You may or may not remember my blog about brewing a lager – a style which I’ve had plenty of experience drinking but not much experience brewing. Lager brewing poses a lot of interesting challenges for the homebrewer. The grain bill typically contains a large proportion of Pilsner malt which contains a large amount of the precursor to dimethyl sulfide (DMS), a chemical known as S-Methyl Methionine (SMM). As a result, it is important that you do a long and vigorous boil when making a lager in order to drive off DMS – otherwise you end up with a creamed corn flavour in your beer which can be highly off putting (although it is acceptable to a certain extent in some styles like American lager).
Temperature control is the next big hurdle for most homebrewers as lagers are fermented cold and then cold crashed for extended period. I didn’t have temperature control available when I made my lager so I was relying on the cold British weather to do the job for me.
Related to temperature control is the requirement of a diacetyl rest. Diacetyl is an organic compound that is produced as a natural part of fermentation and imparts a buttery/butterscotch flavour in your final beer. In normal fermentations, diacetyl is cleaned up by the yeast towards the end of fermentation but this process is slower at colder temperatures.
When you pitch your yeast into your wort it begins to build amino acids, one of which is called ‘valine’, the precursor to which is ‘acetolactate’. Not all acetolactate will become valine, some may leak out of the yeast cell and into the beer where it will eventually become diacetyl. Higher temperatures speed up this reaction and certain yeast strains are more prone to producing acetolactate than others. Due to the cold fermentation temperatures of lagers, you may taste a sample and it tastes fine only for you to then bottle the beer and find that diacetyl forms in the bottle, ruining the batch. Once you have removed the beer from the yeast the diacetyl cannot be reabsorbed and converted into compounds with higher flavour thresholds than diacetyl, meaning you will be unable to save the batch if diacetyl forms after you have packaged your beer.
To help with this brewers will typically raise the temperature of their fermentation from 10-12°C to 15-20°C for a period of two days once the specific gravity is around 2 points from your final gravity. This allows the yeast to clean up any diacetyl that is present before you begin your cold crash.
For me, this meant moving the fermenter indoors for a couple of days. I’m hoping to have a fermentation chamber built in time for my cold crash so will be able to better control the temperature for that part of this fermentation. Stay tuned for further updates!
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