Cutting the cost on yeast is one of my new adventures this year in home brewing. I accomplish this by, in a sense, farming yeast.
The process of taking a small amount of yeast and making a large amount that can be split into small batches is actually fairly simple.
A brief look at yeast. Yeast are actually a type of fungus. They are asexual being that require no sun light or oxygen to survive. They love to feed on dextrose / sugar. When they eat this sugar they produce alcohol and release CO2 gasses. The reason they produce alchol is to kill off any competition. Yeast sole purpose in life is to replicate. The sugars they eat give them the energy that is need to multiply. Now I wont go to far into how they do this. I will save that for a future post.
So back on track. Now that we know that yeast sole purpose is to eat, kill , be marry and multiply, How do we get them to do that? We make a small batch of wort ( sugar water for the non brewer ) and we add a small amount of yeast to it. Those yeast then begin to eat up all the sugars and multiply. In a matter of two days they will go from 35 billion to 92 billion. We have almost tripled are count. That is a good starter for one batch of beer but we are not in it for one batch. We are looking to make enough yeast for 4 or 5 batches. When your yeast are all done eating and doing their business they settle to the bottom and go dormant. We want to put them back to work making more yeast. How do we do that? You guessed it, we feed them more sugar. You remove the beer they have created and you replace it with more wort. Your yeast are pretty happy and ready to start back up quickly the second round.
You should now have enough yeast that you can split them up into 4 jars each becoming the next small amount of yeast that needed to make a good starter for you brew day. they say that you should only clone your yeast 5 or 6 times. I have read of people doing it many more and still having success.
For any home brewer looking to do this there is plenty of information on the net to get you started. Of course with all things brewing make sure to sanitize everything your wort and yeast will come in to contact with. Most people boil the mason jars but I find you can get the same results by soaking them in star san.