Sourdough is a culture of yeasts and beneficial bacteria that occur naturally in bread flour and dough. The yeasts are more varied and less concentrated than baker’s yeast, so they raise the dough more slowly. The lactic acid bacteria (LAB) also require many hours of fermentation to work their wonders.
Real sourdough is very simple, as befits a method that’s thousands of years old. You take some starter, refresh it with several times its own weight of fresh flour and water and let this ferment for some hours until the yeast population has grown. You use most of this dough to make bread by adding more flour, water and salt, and keep a little bit back as your starter for the next batch of bread.
(There is no need to fuss over and ‘feed’ your starter regularly: we’re talking fermentation here, not pet-care. Established starters will keep undisturbed in the fridge for days, weeks or months between bakes.)
Time is crucial. When the sourdough is allowed to ferment slowly over several hours, it is able to transform the main ingredient – flour – in ways that together justify sourdough bread’s claim to be the best.
Here’s a summary of the many benefits of sourdough, as revealed by research done in the past fifteen years (1):
Sourdough LAB can modify the bits of gliadin and glutenin protein in wheat flour that are toxic to people with coeliac disease (CD) and non-coeliac gluten sensitivity(2-6). This doesn’t mean CD sufferers can eat all (or even any) sourdough bread. It does mean that there is a time-honoured method for making wheat flour more digestible and that we urgently need to know which types of bread on sale in the shops deploy this to real effect.
LAB (including those commonly found in sourdough bread) produce beneficial compounds: antioxidants (7), the cancer-preventive peptide lunasin (8), and anti-allergenic substances, some of which may help in the treatment of auto-immune diseases (9). Interestingly, these by-products seem able to survive heating, suggesting that baked sourdough bread may have ‘probiotic’ potential (10) by stimulating immune responses in the gut (11).
Bread, especially if made with unrefined flour, is a significant source of dietary minerals such as iron, calcium, magnesium and zinc. But a slice of fast-made wholemeal may be nutritious only in theory if its contents pass straight through the body without being absorbed. The main culprit here is phytic acid, present in the bran layers of cereals, which ‘locks up’ the important minerals. Several hours of fermentation with sourdough is sufficient to neutralise phytic acid and make the minerals more bioavailable (12-13).
Problematic protein fragments are not the only thing in bread that we might want to reduce to a minimum. Acrylamide, a suspected carcinogen, can be found in bread crusts. Long fermentation, typical of sourdough systems, can reduce levels of the amino-acid asparagine that is a precursor of acrylamide formation (14).
Bread is often avoided by those affected by weight-gain and metabolic syndrome – rightly, perhaps, in the case of industrial white loaves with a high glycaemic index (GI). But sourdough LAB produce organic acids that, under the heat of baking, cause interactions that reduce starch availability. The lowest GI breads are whole-grain sourdoughs with a compact texture (15).
That’s a pretty compelling list of benefits even if we ignore the fact that bread-related metabolic complaints have proliferated just as the time taken to ferment most commercial bread has reduced. It’s this interplay of time and commercial advantage that should make us ask searching questions of some of the ‘sourdough’ breads now on offer.
The benefits of sourdough translate across the whole range of baked foods, including bread, buns, pancakes, pastries, cakes and even oatcakes. We are collating a selection of Scotland The Bread’s favourite recipes, which can be found here.