Why, here’s two good reasons. Firstly because you can save a lot of money and secondly because you will always have a supply of fresh yeast on hand which you can share.
Buying yeast for each brew can get expensive especially if you use the “wet” packets from USA.
Here’s a suggestion which I can vouch: for your next ale buy a 6 pack of Coopers Pale Ale , chill and when drinking leave an inch at the bottom to cover the sediment,put the cap back on and put bottle back in the fridge. Next day,you did drink them all? Swirl content and add to yeast starter wort. Cost about $12 plus or minus $1. Compares well with $7 for dry yeast. Not sure about the other Coopers Beers though.
Yeast starters? You definitely need them with the Wyeast and White Labs products and when you’re coaxing dregs to grow. When I do this I use a sterilised plastic bottle which from time to time I squeeze to force the CO2 out to be replaced with oxygen and then shake or swirl. You’ll find plenty of other methods out there on the internet.
But with the dry yeast I just rehydrate in a cup of cooled boiled water which I add with the wort to the fermenter. Remember to aerarate your wort. Yeast starts best on oxygen. Some folks use aquarium oxygen stones, others stir with paddles or power drills. I just tip between a couple of buckets a few times.
Right, the yeast is at work and you must consider the best time and way to capture them for next time. Old school advocates doing this at high kraesen(ie when foaming is at its peak) by scooping and putting away in sterile container. The theory is that this is the freshest yeast. However this is beyond people who have a small opening in their fermenter. All not is lost as Jamil Zainasheff suggests this process limits selection to the early attenuators to the disadvantage of the ferment. Therefore better to leave (he doesn’t do secondary ferment) and harvest at bottling/kegging. If you do secondary(I do) just bottle, set aside the sediment in the fridge and then add end dregs to them.
I keep my yeast in plastic bottles with sufficient beer to cover sediment plus about 200ml. It pays to check the bottle the first week and let off any pressure building up. I find that kept in a fridge the yeast is good for a few months which I test by tasting the beer on top. If its marmitey or not to your taste tip it out. I like to have a choice of yeasts on hand but it pays to keep a record so that you use regularly. I have used one batch of Nottingham in 14 brews before I lost my nerve and started again. It’s no different than using the Coopers sediment mentioned above. Which is the really old school method of getting yeast.
Alternatively, if you’re organised tip the slurry from your fermenter straight to the next brew and stand aside for yeast action! Best to keep for similar worts,ie ales to ales not to stouts.
What? No measures, calculators, etc. Sorry counting cells too hard for this old fellow. Just spoon in what seems appropriate. Or google it. You can make it as precise(complicated) as you like.
Ps. you can use surplus yeast for bread and pizza making!