Leavening itself is the process of "raising" the bread for it to become spongy/soft. This is brought about by CO2 (carbon dioxide) trapped in the dough. Various agents are utilized to generate this CO2 for us.
- Baking soda/powder. This is just a chemical Na2CO3 (sodium bicarbonate) which when either combined with acid or brought to a certain temperature in your oven starts bubbling by breaking down and producing CO2. This mixed in the dough when baking gets trapped in the dough and expands the dough thus raising it.
- Commercial Yeast. This is the Saccharomyces cerevisiae species of yeast which is cultured and is commonly available in granular form as Active Dry Yeast or Instant Yeast. Also available as Compressed yeast/Cake yeast. This kind of yeast when hydrated gets active and starts to eat the sugar in the flour and starts "farting" CO2! ;) Very desirable farts indeed!!!
- Wild Yeast. A sourdough starter is a stable symbiotic culture of bacteria and yeast present in a mixture of flour and water. The yeasts Candida milleri or Saccharomyces exiguus usually populate sourdough cultures symbiotically with lactobacillus bacteria. The "farts" of the yeast are utilized the same way as above. So, in other words, sourdough refers to the process of leavening bread by capturing wild yeasts in a dough or batter, as opposed to using a commercial yeast.
So what is so much better about sourdough?
Sourdough likely originated in Ancient Egyptian times around 1500BC, and was likely the first form of leavening available to bakers. Sourdough remained the usual form of leavening down into the European Middle Ages.
The symbitic culture of lactobacilli and wild yeasts in the sourdough, give a distinctively tangy or sour taste (hence its name), due mainly to the lactic acid and acetic acid produced by the lactobacilli. I can literally eat sourdough bread all day long and not feel too "breaded out!" It somehow feels gentler on the stomach and guts and healthier compared to if I make bread using commercial yeast. But most importantly it's the taste and the aroma that I love!
Sourdough bread is made by using a small amount of the starter dough, which contains the yeast culture, and mixing it with new flour and water. A small part of the starter dough is saved for the next batch of baking bread and is fed on flour and water and goes in the fridge to slow down it's consumption of the food on the flour until the next bread baking day (which shouldn't be more than a week away.)
As long as the starter is fed flour and water daily, the sourdough mixture can stay in room temperature indefinitely and remain healthy and usable. It is not uncommon for a baker's starter to have years of history, from many hundreds of previous batches. As a result, each bakery's sourdough has a distinct taste. The combination of starter, yeast culture and air temperature, humidity, and elevation also makes each batch of sourdough different.
A fresh culture begins with a mixture of flour and water. Fresh flour naturally contains a wide variety of yeast and bacteria spores. When wheat flour contacts water, naturally-occurring amylase enzymes break down the starch into complex sugars (sucrose and maltose); maltase converts the sugars into glucose and fructose that yeast can metabolize. The lactobacteria feed mostly on the metabolism products from the yeast. The mixture develops a balanced, symbiotic culture after repeated feedings.
These symbitic starter cultures are very stable due to their ability to prevent colonization by other yeasts and bacteria as a result of their acidity and other anti-bacterial agents. As a result, many sourdough bread varieties tend to be relatively resistant to spoilage and mold.
The yeast and bacteria in the culture will cause a wheat-based dough, whose gluten has been developed sufficiently to retain gas, to leaven or rise. Obtaining a satisfactory rise from sourdough, however, is more difficult than with packaged yeast, because the lactobacteria almost always outnumber the yeasts by a factor of between 100 and 1000, and the acidity of the bacteria inhibit the yeasts' gas production. The acidic conditions, along with the fact that the bacteria also produce enzymes which break down proteins, result in weaker gluten, and a denser finished product.
There are several ways to increase the chances of creating a stable culture.
- Always use freshly ground whole-grain flour (wheat/rye/spelt) to make the starter.
- Use unclorinated water.
- Some people also use water from washing the skins of grapes/plums etc. (due to the wild yeast found on the skins of these fruits.)
- I have learnt from various sources that fenugreek seeds and cumin aid in help capture wild the wild yeast! This is what I want to experiment with this time. I am sure it will turn out fine without these seeds too as mine have in the past...
Here are some starter tutorials... It will take 6-7 days to get your starter ready and performing! So get going :)
Sourdolady's starter: This is the method I followed for making my starter, except that I kept it whole grain/whole wheat throughout and didn't switch to white/All purpose midway through. An excellent tutorial.
It's very-very important to stir the starter briskly for a minute or two to stir in a lot of oxygen in the dough.
Another great Sourdough starter tutorial using rye, but again I would just keep to 100% whole grains.
Some more good articles:
When Yeasts Attack: A First Experience with Naturally Leavened Bread
More about Sourdough