Wednesday 15 February 2012

Don’t fear the filter

Filtration is one of the things which I never understood before I got to see it happen a few days a week. As I didn’t understand it, I say it as a bad thing, something only the big breweries did as part of their flavour-reduction mission, but I was wrong.

This is the filtering process I know (there might be other ways of doing it): unfiltered beer from the tank mixes with kieselguhr, a diatomaceous earth (which Wikipedia explains better than I can). The kieselguhr catches the yeast still left in the beer, clumps it together and this then gets left behind on a gentle plate filter (you can choose how fine or coarse you want the plates to be) which the beer passes through. So it goes in cloudy and comes out clear and the process is there just to remove the haze. But it’s about the flavour and that’s important.

Filtration inevitably removes flavour as it passes through the filter, but what is left behind is just yeast so what you find is that the flavour changes, the body changes and your perception of the beer changes. I can give the three examples I know: a lager, pale ale and wheat beer.

First, the one that’s unfiltered: wheat beer. It’s all about the yeast in the beer and you can taste and feel it when you drink: it’s got a fullness to it, a fatness, a juicy roundness. The yeast is there to give texture, flavour and aroma – it’s very different when it’s not cloudy and it becomes drier and crisper, lacking the creaminess that you want from wheat beer. The beer is tastiest cloudy.

Lager is the best way of showing filtered beer: unfiltered it has a rich fullness to it, a rounded flavour profile, a softness; filtered it becomes dry, crisp and sharp. Lager should have a snappy finish to it and so filtration is key to get that refreshing quality (though I do totally adore unfiltered lager, it’s just a different drink to the filtered stuff). It’s similar with pale ale: you want the hops to be bold but in unfiltered beer you’ve got other stuff softening the bitterness and wrapping it in the roundness of yeast. By filtering you take the roundness and make it sharper. It’s like a sentence which ends with a dash or one which ends with an exclamation mark.

Then there’s fining. Have you ever seen a pint of isinglass – the dried swim bladders of fish – before they go into a cask? The first time I saw it I almost posted back my CAMRA card. I’m okay with knowing that there’s something in my beer which makes it clear, but it looks like a pint of body fluid and makes me think of a story my mate Matt often tells: he was on a big night out and one of the group was sick into his pint glass. A bit later he was back drinking from the same pint: “It’s ok,” he said, “it’s sunk to the bottom.”

Drinkers don’t want opaque beers. Every beer is different: some styles are better filtered, others are better unfiltered, some work best with isinglass pulling the yeast to one side (though only in cask beer). Sometimes I want my lager to be unfiltered but it’s doing a different job to if it’s the crisp and refreshing filtered version; the flavour changes, the body is different, the beers are different, but both are good.

Filtering isn’t beer’s F-word. 

UPDATE: In trying to talk about the good side of filtering, I left out a whole paragraph about the bad side of filtering, because not all of it is gentle and not all of it is done as described above (the DE filtration described is the only one I have experience of). See comments below for more where I stopped. As for the 'triple filtered' Stella Artois, I suggest they need a new filter that does the job once.


  1. I still think it is an F word :) You will loose taste as well as some vitamins. And you need to pay a lot more for the filtration device.

  2. Mark - I think you have over simplified this and slightly missed the point.

    Filtration can be so many different things and ranges between sterile filtration (which for me fundamentally damages any beer)and a light filtration which we do at BrewDog.

    Some brewers only use a DE filter, others back it up with a sheet filter whilst some of the best craft beers coming out of the US at the moment are not filtered but centrifuged.

    The key number is the micron rating which is the beer is filtered too. Sterile filtration is 0.45 microns and as stated above, for me this damages beer and strips out so many of the flavours we all work so hard to put in there. We filter our beers very lightly so as to leave as much of the flavour in as possible.

    Our beers are often slightly hazy and this is because we filter them so lightly. Sterile filtration (an aggressive filtration practised by many brewers) strips out flavour, aroma and mouthfeel from a beer. These are things that we work really hard to impart in our beers, the last thing we want to do is remove them before bottling! We prefer taste over absolute clarity. The shelf life in our beer comes from the sheer amount of hops (which also act as a natural preservative) and the cold conditioning time.

