Active dry yeast undergoes a few more processing steps than compressed yeast. After the live yeast is spun out of the fermentation vat and a good deal of the water is removed, it’s mixed with a small amount of oil and extruded in extremely thin little ribbons. Those ribbons are cut up into granules, then the granules are tossed in a powder of some, shall we say, “detritus”…dead yeast cells mostly, to give them a protective coat. At that point they’re fully dried, packed and shipped.
Also called “cake” yeast, this form of yeast is a living culture, taken straight from the fermentation vat — actually spun out via a centrifuge. Water is removed, then the live yeast is mixed with a little cottonseed oil, a few emulsifiers, then pressed to shape. It’s available in most larger supermarkets and is usually found on an upper shelf near the cream cheese (in the States).
The nice thing about fresh yeast is that it’s active when you buy it. It doesn’t need to “wake up” in order to be used, and a lot of people find that reassuring. Add it to a dough and you get a very fast and lively rise with it.
Being a living thing, yeast has needs if it’s going to survive in the kitchen or anywhere else. Obviously it needs food (simple sugars) and water. Beyond that it has temperature requirements. It grows and produces CO2 most prodigiously at about 92 degrees Fahrenheit. It slows to the point of dormancy at 40 degrees and goes completely dormant below the freezing point of water. If it’s frozen for long, some of its population dies, about 10% per month. Similarly, yeast activity starts to slow down when the temperature gets over about 100 degrees Fahrenheit and stops completely at 135 degrees, at which point it dies.