The importance of the yeast in the wort, which requires adequate dosage and oxygenation
By Michael Trommer
The process of fermentation occurs with the addition of yeast in the primed wort. From storage to the beginning of metabolic activities and propagation, it takes a certain period of time (induction stage). In a time span of 10 to 14 hours, the yeast starts to act on the wort (fermentation or respiration).
For the transformation of wort into beer, the contact area of the yeast with the culture medium is crucial. With the increase of the yeast cell concentration, the contact area is expanded. The amount of yeast is determined in millions of cells per milliliter. On the stage with the highest growth, the cell concentration is 30 to 40 million cells per milliliter, but this concentration can reach 100 million cells/ml in some processes.
Normal dosage is around 0.5 l of thick spongy yeast per 100 liters of wort at 12°P. The dosage can also be made using the amount of factory malt: 100 kg must get between 2 and 3 liters. In worts with well-dissolved yeast, that corresponds to 15 to 20 million cells in 1 milliliter of wort. This quantity depends on the conditions in which the yeast will act in the wort.
Nowadays a dose contains between 15 and 20 million de cells per milliliter, which corresponds to 0.5 to 0.7 liter of thick spongy yeast in 100 liters.
The yeast can be transferred from container to container. For the removal of trub, cleaning can be performed using sieves and vibrators, although this method is hardly used now, due to danger of yeast contamination when exposed in the sieving.
In modern cooling systems, the yeast is dosed right after the heat exchanger of the culture medium (wort). Through the piping, the wort goes from the cooler to the receiving tank. The yeast is dosed preferably at the beginning of the course (close to the wort cooler), in order to ensure a better homogenization.
The removal of the yeast from a storage tank to the piping is performed using pumps with adjustable speeds, with gearing, membranes etc. The dosage can also be made via Venturi tubes.
Nowadays the dosage is controlled via CPU, according to the reading of live yeast cells using light beams.
With the parameters entered in the CPU, the data is used to increase or decrease the dosage, in order to reach the desired number of live cells. This step is necessary to guarantee that the fermentation always occurs in the same period of time. The fermentation must take the same amount of time in each fermenting tank to produce the standard finished beer, free from delays caused by low dosage of live cells in one or more tanks.
Air or oxygen dosage in the wort
It is a known fact that yeast needs oxygen to propagate. In beer making, a large amount of yeast is required for the propagation of wort inside the tank. Therefore, it is necessary to activate the yeast for fast propagation and, after that, to ferment the wort. To that end, a dosage of plenty of air or oxygen is needed.
Cold wort oxygenation for the yeast is the only time that oxygen is dosed into the wort. The yeast consumes that dose of oxygen within a few hours only and does not alter the quality of the wort.
The oxygen must be pulverized and turbulently mixed in the cold wort, in order to dissolve. Following these steps, it is possible to achieve 8 to 9 mg of oxygen per liter easily.
Theoretically, 3 liters of air must be injected per 100 liters of wort, enough to obtain that amount of oxygen. However, in reality, only one tenth of that air is effectively used, since part of the air bubbles does not dissolve in the wort.
The air dosed in the wort must be sterile, so it is necessary to send this air through a battery of filters, in which the last filter must be absolute-rated, with pores of at least 0.45 microns (0.20 microns is the ideal size).
Methods to oxygenize the wort
The following processes are used for wort oxygenation:
· Sintered plug;
· Dosing system with Venturi tube;
· Dosing system with static mixer;
· Dosing system with centrifugal mixer;
It can be made of ceramic or sintered metal, with 5-micron pores. Alternatively, it can be used small metal sheets with fine orifices, through which small bubbles pass and are homogenously distributed in the wort.
These oxygenation systems must be mounted right at the outlet of the cooler, in order to make sure that the oxygen remains in the wort counterflow, allowing a homogenous mixing.
It is important that the carbon dioxide is removed from the yeast is made as soon as possible, as well as the injection of air or oxygen into the yeast to accelerate metabolism. To that end, the yeast must be oxygenated in the storage tank and injected with the oxygen-rich wort.
The advantages of this process are fast fermentation start, good wort propagation, and fast fermentation.
1 – laminar flow;
2 – throat for increasing speed;
3 – sterile air or oxygen inlet;
4 – turbulence area;
5 – sight glass.
In a Venturi tube, the throat causes an increase of the wort flow speed. At that point, air is injected into the wort through an inlet in the turbulence area, and is dissolved in the wort afterwards, in an area with greater diameter.
Dosing system with static mixer
In this system, the air–wort mixture is obtained through a reaction stretch with many angles, which forces the wort to change direction, ensuring proper air or oxygen dissolution.
Dosing system with centrifugal mixer
In this system, the air or oxygen is dissolved in the wort with the use of centrifugal force.
HEYSE, Karl-Ullrich. Handbuch der Brauerei-Praxis. Nuremberg, Germany: Hans Carl.
KUNZE, Wolfgang. Technologie Brauer und Mälzer. Berlin, Germany: VLB.
NARZISS, Ludwig. Die Technologie der Würzebereitung. Stuttgart, Germany: Enke.