The Microbiology of Sourdough
A sourdough starter is a stable symbiotic combination of bacteria and wild yeast in flour and water. When flour is mixed with water, enzymes in the flour split the starch into sugars. These sugars then provide the diet of the microorganisms that colonise the starter. The bacteria comprise lactic acid bacteria, principally Lactobacillus species, which are anaerobic (they do not require oxygen to multiply) and which metabolise hexoses (sugars formed by the action of enzymes in the flour) to produce lactic acid and acetic acid. The production of these acids lowers the pH of the starter to around 3.8, and it takes on a sour taste which gives sourdough bread its characteristic flavour. The wild yeasts can tolerate the low pH conditions in the starter and include Saccharomyces and Candida spp. These yeasts ferment the sugars into ethanol and CO2 and the CO2 is what we need to make our bread rise. So far so simple? Well yes and no. A healthy starter is a perfect example of survival of the fittest, if you begin your starter from scratch then the initial population of microorganisms comes partly from the flour used and partly from your kitchen environment. The population that you will have when your starter stabilises however will depend on many factors, including ambient temperature, how often you refresh the starter and what you feed it with. If you 'buy in' a starter, it will reflect the microorganisms present in the environment where it was created. Whether it will retain its original characteristics is a matter of conjecture and will be discussed elsewhere. In either case the type, number and proportion of bacteria and yeasts will be optimised for the conditions prevalent in your kitchen. Each time you open the jar of starter fresh organisms will fall into the mixture and start multiplying (and competing with those already there), the same thing will happen each time you feed the starter with fresh flour. As long as the feeding intervals are kept constant, the pH doesn't vary and the type and source of flour remains the same the population will be stable. Does any of this matter? If you have an active starter that leavens your bread, adds a nice flavour and remains active during subsequent refreshments, then I would say not! However, if you have difficulty getting a starter to become active or it doesn't seem to be working as it should then you might need to pay more attention to conditions.