Sourdough baking

15 March 2010

Sourdough loaves
Traditional breadmaking uses fast-growing cultivars of yeast derived from brewing to leaven the dough. The yeast introduces gases into the mixture, a by-product of the its consumption of natural sugars in the flour. These gases lift the dough and lighten (or leaven) the finished loaf, so that it has that light, crumby texture.

Over last two or three hundred years, strains of yeasts have been bred stronger and stronger, so that only 6g of modern dried yeast will activate in 10 minutes and lift 2kg of dough in about an hour. Many now claim that although this style of baking is convenient for the baker, who can produce industrial quantities of uniformly textured bread in a working day (or more commonly, night), it’s not so good for the consumer, whose daily bread it actually is.

Although it has a pleasant aroma, it is not modern brewers’ yeast that gives older-style breads their peculiarly beautiful taste. For that you need two specific micro-organisms – naturally-occuring wild micro-yeasts and a bacterium called lactobacillus. You also need time.

Wild yeast strains raise the dough in much the same way as modern yeasts, but because they’re much less strong, they also take much more time – perhaps 12 to 24 hours. This extra time allows the lactobacillus a chance to grow and proliferate in the dough, allowing the so-called ‘sourdough’ to develop its characteristic tangy or sour flavour – which simply can’t happen in modern bread.

So how does a traditional baker find the yeasts and lactobacillus needed for sourdough? A sloppy mixture of rye flour and water left out in the fresh air or in the kitchen for a few days will probably start to bubble and ferment, showing that there is microbial activity. If you’re lucky, the mixture will become active enough to raise your loaves and give them a wonderful sharp taste – there’s nothing like it. This is your ‘starter’ – and if it’s a good one, it will be highly valued and carefully looked after.

Because here’s the thing: you don’t have to begin with a fresh ‘starter’ each time you bake. Sourdough bakers use a piece of dough carefully saved from the last batch to inoculate the new dough and start the whole process off again. The yeasts quickly spread through the new dough, making it double in size with the gas they give off. And then a few hours later, just before baking, the baker remembers to pull off a small quantity of raw dough (about an egg-sized piece, they say – or about 80g if you prefer, like me, to measure) which forms the next starter. With each generation the starter gets stronger, and is then ‘diluted’ again with fresh flour. Dilution in this sense also means strengthening because the new flour contains the sugars that the wild yeast needs to multiply and inoculate the next batch – a circular process known as ‘refreshing the starter’.

In this way, the starter goes round and round the process, never getting older, but having its origins in a much earlier baking, replenishing and re-inoculating itself. It is credible that there are starters in use today which are hundreds, if not thousands of years old. A vigorous and strong-flavoured starter is much prized among bakers.

Although the process is slow, it is also in its way very efficient. When I put my two 1kg loaves in the oven, the starter I began with 24 hours earlier by now constitutes only 4% of the whole. The starter which inoculated the batch before that is still present – at 0.16%, and the one before that, at 0.0064% and so on, recursing back into infinity – or to the dawn of history and agriculture, anyway.

Two very good books on sourdough baking if you want to have a go: Bread Matters by Andrew Whitely and Classic Sourdoughs by Ed Wood. Both links are to

two sourdough loaves


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