Yeast (Llevat)

My grandfather used to be a baker. But he died before I became interested in baking. I wish he was here to teach me!.I am completely new on that, but I want to be able to bake bread. So I have started today.
The first to learn about baking is the Yeast.
I did not expected that yeast was that tricky...

I learnt about yeast when I was studying my degree in Biology. Yeasts are unicellular microorganisms and what they do is converting fermentable sugars into carbon dioxide and ethanol. Saccharomyces cerevisae is the yeast used for baking and it is the same species commonly (but not the only one) used for doing alcoholic fermentation too (that is why the name cerevisae, beer in latin).

There are different types of yeast for baking:

-Baking powder or chemical yeast- is a dry chemical raising agent used mainly for sweet cakes (muffins, cakes..). Most commercially-available baking powders are made up of an alkaline component (typically baking soda), one or more acid salts, and an inert starch. You need the oven (temperature) for the increase of the volume. It lasts around a year or more. 20 gr/ 1kg flour (like Royal for example)

-Fresh Yeast or compressed yeast (also Paris Yeast): Normally it is the one you can get in the bakery. But now you can also find them in the supermarkets. It lasts some days. It needs time and room temperature to grow, so don´t´put it in the oven straight to the oven like the baking powder!. It needs flour (flour contain fermentable sugars) and water to react with and grow. I recommend add some water and leave it for 10 minutes before you start baking the bread. Mainly used for bread. 20 gr (for regular bread)-40 gr (sweet bread) /1 kg flour. (Like Fleischmann’s Yeast or Levital, for example).

-Active dry yeast is the form of yeast most commonly available to noncommercial bakers in the United States.Under most conditions, active dry yeast must first be proofed or rehydrated. It can be stored at room temperature for a year.

-Instant yeast: appears similar to active dry yeast, but has smaller granules with substantially higher percentages of live cells per comparable unit volumes.It is more perishable than active dry yeast, but also does not require rehydration, and can usually be added directly to all but the driest doughs. (LeSaffre's "SAF Instant Gold" for example). 1gr of instant yeast equals 3 gr of fresh yeast, so use less. 8 gr /1kg flour

Others:

-Home-made Yeast (Wild Yeast)
-Cream yeast: Its primary use is in industrial bakeries with special high-volume dispensing and mixing equipment, and it is not readily available to small bakeries or home cooks.


If you have any comments or suggestions please let me know!

References

http://en.wikipedia.org/wiki/Yeast
http://www.jesusquerol.com/demos.php?t=3&idm2=1&x=224
http://panarra.wordpress.com/el-llevat/
http://www.serveidols.com/index.php?apartat=subfamilia&id=1&sec_vis=1&ap_vis=2
http://www.alrasa.com/pa/llevat.htm
http://en.wikipedia.org/wiki/Baking_powder
http://www.alrasa.com/pa/llevat.htm
http://notsohumblepie.blogspot.com/2010/04/wild-yeast.html
http://www.thefreshloaf.com/faqs/baking/yeast