a little about how it works

For all practical purposes there is only one yeast, saccharomyces cerevisiae. Meaning "sugar loving mold".If you notice the second word is very close to the spanish word "cerveza", which means beer. Wether one bakes with compressed, dry or instant yeast, its all the same yeast. Again, in God's wisdom, he created yeast complete with it's own enzymatic powers to breakdown compounds into it's own digestive process. This is an enzymatic transformation. The benefits we bakers realize is alcohol, acidity and carbon dioxide. Alcohol is responsible for the aromas and flavour. The acidity is a component in creating gluten strength. And the carbon dioxide is what causes the dough to rise. Bakers yeast, has millions upon millions of yeast cells. It is a lump of highly concentrated yeast cells. The dry stuff, is the same, except it has been dehydrated. Once hydrated in the mixer, it becomes the same stuff. Old bakers will tell you "it's not the same". Fortunately, I'm a few years away from being able to share in that belief.

Sourdough yeast, is the same yeast. It is a different species. The biggest difference is it's metabolism. Sourdough yeast relies on bacteria to transform compounds into digestible form. The concentration of yeast cells is not nearly as high as baker's yeast. The wild yeast spores are in the air, and the bacteria is everywhere. When you build a sourdough starter from the beginning, you blend flour and water, allow it to sit. Take a small piece of that, add more flour and water, repeat and repeat, and you continually build up the concentration of yeast cells until it is strong enough to leaven a bread dough.

In the proper environment, sourdough yeast cells will multiply quickly. I'm not sure of the ratio, but the bacteria breeds as well. Yesterday I mentioned keeping the levain in balance. That means keeping the proper ratio of yeast cells to bacteria. This is done by controlling time and temperature. I mentioned that we keep our levain, I think from now on we will call it "culture", at night in a 55' fridge. Between 50 and 60f, the ratio of yeast to bacteria grows at a very balanced ratio. If the culture is held to warm, the yeast cells take over. The concentration of bacteria goes down, and more acetic acid is created. The culture becomes very active, will leaven bread just fine, but it won't yield much flavour or aromas. If the culture is kept to cold, bacteria will multiply to fast, it won't take over, but over time it will create more lactic acid than acetic. It will bake bread with very sharp acid tones. It also will become very active when warmed up, but again not much flavour. If you ever saw the movie "Karate Kid", Mr. Miagi said it best, "must have balance".

When baking with traditional yeast, either compressed or instant, there is a second distinguishable difference. When one adds this yeast to a dough, it immediately takes off, multiplying like crazy. This fertilization continues until the acids created, lower the ph of the dough to a certain point. Not sure what that number is. But it's not very low. Sourdough holds under 5, on a ph scale. I'm thinking traditional yeast will start shutting down as it approaches 6. If you've ever seen a piece of old dough, three, four days old, under refrigeration, it appears dead. It stopped moving. It is very much alive. The yeast in there has been fed well. Add a small piece of that to some flour and water, and it will take off again, because the fresh flour has diluted the ph. It has climbed up towards 7. The ph will drop fast as the dough sits. The yeast in that old dough has been pent up, it's aching to get going.

Enough for today. Carla, I saw your comments. I'll address them tomorrow.

Irish soda bread. It is popular because the Roman Empire never conquered Ireland. The Roman army traveled with their own bakers, and they baked the bread with sourdough yeast. There wasn't any other choice back then. The Romans never spread their technique thru northern Europe.

Now you know.