Everyone wants to brew a great beer. There are numerous factors involved, no matter what your setup. Extract, Partial Mash, Full Grain, there are a few things that are common to all.
First priority is cleaning. The most important part of brewing is cleaning your equipment. Everything in your brewery needs to be cleaned. Any surface that touches wort or beer needs to be spotless. Otherwise, your equipment becomes a breeding ground for microorganisms which can contaminate your beer.
Logically, something that goes hand and hand with cleaning is sanitization. The most important advice about sanitization is to use a sanitizer only after everything is clean, and only at the proper concentrations. Anything that goes into the wort should be clean and sanitized. Wort is a rich growth medium for microorganisms. Brewer's yeast is the only microorganism you want growing in yours. If bacteria or wild yeast begin growing in your wort or beer before your brewer yeast has established its dominance, sour, acetic, phenolic, other off flavors and aromas, or improper alcohol conversion may result.
Since bacteria can grow as much as 6 times faster than yeast, it is vitally important to pitch the proper amount of viable yeast. Dry yeast packets generally do contain enough cells in them, but I would always make a starter first and pitch that. If you want to make a style of beer that there isn't a dry yeast for, that is where the problems can arise. Most homebrewers start out pitching a 50ml Wyeast packet. Assuming all the yeast in a Wyeast packet are viable (practice has shown that less than 50% are viable), according to my last Wyeast packet, your adding 50ml of roughly 500 million/cells per ml. Or roughly 25 billion yeast cells. As you'll see below, that underpitching by roughly a factor of 10 for a 5 gallon batch.
When you read below, understand that I’m NOT saying you need to follow these guidelines to make great beer! Just remember, that you want very little yeast growth to go on in the fermenter. If underpitched the yeast will spend much more time trying to grow adequate quantities, during which the yeast can secrete more esters and fusel alcohols. They may not have sufficient number to digest all the fermentable sugars. You’ll get better beer if your yeast spends most of their energy making alcohol, and not reproducing in the fermenter.
The general consensus on pitching rates for an ale is that you want to pitch at least 1 million cells of viable yeast for every milliliter of wort, for every degree plato. A degree plato is about 1.004 of the original gravity.
(1 million cells) * (milliliters of wort) * (last digits of O.G / 4) = Cells needed
For a 1.048 wort (48 / 4 = 12 degrees plato), pitching into 5.25 gallons (3785 milliliters in a gallon), you need:
(1 million cells) * (20,000 ml of wort) * (48/4) = 240 billion cells
That's about 12 million cells/ml in your fermenter.
For a lager, I’d recommend double that amount.
How do you get that many cells? By always using an adequate starter. There are numerous resources to tell you how to make a starter, but not enough to tell you how big a starter to use. A lot depends on what you do with your starter.
Oxygen: Oxygen or aeration is essential for yeast growth. Oxygen is quickly absorbed by yeast and is used by the cell to form (along with a few other things) a healthy cell membrane. Since oxygen directly correlates with rapid grown and cell numbers, your starters should be well-aerated.
Lets look at an example of the approximate starter volume needed for an 1.040 wort:
(1 million cells) * (20,000 ml of wort) * (40/4) = 200 billion cells
That’s 10 million cells per ml. In order to achieve 10 million cells/ml in your fermenter:
A traditional starter with an airlock in place and ignored will produce roughly 20 million cells per milliliter of starter. That’s roughly 10 quarts of starter for a 5 gallon batch! Pitch less than this, and you may not see any appreciable activity for days.
Pick it up whenever possible, and give it a shake to introduce oxygen and re-suspend the yeast, and your probably up to 60 million cells per milliliter of starter. Better at roughly 3.3 quarts for a 5 gallon batch! Pitch less than this, and you may not see any appreciable activity for 24 hours.
If you use an oxygenation system on your wort, use it on the starter. Intermittently inject sterile air or oxygen into your starter through an airstone, and you're up somewhere around 92 million cells per milliliter of starter. That’s 2.2 quarts for a 5 gallon batch, which is much more manageable. Pitch less than this, and you may not see any appreciable activity for 12 hours.
The best you can do is a stir plate. Do a Google search; there are various resources available on the internet for building your own for relatively little. Search auction sites for deals (buyer beware, read carefully, and ask questions). Using a stir plate, you can continuously aerate your starter, and achieve something around 240 million cells per milliliter. That’s roughly 0.75 quarts for a 5 gallon batch! Pitch this, and you should see activity within hours. A stir plate can make just about any size starter in 24 hours. Pitch the entire amount of yeast right away, if you are ready for it.