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.
The down side is that it only lasts in the fridge for about a week or so before it starts to lose its potency. Being a living culture it’s also extremely sensitive to temperature, salt and sugar. If you drop it in the mixing bowl on top of a pile of granulated sugar or a few teaspoons of salt, it will quickly start to die (and you can tell that by the changing color…it goes from tan or grey to a dark, well…tan or grey). Also it dries out fast in the fridge once you peel off the wrapper.
It’s important to remember when using fresh yeast that it’s only half as potent as active dry and a third as potent as instant by weight. So, if you want to convert a recipe from, say, active dry to fresh yeast, you’ll need to use twice as much. Going from fresh to active dry you use half as much. Seewadimean?