We all know that brewer’s yeast is an aerobic-anaerobic microorganism, which reproduces under aerobic conditions and ferments under anaerobic conditions. The yeast needs to reproduce to a certain amount of oxygen before entering the fermentation stage, which is why we need to oxygenate the cold wort to reach a dissolved oxygen level of 8 to 10 mg/L.
In order to promote the dissolution of oxygen, the air must bubble up in very small bubbles and mix with the wort in a vortex fashion. Theoretically, 3 liters of air per 100 liters of wort are needed to achieve 8~10mg/L of dissolved oxygen, but in practice, 10 times are needed in the actual production process. Because part of the air that is not dissolved will be expelled, the bubbling air is not even useful. In addition, the higher the temperature of wort, the lower the dissolved oxygen will be.
Oxygenation device is usually installed at the outlet of heat exchanger, and air enters the oxygenation device after passing through air flow meter and air filter. The wort is completely mixed with air and then enters the fermenter. The commonly used oxygenation device is venturi.
The ends of the venturi are larger in cross section than the middle part (bleed section). When the wort flows through the outgassing section, the flow rate will be higher due to the smaller cross section. The sterile air bubbling in the deflated section will soon be drawn into the wort, forming many tiny bubbles and dispersing them evenly in the fast wort. Swirls will form in the larger section, which facilitates complete contact and dissolution of the wort with these bubbles.