclog pores), and through chemical cleaning, the processes are time-consuming and add mechanical and chemical l stresses to the filtration devices, resulting in a loss of capacity and efficiency of the equipment.
  •  Research by Ulbricht et al (2009), focused on external fouling caused by the deposition of macromolecules and large particles on the top of the membrane surface, as this is the primary reason for the loss of throughput. They noted [31]:
    • Pore size and pore distribution of the membrane seem to have the main effect on fouling.
    • The macromolecules congealing on the surface of the membrane are predominantly polyphenols, polysaccharides and proteins, as well as yeast, bacteria and cell debris.
    • Adsorption of the membrane fouling macromolecules occurs less on hydrophobic membranes than on hydrophilic membranes.
    • Proteins have the strongest fouling interactions.
    • Polysaccharides have a “less strong” fouling tendency, but can cause significant flux reductions.”
  • Van der Sman et al., 2012, in a literature review of fouling during beer clarification using membranes, noted that the fouling mechanisms are similar, and in particular the specific major fouling polysaccharides in beer were arabino-xylans, beta-glucans, and when combined with either tannins or with zein, a proline-rich protein, can form ternary aggregates. Additionally, they hypothesized that yeast cells are retained at the membrane surface and form a cake layer, assuming the yeast are not pre-filtered out.
Other factors impacting the filterability of a membrane filter include:
For more on the mechanisms of membrane fouling in wine, read:
Youssef El Rayess, Claire Albasi, Patrice Bacchin, Patricia Taillandier, Martine Mietton-Peuchot, et al. Analysis of membrane fouling during cross-flow microfiltration of wine. Innovative Food Science and Emerging Technologies, Elsevier, 2012, vol. 16, pp. 398-408. Retrieved from:
hal.archives-ouvertes.fr/hal-00787969

For more on membrane fouling in beer, read:
Van der Sman, R. G. M., Vollebregt, H. M., Mepschen, A., & Noordman, T. R. (2012). Review of hypotheses for fouling during beer clarification using membranes. Journal of Membrane Science, 396, 22-31. www.academia.edu/17635905/Review_of_microfiltration_and_fouling_models_for_beer_clarification

Membrane Materials [32]​

​Polymeric membranes form a complex network of fine, interconnected channels.33 The most common are:
  • Polyethersulfone (PES)
  • PVDF (Polyvinylidene fluoride)
  • Inorganic Membranes
  • Membranes for Water Filtration
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Polyethersulfone (PES)34
Usage: Sterile Filter
Affinity for Water: Hydrophilic

Benefits35:
  • Extremely low protein binding properties, and therefore better color retention, especially compared to nylon.
  • Good physical robustness.
  • Higher flow rates than nylon membranes.
  • The asymmetric version of PES is more often used in wine, whereas the symmetric version of PES works better on beer and other more colloidally challenging beverages (Patterson, 2022).

Drawbacks:
For a technical understanding of PES membrane filtration, read:
Alenazi, N. A., Hussein, M. A., Alamry, K. A., & Asiri, A. M. (2017). Modified polyether-sulfone membrane: A mini review. Designed monomers and polymers, 20(1), 532-546. doi.org/10.1080/15685551.2017.1398208
PVDF (Polyvinylidene fluoride) [36]
Usage: Sterile Filter
Affinity for water: Hydrophilic

Benefits:
  • PVDF has the lowest protein and color binding of any common membrane used in cartridges.
  • May be hot-water sanitized, and is compatible with a large range of cleaning chemicals with a pH ranging from 1-14. Has a service life of six to eight years, depending on the membrane geometry, and may be back-washed vigorously.

Drawbacks:
  • Can be more expensive than PES membranes.
  • PES membranes have roughly 3 times the maximum flow rate specification of PVDF membranes, given the same membrane area. However, beer filtration must be sized to achieve the most total throughput, not flow rate.
Inorganic Membranes
Instead of using a rotary vacuum filter, inorganic membranes can be used in cross-flow filtration to remove high amounts of solids. In the alcoholic beverage industry, lees filtration is the primary usage case.
In winemaking, Pall notes the types of lees generated during the process, including [40]:
  • Juice lees (10 -15% of juice production): pre-fermentation solids generated during harvest.
  • Fermentation lees (1-2% of wine production): mainly yeast cells and other particles left after fermentation.
  • Fining lees (2-10% of wine production): solids resulting from wine treatment with additives like bentonite.
  • Crossflow system concentrates (less than 1% of wine production): the residual solids left over after wine clarification crossflow filtration.

