The Care and Feeding of Your Yeast
There are several variables that can affect the outcome of your beer. The type of water, the temperature of the mash, the temperature of the fermentation, and the amount of yeast pitched, can all have a compounding effect on the final product. Controlling these variables is vital to producing an exceptional beer. One of the most commonly overlooked variables by homebrewers is the yeast pitching rate. Pitching yeast in the proper amount reduces the lag time before active fermentation begins, helps promote more complete fermentation, reduces the risk of infection and improves the overall quality of your beer.
There are two main types of beer, lagers and ales, and each requires a different pitching rate. For ales, one commonly used formula is 0.75 million yeast cells per Degree Plato per milliliter of wort. For lagers it's doubled to 1.5 million yeast cells per Degree Plato per milliliter of wort. (Degrees Plato is roughly equal to specific gravity divided by four.) Using this calculation on a variety of wort sizes and gravities, the recommended pitching rate for a 5 gallon batch will vary from anywhere between 150 and 600 billion yeast cells. The vials and smackpacks we use in our homebrews contain ~100 billion cells; using the pitching rates stated above, this is barely enough yeast for 5 gallons of wort at a specific gravity of 1.030. Yes, you can ferment 5 gallons of wort with 1 vial of yeast, and produce an adequate beer, but using the proper pitching rate can make the difference between an adequate beer and an award winning beer.
There are a couple of options available for achieving proper pitching rates. For example, using the above formula; 5.5 gallons of ale with an original gravity of 1.052 would require a pitching rate of 9.2 million cells/ml or ~200 billion yeast cells. One option for producing ~200 billion yeast cells would be to just purchase 2 vials of yeast and dump them both into your wort, but at $7.00 per vial this can get expensive, especially for bigger beers or lagers. The second option is to Make a Yeast Starter; a yeast starter is simply a small volume of low gravity wort that the yeast are pitched into and then allowed to ferment out in order to a) improve the health and vitality of the yeast cells and b) increase their numbers exponentially.
As a general rule of thumb a 2 liter starter without any type of aeration will double the cell count in 1 vial or smackpack from ~100 billion to ~200 billion cells. The amount of Dry Malt Extract needed for a 2 liter starter is about 7 ounces, this is about $2.00 worth of DME. You can see that making a yeast starter has several benefits, not only do you now have an extremely healthy pitch of yeast for your wort, you've also saved a few bucks on the total cost of your brew. When brewing high volume or high gravity ales and lagers this can add up to a significant savings. Let's suppose you wanted to make 5 gallons of lager with a gravity of 1.057, this would require almost 400 billion yeast cells to achieve the optimum pitching rate of 21 million cells/ml, that's $30.00 worth of yeast! If you were to try growing 1 White Labs vial in a single starter to 400 billion cells; you would need about 8.5 liters of starter wort (without aeration) that's about 2.25 gallons of starter wort. One thing to remember about yeast starters is that the larger the wort volume to yeast count is the smaller the inoculation rate becomes.
Here's what Chris White has to say about inoculation rate:
"One of the most important things to know about starter size is that the inoculation rate affects the rate of growth. In other words, the 'pitching rate' of your starter has a big effect on the amount of new yeast cells you will see from any propagation. It is not the volume of the starter that is important, but how many cells you add in relation to that volume. Too high an inoculation rate, and you get very little growth. If you use too low an inoculation rate, then you are not really making a starter, you are fermenting beer. Just as the pitching rate affects growth in a batch of beer, which is important to beer flavor, it also affects growth in a starter, although flavor does not matter."
Based on the above statement, there is a limit to the amount of growth possible from a given volume of wort. You can go bigger, but when growing a starter we are trying to optimize the health of the yeast. When a large amount of yeast is needed it is best to use a technique known as stepping. When stepping a starter we start with a much smaller volume of wort; this is allowed to ferment out, the liquid is then decanted off and a larger volume of fresh wort is added to the yeast propagated from the first stage.
