Dear Mr. Wizard:
I’m planning to brew some smaller (1 and 3 gallon) experimental batches and wanted to know if I should still boil these batches the standard 60 to 90 minutes. Or should I set the boil time in relation to the amount of wort (1 gallon = 12 minutes, for example)? Would using Wyeast (with a starter) or White Labs yeast vials cause me to over-pitch these smaller batches? I also wanted to know if you have ever used pure oxygen in your homebrewing wort before you pitched your yeast and if it made a dramatic improvement in the taste of your finished beer.
Mr. Wizard replies: Brewing small experimental batches is a good way to determine the effect of a particular brewing variable. By changing one variable at a time, you can see how yeast strain, hop variety and fermentation temperature affect beer flavor.
Homebrewing techniques are really not much different than those used by very large brewers. The main difference is the batch size. A large commercial brew kettle may contain 30,000 gallons of wort, but the timing of hop additions and length of boil are the same as for smaller batches. The duration of boiling affects chemical reactions, like hop acid isomerization, protein precipitation and destruction of microorganisms. These reactions are dependent on time and are not a function of wort volume. In other words, don’t reduce your boil time in relation to batch size.
When it’s time to add yeast, you should scale down the amount of yeast added to the smaller batch just like you scale down the malt, hops and water. If you don’t scale down you will over-pitch the wort and the results of your experiment may be difficult to interpret. The most accurate language used to discuss pitching rates is “yeast cells per milliliter” of wort. This language is not practical for homebrewers because most of us don’t have a microscope lying around the house.
A good rule of thumb for dried yeast is 0.5 grams of yeast per liter of wort. This translates to about 10 grams (about 1/3 ounce) of dried yeast for a 5-gallon batch. Liquid yeast is a bit trickier because cell density varies more in liquid yeast suspensions. When I’m growing yeast for pitching, I use a 10X factor. For example, if I want to brew 5 gallons of beer I will use a 0.5-gallon starter culture. Usually, I will let the starter complete its fermentation and discard the liquid above the yeast to reduce the volume of propagation liquid added to the wort. If you choose to re-use yeast from a previous fermentation, use about 1 cup of thick yeast slurry per 5 gallons of wort.
I have used both pure oxygen and air for aeration and have had good results with both. The greatest advantage of oxygen over air is that wort will hold more oxygen when the gas source is pure oxygen as opposed to air. The only time this really makes much difference is when you are brewing high gravity beers. Another advantage of oxygen is that you don’t have to worry much about contaminating your wort with bottled oxygen. With that said, I typically use air that is run through a sterile filter before it’s injected into the wort stream. Good luck with your experiment.
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