Most brewers know that oxygen is a crucial part of yeast health at the start of the brewing process. We always take measures to make sure that we get as much oxygen as possible into the beer prior to pitching the yeast. Some of us even use oxygen tanks to force pure oxygen into our wort. But exposing your beer to oxygen later into the brewing process can result in an undesirable or even undrinkable beer. Why is this? How can we prevent oxygen from ruining our beers?
How does oxygen affect beer?
Yeast cells need oxygen during their first stages of fermentation. The yeast cells consume the oxygen and use it to make cell walls, sterols, and unsaturated fatty acid which in turn makes them grow and multiply. They need about 10-12 ppm of oxygen to thrive in their environment. Once the yeast cells are finished consuming the oxygen and are done growing, they switch from an aerobic to an anaerobic state. This is where the cells start producing CO2 and ethanol. Or in simpler terms, this is when they start to make beer.
When you're aerating your wort you want to be cautious of what temperatures you are at before you start. Make sure your wort is cooled to below 80º F (27º C) before you start introducing oxygen. Doing this above 80º F (27º C) can cause the oxygen to bind to various wort compounds, resulting in off flavours.
How do you know if your beer is oxidized?
Most of the time when a judge tastes oxidized beer, they can pick it out like a sore thumb at any level. But at lower levels, oxidation can be hard to detect by the untrained palette. Descriptors of oxidized beer include:
How do I avoid oxidation?
Some tips to avoid oxidizing your beer include:
- Aerate your wort, after it is chilled. Shake your fermenter, or manually add oxygen once your beer is chilled below 80º F (27º C).
- Avoid splashing when siphoning into another vessel. This may seem obvious, but taking extra care when siphoning can sometimes save your beer.
- Refrain from removing the lid of your fermenter. This can increase the chance of oxygen sitting on top of your beer.
- Leave the proper amount of headspace in your bottles when bottling. Be sure to leave no more than 1-2 inches of head space in your bottles.
- Always purge kegging equipment with CO2. Once you transfer your beer into your keg, connect your CO2 and purge the keg from the pressure release valve to push out all oxygen.