Wort Cooling, Clarification, Aeration
Cold Wort Aeration
During the long and vigorous boil most of the oxygen that was dissolved into the wort solution will have been driven off. Unfortunately, to properly ferment wort and turn the sugars into alcohol creating the beer, yeast needs oxygen. Because of this there is one (and only one) point in the brewing process where oxygen contact is allowed and actually desirable: after the wort has been boiled and chilled down to fermentation temperature, ready for the yeast to be pitched. Yeast cells need the oxygen to produce sterols and unsaturated fatty acids, which are essential components of the cell membrane. There is a strong correlation between oxygen supplied to wort, the quantity of sterols synthesized, and fermentation performance (Boulton et al., 2006).
Oxygen Requirements
The oxygen requirements for individual brewing yeast strains can range from 3 to 30 mg O2/L but usually it is in the range of 7 to 18 mg O2/L. A working rule of thumb is 1 mg/L oxygen for each degree Plato to be attenuated (Barnes, 2006). Yeast strains with low oxygen requirements can be aerated using sterile air since it contains approximately 8 mg O2/L, while strains with high oxygen requirements must be aerated with pure gaseous oxygen.
Point of Injection
The point of injection of oxygen has been the subject of many debates, and the choice is largely a matter of tradition. Many brewers believe that gas injected into the hot wort prior to cooling improves cold break formation.
Methods of Wort Oxygenation
Oxygen is normally added after wort cooling but before the yeast. There are many different methods to add plant air or oxygen to the wort. These methods include: (1) pumping the cool wort into an open vessel and allowing air to dissolve into the wort, (2) splashing or spraying the wort into a vessel that is full of air, (3) passing the wort through a Venturi aerator, (4) adding air or oxygen via ceramic or sintered metal elements, or (5) bubbling oxygen gas into the fermenter.
Venturi Aerator
Some brewers using a Venturi aerator to oxygenate the wort. The Venturi aerator is a tube that has a restriction in the flow of the wort by changing the pipe diameter. As the wort enters the smaller pipe diameter, the velocity of the wort increases and the pressure of the wort decreases.
Ceramic or Sintered Metal Elements
The best way to inject oxygen into the wort, however, is by bubbling it directly into the fermenter using ceramic or sintered metal elements, air is injected as very small bubbles through fine pores into the flowing wort (Figure 11.3). This is a very simple and effective method, but care should be taken that pores are kept clean. Oxygen stones are most effective from 0.5 to 3 microns (µm).
Measuring Oxygen Levels
Measuring dissolved oxygen in the beer brewing process can either be done in-line (direct, continuous measurements in the piping) or sampling at each step. The oxygen meter should be located in the wort line downstream from the gas injection point located as near to the fermenter as possible to confirm the level of dissolved oxygen. It should be noted that the dissolved oxygen probe must be located at a point before yeast addition in order to avoid errors of underestimation due to yeast oxygen uptake.
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