Yeast - My new Pet Friend

Do you remember the post about rising agents? Here I continue the chat on yeast. It was the one thing I was interested in the most. Besides eggs that is, I really want to know what eggs do too! Yet again this is a completely different post than the previous ones. I really wanted to accentuate the way yeast works, but will try to answer some practical questions as well!

The way it works

"One day, in a bakery far away, there was a little Fungi named Saccharomyces Yeast. The bakers gave him lots of food and water, which Yeast would happily eat all day. He'd make loads of friends and loads of babies and he'd burp and fart all day. The warm baker's hands would make him jump of joy and eat some more until the heat oven the oven took it all away."

Yeast is a living organism, which requires sugar and moisture to activate (food and water). A warm temperature speeds up the rising process, which generally takes 1-2 hours, however a fridge will give the same results over more than double or triple the time. In this time the yeast will duplicate itself (making friends&babies) and convert sugars into gas (burping and farting), which create the leavening we need for the texture of our breads. These sugars can be naturally present in the flour or are added manually. More precisely yeast converts glucose into carbon dioxide, alcohol and other organic compounds. The carbon dioxide expands and creates air pockets within the dough. When the dough is baked the yeast dies and the process is set. The air becomes locked into the gluten resulting in the 'fluffy' texture of the bread. The alcohol and other organic compounds effect the flavour (and texture) of the dough. This results in a pronounced taste, which is why you will most often find yeast in savoury items.

Dough with fast-action dried yeast after it's first rise

Different sorts of yeasts
There are many many many many many different kinds of yeasts. For instance, you have a different kind of yeast for every wine or beer you can imagine (can you imagine?!). Every sort of yeast will have different properties (such as resistance to alcohol or heat) and each will call for a different method. In essence all yeasts work the same as they need feeding, heat and time to rise and they can -to an extent- be used interchangeably. In baking we have three main categories:
- Dried Yeasts
There are several types of yeast in this category: dried, fast-action dried and easy blend dried yeast. Most home bakers will use a fast-action dried yeast (also: rapid-rise or instant) as it is most commonly available in stores. As the name suggests this yeast will rise faster than fresh yeast. It is also more resistant to high temperatures and is easier to keep. This is also the only yeast that doesn't need to be activated in water first and can directly be added to the flour.
- Fresh Yeasts
These yeasts must be stored in the fridge and also come in different types. Most commonly you will find Compressed/Cake yeast. A fresh yeast is considered to be superior in flavour, but home bakers will most often turn to driest yeast for the aforementioned reasons.
- Natural yeasts
Yeast spores are all around us: they occur naturally in the air, on the ground, in our dairy products, plant matter, flour and even on our skin. Sourdoughs rely on these wild yeasts. With enough flour and water a batter will start to ferment spontaneously if it is given sugar or starch to feed it. There are many variations of sourdough, but all require a starter, time to ferment and the addition of water and flour to keep the yeasts active or to refresh the sourdough when part of it is used. Sourdoughs tend to become more flavourful given time. Note: Most sourdough startes now require the use of fresh or dried yeast to 'start' the process as waiting for natural yeast spores to activate can be time consuming and ineffective. Another Note: Even the wine industry sometimes makes use of 'starters', as just like bread, wine can be made with the wild yeasts naturally present in the air.

Different kinds of breads might call for different kinds of yeast

How do we use yeast? (Can we go wrong?)
- Temperature: These are so many different kinds of yeast that the perfect temperature for a yeast to grow or ferment is hard to find. Yeast will generally work at any temperature between -2°C and 45°C (28°F and 113°C). Beyond a point of 48°C-60°C (120-140°F) the yeast will definitely die. Ideally you will be using 'room temperature' which is around 21-25°C (70°F-77°F). At lower temperatures the yeast will become slow or even inactive.
- Changing the type: Dried and fresh yeasts can be used interchangeably, but will need to be converted. For example, you will need far less dried yeast than fresh yeast for a dough. Easiest is to look at the package and the amount of flour you're using and scale the yeast accordingly.
- Scaling:  The amount of yeast you need in a recipe, does not scale proportionally with the amount of flour you need. Meaning you will need more than half the yeast when you halve the amount of flour and less than double when you double the amount of flour.
- Using too much or too little yeast: When one uses too little yeast a dough needs a longer time to rise, but will most often barely rise at all. This will create a very dense, dry and tough bread and could very well resemble a brick. Too much yeast will result in a dough that is likely collapse. You might get a very crumbly or coarse texture, very large holes in the bread or a very dense and tough texture after it's collapsed. 
- Feeding it too much: Note that there is a limit to the amount of food a yeast can handle. Too much sugar or moisture will cause the yeast to die. This is one of the reasons you rarely find yeast in sweet goods.
- Feeding it too little: Unless you forget to add water, this is nearly impossible. Flour naturally contains sugars which the yeasts can convert. However, adding sugars (in the form of sugar, milk, fruits or other) will speed up the process as these sugars are more easily accessible to the yeast.

Just to summon it all up shortly: there are several types of yeast with different properties that require different methods. You'll find that bakers use either dried yeast, fresh yeast or natural yeasts. All these yeasts, in essence, work the same as they all feed on sugars and moisture, need a warm temperature and time to do their job properly and and exert gas (among other things). It's exactly this gas that causes the leavening in our breads. Thus yeast has an effect texture, but also on flavour (remember the other things). There are several points you can take into account when using yeast, such as the ideal temperature, the amount of yeast you need and how much you need to feed it.

Yes, I thought I'd stop here since it's starting to become a very long story again. If you want more information on yeast I'd invite you to read one of the following references. Some are very basic but some will give you even more useful (and technical) information on yeast I couldn't possibly all include into this one post.

My References:
Bread by Christine Ingram and Jennie Shapter - recommended
Zelf wijn maken by C.J.J. Berry
baking 911 - Yeast  - recommended
baking911 - Preferments
Culinate - Kitchen Chemistry
BBC GoodFood - Yeast
GermanFood - All About Yeast

* Please be critical towards the information you find on the internet. For instance, one of the authors in my references explained the process of yeast very well, but insists on using half of the yeast, as to minimize the amount of alcohol in her bread. I'm astonished she failed to understand that the yeast will duplicate when given time and that halving the amount of yeast will effect the taste and texture for the worse. (I could go on but my point is you really shouldn't worry about the >0,1% alcohol! Oh yes, and to read carefully: are you sure what I wrote is right?)