Ceramic membranes

Usage: Product recovery from lees

Affinity for water: Hydrophilic [41]

Benefits:
  • More durable than organic membranes, making them ideal for extreme filtration processes, including clarification of high quantities of solids, and filtration at high temperatures or pressures.
  • Back-flushable, and may be cleaned and sanitized with hot water and various chemical agents, including caustic.
  • The capillary diameter and cross-sectional area is medium to high.
  • More easily sanitizable and sterilizable. [42]

Drawbacks:
  • Sensitive to physical and temperature shocks.
  • The membrane cost is up to four times higher than other options.
  • If used in a cross-flow manner, large pumps with a high energy input are needed to create the high velocities required for filtration. As a result, while many suppliers offer ceramic membranes and membrane systems for other applications, few offer them for wine clarification.
  • The potting material (the material holding together the filtration membrane layers) makes it difficult to use ceramic membranes for spirits because it cannot handle the higher proof (Peterson, 2022).

​Stainless steel can also be used as a crossflow membrane for spirits, as the potting material is stainless steel with a titanium coating. Big distilleries still use a lot of pressure leaf filtration (Patterson, 2022).
Membrane materials for water filtration
Many distilleries will use reverse osmosis for proofing water to ensure that there are no ions present in the water that can cause faults like hazes or casse. Unlike in the filtration of colloidal liquids, more membrane materials can be used. These include:
  • Nylon (polyamide) which can bind greater amounts of macromolecules than do the PES membranes. However, this can cause color and flavor stripping in wine, so while it is still widely used in Europe and Canada, it is rarely used anymore in the US. It works well on water filtration (Patterson, 2021).

Membrane Pre-filter materials

  • ​​Glass Fiber
  • Cellulose Acetate
<
>
​​Glass Fiber
Usage: Pre-filter
Affinity for water: Hydrophobic

Benefits [37]:
  • Used to protect downstream membrane filters.
  • High filtration efficiency and dirt capacity.
  • Retains positively charged, small particles, due to its negative charge. This enables the retention of particles even smaller than the nominal filter retention rate.
Cellulose Acetate38
Usage: Pre-filter
Affinity for water: Hydrophobic

Benefits:
  • Low binding affinity for most macromolecules, including proteins.
  • Ease of manufacture, therefore cost effectiveness.
  • Resistant to degradation by chlorine and other oxidants.
  • Can be regenerated depending on the manufacturer specs.

Drawbacks:
Not integrity testable. The last supplier offering these as absolute membrane has stopped supplying the food and beverage market altogether (Patterson).
​For more on membrane materials:
Baker, R. W. (2012). Membrane technology and applications. John Wiley & Sons.

​Types of membrane filters

  • Cartridge Filter
  • Crossflow Filter
<
>
Cartridge Filters

Design:
Can be used for both depth filtration (as a pre-filter) or as a membrane filter, depending on the type of filter cartridge installed.

A cylindrical housing in which the liquid being filtered flows perpendicularly through the cartridge (dead-end filtration). For an insightful video on cartridge filter setup and usage, watch Scott Laboratories Cartridge Filter Setup and Usage: www.youtube.com/watch?v=2T38rdaVae0

Reverse osmosis filtration system
A series of pre-filtration steps before a membrane filter. In the process, water flows from the tap and then:
  • An activated carbon filter removes any color, flavor and taint precursors.
  • A prefilter(s) captures any solids including activated carbon particulates.
  • Reverse osmosis: 0.0001 μm filter removes ionic compounds.

Benefits:
  • Low cost for filter housing.
  • Can also be used for depth filtration, if using a polypropylene depth filter cartridge rather than a membrane filter.

Drawback: Limited residue holding capacity.


Crossflow Filters
Design: Crossflow filtration not only refers to the direction of flow, but is also the name of the system that employs the methodology. The system is composed of cylindrical housings of the crossflow filtration membranes.

Benefits
  • Back-flushable. This allows regeneration (degunking) of the filter by rigorously pumping a small proportion of filtered wine through the membranes in the reverse direction when filtration is reduced by a designated degree. Maintaining a high flow rate with minimal buildup is crucial because:
    • The longer wine remains in the recirculation loop, the more temperature and oxygen are likely to increase.
    • The filter buildup effectively decreases the membrane filtration size, not allowing some desired compounds through the filter. (Reeves and Wyllie, 2005)
    • Back-flushing also extends the life of the filter, due to less filtration stress.
  • The combination of clarification, microbiological stabilization, and sterile filtration in one single, continuous, highly automated operation; and the elimination of the use of diatomaceous earth reduces production costs and the problem of waste disposal, leading to an improvement in work safety and production. [43] 

Drawbacks
The AWRI does not recommend crossflow for sterile filtration. [44]

For a great video on a cross flow filtration system watch this video by Della Tofolla:
www.youtube.com/watch?v=xbUsv1mTefE
What Filtration Does
While the methodologies of filtration are similar across beverage types, what is being filtered varies significantly. For this reason, we will be discussing filtration as we discuss the production process of each beverage. For example see page X for the discussion on filtration of wine.

