Like a horse after a long ride, you may find yourself racing to the finish line at the end of your brew day. If you cross the finish line too soon, you may create more problems for yourself. Here are six questions you should answer before pitching your yeast into the beer.
Is your wort cool enough for happy yeast?
Yeast does not respond well to higher temperatures. The metabolism of yeast changes as the temperature changes. If your wort is too hot, the yeast will be shocked and most likely will take a long time to start fermenting. You can kill your yeast en masse if your temperature is not low enough. If your temperature is not boiling but still well above the recommended temperature range, you may not kill the yeast but you will unnecessarily stress your yeast. Stressed or shocked yeast can lead to bad flavors, and slowed or stalled fermentation. Make sure your beer is within the recommended temperature range for the yeast you are using.
Does your wort have enough oxygen?
Like most organisms, yeast needs oxygen. The oxygen is needed for cell-membrane components which are used in reproduction. Unfortunately, hot wort contains little oxygen. If you just cool your wort and pitch, your yeast may struggle. Poor aeration can also cause higher than normal ester production in your beer.
Ale yeasts need about 5 parts oxygen per million in wort, while lagers can need 10 to 12 parts oxygen per million. To make matters worse, the maximum dissolved oxygen you can achieve in your wort using air is around 8 parts oxygen per million. Splashing your wort by rocking the carboy will help, but it most likely will not come close to the theoretical maximum. White Labs states shaking will achieve about 10-30% of the needed oxygen levels.
You should aerate your wort for higher gravity brews. The reason is higher gravity brews decrease the solubility of oxygen. You would need to shake very hard to get enough oxygen into your high gravity wort
You can get an aeration stone from an aquarium shop or your local homebrew store. There is some debate among homebrewers if you really need to use oxygen, or if air will suffice. Personally I use air, and have had no problems.
Are you pitching the correct amount of yeast?
Long before pitching your yeast, you need to know how much yeast you need to pitch. Homebrewers notoriously under pitch their yeast. Commercial brewers pitch at least 10 million yeast cells per milliliter of wort.
According to Brewing Classic Styles by Jamil Zainasheff and John Palmer, you need to pitch 0.75 million yeast cells per milliliter of wort per degrees Plato. Wyeast and White Labs recommend 1 to 2 million yeast cells per milliliter of wort per degree Plato if you are reusing your yeast. This yields the following equation:
(0.75 million yeast cells) x (milliliters of wort) x (degrees Plato)
One degree Plato is about 1.004 of specific gravity, so to get degrees Plato divide the gravity of your wort by 4. If your wort is 1.060, your degrees Plato would be 15°P.
1 US gallon equals about 3785 ml, so a typical 5.5 gallon batch is about 20,817 ml. If you enter these values into the above equation yields 234,196,875,000 yeast cells. Your typical smack pack is 100 billion cells. For a 1.060 beer, you need three smack packs to properly pitch. Your best bet is to create a starter.
You can over pitch too, but that usually is a problem only when you pitch 400 billion cells in a typical 5 gallon batch.
Do you have enough nutrients for your wort?
Yeast needs three things to be happy: a happy temperature, oxygen, and nitrogen. The nitrogen can be a bit nebulous for the homebrewer. The best advice is to look at your ratio of grain to adjuncts. Most of your nitrogen will come from your malt. If you are using more than 25% non-barley adjuncts, you need to consider adding yeast nutrient.
What temperature are you fermenting at?
As I mentioned before, higher fermentation temperatures can have a negative impact on the flavor of your beer. Higher temperatures can cause the yeast to produce a fruity flavor. If your temperatures are too low, the yeast will be sluggish and take longer to ferment. The longer the fermentation, the higher the possibility something else (like bacteria) can take hold in your beer. Make sure that you can ferment your beer within the recommended temperature range.
What’s your starting specific gravity?
You should measure your starting gravity before you pitch your yeast. You still have some chance to adjust your starting gravity before your yeast is pitched. Ideally you want to know your starting gravity before you boil, but checking at the end of your boil is important. The initial gravity will let you know how the boil affects your gravity (how much water is lost to evaporation). This metric is also important to determine the attenuation of your yeast, and help you estimate how much alcohol is in your beer.
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