Stuck fermentation on wild fermented cider?

Question

I pressed about 40 L worth of apple juice 12 days ago. Sweet cider was put into a carboy and foamed heavily after 3 or 4 days for about a week with consitent airlock activity.

In the past two days, the thick foam has gone down to the point where it has disapeared, which I guess is normal in wild fermented cider as it starts secondary fermentation. However, airlock bubbling has stopped, which I'm unsure if it's a normal symptom.

1) Does airlock activity diminish when transitioning from primary to secondary fermentation? Should I expect airlock activity to come back in the next following days?

This is how my brew is looking today:

2) I tried the cider. I think it's a bit low in alcohol content. I previously red that ca. 70 % of alcohol content is provided by primary fermentation. In case my cider ends up a bit short in alcohol, can I add some sugar then or is it better to do it now? Could that be the cause of not having any airlock activity?

I'm a beginner, and I hope this questions are still interesting to some of you that want to attempt a wild fermentation. Please forgive me if all this is too obvious for some of the most experienced brewers here!

Answer 1

It is absolutely normal to have a vigorous fermentation for a few days and have it slow down afterwards. The activity of the airlock is by no means a reliable information to know when the fermentation is completed. You really should get an hydrometer (fairly cheap) and measure residual sugar content (gravity). An hydrometer will at least tell you if the fermentation is completed. Knowing the alcohol content just by tasting is unprecise, you can try a vinometer, but that tool requires the wine to be fermented dry (no residual sugar) or it wont work as well.

The thing with wild yeast is that you never know how it is going to ferment. What you are referring to as a secondary fermentation is often just the process of clearing the cider or stabilization, although there is sometimes a malolactic fermentation that occur after the primary fermentation in wines, it is not always the case. In any case, it will never be as vigorous as the first fermentation.

Once you know that fermentation is completed, you can add more sugar to increase the alcohol content if you like, but doing it before and without measuring is risky.

I previously red that ca. 70 % of alcohol content is provided by primary fermentation.

Yes, maybe it was written that 70% of the alcohol production occurs in the first vigorous phase of the fermentation, not to confuse with a total of 70% of ABV in your final drink...