Your sourdough starter is an amazing microbial consortium, a living community, of both bacterium and yeast.  Many of us are familiar with the commercial yeast used in baking bread, which is called Saccharomyces Cerevisiae or “sugar eating fungi”, but your sourdough starter is a combination of wild yeasts, like Saccharomyces exiguous, and lactic acid bacteria or LAB. This bacteria group consists of lactobacilli, which is a species that belong to the Lactobacillus genus. These tiny LAB single cell microorganisms are the main contributors to your sourdough’s flavor development (Schieberle 1996),(Thiele C, Ganzle MG, Vogel RF 2002) improved nutritional quality (M. Gobbetti et al. 2005)  and they even help prevent your bread from going stale. (1) , (A. Corsetti, L. Settanni   2007) 

One of best known LAB is Lactobacillus sanfrancisco, but there are many subspecies and close relatives like Lactobacillus brevis and Lactobacillus plantarum commonly found in your starter; yes, it’s a lively little community.  (Corsetti, A., et al. 2003) (Corsetti, A.,et al. 2001)(Gobbetti, M. 1998) Depending on the temperature, there are roughly 100 bacteria for every yeast cell in your starter. (2)

So how did they first figure out what’s in this gooey mess?   Microbiologists use a genetic marker to identify the species of LAB, and it’s not as easy as it sounds. Lactobacillus sanfrancisco, was only discovered in 1971 and is named after its place of origin.  (Sugihara et al., 1971) In scientific terms, researchers made a phylogram based on rRNA gene sequences which show the relatedness between organisms, like our sourdough Lactobacillus sanfrancisco, evolved in time, thus providing the framework for modern classifications of microorganisms. 

To truly characterize your starter, you would need to study organic chemistry, microbiology, phenotypic and molecular identification (genetics), all of which have a place in understanding the sourdough ecosystem that is defined as the microflora lactic acid bacterium and its interactions with wild sourdough yeasts.  Fortunately, we can take from these disciplines the understanding of the makeup of sourdough bacteria and yeast to help explain fluctuations in a starter and how to develop a successful starter for baking.

Many studies have looked at the effect of both temperature and moisture on the microbial growth and activity of bacteria and yeast. The importance of temperature has been emphasized due to global climate change since microorganisms are the main group producing carbon dioxide during decomposition of organic materials. Temperature is an important factor regulating the bacteria and yeast making up your sourdough starter. Research studies have shown that temperature affects different groups of microorganisms, such as bacteria and yeast, in different ways. Microbiologists have observed, given the same ecosystem and organic material, that yeast (or rather fungi) growth rates are predominant at lower temperatures, while bacteria growth rates tends to dominate at higher temperatures. (Janna Pietikainen 2005) Temperature is one of the most important environmental factors regulating cellular activity and determining the composition of the bacterial community in your starter.  Bacterial communities inhabiting environments with different temperature regimes therefore have different ratios of bacteria to yeast.  

So, what’s the right temperature for your starter?  Gobetti et.al. (1997) found that the enzymes necessary for LAB fermentation maintain high activity in temperature around 86 degrees Fahrenheit (F) Additional studies showed that bacteria can live for up to a week in refrigeration around 40 degrees F. Therefore, if you store your starter in the refrigerator you should take it out and feed it weekly to keep the bacteria alive.  In our kitchen, we keep a master starter in the refrigerator and feed it weekly. However, we keep a second starter out on the counter and feed it every 12 to 24 hours depending on our bread-making needs. 

Our local Northern California ecosystem of wild yeast and bacteria had a great deal of study in the 1990’s. The relationship between bacteria and yeast has shown that the starter’s bacterial growth can be related to specific amino acids, and what we call peptides, excreted by the yeast. These amino acids can keep the bacterium alive even when all other nutrients, like the typical maltose in flour, have been depleted.  What has also been found in these studies is that the type of carbohydrate (found in your flour) and the bacteria energy growth are greatly influenced by the type of sugars present. The final flavor of leavened bread is complicated it is influenced by its raw materials, sourdough fermentation, proofing, baking and by the type of starter used. 

Since the bacteria both multiply and maintain existence at a higher temperature, I choose to keep my main baking starter unrefrigerated.  In the studies I reviewed it was the bacteria that has been shown to be a major contributor to your bread’s taste and smell. (Gobbetti et al 1995B)  They call it bacterial proteolysis during fermentation which contributes to flavor. (3) However there are many compounds which will affect your bread’s flavor. Those noted in the literature are organic acids, alcohols, esters and carbonyls.(4) The main compounds produced by LAB are ethyl acetate, alcohols, aldehydes and acetic acid.  Research has shown that by raising the temperature of fermentation to 86 degrees F more volatile compounds are produced.  After 3 hours of fermentation the main characteristics are iso-alcohols. Increasing the fermentation up to 9 hours gives 3 times more compounds. (Gobbetti et al 1995B)  It takes on average 12 to 24 hours to generate these compounds, so I refresh my starters daily and allow my fermentation to run for about the same length of time.  There are so many factors affecting these living microorganisms that we need a PhD thesis, not a blog, to explain. For now, I will provide a few simple rules of thumb.

Three rules of thumb for your Sourdough Starter's Maintenance

(1)  Use the same flours and similar ratios for your starter as you will use in your bread-making.  This will maintain the same active colonies.

(2)  For increased flavor from bacteria keep your starter around 65 to 85 degrees F and ferment your breads at the same temperature. Fais comme les Français. Do as the French do.

(3)  Lastly, to maintain an active healthy LAB colony literature has suggested that at 65 degrees F you should feed your starter every 12 to 24 hours, and at 85 F degrees every 4 to 8 hours. Observe your starter from a clear container. Allow the starter to rise and then fall again before feeding it.  

By providing a few simple rules of thumb and not the mathematical modeling of microbial growth kinetics and dynamics like  (Michaele Ganzle et al 1998), your baking experience is one of enjoyment and satisfaction.  In future blogs I will discuss building levain and the addition of pre-ferments like a biga or poolish and its effects on fermentation and your bread.  However, in natural leavening the most important thing to understand is your starter. This means you should have a general understanding of the bacteria yeast ecosystem in your starter, and through your own personal observation you can identify when your ecosystem is healthy or when its death is imminent. In my personal baking experience, I believe understanding science can improve my baking, but it is my failures and successes that helped me become a better baker.

References for this post:

1. Enzyme applications in baking - Henrik Lundkvist og Hans Sejr Olsen , Publiceret Oktober 2007 Biokemisk Forening – Danish Society for Biochemistry and Molecular Biology

2. (Gobbetti M et al 1994)(Ottogalli G et al 1996)(Meignen B et al 201)(stolz,P 2003)

3. (Gobbetti et al 1998) (Gobbetti et al 1994) (Thiele et al 2002)(Hoffmann 1989a)(Barber et al 1992)

4. (Barber, Torner,Martinez-Anaya, & Benedito de Barber, 1989),(Czerny& Schieberle, 2002), (Hansen & Lund, 1987), (Hansen et al., 1998)