Disclaimer: I'm not a chemist, so some of my views at the end, are just my way of looking at yeast and what they do or don't do. I'm sure those here that are more knowledgeable of the chemistry and metabolic aspect of yeast will chime in and correct, or at the very least, explain things in a more accurate way. If I were writing a book, then I'd do more research and agonize over every detail of my thoughts about yeast metabolism and chemistry. I have a lot to learn, but I'm getting their slowly.
This is how I would do it.
- I'd begin my starter 48 hours prior to brewing my beer.
- I'd start by stirring my dry yeast into 95-105F water to start rehydration.
- As soon as the yeast has started rehydrating, I'd bring a separate 2-1/4 (1/4 pint extra for boil off) pints of water up to a boil. You want two pints of starter wort when the boil is finished.
- As soon as the water starts to boil, I'd turn the heat off and then stir in 1/2 cup of DME and 1/4 teaspoon of yeast nutrient. Some brewers will throw a little hops in (I don't).
- Then bring it back up to a boil and slowly simmer for 10 minutes. You just need a little rolling action to the boil, not a frothy, oh my god, it's going to boil over type of boil.
- After the 10 minute boil, turn off the heat and transfer your wort filled pan to a ice water bath. Gently stir it until the temperature drops to 70F.
- Transfer the boiled and cooled wort to your flask. I give my starter wort a shot of oxygen. Oxygen promotes healthy yeast growth and carbon dioxide inhibits it. My oxygen source is an aquarium pump with a sanitized diffusion stone. More on oxygen and CO2 after the suggested procedure.
- When I'm adding oxygen in an actual batch of beer (1 gallon +), not a starter, I give it at least 25 minutes. For a starter, I give it the aquarium pump treatment until the foam reaches the top of the flask. The stir plate will keep adding more oxygen, so that the yeast have plenty in a starter.[/i]
- Pitch your rehydrated yeast into your starter wort. Don't worry if it just stays towards the top of the foam, your stir plate action will draw it down into the wort and mix it in really well.
- Put a sanitized piece of aluminum foil on top of your flask to keep contaminants (wild yeast and bacteria) from falling into your wort. You don't want an airlock, because that would stop your stir plate from having access to oxygen.
- Put your flask on your stir plate and start it up. I set the speed on my stir plate to create just enough of a vortex to barely break the surface of the wort. It does not need to be spinning like crazy, it only needs enough action to draw in oxygen and de-gas CO2.
- At the start of brew day, turn off your stir plate and let the yeast settle out of suspension. By the time you're done brewing and ready to pitch, you should have a nice clear liquid (beer) on top and a yeast slurry on the bottom.
- Have 4 pint canning jars of boiled sterilized water on hand and a 1/2 gallon canning jar of boiled sterilized water on hand. You don't want to prepare this sterilized water on brew day, because it will take too long to cool. I always have about 8 half gallon canning jars and 24 pint canning jars of sterilized water prepared and in a cabinet in my brewery/garage.
- During the mash or during the boil when you have free time, take the flask of now separated beer/yeast slurry and pour off the beer, leaving just the slurry behind in the flask.
- Open your 1/2 gallon canning jar and pour enough sterilized water into the flask to double the volume of the yeast slurry and then swirl it up good to get an even mixture. Pour the rest of the sterilized water in your 1/2 gallon canning jar down the drain.
- Pour your yeast slurry into the now empty 1/2 gallon canning jar.[/i]
- Open the 4 pint canning jars and pour each one into the 1/2 gallon canning jar until you fill it up. Put the lid on your 1/2 gallon canning jar and shake it to mix it up well. Open it back up carefully, as it will foam and spritz like a carbonated beverage.
- You should have one pint canning jar with a little bit of sterilized water left in it. Pour that water out, then carefully fill all four pint canning jars to almost the rim. Put the tops on and seal them.
- Put three jars into your refrigerator (don't forget to label the top with the yeast strain, date and approximate yeast count) for later brew days and leave one out for your brew that day. It will settle out so that you can pour the water off if you'd like to do that. On a one gallon batch, I'd pour off the water. On a 5 gallon batch or larger, I don't worry about the little bit of extra water. It's been sterilized, so contamination isn't an issue either.
Some thoughts on oxygen, CO2, and how they affect yeast depending on whether we're making a starter or making beer.
- If you had any yeast slurry left in your 1/2 gallon canning jar, after filling the pint jars, don't waste it. Put it into the beer that you're brewing that day, along with your pint jar of slurry. It will up your cell count a little bit, so no worries.
I don't use pure oxygen, so I can't speak too intelligently how long to give it pure oxygen, but I know that it's less than a minute of pure oxygen.
When yeast are in an oxygen rich environment (and thus a carbon dioxide poor environment), most of their energy goes into budding and multiplying, with some of their energy going into getting healthier.
When yeast are in a carbon dioxide rich environment (and thus an oxygen poor environment), their energy goes into growing and maintaining health.
With those last two statements being said, our goals with yeast management are different, depending on if we want them to make great beer or make more healthy yeast.
At the beginning (when first pitched, whether from dry or liquid), yeast must have oxygen, to get and/or stay healthy and multiply. If we continually feed them oxygen, they stay in the get or stay healthy and multiply mode. Basically, when yeast are in an oxygen rich environment, they're eating the sugar and having sex with themselves (yeast bud, so there aren't male and female versions). They're still producing CO2 and alcohol, but the CO2 is degassing, because of the stir plate action, which is bringing in oxygen. With oxygen, the environment is ideal to stay healthy and propogate, making more healthy young off spring.
If we put them on a stir plate and keep feeding them oxygen, they will keep budding, and each subsequent generation will bud and make offspring. They're still making alcohol and CO2.
Once we put them into the wort that we want them to transform into beer, we need to give then an initial dose of oxygen and nutrients to get them off to a great start. Think of it like a soldier, training for a career in the military. When we give yeast oxygen they're getting to their optimum health and fitness for the task ahead. They are preparing themselves for the task ahead.
When we make beer, we put an airlock on top of our fermenter and cut off that supply of oxygen. They have that initial 8-12 ppm of oxygen that we gave them when we pitched the yeast into our wort. They use that last shot of oxygen to prepare for the war ahead. As the oxygen supply dwindles as the yeast use it up, the yeast change to a fight for life conservation mode. This conservation mode means that they have to cut back their energy usage someplace and that place is reproduction. They don't stop reproducing ( I don't think so, anyhow), but they drastically slow down they're reproduction. They're consuming the sugars and nutrients, and converting them mainly into CO2 and alcohol. They make other bi-products, but that is beyond the scope of this discussion.