- Sulphur is an important flavour compound
- Sulphur left in the yeast cell when growth slows becomes H2S or SO2
- Reduce sulphur at that critical point by :
- controlling mineral sulphur,
- increasing FAN or
- increasing the temperature earlier in fermentation
At some point, every brewer will make a beer that either starts out or ends up containing far more "rotten egg" smell than desired. Usually that desired level is "0", though sulphur containing compounds are extremely flavour active and show up in a lot of places. "0" sulphur is not really achievable nor desirable; that's because sulphur is responsible for the vast majority of flavours we experience. What we want to influence then, is how that sulphur is transformed by the yeast.
This is extremely strain dependent. Lager yeasts and wheat yeasts seem to be the worst offenders, so if you aren't making a lager or a wheat beer, yeast selection will have the biggest effect.
Once that's taken care of lets delve into yeast biology a little bit and figure out what variables we can influence to reduce those rotten egg flavours.
First, yeast need sulphur-containing amino acids. If these exist in the wort, the yeast will take them directly from there. If they don't exist, or the yeast ran out of them, it's going to start making them out of the sulphur that's left in the cell from other processes. If it runs out of sulphur there, it's going to start taking it up from minerals in the wort, usually in the form of sulphate.
In an easy to read format:
- If sulphur-containing amino acids exist, take them.
- If not, make them out of sulphur hanging around the cell.
- If no sulphur in the cell, take minerals from wort.
When the yeast is growing, there's no problem. Sulphur is removed from the cell through the manufacture of amino acids. When growth slows, however, (usually after the first 24 hours), sulphur can start to accumulate in the cell. This is because there's a lag between the signal that tells the genes they've got enough sulphur-containing amino acids (in this case S-adenosylmethionine) and the signal to stop taking up sulphur (usually mineral based by this point.)
Your job as the brewer is to manipulate the conditions so that either:
- The wort runs out of sulphur before growth slows and there's little, if any left when this happens
- Extend the period of growth so that it uses all the sulphur.
In lagers, you want strategy number 1, which means making sure that there is very little sulphate in the water before brewing, especially if it's an adjunct lager. Why?
- Low FAN means the cell is taking up sulphur, rather than sulphur-containing nutrients directly
- Sulphur in the cell when growth slows becomes H2S or SO2.
Ideally, at least from a reduction of rotten egg flavours standpoint, you want growth to slow while there are still sulphur-containing amino acids to take directly from the wort so there is little, if any sulphur remaining in the cell, though that's never going to happen.
For a wheat beer, FAN is generally not an issue as wheat contains plenty. Wheat strains just like to pump out sulphur. For this, you need strategy number 2, so you're going to want to maximise yeast growth and use up all that sulphur in the cell. That means - high oxygen levels and a healthy pitch rate.
What happens if you want an estery hefeweizen (like most people)? In that case, temperature is your friend. Keeping the temperature on the higher side will lift the restrictions on growth and the cell will run through the sulphur before it hits the deceleration phase. Think of it as a diacetyl rest, just earlier in fermentation.