Most of my baking recipes are based on Fresh Yeast. Fresh Yeast is also commonly known as compressed yeast or cake yeast (in the USA).
The San Francisco baking institute suggest to use the following conversion formulas
1gr Fresh Yeast = 0.5gr Active Dry Yeast = 0.4gr Fast Action Yeast
100% Fresh Yeast = 50% Active Dry Yeast = 40% Fast Action Yeast.
This is an extract from the San Francisco Baking institure
Fresh Yeast vs Instant Yeast
It is common at our school to encounter the opinion that fresh yeast is superior to dry yeast. One baker even told us that when he switched from dry yeast to fresh yeast, the flavor of his bread improved. We would like to dispel the myth that fresh yeast produces better bread than dry yeast. In fact-if used properly-dry yeast will produce the exact same bread as fresh yeast. As long as the amounts are correct, the process is the same. It may even be the better choice in some situations, especially when you do not have a reliable source for fresh yeast. And, even if you do, instant yeast is a good back-up to have on hand in case you run out of fresh yeast. An unopened package of instant yeast has a shelf life of up to two years.
Active dry can be used at 50% of the weight of fresh yeast and instant dry can be used at 40% of the weight of fresh. Based on the recommendation of the yeast manufacturers, most people are under the impression that 33% is the proper conversion for instant yeast. This is true for an industrial process, but 40% is better in the artisan process, when dough temperatures are generally lower.
The instant form is the easiest to use since it does not need to be re-hydrated before adding to the dough. The only precaution is that it should not come in direct contact with cold temperatures and therefore should be mixed into the flour before adding water or added after the flour and water is incorporated.
As the San Francisco Bakery Institue says, there should be no difference in flavour using the fresh Yeast (compressed yeast) as opposed to fast action if the latter is properly used.
In addion, where I leave, in Dublin is kind of difficult to find fresh yeast. The only place where I can find it is in Polish shops. Polish people love to use fresh yeast in their baking.
However, I must say that it does not seem to me very fresh.
On the oppisite side, Fast Action Yeast is readily available across the Country, it is easier to store, and it's no perishable, its shelf-life can be of 2-years if the sachet is not open. It comes in 7gr bags.
I couldn't find anywhere Active dry yeast. Aside the number of active cell, which is higher in the Fast Action Yeast, the main difference between the Active dry and the Fast Action is that the Active dry yeast needs to be activated with hot water, while the Fast Action goes into the dry ingredients - you should avoid to put it directly in contact with cold water.
The use of Fast Action Yeast bring about the problem of how to weigh it accurately.
As an example let's suppose that we need to produce a 12hr poolish for 250gr of flour
0.2% Fresh Yeast
Temp 21 C
Doing a bit of math we need 0.5gr of Fresh Yeast for this poolish, which means
40% * 0.5gr = 0.2 gr of Fast Action Yeast!
This is really a tiny amount to measure.
To try to sort this problem out we can use measuring spoons. Since the volume of the Active dry yeast cell is pretty much constant from sachet to sachet, a teaspoon should be a reliable way to weight the Fast Action yeast for tiny amounts, at least more reliable of a digital scale with a minimum step of 1gr.
Tonight I did some math and I came up with these two inequalities
tsp = tea spoon
3tsp > 7gr
2tsp + 1/2 tsp + 1/4 tsp < 7gr
which tells me that
2.33 < tsp < 2.54
So a teaspoon of Fast Action Yeast must be something around 2.33 / 2.54 gr.
After some other investigation I believe that it should be pretty close to 2.47 grams.
In practical term we can assume
1 tsp = 2.5gr Fast Action Yeast
1/2 tsp = 1.25gr Fast Action Yeast
1/4 tsp = 0.62gr Fast Action Yeast
So to make our 12hr poolish we will need 1/3 of 1/4tsp!!!
I will give this a try and let you know how it goes.