Secondary fermentation in the bottle does not exist

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This article is written by René Schokker for Biernet. He argues that secondary fermentation in the bottle does not exist. In this article he explains all about how he came to this conclusion.

Secondary fermentation in the bottle

Still way too often you’ll hear brewers claim that they do ‘secondary fermentation in the bottle’. I would love to know how exactly they do this, as secondary fermentation is nothing more than the fermentation that takes place after the large fermentation peak (main fermentation) which lasts only for a couple of days.

Secondary fermentation lasts a bit longer, until the yeast reaches it’s maximum fermentation level. What people usually mean with secondary fermentation, is bottle conditioning. This is the process of adding ‘bottle sugar’ to your bottles during the bottling process, which increases the SG value and therefore the present yeast can get to work again. On very rare occasions brewers add ‘bottle yeast’ to the bottle as well. This is almost never needed, unless your yeast is completely exhausted or you waited too long with bottling (several weeks).

When does secondary fermentation start?

Directly after the main fermentation, the secondary fermentation starts. This is the moment where the peak of the main fermentation slows down and goes on in a lower speed. When this is exactly, is hard to say. This depends on several variables such as the type of yeast, the quantity of yeast, the temperature, the condition of the yeast and so on. Usually secondary fermentation starts when you reach about 70-80% of the fermentation rate. You’ll notice that at this stage the water seal will “blub” slower. The secondary fermentation is done when you reach the maximum fermentation rate of the yeast you’re using.

When is it safe to start bottling?

Firstly the lack of activity in the water seal is a good indicator that the secondary fermentation is coming to an end. Secondly you have to be absolutely sure that the fermentation barrel is absolutely airtight and there are no (significant) differences in temperature that can mislead you. No no activity at all in your barrel? Good, time to measure. By measuring the SG value in your beer you will know for sure if your fermentation process is finished. When measuring your SG value, you use the following formula:

(start SG – end SG) / (start SG – 1000) * 100

If the conclusion corresponds with the fermentation level of your yeast type, the fermentation process is ready and so the beer can be bottled. Are you still uncertain after measuring? Let the beer rest and measure again after several days. If you’re close to the fermentation rate and there is no difference in SG value, you’ll be certain that the fermentation process is ready. Before you are starting the bottling process you have to be absolutely sure the fermentation has ended. It is advisable to increase the temperature in the last 2 days, so you are certain even the last sugars are fermented.

When you bottle the beer with a too high SG value (so before the fermentation process is fully done) you risk exploding bottles or gushing (foam will flow out of the bottle once opened). Of course nobody wants exploded beer bottles or gushing: serious waste of your beer!

Another tip for during bottling: it’s fine if your beer is still slightly cloudy, but avoid having too much yeast in your bottle. Yeast dies at a certain moment. Dead yeast cells give a special taste to the beer, which you really don’t want to have. It can also have a negative influence on the expiration date of the foam in your beer.

Bottle conditioning

After bottling, either with or without bottle yeast, the bottle conditioning starts. It is advisable to go through this process (which lasts abour 2-3 weeks) in the same temperature as the main fermentation. Yeast does not take temperature changes too well and every yeast has an ideal fermentation temperature. Furthermore the yeast might work less or stop working at all. After several weeks, when the SG value is back to the level it was before adding the bottle sugar and the beer contains enough carbon dioxide, you can drop the temperature significantly.

At a low temperature the aging process will still continue, but the present yeast will slowly go to the bottom, which gives you a nice and clear beer. Some brewers cold-crash (put it away in 0-4 degrees) their beer to create a nice and clear end result. Cold-crashing can happen during lagering, but also during the bottle condigioning. After the aging process (which takes several weeks) the lid can be taken off.

Read the full original article (in Dutch) here.

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