Winemakers continue to learn how to best apply filtration. As research continues and technology improves, the options available have grown with the industry over the past several decades. Depth filtration, also referred to as dead-end filtration, has been utilized by winemakers for centuries.
During the process of depth filtration, wine is directed in a perpendicular flow towards the filter medium (directly into it, thus the term 'dead-end'). The retentate, particles removed, is captured inside or against the filter medium, while the clean wine passes through. The retentate gradually builds up over time. Pressure inside the filter increases as the flow rate decreases. The filter eventually becomes plugged, forcing the operator to stop filtration and clean the filtration setup before re-starting.
Pressure-leaf, plate & frame, lenticular, and cartridge are the most common forms of depth filters used in winemaking.
The basic design of these filters is a bell housing containing a metal screen. Free-floating diatomaceous earth (DE), or an alternative product such as Perlite, is layered onto the screen to create the filtration medium.
Various coarseness of DE allows these filters to be used for a range of porosity, though they cannot be used for sterile filtration. Pressure-leaf filters are quite popular with medium and large wineries due to their high flow rate and ability to handle wines with higher solids levels. The relatively high cost of the equipment and volumes of wine loss during operation make this a poor choice for smaller wineries. Pressure-leaf filters also require properly trained staff to operate the machine. Inhalation of DE is another concern as this can pose a health risk, so proper handling is essential.
Plate & Frame Filters
Commonly referred to as pad filters, plate & frame filters consist of a stainless steel frame that holds a varying number of plates (usually 20, 40, or 60). Filter pads are sandwiched between the plates to allow wine flow through alternating plates/pads. They can also be fitted with a crossover plate to allow two different levels of filtration on a single pass.
Most pads available today utilize DE as the filter medium (there are also cellulose-based sheets), meaning handling is significantly easier and cleaner than with free-floating DE. Pore size can be chosen based on the level of filtration desired, and plates/pads can be added or removed based on the amount of wine to be filtered.
Plate & frame filters are designed for small-batch filtration, making them an excellent choice for smaller wineries or smaller filtration runs. They are often considered labor-intensive when compared to lenticular filters due to more technical and time consuming setup. They are also associated with higher oxygenation, or wine loss depending on winery protocol, since they lack an external housing and inevitably leak.
Lenticular filters operate very similarly to plate & frame filters, but pads are replaced with discs and are stacked vertically as opposed to horizontally.
Lenticular filters are typically considered the next step up from plate & frame filters as they are also designed for small-batch filtration, but have a much smaller footprint. Instead of being open to the air, the discs are completely enclosed in a housing; oxygenation is not a concern, and discs can also stored and re-used (pads must be discarded after a single use). Lenticular filters are also much easier to setup, operate, and clean than pad filters, which saves time and decreases the need of a skilled operator.
The price difference in equipment is pretty negligible, though the discs themselves are more costly than the equivalent in pads. A cost-benefit analysis is really required based on the specific winery's organization and staffing.
Cartridge filters are the best suited for home winemakers. The filter housings and cartridges are relatively inexpensive, but low flow rates and poor ability to handle wines with high sediment levels make them unfeasible in most production settings.
That being said, they are very commonly used by small and large wineries as polishing filters, in-line from the storage vessel to the bottling line. This is often a method of ensuring sterile filtration up to the filter cartridge (basically, everything until the bottling equipment itself).