There are few foods that necessitate tourism. Sure, an authentic Asian style can be difficult to replicate in North America, and locals often have no way of telling the new from the traditional, but it is at all times possible to make most foods in most places. Sourdough, however, cannot be moved. You want real San Francisco sourdough bread? You’ve got to go to San Fran.

The reason is that sourdough bread is made with wild yeast, not the cultivated baker’s species that dominate the baking aisles at the local supermarket. A sourdough “starter” is simply a mixture of flour, water, and sugar left out as easy sustenance for any yeast that happen past on a stray gust of wind. As a result of this process, yeast are not the only organisms that enter the starter. Just as wild yeast colonize the starter and begin their bread-rising fermentation process, so too do other micro-organisms set up camp.

In particular, Lactobacillus sanfranciscensis works to give sourdough bread its distinctive taste. As you might imagine from the name, this strain of bacteria was discovered in the famously unique sourdough starters of the San Francisco area. Though not unique to that region, the bacteria’s strength and abundance makes it definitively the most sought-after variety of sourdough in the world.

That’s not to say that San Fran sourdough is necessarily better than others, and every locality produces sourdough with a distinct taste arising from the particular assortment of bacteria in the area. Much like wine experts, true sourdough connoisseurs can identify the origin of a sourdough bread just by tasting it. The chemistry of different sourdoughs can be classified scientifically according to acidity and the temperature of greatest activity.

Additionally, sourdough can even vary between individual bakeries, as many claim to be using the same starter for years, or even decades! Various parties claim to maintain the oldest starter in the world, and while there is no definitive winner, all of the competitors have been maintained for well over a century. These colonies have been nurtured and grown on a daily basis, fed a steady diet of water, sugar, and heat, and its unique characteristics remain identifiable by true sourdough fanatics.

As a result, cities with traditions in bread often run dedicated sourdough tours. San Francisco, particularly, has a cluster of historic bakeries allowing an easy walking tour – though it becomes steadily more difficult as the stomach fills. Paris, Berlin and most other old-world cities have at least some sourdough tradition, since it is the simplest form of bread.

Sourdough is a living food, much like cheese, and as science progressed the jumble of life-forms used to create bread came to be seen as haphazard and imprecise. Increasingly, the food world is coming to realize that by standardizing our bread-making species across the planet, we have given up on local flavour – literally. A good sourdough bakery represents the local environment through the metaphor of taste – the distinctive bite of a San Francisco sourdough is seen as a reflection of the place itself.