If you look at my UnTappd account you will see that I have been bitten by the sour beer bug. I love their complexity and variety as well as the tradition and unusual fermentation process and their very rich history of production. Having talked to a large number of people it looks like this is a beer style which is having a rapid increase in demand. I was fortunate enough to be sent some samples from Petrus and have used them as the basis for this post, with a focus on blending.

The production of sour beer requires a large amount of patience with the beer ageing / fermenting in oak casks or foeders for a number of years to give enough time for the micro-organisms, lactobacillus, pediococcus and brettanomyces to name a few, to be able to give the beer its unique sour and funky flavour characteristics. Why do sour brewers use wooden casks? Well they create a great environment for the micro-organisms to replicate in as well as allowing some oxygen into the container to slowly oxidize the beer and change the flavour profile. 
Choosing to use either casks or foeders comes down to a number of choices.
1. Do I have space for 220 HL foeders?
2. Assuming you have space, how much oak character do I want in the final product. By using foeders you reduce the amount of beer in contact with the oak and therefore, reduce the oak character in the beer.
3. Am I going to be able to brew enough beer to fill a foeder, a foeder holds in excess of 3500 pints! So that is a lot of beer to brew at once.

With a fermentation and ageing that can take a number of years you can imagine that the flavour of the beer is going to differ each time you brew and age it. This makes getting a consistent product pretty hard to do. Sour beer brewers have come up with a clever way of getting round this, blending. This is where the brewer will blend different amounts of a number of casks to get the desired flavours which they are after. By doing this they can really dial in on a consistent product by using beer of different ages and from different containers and their excellent palates.

So hopefully you can now see why blending is such an important part in the production of sour beer. Petrus from Belgium have since 1894 been creating sour beers using Saaz hops, pale malt and house yeast aged in 220 HL foeders for two years. Since starting their sour beer program Petrus have created a unique mixture of wild yeast and bacteria in the wooden walls of the foeders which they use to produce three core range beers, Aged Pale a 100% foeder beer, Aged Red which is 15% foeder beer and 85% double brown with cherries so very kriek in style and an Oud Bruin 33% foeder beer and 67% young brown beer. What you notice straight away is that apart from the Aged Pale both the Aged Red and Oud Bruin are blended beers. What makes Petrus a little bit different is that they want you to make your own blends at home!  They recommend you start with the master brewers favourite of 50/50 Aged Pale and Aged Red before getting mixing yourself. As a scientist I really like to idea of a bit of an experiment and so I went a bit mad on the blending front before settling on my favourite blend of 75/25 Aged Pale and Aged Red which I felt had a good sour hit tamed slightly by the sweetness from the cherries making a great summer weather beer. Go ahead and get your hands on some Petrus sour beer and let me know what your own personal favourite blend is.

If your interest has been piqued but are new to sour beers and haven't drunk one before please follow the three sip rule so that you don't try it and decide you don't like it. In short the rule is have three sips of the beer as the first one is going to be a shock to your taste buds and will make you pull a pucker face. Sip two is where your tongue starts to get used to the acidity / sourness and sip three is where you can now distinguish between the flavours present and really start to enjoy this excellent style.

In the UK Petrus Sour Beers are available from James Clay, the independent importer of specialist beers and through Boutique Bar Brands, London's forward thinking premium drink's management agency.