This is part three of a three part series on quality control in beer production.

Introduction

Oxygen is everywhere. We need it to live and it is a necessary component to making beer. Too much exposure to oxygen during brewing, though, causes off flavors and staleness in beer. Not enough is even worse, because yeast will die without oxygen, and then, there is no beer at all. For most parts of brewing, except the beginning of fermentation, the contact between beer and oxygen should be limited. This is to prevent oxidation of compounds that impart positive flavor attributes and/or the formation of compounds that cause negative attributes. Oxidation is when an atom, molecule, or ion loses and electron, often due to the addition of oxygen. Oxidation can drastically change the way something tastes. During fermentation, though, it may be necessary to increase oxygen levels in wort to support yeast activity. Oxygen levels during fermentation still need to be low enough to avoid oxidation of flavor compounds and therefore should be monitored and controlled continuously. Oxidation before fermentation is often referred to as Hot Side Aeration (HSA) while oxidation during and after fermentation is referred to as Cold Side Aeration (CSA).

Mashing

Extra care needs to be taken in monitoring and controlling oxygen during the heated mashing stage, because increased temperature tends to increase the rate at which chemical reactions, like oxidation, occur. If too much oxygen is allowed to contact the brew during mashing the result is the formation of stale tasting aldehydes such as trans-2-nonenal, trans-2-butenal, and 2-pentyl-2-butenal (all of which result in paper or cardboard-like flavor in the finished product). Also Acetic acid can be formed by oxidation during mashing resulting in a sherry-like flavor at low concentrations and a vinegar flavor at higher concentrations.

One might think the obvious answer would then be to eliminate all oxygen from the mashing process. This is not the best answer though, as studies have shown that completely eliminating oxygen during mashing and wort boiling decreases the beer’s colloidal stability and tends to induce chill hazing. Opposite studies show that by inducing massive oxygen uptake prior to sparging dramatically increases the colloidal stability of the finished beer. The latter study, though, did not account for the final flavor of the beer. These studies, among others, resulted in breweries developing systems in which exposure to air is minimized to avoid problems with HSA.

Mash Separation

Separating the mash from the wort is referred to as lautering and there are two common practices for lautering. The first is known as batch sparging where wort is drained from the solids in the mash then more water is added to the leftover solids to extract more sugars, proteins, and flavor compounds. Then the water is drained away again. This is repeated until the desired pre-boil volume is reached. This is often done in three batches.

The other method is known as continuous or fly sparging. During continuous sparging most of the wort is drained from the grain. When there is a small amount of water remaining above the grain bed in the mash tun brewers begin to add water to the mash at the same rate that it is being removed.

During traditional batch sparging the water is completely drained from the mash leaving the solids behind exposed to air. This causes oxidation between batch sparges. During continuous sparging the solids in the mash are constantly covered by water minimizing exposure to air and oxidation.

There is a popular modification to batch sparging which is done to help minimize the exposure of the solids in the mash to air. When draining the water from the mash enough water is left in the mash tun to completely cover the remaining solids. More sparging water is then added to the mash tun. By doing this the solids avoid exposure too air and oxidation is minimized.

Boiling

The oxygen levels during boiling are relatively unimportant. Oxygen levels are only appreciable during the very first part of the boiling process because as the temperature of the water is increased the solubility of oxygen decreases. By the time the wort reaches boiling temperatures the amount of dissolved oxygen is negligible.

Fermentation

Fermentation is the only stage in brewing where oxygen is desired. Yeast need oxygen to survive. They use it to make cell walls, sterols, and unsaturated fatty acid. Yeast need about 10-12 ppm oxygen in their environment to thrive. The oxygen levels left in the wort post boiling are not sufficient to support the survival of yeast. Adding oxygen is then necessary.

Oxygen should only be added after wort is cooled. If it is added before wort is cooler than 27°C (80°F), then it will bind with various compounds which can cause off flavors. These new oxygenated compounds can also give off oxygen that can cause different oxidized flavors later on.

