Bread is nothing short of awesome; you can have it as companion to a number of other foods, or make it the only food staple you’ll ever need ¹. The fact that bread has been around since the dawn of civilization is as fascinating as the biochemistry behind its making. And all you need is water, flour, and a little help from microscopic friends.

Special microorganisms are responsible for half the bread’s charm. The carbon dioxide produced by yeast fluffs the dough up, giving this starchy wonder its characteristic loafy shape. The common bread that we buy in plastic bags at the grocery store usually employs commercial leaven containing a single strain of Saccharomyces cerrivasae. On the other hand, we can make our own leavening agent with a naturally-ocurring blend of yeast and bacteria. According to the weblog Cultures for Health, they thrive naturally on the surface of grains, fruits, vegetables, and even in the air and soil.

This colony of wild microorganisms is the building block of a special kind of bread called sourdough, which, as the name implies, has a sour, delicious aftertaste. This leaven is known as sourdough starter. Don’t get me wrong, there’s nothing wrong with using commercial leaven from the nutritional point of view, but I’m all for the DYI culture. So here are my musings and attempts of making my own sourdough starter.

Throughout this note, I try to answer the following questions:

• Is the sourdough starter quality affected by its environmental factors (like temperature)?
• What about different substrates instead of wheat flour?
• What is the optimum feeding interval?

Which species compose the sourdough starter?

Gänzle et al. (1998)² explains that

In sourdoughs with a tradition of continuous propagation, Lactobacillus sanfranciscensis ("Lactobacillus brevis subsp. lindneri"), Candida milleri, and Saccharomyces exiguus (anamorphic form: Torulopsis holmii) are the predominant microorganisms. The process conditions of traditional fermentations ensure a high level of metabolic activity for these organisms and permit the production of breads with sensory qualities superior to those prepared from pure cultures of lactobacilli or yeasts.

The wild yeast and lactobacilli that form the leaven for the sourdough thrive in a symbiotic relationship. The yeast uses carbohydrates to produce ethanol and carbon dioxide. Bubbles of carbon dioxide are responsible for the fluffing up of the dough. The acidity created by the lactic acid from the lactobacilli is tolerable for the yeast (Gänzle et al. (1998) reported growth in pH as low as 3.9), but inhospitable to other organisms.

Does the environment affect the sourdough starter’s microflora?

Despite what some webloggers preach, different environments affect the ratio of yeast and bacteria species, but the biodiversity remains the same, according to Discover Magazine’s “The Biology of Sourdough”.

Metabolic pathways \(\require{mhchem}\)

More specifically, the yeast carries out alcoholic fermentation and converts one mole of glucose (\(\ce{C6H12O6}\)) into two moles of ethanol (\(\ce{C2H5OH}\)) and two moles of carbon dioxide (\(\ce{CO2}\)), producing two moles of ATP in the process:³

\[ \ce{C6H12O6 -> 2 C2H5OH + 2 CO2} \]

Wheat flour is basically composed by starch, \(\ce{(C6H10O5)_{n}}\), which can’t be used by the yeast in its original form and must be broken down into the sugar glucose through the naturally occurring enzyme amylase. The process also creates sucrose, galactose, and raffinose, which the sourdough’s natural yeast can metabolize. Therefore,

\[ \ce{ (C6H10O5)_{n} + nH2O -> nC6H12O6 } \]

Recipe

Ingredients

• 460 g strong white flour
• 300 g sourdough
• 10 g salt
• 230 ml warm water

Instructions

1. Mix flour and sourdough and salt and warm water.
2. Then stir for 2 minutes stretch the sourdough for 10 minutes.
3. Wait 30 minutes for the first rise.
4. Stretch into a retangle make three marks and fold.
5. Dust the basket with flour and sprinkle a little flour onto the dough.
6. Cover the basket and leave it overnight to double in size.

The next day, pre-heat the oven to 230C and put it on a stone or tray. Make cuts and sprinkle water in the oven. Leave it in the oven for 30 minutes.