Today learn the importance of yeast in German Baking. It is all about Baking yeast today! Yeast is using a lot for many baking recipes and many recipes require fresh yeast. In the USA we never saw any fresh yeast in the super markets, it is very rare.
In this article we explain everything there is to know about yeast for baking.
In any case you should have questions about using yeast for baking, this post will clarify it or go to the page below to find more information.
(Source: www.foodsubs.com/LeavenYeast.html)

General Information 

All About Baking Yeast

Yeast is a one-celled fungus that converts sugar and starch into carbon dioxide bubbles and alcohol.   This has made it a useful ally in the production of bread, beer, and wine.   There are many varieties of yeast.  Bread is made with baker’s yeast, which creates lots of bubbles that become trapped in the dough, making the bread rise so it’s light and airy when baked.  A small amount of alcohol is also produced, but this burns off as the bread bakes.   Beer yeast and wine yeast are used to convert sugar into alcohol and, in the case of beer and champagne, bubbles.  You should never eat raw active yeast, since it will continue to grow in your intestine and rob your body of valuable nutrients.  But once deactivated through pasteurization, yeast is a good source of nutrients.  Brewer’s yeast and nutritional yeast, for example, are sold as nutritional supplements, and Australians are fond of yeast extracts–like Vegemite, Marmite, and Promite–which they spread like peanut butter on bread.

 Active dry yeast or dry yeast
Equivalents:  One package = 2 1/4 teaspoons = 1/4 ounce
Information:  This is the yeast called for by most bread recipes.  It’s largely displaced the fresh yeast our grandparents used since it has a longer shelf life and is more tolerant of mishandling.   To activate it, sprinkle it on water that’s 105° – 115° F and wait for it to begin foaming (about five minutes).  Look for it in the dairy case–it’s usually sold in strips of three packages or in 4-ounce jars.  Always check the expiration date to make sure it’s fresh.  Dry yeast can be stored at room temperature until the expiration date–or within 4 months of opening–but it lasts even longer in the refrigerator or freezer.  Always bring yeast to room temperature before you use it.   It’s important to keep stored yeast away from air and moisture, so use the smallest container you can find and seal it well.
Substitutes:  fresh yeast (Substitute one cake for each package or 2 1/4 teaspoons of active dry yeast.) OR instant yeast (Substitute measure for measure, but don’t dissolve it in liquid first.  Your bread will only need to rise once.) OR bread machine yeast (Substitute measure for measure, but don’t dissolve it in liquid first.  Your bread will only need to rise once.)

Baker’s yeast
Or baking yeast, bread yeast
Equivalents:  1 tablespoon = 1 package = 1 cake
Information: This is used as a leaven in breads, coffeecakes, and pastries like croissants and brioche.  It works by converting sugar into carbon dioxide, which causes the dough to rise so the bread will be light and airy.  Yeast comes either as dry granules or moist cakes.   It becomes less potent after the expiration date stamped on the package, so dough made with it may take longer to rise, or not rise at all.  If the potency of the yeast is in doubt, test or “proof” it by putting some of it in warm water (105° – 115° F) mixed with a bit of sugar.  If it doesn’t get foamy within ten minutes, you’ll need to get fresher yeast.

Beer yeast or Brewer’s yeast  
Information: This is used to produce alcohol and bubbles in beer.  There are several varieties, each matched to specific varieties of beer.   It’s available either as a liquid or powder at beer-making supply stores.  Don’t confuse this with the brewer’s yeast that’s used as a nutritional supplement.  That type of yeast is deactivated, so it won’t produce any alcohol or bubbles.

Fresh yeast
Or compressed yeast, active fresh yeast, cake yeast, baker’s compressed yeast, wet yeast    
Equivalents:   2-ounce cake = 3 X 0.6-ounce cakes
Information:  This form of yeast usually comes in 0.6-ounce or 2-ounce foil-wrapped cakes.   It works faster and longer than active dry yeast, but it’s very perishable and loses potency a few weeks after it’s packed.  It’s popular among commercial bakers, who can keep ahead of the expiration dates, but home bakers usually prefer dry yeast.  To use, soften the cake in a liquid that’s 70° – 80° F.  Store fresh yeast in the refrigerator, well wrapped, or in the freezer, where it will keep for up to four months.  If you freeze it, defrost it for a day in the refrigerator before using.
Substitute:  active dry yeast (Substitute one package or 2 1/4 teaspoons for each .6-ounce cake of compressed yeast) OR instant yeast (Substitute one package or 2 1/4 teaspoons for each cake of compressed yeast) OR bread machine yeast (Substitute 2 1/4 teaspoons for each cake of compressed yeast)

Bread machine yeast
Equivalents:  One package active dry yeast = 2 1/4 teaspoons bread machine yeast  
Information:   This type of dry yeast is highly active and very finely granulated so that it hydrates quickly.  Breads made with this yeast require only a single rise, so this yeast is handy to use in a bread machine.  Most machines will have you add this yeast last, on top of the dry ingredients.  If you’re not using a bread machine, add this yeast to the flour and other dry ingredients.   It’s often sold in 4-ounce jars.  You can store unopened jars at room temperature until the expiration date stamped on the jar, but the yeast lasts even longer in the refrigerator or freezer.  If you freeze yeast, let it come to room temperature before using.
Substitutes:  instant yeast (This is very similar.  One envelope active dry yeast = 2 1/4 teaspoons bread machine yeast) OR active dry yeast (One envelope active dry yeast = 2 1/4 teaspoons bread machine yeast.   Ordinary active dry yeast needs to be dissolved in water first, and the bread will need to rise more than once.) OR compressed yeast (Substitute one cake for each package or 2 1/4 teaspoons of dry yeast.  This needs to be dissolved in water first, and the bread will need to rise more than once.)

