SOURDOUGH BREAD

Brief diversion on the history and microbiology of sourdough Since antiquity, people have sought to leaven a simple mixture of flour and water by leaving it to ferment over many days. As this mixture fermented, yeasts (either introduced or present on the surface of the utensils and ingredients) would multiply and release carbon dioxide, aerating the final dough, producing a crumb that was soft and pleasant to eat. The remains of loaves leavened with these ferments have been found in the Pharaoh's tombs in the pyramids in Egypt. Similar breads are still crafted in the region. Yeasts and bacteria are essential to our life on earth. They have helped shape what we eat and the way we eat for as long as man has cooked food. We have used these organisms to change the taste and texture of the foods we eat, and to preserve our foods through the leaner winter months. In baking, they are essential to many of the breads we cherish. For all of us, our ethnic baking practices are part of our cultural heritage, and replicating the traditional methods used to create them helps us maintain those links with our past. One of the most famous breads, and the loaf that is held as the symbol of artisanal baking excellence, is the San Francisco Sourdough. The sourdough culture used to leaven this bread, and to give it its unique texture, aroma and flavour, is extremely stable. The method used to initiate and maintain the sourdough culture of yeasts and bacteria used in San Francisco Sourdough is typical of early, traditional baking methods. The bread made today is very similar to those ancient breads. Its stability made it the yeast of preference to take into new and remote places. Miners took it along on the Yukon Goldrush and early homesteaders of the US carried on their long treks to new territory. If we provide the right environment, this culture will grow and thrive, and make our bread rise. Sourdough culture is a yeast living symbiotically with a friendly lacto-bacteria. We need to start with enough of the right organisms so that they can become the dominant culture, food and water and the right temperature. Given the right organisms, the optimum temperature is just over 80F/27C. Much hotter and the activity of the yeast declines. Above 95F/35C the yeast is effectively dormant or dead. The bacterial activity peaks at 93F/34C, so some bakers choose to ferment at 90F/32C to get a sourer bread. At 70F/21C the activity of the yeast has roughly halved, so the fermentation will take twice as long. You can chose to ferment at cooler temperatures - you may not have reliably warm place available - but if you do so you will need to compensate by increasing all the times in the recipe correspondingly. Temperature control is critical (within a few degrees) for optimum results. A thermometer is a baker’s best friend. Our task is to create an environment that promotes optimal growth, but then starts to slow just before we put the bread into the oven. This is why the starter culture is built up in stages, (starter, then dough) roughly tripling in volume each time. (Some recipes for bulk production call for more stages: Starter, Clef, first leaven, second leaven, bulk fermentation, proof.) Why not dump all the flour and water in at once? We want to ensure that our culture is the dominant species! Our kitchens, our flour and our water are not completely sterile. They, too, may harbour spores and if our culture is too diluted, some of these less desirable cultures could get a foothold. Roughly tripling the volume each time (1 of starter, 1 of water, 1 of flour) ensures that our starter is dominant. Varying the hydration and temperature of each stage gives some control over flavour as well. A sourdough culture consists of a whole lot of little critters. There are maybe a several hundred different strains of yeast and friendly bacteria in there, some from the starter, and some carried in on the air and via the environment, or from the water or flour. In studies by Sugihara and colleagues on San Fransisco sourdough cultures, the dominant yeast was found to be a version of bakers yeast called Candida milleri species. They christened the dominant lacto bacteria (called a lacto bacteria not because it lives on milk, but because it secretes lactic acid, which is also found in milk) Lactobacillus sanfrancisco species. These two have learned to live well together, and like the best marriages they form a symbiotic relationship, supporting each other and keeping strangers out. The yeast has evolved to live in a highly acid environment, and breaks the starch in the flour down into sugars. The LB keeps the environment acid, breaks down some of the more complex sugars, such as maltose into simpler sugars that the yeast can feed on, and secretes a sort of anti-biotic to kill competitive bugs. You can read more in the Web resources referenced in the lesson. The stability of the starter means that, providing we provide about the right environment, the culture will grow and thrive, and make our bread rise. Sourdough culture is a yeast living symbiotically with a friendly lacto-bacteria .We need - To start with enough of the right organisms so that they can become the dominant culture. - Food and water. Normal dough provides more than enough. - The right temperature. The right temperature is the single most critical variable. Michael Ganzle and his co-workers did some studies on this. They found the following growth rates of L. sanfranciscensis and C.milleri as function of temperature. Growth rate is ln2/generation time, i.e. a growth rate of 0.7 is a generation (doubling time) of about 1 h. The generation times measured in laboratory media are different from that in rye / wheat / white wheat dough. If the generation time at 20 C is 1/2 of that at 30 C in my medium, the organism will also grow 1/2 as fast at 20 C compared to 30 C in dough (we checked). So, it's not the absolute numbers that matter, but the ratio of growth rate to growth rate at optimum temperature. here is a table illustrating growth rates at varying temperatures. When the environment changes, the culture does not react immediately but after a lag. Spicher defines the phases of growth as 1. Lag Phase: The organisms need to adjust to the new environment before growth can resume. This depends on the vitality of the inoculation. The time can be significant with dried cultures, which need to re-hydrate. In this phase, the greatest danger exists that a spontaneous flora from the organisms present in the flour will be able to alter the existing flora. 