    After we condition our beers for 7 days, the still hazy beer goes to our DE filter for a light filtration to around 5-6 microns, which still leaves some yeast and still leaves all the flavour and mouthfeel in the beer.
    Does this make our beer real ale? Probably, but who really knows anymore. And who actually cares? A new way has emerged with the craft brewing wave that transcends these out-dated conventions.

    So you can't really say filtration is good or bad because it is so many different things. However, for us, sterile filtration is never, ever good.

    1. James, you clearly have no idea about filtration. Judging by the taste of your MANY infected products sterile filtration would at least mean there is chance a customer could taste how your beer was intended to taste rather than the overwhelming taste of Brett or Lactic infection! even a pasteurised beer would be better than an infected beer!

  3. I think your comment in the lager section sums it up - different flltration gives/leaves different flavours and as James says, sterile filtration results in almost no flavour.

  4. If you've not tried them, and you can find them, I totally recommend trying some of the unfined Moor ales. Was lucky enough to be able to compare fined and unfined at Cask about a year ago, and in that case the unfined ale was markedly tastier. However, I wouldn't expect every beer to be unfiltered - horses for courses!

  5. "Sometimes I want my lager to be unfiltered but it’s doing a different job to if it’s the crisp and refreshing filtered version; the flavour changes, the body is different, the beers are different, but both are good."

    You've never spent any time drinking unfiltered lager in German brew pubs have you?

  6. I have never given two seconds thought about the degree of haze in a beer. This is not to say it is wrong to filter but give me mud if it is tasty mud.

  7. James - Fair comment and thanks. I've updated the post - in trying to show the side which needn't be feared, I neglected the side which should be!

    Scissorkicks - I think the unfined Moor beers are great and I'm all for cloudy beer!

    Tandleman - Not in Germany, but in Czech. Unfiltered lager is one of the best tastes in the world but sometimes it needs to be filtered and sharp, for me.

  8. Sterile filtration is used by many decent breweries over the world and is much preferable to pasteurisation. Bottled beer is often either sterile filtered or pasteurised to ensure there is no bacteria or wild yeast which could grow overtime and ruin the product. Only breweries who are very confident with their processes and sterility would go without either especially if they are able to turn their beer around in trade relatively quickly. Or they are quite frankly stupid and don't have any control where their products are being packaged, with no positive release QA system in place.

    Sterile filtration does strip some flavour but is much less damaging to the product than pasteurisation, even with low dissolved oxygen levels pasteurisation oxidises the beer. Beer geeks should take their hats off to any brewer who uses sterile filtration, as its costly and riskier than pasteurisation.There are virtually no big guys in the UK who use sterile filtration. They almost all pasteurise.

    Please can people stop making uninformed comments about subjects where they only have limited knowledge.

    1. Sterile filtration is still an unnecessary step if you have good micro control upstream though. Beer geeks should deplore it just as much as pasteurization. Both are either the domain of larger brewers who aren't looking for the best flavor, or lazy brewers who want an easy way to control contamination. Either way , you are compromising flavor.

  9. Anomymous - i totally agree

  10. anonymous - I actually think James is right on some of this and his point on centrifuging is spot on - difficult for me to comment on what others do but I'm sure all craft (who knows what that means!) brewers aim to extract the best possible flavour from their beers - at Thornbridge we don't filter or pasteurise, we centrifuge, like the US Brewers JAmes mentions, and make full flavoured beers

  11. Mark, I think your description of the process of DE filtration is either a little off, or just a way I have never seen. The metal plates or screens don't do the filtration. they just give the DE a base to build up on. You first have to build up a base coat of DE that creates a depth filter that is like a maze that the beer passes through, catching yeast, proteins, and other compounds. You then slowly dose in more DE from a slurry tank into the beer before it hits the filter, building up the filter layer as your run continues, which keeps the DE layer form getting overloaded and ceasing flow. Of course, there is a limit to how deep your filter bed can get and that limits how much beer you can filter at a time without breaking the filter down and starting over. DE doesn't give you an absolute size of filter, which is why it can't be sterile.

    This is how I learned filtration in school and what we do where I work at least. We tried centrifuging a while back but it beat the beer up too much for us. I don't think sterile filtration or pasteurization are ever positive for flavor, but some brewers do what they have to...