A "compressed yeast cake" is called for in each of my great-grandmother's bread recipes. Can I use active dry yeast as a substitution for one? If so, how much active dry yeast should I substitute per compressed yeast cake?
They are completely equivalent, with a strict conversion ratio of 1:3 (so if you have a recipe calling for 20 g of fresh yeast, you would use 6.7 g of dried, rounding up is OK). It is the same thing you are using, living organisms of the species Saccharomyces cerevisiae. The difference is in the nutritious medium in which they are packaged, it is fresh in one case and dried out in a special non-yeast-killing method in the other. Once they "wake up", they act the same way in all recipes.