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 (see the Real Bread Campaign’s Honest Crust Act for more information on the call for loaf labelling transparency).
- 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. This is particularly true of bread made from our unusually nutrient-dense flour. 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.
References 1. Katina, K et al, Potential of sourdough for healthier cereal products, Trend Food Sci Technol, 2005; 16: 104–112. 2. Gänzle, MG et al, Proteolysis in sourdough fermentations: mechanisms and potential for improved bread quality, Trend Food Sci Technol, 2008; 19: 513-52. 3. Di Cagno, R et al, Sourdough bread made from wheat and non-toxic flours and started with selected lactobacilli is tolerated in coeliac sprue patients, Appl Environ Microbiol, 2004; 70(2): 1088-96. 4. Rizzello, CG et al, Highly efficient gluten degradation by lactobacilli and fungal proteases during food processing: new perspectives for celiac disease, Appl Environ Microbiol, 2007; 73(14): 4499-507. 5. Di Cagno, R et al, Use of selected sourdough strains of Lactobacillus for removing gluten and enhancing the nutritional properties of gluten-free bread, J Food Prot, 2008; 71(7): 1491-5. 6. Di Cagno, R et al, Proteolysis by sourdough lactic acid bacteria: effects on wheat flour protein fractions and gliadin peptides involved in human cereal intolerance, Appl Environ Microbiol, 2002; 68(2): 623-33. 7. Coda,R et al, Selected Lactic Acid Bacteria Synthesize Antioxidant Peptides during Sourdough Fermentation of Cereal Flours, App Environ Microbiol, 2012; 78(4): 1087–1096. 8. Rizzello, CG et al, Synthesis of the Cancer Preventive Peptide Lunasin by Lactic Acid Bacteria During Sourdough Fermentation, Nutri Cancer, 2012; 64: 1, 111-120 9. Nonaka, Y et al, Antiallergic effects of Lactobacillus pentosus strain S-PT84 mediated by modulation of Th1/Th2 immunobalance and induction of IL-10 production, Int Arch Allergy Immunol, 2008; 145(3): 249-57. 10. Poutanen K et al, Sourdough and cereal fermentation in a nutritional perspective, Food Microbiol, 2009; 26: 693–699. 11. Van Baarlen, P et al, Differential NF-kB pathways induction by Lactobacillus plantarum in the duodenum of healthy humans correlating with immune tolerance, Proc Natl Assoc Sci, 2009; 106: 2371–2376. 12. Leenhardt, F et al, Moderate decrease of pH by sourdough fermentation is sufficient to reduce phytate content of whole wheat flour through endogenous phytase activity, J Agric Food Chem, 2005; 53: 98-102. 13. Lopez, H W et al, Making bread with sourdough improves mineral bioavailability from reconstituted whole wheat flour in rats, Nutrition, 2003; 19(6): 524-530. 14. Fredriksson, H et al, Fermentation Reduces Free Asparagine in Dough and Acrylamide Content in Bread, Cereal Chem, 2004; 81(5): 650-653. 15. Östman, E.M. et al, On the Effect of Lactic Acid on Blood Glucose and Insulin Responses to Cereal Products: Mechanistic Studies in Healthy Subjects and In Vitro. J Cereal Science, 2002; 36: 339-46.