A note to Hawaii’s alcoholic beverage producers
For Hawaii’s breweries, distilleries and wineries, Maria Peterson of Scott Labs noted that due to shipping costs, regenerative media can be significantly more cost-effective, even though it has a higher up-front cost.

​Sources and Suggested Reading

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2. Wine filterability index. Oenolab Diagnostics. (n.d.). Retrieved May 16, 2022, from https://www.oenolab.com/en/filtrability/30-ftu-eco.html

3. Filter Media Grade Selection. Scott Labs. (2021, June). Retrieved May 16, 2022, from https://scottlab.com/filter-media-grade-selection

4. Scott Labs & Scott Labs. (n.d.). Enzymes Can Make Filtration Easier. Scott Labs. Retrieved May 27, 2022, from https://scottlab.com/enzymes-can-make-filtration-easier

5. Antony, A., & Leslie, G. (2011). Degradation of polymeric membranes in water and wastewater treatment. Advanced Membrane Science and Technology for Sustainable Energy and Environmental Applications, 718-745.

6. Patterson, T. (n.d.). If Filtration 'Strips' Wine, What's Getting Stripped? . Wine & Vines. Retrieved May 26, 2022, from https://winesvinesanalytics.com/sections/printout_article.cfm?content=58981&article=column

7. Gusmer Enterprises. (2018). Wine Filtration: Membrane Filtration Final Bottling [Video]. https://www.youtube.com/watch?v=Ze5qcPCod40

8. Zoecklein, D. B. W. (n.d.). CONTROLLING MICROBIAL GROWTH IN WINE. Retrieved May 26, 2022, from https://www.apps.fst.vt.edu/extension/enology/downloads/wm_issues/Wine%20Microbiology/Micro3.pdf

9. Sterile Membrane Filtration. Pall. (n.d.). Retrieved May 26, 2022, from https://www.pall.com/en/laboratory/life-science-research/sterile-filtration-clarification/sterile-filtration.html

10. Cellulose. Hollingsworth & Vose. (2021, November 23). Retrieved May 16, 2022, from https://www.hollingsworth-vose.com/products/cellulose/

11. Seitz 40 x 40 cm zero-de filter sheets. Scott Laboratories . (n.d.). Retrieved May 16, 2022, from https://shop.scottlab.com/filter-media/seitz-40-x-40-cm-zero-de-filter-sheetsstzpad40zd?returnurl=%2Ffilter-media%2F%3Fcount%3D32

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18.Kyselová, Lucie & Přinosilová, Šárka & Riddellová, Kateřina & Hajšlová, Jana & Melzoch, Karel. (2012). Changes in Quality Parameters of Vodka Filtered through Activated Charcoal. Czech Journal of Food Sciences. 30. 10.17221/361/2011-CJFS. www.researchgate.net/publication/268269931_Changes_in_Quality_Parameters_of_Vodka_Filtered_through_Activated_Charcoal

​19. Bowling, T. (n.d.). Tennessee General Assembly legislation-HB1084/SB1195. Tennessee General Assembly legislation. Retrieved May 17, 2022, from https://wapp.capitol.tn.gov/apps/BillInfo/default.aspx?BillNumber=HB1084&GA=108

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29. Wikipedia contributors. (2022, April 2). Osmosis. In Wikipedia, The Free Encyclopedia. Retrieved 00:52, May 26, 2022, from https://en.wikipedia.org/w/index.php?title=Osmosis&oldid=1080674618

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32. Ulbricht, M., Ansorge, W.R., Danielzik, I., König, M., & Schuster, O. (2009). Fouling in microfiltration of wine: The influence of the membrane polymer on adsorption of polyphenols and polysaccharides. Separation and Purification Technology, 68, 335-342. https://multimedia.3m.com/mws/media/1428688O/membrane-polymer-on-adsorption-of-polyphenols.pdf

​33. Reeves, G. (2005, April). How to choose a wine crossflow microfiltration system. Pall. Retrieved May 17, 2022, from https://www.pall.com/content/dam/pall/food-beverage/literature-library/non-gated/How_to_choose_a_wine_crossflow_microfiltration_system.pdf

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