What is a leaven?
A leaven is simply the agent that causes a bread or other baked item to rise. It can be a chemical agent like baking powder, or it can be a type of yeast, which is encouraged to fart out carbon dioxide to fill your dough with pockets of gas – the holes in the crumb.
The yeasts (products, not species) most commonly used in bread-making are the powdered, instant or easy-blend yeast; the granular active dried yeast (ADY) aka dried active yeast; and fresh yeast, compressed bakers’ yeast, which is known as lievito di birra (beer yeast) here in Italy, a reminder of the relationship between baking and brewing. On this blog, however, I also talk a lot about natural leavens, which I often just refer to as leavens.
Yeast, bacteria and a long, strange symbiosis
A natural leaven is also known as sourdough, or lievito naturale or madre (mother) here in Italy (where it’s generally a much lower hydration), or a levain (if you prefer to use a French term to sound extra sophisticated).
The natural leaven is a culture containing both yeast and bacteria strains. So while bakers’ yeast is the strain Saccharomyces cerevisiae, a natural leaven will contain a colony of various strains of wild yeasts and lactobacilli. The latter is a remarkable genus of bacteria that humanity has had a relationship with for millennia. Lactobacilli play a part in the production of many fundamental consumable delights, such as beer, wine, cheese, yogurt, cocoa and, of course, bread.
It’s a fascinating symbiotic relationship, and one I enjoyed exploring while living on a smallholding in New Zealand years ago, making some crazy cheeses. And who’d have thunk chocolate involved fermentation? Well, it does: the pulp and seeds of the harvested cocoa pods are fermented before drying. Gotta love Theobroma cacao – that is “Cocoa: food of the gods” in Greek.
Anyway, I digress. (As usual.)
When humanity first started making leavened bread, it was thanks to lactobacilli and wild yeasts. In a bread natural leaven, the lactobacilli and yeasts live alongside one another in a sludgy slurry of flour and water, the starter. This was the chief way of making leavened bread from around 1500BC (or earlier) until the Middle Ages, when bakers starting using beer barm, the froth on the fermenting brew.
In the 19th century, when the likes of Louis Pasteur were not only overhauling our understanding of disease but also revolutionising the production of fermented foods, Saccharomyces cerevisiae was finally identified and became, effectively, a crop: brewers’ yeast.
When you mix yeasts with flour, the sugars in the cereal grains (notably maltose) provide them with food. The result of this feeding is lactic and other acids; the by-product is the carbon dioxide. These acids are what can give sourdoughs a pronounced, even tart flavour. Rye flour gives an especially sharp flavoured bread, but some wheat-based sourdoughs can be as mild as breads made with bakers’ yeast.
How to create a leaven culture
The easiest way to start a leaven may well be to buy a good book, like Dan Lepard’s The Handmade Loaf. This gives a great account of how it’s done with plenty of lovely pics. Or you could read on, for a somewhat less professional lesson.
If you’re using a good quality flour, one where the character of the grain hasn’t been brutalised too much by industrial processing, it may well contain have suitable wild yeast strains already. The yeasts are also on and in your body, and even in the atmosphere. Some people collect them by leaving a jar of flour and water outside but if you live in a polluted city you might want to control the process a little more carefully.
You can do this by putting a handful of raisins and a few teaspoons of live yogurt in a mixture of equal proportions of water and flour. Another baker here recommends rhubard. While Sandor Ellix Katz says “organic plums, grapes, or berries.”1 The fruit will (hopefully) have just the right yeasts on their skin, the yogurt will introduce the lactobacilli. Put the jar, covered with a cloth or with the lid on loosely, in a warm place.
Over the following few days, feed more flour and water to the leaven, stirring it well. To keep things at a reasonable domestic scale, I tend to feed mine around 20g each of flour and water, morning and night. Equal proportions of water and flour mean you have a “100 per cent hydration” leaven2. Lepard suggests increasing the proportion of flour slightly to slow down the fermentation.
It should start to bubble – not like a fizzy drink of course, but you’ll see gas in the sludge and perhaps some foaminess. The smell will become slightly alcoholic too. If your leaven is nice and active and bubbly, it’s happy – so strain out the raisins, their work is done. You can now use some for baking. (Instead of straining out the raisins at this point, you can just soak them overnight at the start and use the liquid. Watch the video here on Viva La Foccacia. It’s in Italian, but even if you don’t understand, the process is clear.)
If you just keep feeding the leaven, without removing any to use, it can become unhealthy, with too much ethanol (another yeast by-product) in the mix. If this happens, simply take a small amount, and, in a fresh jar, start feeding it again.
Your pet slurry
When you have a natural leaven, you have to think of it almost like a pet: it’s a living thing that needs care, needs feeding. But that doesn’t mean it’s hard to maintain. If I’m not baking much, or not making sourdoughs, I leave my jar of the leaven in the fridge for weeks, even months occasionally. This retards the fermentation process, as the culture prefers a temperature of 25-35C, not the 4C or so of a fridge.
After a while, the leaven will divide into a grey, putty-like goop, with a watery liquid on top. It’s not dead though, just a bit unhappy. To revive it, you can take a spoonful of the goop, put it in a clean jar and start feeding it again, as described above.
If you change the flour, the yeast strains may well change, and your bread may well taste different, but to me that’s all part of the fun. My leaven is about four years old now, had moved house three times and even moved from Britain to Italy. During this time, it’s been fed on white wheat flour, wholewheat flour, durum wheat flour, emmer flour, spelt flour and rye flour.
For a while I had two parallel cultures, one white wheat, one rye, but it didn’t really seem necessary. If you plan ahead, you can simply take some leaven and feed it with a different flour, creating another strain. This may take around a week of regular feeding.
Be nice to your pet slurry and it could last you a lifetime. Or several lifetimes. There are stories of prospectors in the American gold rush who carried their sourdoughs, which may well have originated in Europe, across North America. I can well believe this as, years ago, I knew an Austrian guy in New Zealand who’d transported his culture around the world in the form of dried flakes, which he rehydrated when he wanted to bake. (He was basically smuggling. Introducing food, vegetable matter and organisms to the Antipodes is a serious no-no these days as they have major problems with foreign species. Don’t do it kids!)
Anyway, some of the US immigrant natural leaven cultures may well still be used today in the thriving sourdough bakery scene of the West Coast US. Certainly some cultures still in use date to the mid-19th century. Gotta love that symbiosis.
1 Katz is a respected US fermentation expert. Something of a guru in fact, who treats his HIV with fermented foods. His book Wild Fermentation includes a recipe for Basic Sourdough Starter (page 95), which says this about the inclusion of fruit: “One effective technique for speeding up the introduction of wild yeasts into your sourdough is to drop a little unwashed whole fruit into it. Often on grapes, plums, and berries you can actually see the chalky film of the yeast (‘the bloom’) that is drawn to their sweetness. These and other fruits with edible skins… are great for getting sourdoughs bubbling. Use organic fruit for this. Who knows what antimicrobial compounds could lurk on the skins of the fruits of chemical agriculture?”
2 Bakers’ percentages are based on the amount of liquid (usually, but not always, water) as a proportion, a percentate of the flour in the recipe. There’s more info here. Leaven percentages are considered in the same way as dough percentages.