What are the alternatives?

Filtration using diatomaceous earth has been a bastion for many wineries in the UK and around the world for decades. It has relatively low capital costs and running costs, does the job – and at a fairly high flow-rate – is gentle on the wines and pretty forgiving. Vineyard asks if the earth filter still has a place in a modern winery? If not, why not? And what are the alternatives? 

Filtration is an important winemaking tool that is used to clarify wines and ensure that they are microbiologically stable to avoid the risk of re-fermentation and spoilage in bottle. However, there are now countless different filtration methods available, so how does a winemaker decide which filtration tools best suit the wineries production scale and budget? As with most winemaking questions the answer is usually, and frustratingly, ‘it depends’. There are many considerations, including the impact on the environment. 

Filtration technology is constantly evolving and past criticisms that some methods result in flavour losses or strip colour are now generally unfounded. So, with a range of alternatives, should earth filtration still have a place in a winery? Some of the reported disadvantages of earth filtration are that the handling of the dry earth can be considered a health hazard, the disposal of the waste earth post-filtration has an environmental impact, that manual requirements are high as an earth filter is quite complex to operate, it is easy to make mistakes and the cleaning is also slow. In addition, wine losses are often reported to be relatively high on small volumes compared to other filtration methods. 

However, earth filtration has a higher dirt holding capacity than other forms of filtration and a larger area of media compared to, for example, cellulose-based sheet filters. Using sheet filters the flow rate will start high but drop off. However with an earth filter it remains constant and it is possible to get relatively high flow rates. 

 “Wine filtering is not like filtering through a sieve, as the filter method and media uses the electrostatic charge on particles to remove them from the wine,” explained Sam Lindo, from Camel Valley in Cornwall, speaking at a recent WineGB webinar. “At Camel Valley we have now used just about all types of filtration. We started with a sheet filter, have used diatomaceous earth filters, rotary drum vacuum filters and now have a new cross-flow filter, as well as a membrane filter,” Sam added. “We have moved to cross flow filtration as I was unhappy with the dust from the diatomaceous earth, as without sufficient safety information I don’t feel I can trust the product. I wouldn’t want to use it, so wouldn’t expect any of my team to use it,” commented Sam. 

Simon Roberts, Winemaker at sparkling wine producer, Ridgeview Wine Estate, in Sussex, has been using diatomaceous earth for many years. Like Sam Lindo, he is concerned about safety when handling the product and also about the sustainability issues with the waste. “We have been looking at alternative filtration methods for a while and decided we will move to cross flow filtration in the next few months, as the technology is now at a level to make it worthwhile switching.”

Emma Rice, Winemaker at Hattingley Valley in Hampshire, also mainly produces sparkling wine but uses lenticular filtration. “I looked at the earth filter with so many levers, valves and moving parts and decided that the lenticular was a viable option and simple. They are great but less flexible than cross flow or earth filters,” she added. “If the wine is too cloudy, for example if the lees have been disturbed, then we would probably call in David with his mobile cross-flow!” 

Lenticular filtration is nominal grade so a membrane filtration is still required if sterility needs to be guaranteed. “Although not all our wines for blending go through malolactic fermentation, I’m not really concerned that we do not sterile filter for sparkling as the pH is low and pressure in the bottle high, which reduces any risk,” explained Emma. “However, if my contract winemaking clients want to use encapsulated yeast, then sterile filtration is necessary.”

“On first sight, there do appear to be a lot of moving parts to an earth filter and incorrect use can result in depositing unfiltered wine into a clear filtered tank but, like anything in the winery, it becomes second nature as the user learns to open and close the valves in the right sequence,” explained consultant winemaker David Cowderoy. “I do still use an earth filter occasionally for small volumes of wine or a wine that is particularly cloudy. Also earth filters are nominal filters and are not able to guarantee sterility. For sterility you need a 0.45 μm (micron) absolute membrane filter and you may need to carry out another filtration step in order to achieve a filterability index suitable for the wine to pass through the sterile membrane,” he continued. 

When to sterile filter

When deciding if sterile filtration is necessary consider the inherent stability of the wine, advised David Cowderoy, during the recent WineGB webinar, “in particular the potential available metabolites, such as residual sugar and malic acid and also what factors are inhibiting the microbes – such as alcohol level, free and total sulphur dioxide, and the wine’s pH. A red wine with no residual sugar, no malic acid, an alcohol level of 14.5% and pH of 3.1 is essentially stable and probably does not require filtration. However, a white wine with 10.5% alcohol and 12g of residual sugar, like most made in England, would be at high risk if not filtered.”

“A wine may look clear but yeast and bacteria are not visible to the naked eye. In order to achieve sterility a wine needs to pass through a 0.45 μm membrane so that yeast and bacteria cells are removed before bottling,” added Emma. 

