Brewers Yeast
Yeast Propagation and Scale-up
The objective of propagation is to produce large quantities of yeast with known characteristics for the primary role of fermentation, in as short a time as possible. Most brewers use a simple batch system of propagation, starting with a few milliliters of stock culture and scaling up until there is enough yeast to pitch a commercial brew. Scale-up introduces actively growing cells to a fresh supply of nutrients in order to produce a crop of yeast in the optimum physiological state.
Laboratory Phase
The process initially begins in the laboratory when cultures are taken from the "working" master culture and grown in a progression of fermentations of increasing size until enough yeast is produced to transfer to the propagation plant. The number of transfer steps in the laboratory varies according to the final weight of yeast required for the propagation plant. Of course, the more transfers, the greater the risk of infection. Most yeast culturing is done in a laminar flow hood as shown in Figure 4.1.
Plant Phase
After rigorously cleaning the yeast-propagation vessel (in the case of smaller breweries, a production fermenter) it is then filled with hot or cold wort and aerated with sterile air. Preferably the wort should be of the same quality as that used in fermentation. During propagation, temperature is maintained at a set level and the propagating yeast is intermittently aerated. When the yeast has reached the required cell concentration, it is pitched into an intermediate fermenter or directly to a production fermenter. As in the laboratory, yeast is grown in a progression of fermentations until there is enough to pitch a commercial-size brew.
Aeration
As mentioned, air or oxygen is passed continuously into the vessel through an efficient gas sterilizer to encourage yeast growth. Oxygen is preferable since it is sterile, whereas an air supply may contain impurities that must be removed before the air enters the vessel. The optimum rate of oxygenation for a system must be found by experiment, as the rate will affect the total crop produced.
Optimum Propgation Temperatures
There is a wide variety of recommendations in this instance as well. Some brewers prefer to propagate their yeast at temperatures identical to those employed during fermentation in order to prevent temperature shock to the yeast (26).
Propagation Plants
The propagation plant usually consists of anywhere from one, two or more closed stainless steel vessels of increasing volume, which are usually situated in a separate room to minimize contamination of risk.
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