In the April issue (p. 40), I introduced wine filtration as a controversial issue, controversial because many regard filtration as introducing an industrial step in the process of making wine with minimal interference. James Halliday, writing in the Australian Wine Companion, comments that ‘sterile filtration is the most controversial form of what is a controversial subject at best [filtration generally], although the controversy is seldom created by winemakers; it is more frequently due to wine writers who have no practical knowledge of the subject’ (bit.ly/2FWEJlx).
Fighting words from Halliday perhaps. However, given the number of industry articles that address issues such as why and how, I am not sure that it is just wine writers who have an issue with filtration, especially sterile filtration. In essence, there are two stability issues that require filtration – physical stability and microbiological stability – the latter requiring sterile filtration. Matt Holdstock, an oenologist at the Australian Wine Research Institute, has published a succinct note on sterile filtration on the Wine Australia website (bit.ly/2tkqMef). Matt comments on the need for winemakers to know the wine composition when deciding when or if to filter. For example, residual sugar (glucose, fructose) and malic acid are classic food sources for yeasts and bacteria, thereby underlying the need for sterile filtration.
The relationship between sterile filtration and winemaking practice raises a terminology issue. Many winemakers will pass their wine through a 0.45 µm filter and claim that it is a sterile filtration. There are some difficulties with this simple strategy. First, it is essential that the filter used has an ‘absolute’ pore size; that is, it is capable of removing 90–99% (manufacturer-dependent criterion) of particles at or above the stated pore size. Filters with a ‘nominal’ pore size of 0.45 µm may only retain 60% or more of particulate material at or above this pore size.
The second difficulty is that some bacteria in wine are smaller than 0.45 µm and so wine filtered through 0.45 µm cannot be said to be truly sterile. Greg Howell MRACI CChem from Vintessential has described the terminology dilemma rather succinctly (bit.ly/2oQDnAA). Yeasts are generally between 5 and 10 µm, so relatively easy to remove by filtration. Bacteria are much smaller, with size or diameter depending on the shape. Spherical cocci, such as Oenococcus oeni and Pediococcus, have diameters between 0.5 and 3 µm, whereas the rod-shaped bacilli, such as in Lactobacillus, range from 0.2 to 2 µm. Acetobaceter, the acetic acid producing organism, tends to be 0.6–0.8 µm.
Thus, there seems to be a need for a 0.2 µm standard to ensure that filtration produces a sterile product. I am not aware that this is used in routine winery processing and Greg Howell notes (bit.ly/2oQDnAA) that ‘many, if not most, winemakers have a great concern’ about filtration at this size, the argument being ‘that such fine filtration will most likely remove much of the positive attributes from the wine’.
With 13–15% alcohol wines that have low residual sugar, low pH, have been through malo-lactic fermentation and have a good sulfur dioxide regime, the chances of microbial spoilage without filtering are generally low. However, lower alcohol sweet wines with poor sulfur dioxide management are severely at risk of spoilage unless they are filtered before bottling.
Ensuring sterility in the bottled wine does involve more than good filtration. Even with the most careful filtration procedure, microbial contamination can still occur in the final step in preparing the wine for market, the bottling process. Bottling lines are notorious for harbouring yeasts and bacteria and even the occasional mould. Dr Tina Tran and colleagues from the AWRI have discussed the packing process in detail (see bit.ly/2ItTcGk). Good quality assurance practice is necessary to ensure cleanliness before passing the ‘sterile filtered’ wine through the line to the bottle. Hot water above 80°C is reasonably effective in sterilising the lines, as is steam containing caustic cleaning agents, provided all traces of caustic are removed before commencing bottling. Checking places where wine may be trapped, such as joins, seals and
O-rings is always helpful to ensure that old wine has been removed prior to the next bottling run. Water used in the sterilisation process needs to be clean as well. Simple filtration of the water is usually sufficient. While microbial contamination can be checked by plating samples from the bottling wine onto a suitable growth medium and monitoring the growth of colonies, this is a slow process and not an overly useful routine quality check. None-the-less, good QA is far better than having the wine returned as ‘spoilt’ with consequent loss of customer satisfaction.