What is Oxygenation?
Oxygenation is the process of injecting oxygen into the wort during brewing, typically after boiling and cooling but before fermentation. Boiling the wort removes most of the oxygen in the solution. However, during the “log phase” of yeast growth (late fermentation or after fermentation), a significant amount of oxygen is required for yeast to reproduce effectively and complete fermentation.
The yeast species Saccharomyces cerevisiae is a facultative microorganism that grows and reproduces under aerobic conditions and performs alcoholic fermentation under anaerobic conditions. Yeast needs to multiply to a certain number before entering the fermentation stage, and this requires oxygen. Therefore, it is necessary to supply oxygen to the yeast by introducing sterile air into the cooled wort, resulting in a dissolved oxygen content of 8-10 mg/l in the wort.
To enhance the dissolution of air in the wort, very fine air bubbles must be introduced and mixed with the cooled wort in a vortex form. This ensures that the dissolved oxygen in the wort reaches 8-10 mg/l. Theoretically, about 3 liters of air are needed per hectoliter of wort, but in actual production, ten times that amount is required. Some air may not dissolve and overflow, causing uneven distribution of incoming air. Additionally, the solubility of oxygen decreases as wort temperature and concentration increase.
The wort oxygenation device is typically installed at the outlet of the plate heat exchanger, where the cold wort flows. Compressed air is decompressed, passes through an air flow meter and air filter, and then enters the wort oxygenation device. This ensures thorough mixing of air and wort before they enter the fermentation tank. The Venturi tube is the most commonly used wort oxygenation device.
ACE Venturi Tube
How to Properly Control Wort Oxygenation Method and Amount
Oxygenating cold wort may seem like a simple process, but proper control of the oxygenation method and amount can significantly affect yeast fermentation, production smoothness, beer quality stability, and flavor characteristics.
Improper oxygenation can lead to changes in the fermentation process and result in poor fermentation outcomes and flavor defects.
(1) Control of Wort Oxygenation
The amount of oxygenation in wort is controlled based on the oxygen demand of the yeast species and fermentation process used.
Different yeast species and fermentation states require varying amounts of oxygen. The typical practice is to control the dissolved oxygen in cold wort at 5-8 mg/l and make necessary adjustments based on the actual needs of yeast reproduction, generally not exceeding 10 mg/l.
The oxygenation of wort is generally controlled through oxygen flow, pressure, time, and oxygenation equipment.
The oxygenation effect is influenced by the control of oxygenation time. Using unreliable or simple oxygenation devices may fail to achieve the desired oxygenation effect, and the oxygenation amount may not necessarily correlate with the actual dissolved oxygen content in the cold wort. Consequently, some brewers tend to extend the oxygenation time to ensure sufficient oxygenation. However, the dissolved oxygen in cold wort is not only determined by the oxygenation time but also affected by wort concentration and temperature.
With low wort temperature and concentration, the oxygenation effect is better, allowing for shorter oxygenation time. The oxygenation time of wort depends on the oxygenation equipment used, sterile air pressure, and flow rate. Higher pressure and flow rate result in better emulsification and shorter oxygenation time.
However, in large-scale production, the controlled oxygenation time is not solely based on the desired dissolved oxygen content. It also considers the distribution and dissolution of oxygen in the wort. For instance, batches of wort with added yeast are typically oxygenated throughout, while batches without yeast may undergo oxygenation at different intervals, reduced oxygenation, or no oxygenation at all. As long as the required amount of dissolved oxygen for yeast reproduction is achieved, over-oxygenation is not recommended. The budding cycle of yeast is approximately 12 hours, after which further oxygenation is unnecessary. Yeast can synthesize linoleic acid by itself.
(2) Implications of Oxygenation Method Control
The location where cold wort is oxygenated plays a significant role. The ideal oxygenation position is near the yeast addition point, where a device capable of oxygenation, yeast addition, and thorough mixing is installed along the cold wort pipeline. This yields the best results. Breweries commonly use a venturi tube added to the cold wort pipeline. This allows oxygenation of the cold wort before yeast inoculation, ensuring the presence of dissolved oxygen in the tank.
A relatively low flow rate can be used throughout the wort cooling process to evenly distribute dissolved oxygen.
In terms of oxygenation timing, all batches of wort with added yeast can be oxygenated throughout or in stages, while batches without yeast are generally not oxygenated. Considering yeast reproduction needs, multiple-batch full-tank fermentation systems can oxygenate each yeast-added batch throughout or in stages. For subsequent yeast-added batches, the oxygenation amount can be reduced or adjusted based on yeast reproduction requirements. If multiple batches of wort are being transferred to the tank, the final batch entering the tank may not be oxygenated as a general rule. However, if uneven wort distribution is observed, sterile air can be introduced at a lower flow rate and pressure for 5-10 minutes.