There are a lot of benefits to using dry yeast to ferment your beer. Chief among those benefits is ease of use. While it is certainly easy to use dry yeast, it is still important to follow some established best practices to make sure you are getting the most from the magic sugar-munching microbes. With that in mind, here are some very common questions related to using dry yeast, along with answers to help you do right by your single-celled friends. I'm sure there are questions that I have missed, so the intention is to grow this FAQ as necessary to provide a comprehensive guide to best practices when using dry yeast.
How should I store dry yeast?
For long term storage, it is best to store dry yeast in a refrigerator. Over time, dry yeast will naturally lose some viability, but it loses less at cold temperatures than it does at warm temperatures. You can safely store dry yeast for a year or longer at refrigerator temperatures without substantial loss in viability.
When you decide to use the yeast, you should allow it to come to room temperature before rehydrating so it does not experience a thermal shock.
Do I need to rehydrate dry yeast before pitching into my wort?
In a word: yes. If you don't rehydrate your dry yeast before pitching, you may end up killing 50% of the yeast in the package which could increase both lag time and the risk of other bugs growing a stronger colony in your wort. The remaining yeast will be stressed, increasing the chance that you will notice off-flavors in your final product. Further, the remaining yeast will need to divide more times to reach the ideal colony size, virtually guaranteeing a greater presence of ester flavors.
When the yeast cell is first rehydrated, it has no way to control what it absorbs or what leaches out of it. It needs a chance to rebuild its cell walls before it control those things. By adding the yeast directly to the wort (effectively rehydrating it in wort), the yeast cell will be flooded with sugars and other compounds that it isn't ready to process.
How do I properly rehydrate dry yeast?
From “Yeast” by Chris White and Jamil Zainasheff:
How quickly do I need to pitch yeast once it has been rehydrated?
Every yeast strain has its own optimum rehydration process, but the basic procedure is as follows:
1. Warm the dry yeast to room temperature.
2. In a sanitized container, prepare an amount of sterile tap water at 105*F (41*C) equal to 10 times the weight of the yeast (10 ml/g of yeast).
3. Sprinkle the dry yeast on top of the water, trying to avoid setting up large, dry clumps. Let it sit for 15 minutes, then gently stir.
4. Once the yeast has reconstituted, gentle stir once again to form a cream, and let sit for another 5 minutes.
5. Carefully and slowly, adjust the temperature of the yeast to within 15*F (8*C) of the wort temperature.
6. Pitch the resultant cream into the fermentation vessel, ideally as soon as possible.
Once the dry yeast has been reconstituted, you have a window of time to pitch the yeast with minimal risk of contamination that is dependent on temperature. If the yeast is being stored at 25C, you should pitch within 4 hours; at 20C you should pitch within 6 hours; at 4C you should pitch with 18 hours.
Keep in mind, it is a best practice to pitch the yeast as soon as possible. During the first 30 minutes, the yeast is given a boost from vital nutrients that are packaged with it to help energize the yeast as it enters into its growth phase. Waiting longer than the recommended 30 minutes causes the nutrients to be exhausted and the yeast will want to go dormant, increasing the chances that you'll see a longer lag time when you pitch it.
How many cells come in a dry yeast packet?
According to Fermentis, each packet of yeast has an average of 150 billion viable cells. Danstar claims roughly 20 billion live yeast cells per gram. Cell counts done by independent parties also indicate that after rehydrating, between 150 and 220 billion viable cells are present.
Should I make a starter with dry yeast?
It is generally not recommended to make a starter with dry yeast, for a few reasons. The yeast comes packaged with a cell count and nutrient reserves sufficient for most 5 gallon batches. The cost of a packet of yeast is generally less than the cost of making a proper starter. To be large enough for optimal cell growth based purely on the number of yeast cells inoculating the starter wort, the starter would need to be roughly 3L on a stir plate and this is impractical for most and would yield far more cells (380+ billion) than you would need for most 5 gallon ales. Making a traditional 1L to 2L starter would not allow for healthy growth of the yeast cells.
I hear a lot of “I pitch dry and it works fine” stories, so why bother?
In the early stages of your homebrewing evolution, there are 4 parts of your process that will greatly impact the quality of your beer: Sanitation, pitch rate, fermentation temperature, and patience. In any of those 4 areas, you will hear stories of great success from people who say a particular step is unnecessary. Most of those same folks will tell you that when they finally decided to pay attention to any single one of those areas, their beer improved immensely. When you pay proper attention to all of those things, your beer will be consistently outstanding.
The reason for pitching ales at .75 million cells per milliliter per degree plato is to ensure there is a large enough colony to go through a complete, clean fermentation. By pitching less, you are putting your yeast at a disadvantage from the get-go. Lower pitch rates increase ester production. A yeast cell also cannot control what toxins it absorbs or what vital chemicals it leaches out during the first few moments when it first comes into contact with water/wort.
Obviously, you can pitch a single packet of yeast into a 5-gallon batch without rehydrating and it will ferment. You can also ferment your beer at ambient temperatures. You can package your beer on the day it hits final gravity and drink it 7 days later. None of these practices are considered best practices because they don't result in the best beer possible. The fact is, as homebrewers we spend a lot of time on a beer before we get to sit back and pour a bottle or a pull a pint. With all the time we spend on crafting a recipe, selecting the right ingredients, cleaning, sanitizing, mashing, boiling, cooling, bottling/kegging, and whatever else goes into your process, it just seems to make sense that we would spend a few minutes to make sure we are pitching the healthiest yeast colony we can at the proper inoculation rate for the style, volume, and gravity that we are brewing.
Can I re-use dry yeast?
Yes, after fermentation is complete it can be harvested just like liquid yeast. There are lots of resources available on HBT about harvesting and also rinsing yeast, like this illustrated thread
. Just as with any yeast harvested for reuse, it is important to be aware of the viability of your yeast slurry in order to predict an accurate pitch rate. While that falls outside the scope of this FAQ, you may consider consulting pitching rate calculators like Mr. Malty
or use other means as you see fit to ensure you pitch enough slurry or make a starter to propagate the right amount of yeast.
What are the disadvantages of using dry yeast?
The biggest one is limited selection: there is a far bigger variety or liquid yeasts.
by Chris White and Jamil Zainasheff
(Fermentis Tips & Tricks PDF)