There are a few different methods employed to increase dissolved oxygen levels in the wort. One method is to add the wort to the fermenter in a very turbulent fashion to dissolve more oxygen from the air. This method does increase the amount of dissolved oxygen, but it is often not enough oxygen and a different method is needed to make sure the yeast are happy. Another popular method is to slosh, swirl, and mix the wort to dissolve more oxygen. This is a strategy often employed by homebrewers. Another approach is to pump air into the wort using an air pump and filter. By using a sintered stone more oxygen can be forced to dissolve. These two methods increase oxygen levels to around 8 ppm. This is enough for yeast to grow and ferment sugars to alcohols but slightly higher levels are preferred by the yeast. The most effective way to increase the dissolved oxygen content of the wort is to bubble pure oxygen through the wort with a sintered stone. This is able to increase dissolved oxygen levels immensely though (to levels >25 ppm) and great care should be taken when oxygenating wort in this fashion. By pumping pure oxygen through the wort 10 and 12 ppm can be reached. Care should be taken when using pure oxygen as this is the only method capable of increasing dissolved O2 to levels that cause oxidation. Below is a table indicating oxygen levels relative to the technique used.

Method Oxygen levels achievable
Spray filling 4 ppm
Mechanical mixing 8 ppm
Air pump w/ sintered stone 8 ppm
Pure Oxygen w/ sintered stone 26 ppm

Too much oxygen results in two things. The first is undesired oxidation of molecules in the wort creating undesirable flavors. This the reason the rest of the brewing process is kept as oxygen free as possible. The other reason is that the yeast may grow at an elevated rate resulting in too much yeast in the fermenter. This can cause off flavors in the beer but is a minor issue. Dissolved oxygen can be measured easily enough using a dissolved oxygen meter. A decent meter probe combination costs $400-$500. In most cases oxygen should not be introduced to the wort after primary fermentation has begun, because it will result in the oxidation of several compounds resulting in off flavors. Yeast will also use this oxygen to make early fermentation products such as diacetyl.

Another technique which has gotten a fair amount of attention lately is the use of olive oil as an oxygen source. By using olive oil instead of air to feed the yeast their required oxygen. Olive oil is composed of unsaturated fatty acids such as oleic acid. The yeast can use the oxygen from these molecules in replace of oxygen dissolved by aeration. The people at New Belgium brewery have done studies in how the use of olive oil as an oxygen source for yeast influences fermentation and flavor stability of the beer. The results are that the technique is a viable alternative to aeration. This technique, though, may not be very useful for small scale brewing as the amount of olive oil needed is very small and difficult to measure accurately. One gram of olive oil is more than enough to make more than 650 gallons of beer. Too much olive oil can lead to decreased foam stability and in extreme cases oily or soapy flavors in the finished beer.

Packaging and Distribution

Oxygen in the headspace of a bottle, can, or any other beer container post-fermentation is arguably the most critical problem when controlling oxygen. This is because the beer tends to stay in contact with this oxygen for extended periods of time. By exposing the finished beer to oxygen many flavor changes occur. The most notable is formation of the stale tasting aldehydes mentioned during the section on mashing, especially in lighter beers. Often bread-like flavors develop along with metallic notes and a butter-like taste from evolved dimethyl sulfide. Dark beers suffer in that they lose flavors, most notably malty flavors, and become bland. Acetic acid can also be created which in low concentrations adds sherry notes which can be enjoyable in some beers. In high concentrations though acetic acid will cause beer to become sour and vinegary.

Headspace air is controlled during the filling of vessels for some packaging, especially kegs, the vessel is flushed with CO2 which displaces O2. Commercial bottle filling equipment allows as little as 0.2 mL of headspace air per 1/3 L of beer. Headspace air can easily be monitored. Instruments exist specifically designed to find the concentration of oxygen and nitrogen in the headspace of beverage bottles. The device punctures the cap of the beer and pressure from dissolved CO2 pushes the air in the headspace of the bottle through a solution that eliminates CO2. Oxygen and nitrogen are then the only gasses remaining and the volume of these gasses is measured. These devices cost about $900 and are well worth the investment for even the smallest commercial brewer. This measurement should occur as soon as possible after filling to ensure that as little oxygen as possible has dissolved into the beer.

The effect of headspace air on the stability of beer is immense. Also important is the temperature at which the beer is stored. Warmer temperature speed up oxidation. The table below illustrates beer stability relative to headspace air and storage temperature.

Table II (http://www.morebeer.com/articles/oxidation_in_beer)

Conclusion

Control of oxygen is important in many stages of brewing. It effects yeasty health and productivity as well as the oxidative formation and degradation of flavor compounds. Careful monitoring of oxygen levels as well as taking measures to control oxygen levels are essential to quality control during beer production.

Sources:

Zachary Bushman

Chemical Engineer

Zachary is the chemical engineer at Analytical Flavor Systems. He graduated with a degree in chemistry from the University of Wisconsin - Platteville. His upbringing in Wisconsin taught him to love good beers and cheeses. He likes puzzles and solving problems. In his free time Zach likes to go fishing and play rugby.

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