OK, good question. Post fermentation oxidation is being discussed more and more around here and people range from not taking any precautions to being extremely cautions (me) some say to the point of paranoia. Let’s take a second and discuss this.
What is oxidation?
Oxidation is a chemical reaction where there is a loss of elections in an atom. It is the type of reaction that causes rust, apples to turn brown, wine to go bad after opening and the keg of KeyStone Light you bought in high school to taste (more) like **** the next day.
How do we measure Oxidation?
The oxidation reactions consume the oxygen from the beer. If you oxidize beer then place it in a closed system and measure the dissolved O2 (DO) concentration, leave, come back in an hour and measure the DO again it will be reduced. Eventually it will all be removed by reactions. For this reason DO is usually stated as UPTAKE. As in,
x parts per million (ppm) of oxygen can be absorbed during transfer,
x ppm during packing instead of total DO level.
Without expensive lab analysis we can only eminently measure DO levels after a process (still at the cost of hundreds to thousands in instrumentation) or judge by tastes. However, we can calculate potential and mitigate risks.
Of changes caused by oxidation are not always from the immediate oxidation reaction. Several oxidation reactions for precursors to OTHER reactions, therefore the changes take time to develop.
What does oxidation do to beer?
There are several oxidation reactions that take place in beer at the same time. Several of the major reactions are.
Rapid loss of hop aroma and flavor, especially in dry hopped bears.
Sweet flavors may devolp.
Ethanol reacts via oxidation to form Aldehydes. Aldehydes above 10ppm can be tasted in light beers and above 35ppm in other styles as a green apple flavor.
Linoleic acid oxidizes into dihydroxy and trihdryxyoctadecenoic acids that are precursors to 2-trans-nonenal. The "cardboard" flavor. This reaction will take some time to develop.
Melanoidins in oxidize into the characteristic flavors of sherry. This reaction takes time.
During lag and fermentation yeast build amino acids, proteins, ect. Most do not affect flavor but many leak out of the cell. Valine is one of the amino acids produced. An intermediate compound in valine production is acetolactate, however not all of the acetolactate becomes valine. The acetolactate that leaks out of the cell oxidation reaction converts to diacetyl. After fermentation is complete the yeast enter a stationary phase and reabsorb diacetyl converting it to acetoin and subsequently to 2,3-butanediol. However, the intitial conversion of acetoacetate is dependent on the DO availability, if you under oxygenate the wort some acetoacetate may still remain and convert to diacetyl when the new DO is added. However, MOST of the time diacetyl in the beer is not from post fermentation oxidation but from not doing a diacetyl rest and allowing the diacetyl to be converted to 2,3-buetanedoil.
Occasionally some oxidation is GOOD for a style, like a sherry flavor in a barley wine.
How is O2 Dissolved into beer?
Oxygen behaves the same way as CO2 (so keggers should be familiar). The amount of O2 that can be dissolved (at a time) in beer is dependent on the Pressure of the gas (partial pressure of the gas when talking about a gas mixture) and the temperature of the beer. The time it takes the O2 to dissolve is based on surface area of the beer.
How much (little) uptake should I have.
Pre Fermentation, post pitching 5-20ppm.
Post fermentation pickup should not exceed 0.02ppm-0.03ppm. *
Brewing Science and Practice
What happens when I cold crash?
Gases behave by ideal gas laws. The pressure, temperature and volume of gases are all related by the below equation.
So when we cold crash (2) things may happen.
1. The fermenter is completely sealed. As the gas temperature decreases it reduces in pressure and creates a vacuum. 1 pounds per square inch on the outside of the fermenter!! Can your fermenter handle this? Or will it leak?
2.The fermenter is not completely sealed. As the gas temperature decreases air, sanitizer, or other is pulled into the fermenter to fill the space made by the decreasing volume. The oxygen available to dissolve in the beer is 0.171% by volume or
1710ppm!!!! How much dissolves is dependent on time. (I Know of no studies that measure the time, as soon as the oxygen dissolves it begins to oxidize making measurement hard)
*For the above I am assuming 5 gallons of beer in a 7 gallon carboy, dropping from 70°F to 32°F, all air pickup (ie no CO2 pressure, no sanitizer suckup)