2. Acceleration Phase: The organisms are now adjusted to the new environment and start to multiply. The end of this phase is reached when the highest multiplication rate is reached. 3. Exponential growth Phase: The organisms are at their optimum growth rate, repeatedly doubling their numbers by binary fission. 4. Transition Phase: The growth rate decreases from the optimal rate. Possible reasons are exhaustion of nutrients, waste products accumulating acidity increase. 5. Stationary Phase: The number or newly generated and dying organisms is equal and it will not increase any further. 6. Death Phase: Initially, the number of dying organisms exceeds the number of newly generated. Later on, no new organisms are generated. Existing organisms die in increasing numbers for various reasons as mentioned in Phase 4. Yeast food The process of fermentation is the yeast feeding on simple sugars, like glucose and fructose, and turning them to into alcohol and acetic acid (vinegar). The sugars come from the starch being broken down by enzymes released by yeast cells. One enzyme is amylase (by convention, enzyme names end in –ase), which breaks down a component of starch, called amylose, into the sugar, maltose. This process is called amylisation. The yeast in sourdough can’t digest maltose, but our friend, the lactobacteria can, and they break it down into the simpler sugars, which the yeast can digest. Commercial bread flours often contain small amounts of diastic malt, rich in enzymes, which helps break down the starch more quickly and make more sugar available to the yeast. Rye flour is sometimes used, but the rye enzymes, although more heat stable, are inhibited in the acid sourdough environment. Ascorbic acid, Vitamin C, is also used as a yeast growth promoter. If your flour does not already contain Vitamin C (read the small print on the bag) you could consider adding some. 1/2 tsp of Vitamin C powder (not the fuzzy kind) per bag should do it. Ordinary sugar, sucrose, promotes the growth of ordinary yeast in competition with the sourdough yeast. The sourdough yeast can’t utilise it directly, but the lactobacteria convert it to glucose and fructose, and then reduce the fructose to acetic acid (vinegar). The result is a sourer tasting (but not smelling) bread. Ordinary baker’s yeast and sourdough don’t mix well. The sourdough environment is too acid for the ordinary yeast to thrive, and they compete for the available sugars. Some recipes use both yeast and sourdough, but this results in an ordinary yeasted bread, with the sourdough added for flavour, rather than for an acid ferment. A small amount of oil or a knob of butter can be added to the dough. Its effect is to make the strands of gluten slip over each other more easily and the finished bread is “shorter” with a softer crumb. About flour and gluten Gluten is the protein that holds dough together. If you take a piece of raw dough and wash it repeatedly under the tap, or keep chewing it, you will end up with a rubbery substance. This is the gluten. It acts like the rubber in a balloon. When the yeast ferments and produces gas, the gluten blows up into bubbles. It is these bubbles that determine the texture of the crumb in bread. There is some evidence that the bubbles form around the nuclei of tiny bubbles of air beaten into the mixture when it was originally mixed, so beat the dough well when first mixing. If there is not enough gluten the bubbles leak or burst, and the bread does not rise well. Flour with a lot of gluten is called “strong”. Pastry flour is low gluten (weak), since the gluten would make the pastry tough and hard. Gluten content is not measured directly, but some idea can be gained from the amount of protein in the flour. Bread flour is typically around 12%. Acidity The natural acid in the loaf is primarily made up of lactic and acetic acid (vinegar). Acetic acid tastes more acidic, but lactic acid smells sourer. How acidic the loaf tastes depends on many factors, including the ash content of the flour being used. But as a rough rule of thumb, if you want it sourer, ferment the starter for longer, or at a higher temperature. Bakers percentages Bakers express recipes (called formulas) in Baker’s percentages. In this formula, flour is always 100% with the other ingredients reckoned as a percentage of the flour by weight. Thus the starter, which is half flour and half water by volume, in our recipe consists of : 150g of flour = a baker’s percentage of 100, and 225g of water = a baker’s percentage of 150. The main dough consists of: Flour - 450g (100%) Water - 225g (50%) Starter - 200g (40%) Salt - 9g (2tsp) (2%) Total - 884 g Roughly 15% of weight is lost in baking (mostly water) giving a final loaf weight of about 750g Total flour (starter + main dough) 75g+450g=525g Total water (starter + main dough) 112+225 = 337g or about 65% hydration Hydration is a measure of how wet the dough is: Stiff & Dry- 58 to 60% water Content Firm & Tight - 60 to 62% Modestly Firm - 62 to 63% Malleable - 63 to 64% Soft - 64 to 65% Slack - 65 to 67% American style breads usually are about 60 to 62% hydration, French style breads between 62 to 65%, and Italian style (ciabatta) breads upwards of a 68% range. Our dough is on the slack side because it has a relatively high percentage of water. But notice how little extra water is needed to change from firm dough to a soft one: 3% is 15cc or 3 teaspoonfuls for our amount of flour. References Sugihara/Kline/Miller, Microorganism of the San Francisco Sour Dough Bread Process, Applied Microbiology, Mar. 1971, 458 Gänzle et al., Modeling of growth of Lactobacillus sanfranciscensis and Candida milleri in response to process parameters of the sourdough fermentation, Applied and Environmental Microbiology, July 1998 Spicher, G., Handbuch Sauerteig, 5th edition 1999, Behrs Verlag, 61-64