The Science of Sourdough

Faire du pain au levain : un art ou une science ?

Conseils scientifiques pour réussir son propre pain.

En anglais, le terme “sourdough” désigne un pain dont on a confectionné soi-même le levain. Durant leur quarantaine, nos voisins anglais se sont, eux aussi, souvent improvisés apprentis boulangers. Mais réussir son pain est aussi une affaire scientifique. Voici quelques conseils, approuvés par la recherche, à lire avant de se mettre aux fourneaux...

Along with pasta and toilet rolls, flour was among the first products to vanish from supermarket shelves and Covid-19 inspired a home-baking boom. While Google searches for “bread” tripled in the UK in the weeks after mid-March, those for “sourdough” rose sixfold. Sourdough differs from most bread in that it contains no baker’s yeast, relying instead on a fermented “starter” of water and flour to provide lift. This also provides its sour flavour and chewy texture.

2. The sourdough revival has gone into overdrive. Vanessa Kimbell, author of The Sourdough School and regular contributor to Radio 4’s The Food Programme, says she has seen a 50% increase in Instagram followers and a 25% increase in membership of her online Sourdough Club, and that “the phone hasn’t stopped ringing”. However, many sourdough newbies have found that producing beautiful, tasty loaves isn’t as easy as it looks on social media. Maybe it’s time for a new approach. The transformation of dough into a light and airy loaf is, after all, chemistry in action, with a good side of physics.

The transformation of dough into a light and airy loaf is, after all, chemistry in action.

WHAT SHOULD I FEED MY STARTER?

3. To replace the baker’s yeast that most breads need to rise, sourdough requires some starter – a self-sustaining fermentation of flour, water, wild yeasts and bacteria that produce lactic and acetic acids. You might be able to get some starter from a local baker, or you can make your own – by combining small equal quantities of water and flour and letting it ferment for around five days. Once you have established a starter you need to feed and maintain it with additional flour and water. “I’ve had the best luck with wholegrain rye flour, which seemed to stabilise it in a way that has worked very well for us for years.” says San Francisco baker Josey Baker.

HOW DOES MY STARTER’S DIET AFFECT BREAD FLAVOUR?

4. Flours consist mostly of starch (70-80%) and proteins (10-15%). Use of more water and warm weather encourages microbes that will generate a sweeter-tasting loaf. To add sourness, rest your dough in the fridge, possibly overnight, as lower temperatures favour the activity of bacteria that produce acetic acid.

WHERE DO THE MICROBES IN MY STARTER COME FROM?

5. Scientists at North Carolina State University (NCSU) used DNA sequencing to identify the

microbes in starters sent to them by more than 500 participants in their Global Sourdough Project. “We found there is incredible diversity across different sourdoughs, even those from the same neighbourhoods,” says microbiologist Dr Anne Madden, of NCSU. In a follow-up study, the team asked 18 bakers from 14 countries to make starters using the same flour and method, and then assembled them in Belgium to make bread. The microbes in their starters were most similar to those in the flour they used, but were also reflected in those found on the bakers’ hands. “This suggests an intimate relationship,” says Madden. “Either the hands are adding microbes to the starters or the starters are adding microbes to the hands.”

SHOULD I KNEAD, OR STRETCH AND FOLD?

6. “Without a good gluten network your bread cannot rise and you will end up with a brick,” says De Smedt. Kneading adds energy and oxygen to dough, speeding the development of the gluten, allowing speedier baking.

HOW DO I GET A MORE OPEN CRUMB?

7. “To achieve a very open crumb, you need a good flour with a high protein, say 13-16%,” says

De Smedt. The water content of your dough is also key. Beginners often slop in too much too quickly and then panic when their dough becomes gloopy and so throw in extra flour. Kimbell’s top tip is to start with about threequarters of the water in your recipe,

mix slowly and add splashes as you go. If you want larger air pockets, be gentle when handling your dough, otherwise you’ll knock out the gas needed to make them.

WHY DO I NEED STEAM DURING BAKING?

8. Sugars and amino acids on the loaf’s surface start to react at about 150C in the Maillard reaction, a process that gives seared foods a distinctive taste and colouring. It allows the loaf to stretch so that it becomes light and airy. Many cook their sourdough in a Dutch oven or under a cloche to trap water vapour from the dough and create steam.

IS SOURDOUGH HEALTHIER THAN BAKER’S YEAST BREAD ?

9. A study published in December by Prof Marco Gobbetti, of the Free University of BozenBolzano, in Italy, found sourdough to be more digestible than bread made with baker’s yeast. Sourdough fermented for longer periods made healthy individuals feel more full more quickly, and those who ate sourdough had higher amino acid concentrations in their blood.

IS MAKING SOURDOUGH A SCIENCE OR AN ART?

10. That depends who is making it. De Smedt believes it is best approached as a science. Josey Baker believes that to do it properly requires both perspectives. “Ultimately bread is made, not in labs but in homes and bakeries, by people with subjective opinions using their skills, intuitions, emotions and other intangibles that science doesn’t attempt to address,” he says. “If you’re trying to make bread relying only on one or the other, then you’re leaving something important out.”