Sourdough recipes replace the usual processed yeast with a fermented ‘starter’. This takes about five days to develop but once this initial base is made it must be kept alive by feeding it with flour and water. This process can be helped along by the use natural yoghurt's friendly bacteria.

The naturally occurring lactobacilli used to make sourdough produce lactic acid, which gives the bread its namesake sour taste.

Demystifying sourdough ​

The organizers say that the event aims to “demystify”​ what they call the original and best leavened “real bread”​. But aside from a back-to-basics nostalgia, what are the benefits of switching to sourdough?

Annemarie Aburrow, registered dietitian and owner of Expert Dietitian​, told BakeryandSnacks.com that in order to contribute to a healthy, wholegrain starchy diet the sourdough must be made from wholegrain/whole wheat flour. Regular sourdough however is made from white flour.

It is often claimed that the gluten sourdough contains is more digestible than that found in regular manufactured bread. Aburrow said that "it is definitely something to try if you suffer from wheat intolerance or IBS-type symptoms, as there is a chance it could help."​

"Not necessarily always 'low GI'"​​

The fermentation process used in the manufacture of sourdough bread lowers the glycaemic index (GI), releasing glucose steadily into the blood-stream and preventing erratic spikes in insulin. White bread has a GI of 70-73, while sourdough bread's GI is 50-60, depending on the flour used, Aburrow explained.

Yet Aburrow said that on this point white flour sourdough does not beat other wholemeal bread types."The GI of sourdough bread will either be low (less than 55) or medium (56-69), so it is not necessarily always 'low GI'​ - it depends whether white or wholemeal flour has been used. Regular sourdoughs will have medium GIs."​

"There are other breads with low GI levels (in the early 50s) - lower than sourdough. These include granary/mixed grain bread, stoneground wholemeal bread and pumpernickel bread."​

These granary or mixed grain breads are also often cheaper than sourdough bread, said Aburrow.

It has also been claimed that the lactic acids found in sourdough neutralize phytates in flour that would usually interfere with absorption of the flour’s vitamins and minerals. However this may not differ greatly across the different types since most manufacturers of bread fortify their breads with added vitamins and minerals, including vitamin A and B vitamins, folic acid and iron, said Aburrow.

"I always advise clients to check the labels of breads and other cereals to check they are fortified with added vitamins and minerals,"​ she said.

Made of strong stuff​

The dominant antifungal compounds​ produced in the fermentation process mean that sourdough bread has a greater resistance to mold and a longer shelf life than conventionally leavened bread.

Discussing the sourdough campaign, Duncan Glendinning of the Thoughtful Bread Company in Bath said that sourdough is “the only type [of bread] that actually improves with age".​

Commercial possibilities​

According to the campaign organizers, this kind of bread is experiencing a popularity boom in Britain as people rediscover “the delicious delights of real bread made with just flour, water, salt, time and care”​.

Yet the comparatively lengthy process of making sourdough has confined sourdough commercially to artisan bakeries, where sourdough loaf prices tend to reflect the man-hours required.

However, a recent study published in the International Journal of Food Science & Technology​​ found that sourdough cultures can improve the sensory and technical qualities of partially baked breads which could bring the traditional technique onto the commercial playing field.