The Gulong Soy Sauce Factory in Xiamen uses methods that would not have been all that out of place two thousand years ago. The only difference is the scale. Soybeans are steamed and mixed in with wheat flour, and the all-purpose qu fermentation starter, then wheeled in to the drying rooms on trolleys the size of hospital gurneys.

Clad in his uniform grey boiler suit, fetching white wellies and a wide-brimmed straw hat, manager Liu Tuanjie walks me through the process, as his gloved assistants hand-mix piles upon piles of still-warm beans on gurney trays.

“This used to be a family thing,” he says. “Every household would have their own particular recipe, and their own home-grown qu. It was only over time that the process grew larger and larger in scale, until it was industrialised like this.”

The warm beans are left to dry-ferment in a hot room for three days, until they start to go a dodgy-looking yellow, at which point they are whisked away and tipped into child-sized jars of salt water out in the baking sun.

This, I realise, is why Mr Liu is wearing his straw hat, as we march along row after row of the large jars, each topped by an oversize straw hat of its own. They are sitting on open ground, stretching far into the distance – there are 60,000 of them, in an area the size of several football pitches. Each day, as the cool night air recedes, Liu and his assistants dart down the rows, lifting off the straw covers in sets of four, so the brine solution and its bean-mix contents get the maximum amount of sunshine.

I struggle to keep up with him on the next row over. My own hastily issued straw hat, which comes complete with a flowery brim to put me in my place, fails to keep the bright sun from my eyes, and my fingers have trouble gripping the rasping fibres of the heavy straw covers. He is already several jars ahead of me, flipping the covers off like they are bottle caps.

“We take the beans, we cook them and roll them in flour and yeast, we let them dry-ferment for a bit, and then we put them out here for a year. The sun rarely changes. It’s always hot, and that slowly bakes them down into soy sauce.”

“It takes a year,” he continues. “In each season, spring, summer, autumn, winter, we adjust the conditions.” So – covers off for longer in the slightly colder months, on for longer when it’s warmer. But with Xiamen being warm all year around conditions for brewing the soy sauce are not as fiddly as they might be elsewhere. “Each day, we have to optimise the daylight – lift the lids, let the light in… see how the fermentation’s coming along.”

“You couldn’t do this in Beijing,” he scoffs, whipping away another cover with practised ease. “The weather changes too much.”

Jonathan Clements is the author of The Emperor’s Feast: A History of China in Twelve Meals. These events featured in Route Awakening S02E03 (2016).