David also explained that it is a misconception that SO2 is a biocide: ”At levels found in wine it is actually biostatic, so it doesn’t kill, it just inhibits the growth of yeast and bacteria. Yeast cells can remain dormant. The reaction of yeast cells to SO2 is to produce acid aldehyde which binds free SO2. Over time, in bottle, the free SO2 drops, particular with a wine under cork as it allows oxygen ingress, or a screw cap without a tin liner. When the level of free SO2 drops to a critical level, it all ‘kicks off’, the wine is no longer protected and fermentation problems can start in bottle!” 

Cross-flow filters

A commonly asked question is if cross flow filters remove flavours from the wine? However, winemakers comment that wines in England are generally considered to improve with filtration. Early versions of cross-flow filters may have had a poor reputation but the technology has improved vastly. Trials have shown that there is little difference for young white or young red wines. “If you consider that your wines are not tasting as good in bottle after filtration, it may not be the filtration process and it is worth evaluating all the winemaking processes,” advises Sam Lindo. When considering environmental impact, the cross-flow filter produces the least waste and very little solids. 

Simon Roberts plans to filter the wine through a cross-flow straight after cold stabilisation. “With warmer weather at harvest we now do not routinely put the wines through malolactic fermentation. This means that we will use a membrane to sterile filter after the cross-flow filter.”

Sam advises that all filtration methods work better if the wine is properly prepared, so that it is as clear as possible, “if in doubt do an extra racking as it will save time. Also, there is no point in trying to go too fast and a slower rate will usually result in getting more wine through the filter.” As well as going too fast problems can be due to colloidal and turbidity issues,” he adds. “However, the cross-flow really is a filter that can filter pretty much everything,” exclaimed Sam. “There are now options for smaller wineries with single modules.”

When preparing the wine, David advises that: “Pectins can be a problem at filtering so the use of pectolytic enzymes during juice handling is usually advantageous for both filtration and, for sparkling wine, disgorging. If in doubt, do a pectin test. If there is an issue with botrytis and glucans, then using beta-glucanases in the juice can help.”

The membrane is only as good as its integrity test

Essentially all filters, other than membrane filters, should really be considered as pre-filters or nominal filters, as the only way to achieve a microbially stable wine is to use an absolute membrane cartridge as the final filter pass before bottling. “Achieving sterile filtration usually requires a pre-filtration using 1 μm beforehand to avoid clogging the 0.45 μm membrane filter,” explained David. 

The membrane cartridge filter works more like a sieve and the wine passes through a pore size of 0.45 μm. However, membranes can fail, and the damage may not be apparent visually, so the operator needs to check that the membrane is fit for purpose in order to guarantee that it is removing the potential spoilage organisms.

There are two current forms of integrity test that are used in wineries. The first is known as the Bubble Point test, but according to Peter Riddell, from Integrated Processing Technologies Ltd: “The Bubble Point test is not a particularly useful or reliable test.” Peter recommends the Pressure Decay test. “All manufacturers of membrane filters should have validated their filter to provide a maximum allowable diffusional flow value that indicates the membrane is fit for purpose.” The test method uses the fact that a wetted membrane holds water in a very thin layer, due to hydrophilic properties. Peter explains that: “Once test pressure is achieved, the filter housing can be closed off on the inlet side and held for a period of time. The gas, air or nitrogen, will diffuse through the wet membrane resulting in a small loss of pressure. CO2 should not be used as it is too soluble. The extent that the pressure falls can be related back to the maximum allowable diffusional flow specified by the manufacturer, which correlates directly to the filter’s ability to remove micro-organisms,” explained Peter. This pressure loss is the principle of the pressure decay test. 

“Some people just wait for the membrane to block, but what they don’t realise is that if the membrane is damaged, it won’t block. They will be merrily filtering away, completely unaware that the membrane is useless and that they are not actually filtering at all,” exclaimed Peter. 

When it comes to dissolved oxygen (DO) pick up, the filtration process is often blamed, but David Cowderoy comments that: “DO pick up does not necessarily vary with filtration method but is usually due to other factors. It could be simply that the hose connections are only hand tightened and oxygen is able to penetrate or it may be that inert gas is not being used to protect the wine in tank.” However, he does consider that earth filtration does have an increased risk of DO pick up, as the dosing tank on the side is exposed to oxygen. He advised that it’s important to make sure that any filter housing is fully purged with CO2 and that as water is drained after cleaning it is displaced with CO2, not air.

The diatomaceous earth filter works well, it’s been around a long time, but it seems that the safety information for the diatomaceous earth powder is lacking and precaution is advised for the handling and disposal of the powder. However, for a small winery with a limited budget “the earth filter does have the advantage that it is quick and easy, particularly for small volumes of wine. You can set it up with a pre-coat in 30 mins, its only takes about 30 mins to filter the wine, and then another 30 mins to clean up after,” explained David Cowderoy.

Simon Roberts agrees: “I still think that perhaps there is a place for earth filtration for smaller producers and smaller budgets. But sustainability is important for all of us, whatever size, and waste is a big consideration.”