In order to make a starter you are going to need a few basic pieces of equipment. Depending on your budget, this can range from a saucepan and a 1 gallon jug; to a couple of Erlenmeyer flasks, and a magnetic stirplate. The saucepan and jug will work just fine, but if you have the financial means, I highly recommend investing in a reputable brand 2000ml Erlenmeyer flask and a good stirplate (simple DIY project), not only will it greatly simplify the process, it will also reduce the risk for infection, and increase the amount of yeast your starters produce by a factor of 2 to 3.
A premium set up for stepping up starters would include:
1- 2000ml Erlenmeyer flask 2- 46-50mm breathable foam stoppers
1- 4000ml Erlenmeyer flask 1- 1.5 inch magnetic stir bar
1- Magnetic stirplate
How to make (and step) a starter.
- Smack your Smackpack. If your using a White Labs vial break the seal on the cap and place it in an upright position where it can warm up to room temperature. Breaking the seal on the cap allows the Co2 to escape as it warms up, preventing your vial from turning into a miniature volcanoe when you open it.
- Use YeastCalc to determine the size of your initial starter and using either a saucepan or an Erlenmeyer flask, mix the appropriate amount of water and DME, I like to use a ratio of 10 grams DME per 100 milliliters of water. This will give you a starter with a gravity of around 1.040.
- Add some yeast nutrient. Wyeast makes a good yeast nutrient, it’s about 2 bucks a tube, but you only need a tiny pinch; it will reduce lag time, improve yeast viability, and provide consistent attenuation rates.
- Add some anti-foaming agent (Fermcap-S). Trust me on this, it’s the best 2 bucks you’ll spend, and one small bottle will last forever.
- If you’re using an Erlenmeyer flask; insert the foam stopper now.
- Bring the whole thing to a boil, and leave it there for about 1 minute. (yes, a 1 minute boil will kill all the nasties.)
- Remove from heat and chill to around 70 degrees F.
- If you are using an Erlenmyer flask with a foam stopper, it has been kept sealed with the (now sanitized) foam stopper throughout the boiling and cooling processes, and is now ready to receive yeast.
- If you’re using the saucepan method, you should now sterilize, or at the very least sanitize your chosen fermentation vessel and then transfer your wort into it. This is where your yeast is at its highest risk for contamination. Be diligent in your sanitation methods.
- Sanitize the outside of your smackpack or vial, give it a good shake, open it and pitch your yeast.
- If you’re using an Erlenmeyer flask, put the foam stopper back in the neck and go on about your business.
- If you’re using a gallon jug or similar container; loosely cover the vessel with some sanitized aluminum foil, this will promote air exchange and keep out bacteria. Yep, you read that correctly, loosely wrapped foil will keep out the bacteria; one of the biggest misconceptions regarding microbiology is that bacteria can "crawl" into a container and infect it. Airborne bacteria do not have wings or legs. They get transported about by dust particles in the air.
- At this point there are a number of ways that you can increase the cell count of your starter.
a) Stirplate. A stirplate will produce the largest increase to cell counts, and has no risk of contamination.
b) Continuous aeration. A micron filtered airline, attached to a pump, is submerged in the wort and continuously pumps air through it.
c) Intermittent shaking. This involves shaking the wort vigorously for a few seconds every hour or so.
d) O2 injection. The wort is injected with pure oxygen for several seconds. This method produces the least amount of increase to cell counts.
- Let your wort ferment at around 70 to 75 degrees for 24 hours.
- Put the vessel in the refrigerator and let it chill for at least 24hrs. This will cause the yeast to drop out of suspension.
- Carefully decant the spent wort from the yeast, leaving just enough wort to produce a thin slurry.
- If one step was all you needed to reach your optimum pitching rate; when your ready to pitch, let the slurry warm up to room temperature and pitch it into your wort.
- If you need to step it up. It’s time to make your second starter; it’s probably no surprise that the second starter is made in exactly the same way as the first starter. Once the second step has chilled, transfer the wort onto the yeast slurry in the first starter, once again using proper sanitation methods. Let this second volume of wort ferment as before, chill, decant, and pitch.