Rather than thinking about stages of fermentation I like to look at the lifecycle of yeast. There is a great interview with David Logsdon from Wyeast on the April 5, 2007 episode of Basic Brewing radio.
Yeast cells bud in the presence of oxygen. Only yeast cells with a reserve of glycogen have the energy to bud and that glycogen supply is depleted after roughly four divisions. The first thing that happens when you pitch healthy a vial of yeast into well-aerated wort is the consumption of available O2 with about a sixteen-fold (24) growth of the yeast population. There is not much visible activity at this point. It is generally known as the Lag Phase (called the Adaptive phase above).
Once the oxygen is gone and the yeast is out of glycogen it starts consuming sugars to produce more energy. This phase is where it eats available sugar, pees alcohol and farts CO2. Most people call this part primary fermentation and the wort is visibly active with carbon dioxide production, rising temperature and swirling trub.
As the easy to eat sugars are removed from the wort and alcohol potentially rises to toxic levels the yeast switches from energy production to life preservation. It stores glycogen for later budding (when O2 comes back), becomes inactive and drops out of solution. Fermentation activity slows and appears to halt.
Even though most of the yeast has flocculated at this point, some cells are survive and consume the more complex sugars. As they do the beer "conditions" and continues to develop flavors.
I understand reason for transferring beer from primary to secondary, which is to separate the beer from dormant yeast and trub. Under the right conditions, yeast cells will undergo autolysis where the cell's enzymes begin to consume itself. Nasty flavors develop.
I do not transfer for a few reasons.
- I've never experienced autolysis.
- You run the risk of introducing infection
- I often don't have enough vessels
- It eliminates a step
My beers turn out tasty (damn tasty) and (mostly) clear.
I think what most differentiates primary from secondary is the separation of beer from sediment, or at least the effort to do so.
There may be other markers of the move from primary to secondary; like a transfer (which is the only way I know to separate the good stuff from yeast/sediment), additives, bottling, etc.
Or, maybe more simply, the difference of secondary and even tertiary fermentation from primary could just be that a large majority of fermentation occurs in the first stage.
I think one thing to keep in mind is the evolving state of the homebrewing art. I get the feeling that the distinction between primary and secondary fermentation is an older concept, dating from the time when dried yeast was the norm, autolysis was a serious vector of off flavors, and a secondary transfer was practically always part of the fermentation schedule.
As others have noted, fermentation is really a continuous process with several overlapping phases, and not a rigid 2-step procedure that requires transfer to a secondary vessel. Indeed it's beneficial for healthy yeast to be kept in contact with the wort well into their dormant phase, as this is the time that yeast will reabsorb metabolic byproducts that they previously dumped into the beer. As far as I can tell the only good reason to remove the beer from the yeast partway through fermentation is autolysis, which shouldn't be a problem with well grown, healthy yeast.
Having said that, racking to a secondary vessel is a good idea for procedures like dryhopping or spicing, or for high alcohol or dark beers that benefit from some mellowing unrelated to yeast fermentation.
The rule I follow is no more than 4 weeks on the yeast cake, and if the beer needs more finishing beyond that before I keg, then that's the only time my secondary vessel gets used.
I realize most people here are talking about beer...but I've made both wine and beer, and have never used a secondary fermentation (apart from making apple wine into apple beer). If I've added flavors, such as cinnamon, to apple and/or pear wine, I boiled the sticks/other ingredients in the sterilization/flavor extract process, and that was that. Why add a potentially infectious vector to what you've sterilized prior? Many, many people have complimented my wines/beers, and only one (an amateur wine snob) had anything bad to say. Why add another step? If it's not clear, which it should be after bottling, rack it out of the bottle after a day of settling (most wont have to do this), then serve.