Both commercial brewers and homebrewers are working hard to reduce the oxygen content in finished beer. Oxygen in finished beer can negatively impact its flavor, clarity, and aroma. Oxidized beer can develop a papery or even sweet stone fruit smell.
Oxygen in Fermentation and Finished Beer
Oxygen is generally beneficial before the fermentation process. Sufficient oxygen is needed for yeast to grow healthily, which is why many brewers aerate the beer before adding yeast. It is difficult to over-oxygenate the wort before fermentation unless pure oxygen is used. During the initial stages of yeast growth, the yeast actually consumes all the oxygen in the beer for its growth and expansion.
However, once fermentation begins, oxygen becomes a pollutant. When oxygen enters fermented beer, it can spoil the beer within 24 hours. Even a small amount of oxygen is detrimental to finished beer, as it quickly degrades the flavor and affects long-term stability. Oxygen also affects the clarity of beer by interacting with polyphenols and tannins, causing haze or cloudiness.
After fermentation, a protective layer of carbon dioxide, heavier than air, is placed on top of the beer to prevent oxidation. As long as the fermentation tank is not disturbed, this layer protects the beer from oxidation. However, homebrewers may introduce oxygen when transferring beer from one container to another during bottling or kegging. Processes like excessive splashing, siphoning, or leakage can introduce excessive oxygen.
Formation of Papery Off-Flavor in Beer
The main compound responsible for papery off-flavor is trans-2-nonenal. When packaged beer contains high levels of dissolved oxygen, storage at high temperatures (above 40°F) for an extended period can result in this flavor development.
The more exposure beer has to oxygen, the quicker oxidation occurs. Lower storage temperatures slow down this process. Improper handling of beer, such as storing at high temperatures, can lead to increased oxidation.
Unsaturated fatty acids in wort can also react with oxygen to form oxidized flavor compounds called carbonyl compounds. During boiling, these compounds typically bind to proteins, which is not a significant problem. However, further oxidation breaks these bonds, leading to increased carbonyl compounds and flavor deterioration.
Reducing Papery Off-Flavor Production
To minimize papery off-flavor, it is essential to avoid oxygen exposure in finished beer. One strategy is to minimize transfers. Many winemakers skip secondary fermentation altogether and bottle directly from fermentation tanks. Commercial brewers use conical fermentation tanks to remove excess yeast and sediment without transferring beer. Using food-grade wort pumps for transfer and conducting secondary fermentation is also beneficial for commercial brewers.
If you plan to store or age beer for an extended period, use oxygen-blocking containers. Glass and stainless steel are effective oxygen barriers, so opt for glass or stainless steel fermentation tanks. Plastic is somewhat breathable, allowing oxygen to enter through its molecular structure. Therefore, plastic fermentation tanks are not suitable for long-term storage or aging.
Splashing during transfers and bottling is another significant source of oxygen. Poorly sealed automatic siphon devices can introduce oxygen through their seals. You may notice bubbles at the siphon seal attachment. If your automatic siphon is leaking, either discard it or add sterile water to the seal to absorb water instead of air when needed. Commercial breweries should pay close attention to equipment leaks, as brewing should be halted for inspection and maintenance.
When filling barrels, minimize splashing and perform a CO2 purge after filling. Hold the keg upright and release the pressure relief valve a few times to allow heavier CO2 to replace the oxygen in the keg, protecting the beer.
During bottling, minimize headspace (usually about an inch). Avoid splashing while filling and consider using oxygen-absorbing bottle caps if possible. Ensure caps are tightly secured on individual bottles, as even a slight leak can result in undercarbonation and oxidation.
Removing all oxygen from beer, especially during packaging, is challenging. Brewers and packaging line operators must review their processes to minimize oxygen content. Proper storage at low temperatures is the easiest and most critical way to avoid oxidation. Oxidation is a temperature-dependent process, and keeping beer cold extends its freshness. For every 10°C/18°F increase in storage temperature, freshness decreases by 2-4 times.