As home-brewers we do not, as a rule, filter our beers. And so it becomes an article of pride for many of us to have beer that is unfiltered, and yet crystal clear. We like the yeast, we use it to turn our wort into beer, and then we would like it to sink down as quickly as possible and leave our beer clear and transparent.
One of the methods that we use to achieve this lofty goal is Cold Crashing.Cold crashing is a process by which we take the beer once it's done fermenting, and abruptly drop the temperature in the fermentor from the blissful 17-21C of ale fermentation to a blistering 4C for about two days. (Lager, being laggered at 4C anyway is not cold crashed. Or if you will, Cold crash is part of its basic process.) The idea is that the yeast will become dormant and flocculate out of the beer, leaving it clear. A secondary use of cold crashing is to stop fermentation before it is complete, in order to leave more sugar in the beer and a lower alcohol content. (This is kind of a misconception, since you don't stop fermentation, you suspend it, and the yeast will take it back up if you raise the temperature again)
Personally, I can never bring myself to cold crash a beer, and instead I prefer a slower process in which I drop the temperature by about 1 degree C every 12 hours, for about a week. This is enough to bring me from 18 degrees to 4. It just takes longer.
The reason I do it this way is because of what happens to yeast when it gets stressed. When yeast gets stressed (such as, when the temperature changes abruptly) it produces a variety of stress compounds in the form of phenols and other off-flavor chemicals. It's rather like a human being that's suddenly been confronted with a stressful situation: These chemicals are the yeast equivalent of cold sweat. By gradually reducing the temperature in small increments I strive to minimize this effect by letting the yeast acclimate and fall asleep gently, rather than shocking them into a comma.
Is this a better approach than cold crashing? I have no idea. I have heard claims that when you cold crash your yeast falls asleep so fast that it doesn't have time to produce off-flavors in significant quantities. I don't know. One of these days I may try this: Take a batch, split it in two, and let half cool in the house fridge (at 4C) while I cool the other half slowly. It will be interesting to see the difference.
My philosophy of brewing is that you can't rush good beer, and therefore you should avoid doing anything abrupt to it. This is a luxury that we, as home brewers, have over commercial brewers. We can afford to wait the extra few days. We can age a beer for a year before we crack open the first bottle. We can experiment much more readily with our 20 liter batches than they can with their 200 or 2000 Liter batches. Given all that, it seems only natural to avoid doing something fast and brutal to your yeast, and prefer instead a slower, gentler process. But then, that may just be me.