The lowdown on kettle sours | Rob Lovatt, Thornbridge

Conversely, kettle sours are much more straightforward to produce and their dwell times are pretty much that of any standard beer. That’s not to say kettle sours don’t come without their own particular challenges and this article will discuss my approach to producing this style, rather than the more complex barrel aged sour beers mentioned above. I first tasted a kettle sour, or simple sours as I refer to them, when I was in Philadelphia a few years back. The first impressions of the kettle sour I tasted were clean, sour and relatively one-dimensional, and I guess this is where the term simple sour comes from. On my return I did my research and attempted to brew our own kettle sour.

The basis of a kettle sour is that a standard, unhopped wort is produced. The reason the wort needs to be unhopped is because lactic acid bacteria are sensitive to the iso-alpha-acids in hops, depending on the strain to varying degrees. The wort is then inoculated with lactic acid bacteria for 24-48 hours, until the desired pH is reached. The beer is then boiled to kill off the lactic acid bacteria and is then finished off like a normal beer by the process of a standard fermentation.

The method of souring the initial wort can be approached from different angles. A common approach is to hang a bag of malt in the wort which will have natural lactic acid bacteria on the husks of the grains. I was very hesitant to employ this method, as I was concerned that the inoculum of lactic acid bacteria would be too low and this would enable wort bacteria to join the party. Another popular method is to simply pitch in a sizeable quantity of good quality natural yoghurt! While these methods achieve a degree of success for some brewers, the lack of control over the process is not ideal in a brewery like ours.

My preferred method of souring the wort is to pitch a decent size pure inoculum of lactic acid bacteria. This ensures rapid acidification, greatly reducing the risk of wort bacteria joining the party. We initially propagated at the brewery using ‘Homebrew’ vials as starter cultures. Unfortunately, we often found the starter cultures to be pre-infected with yeast and the wort would sour but also ferment in the kettle. This reduced the overall ABV in the finished product. I decided to then buy in pitchable volumes from the USA from a reputable company, which we have found to be the best approach to producing a sour wort quickly.

The second dilemma we came across was the strain of bacteria to employ. We initially used Lactobacillus Brevis. However, this strain is the predominate strain which infects standard beers! It gave us a good flavour profile and an assertive sourness; but I really didn’t feel comfortable with having that strain around the brewery, despite us having robust QA and procedure to sterilise everything involved with the kettle sour production. We trialled various different strains of lactic acid bacteria, but finally settled on L. Delbrueckii. This strain gave us the flavour profile we were looking for and is very sensitive to iso-alpha-acid, so if it was inadvertently introduced to a standard beer it would cause less of an issue.

There was also the problem with the wort residency time in the kettle. If this was the last brew of the day, it wouldn’t be sour enough to finish off the brew the next day and holding up the brewhouse in a production brewery isn’t really an option! So, we came up with a plan to send the wort over to a dedicated souring tank and dedicated transport hoses etc. We could then leave the wort souring in there; when the pH was in specification, then we could fit it back into the brewing plan at our convenience. The other advantage of this method is that we could send the wort through the heat exchanger and make sure the temperature of the wort was optimum for the lactic acid bacteria.

I have to say the main faults I pick up on kettle sours are due to the infection of the initial wort during souring. Two of the main off flavours I often detect due to wort infections with sour beers can be described as cheesy and ‘vomit-like’. These two flavours can be attributed to the presence of two short-chain fatty-acids called butyric and isovaleric acid, which are produced by wort bacteria. In fact, both of these compounds are present in Parmesan cheese and are the very same ‘sweaty feet’ chemicals contained in our vomit and body odour smell. Preboiling of the wort prior to souring and a fast drop in pH are key to minimising these off-flavours. The aim of the process is to produce a clean, lemony, crisp sour beer which is most refreshing, so the main advice I can give is don’t scrimp on your inoculum!

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