Modern mashing production processes require strict protection of the wort from oxygen before cooling. In the initial stage of saccharification, the manhole cannot be opened during normal production to reduce wort oxidation. The process of transferring the wort to the whirlpool tank after boiling is almost free of oxygen, and the cold wort, which is not oxygenated, also has low oxygen content.
Cold wort oxygenation may seem like a simple technological process, but it has a significant impact on yeast fermentation, the smoothness of the production process, beer quality stability, and flavor characteristics.
If oxygen is added incorrectly, it can lead to changes in the fermentation process, resulting in poor fermentation results and flavor defects.
Why oxygenate cold wort?
The fermentation process is anaerobic, meaning it does not require air or oxygen. However, oxygen is necessary for yeast growth, strength, and health. Yeast influences cell count, beer pH, fermentation speed, FAN absorption, and ultimately the flavor of the beer. Therefore, proper oxygenation is crucial.
Oxygen is a growth-limiting nutrient. Yeast absorbs and dissolves oxygen within a few hours, storing it as reserves. Once all the oxygen, including the reserves, is depleted, yeast reproduction and growth are inhibited.
Inhibited yeast slows down the fermentation process and negatively affects beer flavor. Therefore, preserving yeast growth by ensuring an adequate reserve of oxygen from the wort throughout the fermentation process is vital.
The process of providing oxygen to yeast is called “oxygenation”. Oxygenating the cold wort is the least harmful part of the brewing process in terms of introducing oxygen into the beer, as the yeast consumes the added oxygen before fermentation, without affecting wort quality.
Wort aeration is the only opportunity to provide oxygen to the yeast. Yeast can consume the supplied oxygen within a few hours without compromising wort quality.
To dissolve air into the cold wort, very small air bubbles must be introduced and mixed in a vortex to achieve a dissolved oxygen content of 8-9 mg/L.
A large amount of air is required to achieve this level of dissolved oxygen. In theory, about 3 liters of air are needed per liter of wort, but in practice, several times that amount is necessary.
The reasons for this are: (1) some bubbles are insoluble in the wort, and (2) air cannot be evenly distributed.
The challenge lies in ensuring the small bubbles are evenly distributed throughout the wort and dissolved. Bubbles that rise to the surface can create larger inconvenient bubbles that may block the oxygenation process. The gas dissolution depends on temperature and pressure, with each gas having a unique “technical solubility coefficient” determined by temperature.
Methods of wort oxygenation
1. Wort oxygenation equipment
The venturi tube has a narrow section for increasing flow rate. Air is sprayed through the nozzle, creating a vortex in the widened section of the tube to mix the air with the wort. This results in pressure loss.
Some breweries apply pressure to aid saturation before the wort enters the fermenter. Oxygen is more soluble under high pressure, so setting the pressure just before entering the fermenter promotes wort saturation. After passing through the pressure backup valve, the normal dissolved oxygen concentration of 8 to 10 ppm decreases.
Air pump, gas filter cylinder, and bubble stone
The air pump provides a continuous flow of air, which is washed by disinfectant in the gas filter cylinder. The bubble stone generates many small bubbles into the wort. Gas cylinders typically have a flow meter (in L/min) to calculate the required oxygenation time.
2. Timing of wort oxygenation
Oxygenation should commence before adding yeast and after the wort has cooled down. If the recovered yeast is strongly oxygenated, it becomes reactivated but lacks fermentation substances, resulting in the consumption of stored carbohydrates. This weakens the yeast, and fermentation starts without reserves, leading to yeast death. Removing CO2 from the recovered yeast is important.
3. Oxygenation methods in home brewing
It is recommended to use a professional oxygenation pump to avoid wort contamination by bacteria and for convenience. If an oxygenation pump is not available, two fermenters can be used to transfer the wort between each other, increasing the wort’s oxygen content.