Brewer’s yeast  
Information:  This inactive yeast is rich in protein and B vitamins, and it’s used a nutritional supplement.  It’s a by-product of beer-making, which gives it a slightly bitter flavor.  If you object to the bitterness, try nutritional yeast, which is made from the same yeast strain but grown on molasses.  It’s more expensive but has a more pleasant flavor.  You can also buy debittered brewer’s yeast.  Brewer’s yeast comes powdered (the most potent form), in flakes (best for health shakes), and in tablets.  Don’t confuse this with active forms of yeast, like the kinds bakers, brewers, and winemakers use.  If you eat them, active yeasts will continue to grow in your intestine, robbing your body of valuable nutrients.
Substitutes:  nutritional yeast (better, nuttier flavor, lighter color) OR yeast extract.

Instant yeast
Or quick yeast, rapid rise active dry yeast, quick rise active dry yeast, fast-rising active dry yeast, fast rising yeast  
Equivalents:  One package = 2 1/4 teaspoons = 1/4 ounce
Information:   This very active strain of yeast allows you to make bread with only one rise.  The trade-off is that some flavor is sacrificed, though this doesn’t matter much if the bread is sweetened or heavily flavored with other ingredients.  Unlike ordinary active dry yeast, instant yeast doesn’t need to be dissolved in liquid first–you just add it to the dry ingredients.  Look for it in the dairy case–it’s usually sold in strips of three packages or in 4-ounce jars.   Before buying it, check the expiration date to make sure it’s fresh.  Dry yeast can be stored at room temperature until the expiration date stamped on the jar, but it lasts even longer in the refrigerator.
Substitutes:  bread machine yeast (very similar; substitute measure for measure.) OR active dry yeast (Substitute measure for measure. Active dry yeast needs to be dissolved in water first, and the bread will need to rise more than once.) OR fresh yeast (Substitute one cake for each package or 2 1/4 teaspoons of dry yeast.  This needs to be dissolved in water first, and the bread will need to rise more than once.)

Nutritional yeast
Equivalents: 1 tablespoon powdered = 2 tablespoons flakes
Information:  This nutritional supplement has a pleasant nutty-cheesy flavor and is packed with protein and B vitamins.  It comes in flakes or powder and is popular with vegans and health buffs who use it to make cheese substitutes, gravies, and many other dishes.  It’s also a great topping for popcorn.  Nutritional yeast is very similar to brewer’s yeast, which is also used as a nutritional supplement and is made from the same strain of yeast.  The difference is that brewer’s yeast is a by-product of beer production and retains some of the bitter flavor of hops.   Don’t confuse nutritional yeast, which is deactivated, with active forms of yeast, like the kinds bakers, brewers, and winemakers use.  If you eat them, active yeasts will continue to grow in your intestine, robbing your body of valuable nutrients.  Look for nutritional yeast at health food stores.  Get fortified nutritional yeast if you’re taking it as a source of vitamin B12.
Substitutes:  brewer’s yeast (inferior flavor, darker color) OR Parmesan cheese (as a condiment; higher in fat, less nutritious) OR wheat germ (works well in baked goods or sprinkled on cereals) OR yeast extract.

Yeast starter = sponge = levain
Equivalents:  2 cups yeast starter = 1 package active dry yeast 
Information:   A starter is a mixture of flour, water, and other ingredients that’s been colonized by wild airborne yeast and friendly bacteria.  These one-celled immigrants lend the starter–and the breads made with it–a special character.
Sourdough starter, for example, contains a strain of yeast that’s tolerant of the lactic and acetic acids produced by the lactobacilli.  Those acids give sourdough bread its characteristic tang.  The French use a soupy starter called a poolish to make their breads, while the Italians use a thicker one called a biga.  Up until the late 19th century, all yeast breads were leavened with starters, and keeping a starter alive in its crock was a routine household chore.  To keep your own starter alive, wait until it’s established, then store it in an airtight container in the refrigerator.  To keep it healthy, bring it to room temperature once a week and remove all but about 25% of it (either make bread with it or discard it).  Replace what you’ve taken with a mixture of equal parts warm water and flour, stir, then return it to the refrigerator.  Properly maintained, a starter can last for decades, developing an ever more distinctive character as it ages.   To use a starter to make bread, remove some of it (usually about 2 cups), and use it in place of other forms of yeast.  Replace the amount you took with a mixture of equal parts flour and warm water.  Discard your starter if it becomes orange or pink, or if it develops an unpleasant odor.   It’s easy to make starters from scratch, but even easier to borrow some from a friend.  Since sourdough starters must be colonized by strains of yeast and lactobacilli that are particular to certain regions (like San Francisco), a homemade starter might not yield sour bread.   Your best bet is to get a powdered sourdough starter mix from your supermarket or a mail order supplier.
Make your own:   Sprinkle 1 package of active dry yeast on 2 cups of warm water, wait 10 minutes, then stir in 2 cups of flour.  Cover loosely, and let the mixture sit at about 85°.