Kettle/Fast Souring Beers
Fast (wort) souring is a method which has seen a huge increase in popularity over the past few years, especially from commercial breweries who want to produce tart, sour beers quickly and in a controlled manner. Bacteria is added to the wort (usually Lactobacillus) which consumes some of the sugar’s in the wort, producing lactic acid. This lactic acid significantly lowers the pH of the wort and gives the beer a tart, refreshing character.
There are several methods of achieving fast souring but one the most popular is kettle souring. Following this method bacteria is added to the mash run off after the sparge has completed and left until the desired acidity (or pH) is achieved. Once the souring is complete, the wort is then boiled just like any other beer. This pasteurises the wort, kills off the bacteria and locks in the acidity. It also has the added advantage of ensuring there are no contamination issues further down the brew process as all the bacteria is killed. Depending on the strength of the bacteria, souring usually takes around 24 hours which significantly quicker than more traditional methods of sour beer production. The beer will end up at around a pH of 3.4 compared to around 4.5 on a standard beer.
There are several ways of inoculating the wort. A lab pitch of Lactobacillus can be used and there are several available from labs like wyeast, whitelabs and omega labs. Like using fresh yeast, a starter can be made to strengthen the bacteria prior to pitching to speed up the souring process. Live, non fat greek yoghurt is also a good source of lactobacillus. A more traditional method would be to simply add some uncrushed malted barley to the wort as the grain husk carries souring microorganisms but this also the most likely to go wrong due to the risk of other bacteria and wild yeast being present. Most lactobacillus strains work quickest at temperatures between 30c and 40c.
But things than can go wrong. Care must be taken as this warm, sugary wort is a perfectly welcoming environment for other, unwanted bacteria. This can cause off flavours, most common of which are butyric acid or isovaleric acid. These spoilage bacteria create cheese like aromas similar to that of smelly feet or baby vomit. Clearly something to be avoided. Although more research is needed to really determine what causes these off flavours, reducing the oxygen and the starting pH before souring seem to definitely help minimise the risk.
Oxygen can be significantly reduced by first boiling the wort for around 15 mins which also pre sterilises it. Co2 can then be bubbled through which will then sit above the wort, creating a natural barrier against 02 in the headspace. The pH can be lowered by adding commercial lactic acid. Reducing the pH to around 4.5 before adding the bacteria this will create a less favourable environment for other bacteria and also speed up the souring process.
Another advantage of kettle souring over other souring methods is the ability to use hops. Most Lactobacillus strains are not tolerant to the bitterness (IBU) given by hops. Even very low levels of around 5 IBU’s will inhibit their ability to sour. This is actually one of the reasons hops they were originally added to beers. Allowing the bacteria to sour the beer first means that hops can be added to the beer without it adversely effecting the sourness levels reached.
Once the required level of acidity is achieved the wort is boiled, hops added, cooled and fermented with normal brewers yeast, just like any other all grain batch.
Sourness is best measured with a reliable pH meter but can also be measured by tasting. Choice of yeast strain is also important as some are not very well adapted to working in a low pH wort and therefore may struggle to ferment all the sugars, leaving an overly sweet beer.
- Perform a standard mash and sparge (Normally at least 30% wheat is used in the mash)
- Once the wort is collected, bring up to the boil for 15 mins then cool to the desired temperature for your lactobacillus strain. If unknown aim for around 35c as this will work well for all lacto strains.
- Take a small sample and add measured amount of lactic acid until pH of around 4.6 is achieved. Then scale this back up and add the appropriate amount of acid to the whole batch.
- Add your bacteria.
- Put a lid on the vessel and insolate the outside to help maintain 35c for the souring time.
- Bubble a small amount of c02 through the bottom tap of your boil kettle.
- Leave for approximately 24 hrs or until desired sourness is achieved.
- Bring back to boil and boil for 60 mins.
- Cool to yeast pitch temperature (18 – 22c)
- Transfer to Fermentor
- Pitch yeast and ferment as normal.
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