The minibus rumbles up to the red light, stops, and lurches on its suspension as we crowd over to the right-hand windows, gawking at the sheer scale of the city alongside us. More than 3000 food-grade fibreglass tanks – each 10 metres high and algae-green – stretch further than we can see, interspersed with roads, spindly exhaust stacks and blocky white buildings. The residents? Trillions and trillions of tasty microbes that will end up on dinner tables all over the world.

This is Xinhui Production Base, built in 1996 and still one of the world’s largest soy sauce factories. Located in the bend of the mighty Tanjiang River, a couple of hours’ drive from Hong Kong, it’s one of three Lee Kum Kee factories in mainland China. In addition to those, there’s one each outside Los Angeles, outside Kuala Lumpur and in Hong Kong proper, where the company began in 1888.

We’re here to see how the sausage is made – or in this case, the sauce. I’m not the only giddy journalist on the bus. The company’s founder, Lee Kum Sheung, is credited with inventing oyster sauce in the late 1800s. The story goes, he accidentally left a pot of oyster soup on the stove and returned to find it had boiled down to the sweet, syrupy sauce that today forms the backbone of many southern Chinese dishes. It’s legendary stuff.

LKK oyster sauce is always in my pantry, as are the brand’s light and dark soy sauces. But my biggest crush is LKK’s doubanjiang, or spicy fermented broad bean paste. I started buying it to make classic Sichuan dishes like mapo tofu, but gradually found myself adding it to fried eggs, steamed veggies, toasties and all sorts of other foods. Like the king of Mexican cuisine, chipotles in adobo sauce, doubanjiang seems to go with everything and has an absurd depth of flavour from those broad beans and chillies, plus a hit of soy sauce made at a factory like this one.

So yes, I’m excited to be here.

The light turns green and the bus makes a right. We motor up a wide, surprisingly regal boulevard. Workers in spotless white uniforms and white hard hats move between the silos on bikes and on foot. Shipping containers emblazoned with the words “Zhonggu Shipping” in English and Chinese roll past on trucks. There’s a touch of Willy Wonka to it all.

We hop off at the main admin building and briefly marvel at a scale model of the 133-hectare site (328 acres, if you need the conversion) before we’re issued hairnets and a mic’d up tour guide. Back outside, we pile into stretch golf buggies and head for a building where soybeans are washed, soaked, steamed under pressure and inoculated with a mould known as aspergillus, or koji in Japanese.

“Many cultures borrow. Japan transforms,” the New York Times wrote last month, pointing to Buddhism, sushi and tea ceremonies as evidence of the country’s flair for adopting. Soy sauce, invented in China at least 2000 years ago, strangely didn’t rate a mention. The former cottage industry has scaled up over the past century or so, with Japan predictably at the technological forefront, pioneering the use of defatted soybeans (i.e. soybean meal with the oil removed) to speed up production.

Another Japanese innovation was vast rotary machines to integrate cooked soybeans with wheat flour and koji. We peer into the one at Xinhui – 20 metres in diameter and the largest of its kind in the world – through a condensation-fogged window. It’s fully sealed, with temperature, humidity, air volume and air pressure monitored from outside during the 40 hours it takes to cultivate each batch. The computer terminal where this happens is cartoonishly complicated, like an ’80s movie director’s idea of the nuclear power plant control centre where Arnie would finally defeat the bad guy. Two workers monitor 16 screens displaying live interior video feeds, engineering diagrams and various fluctuating numbers.

High technology notwithstanding, this is the beginning of making soy sauce the traditional way, also known as natural, brewed or fermented. LKK’s commitment to this time-intensive process is the main reason I keep reaching for its bottles at the supermarket. This method takes at least three months to come to fruition, and years for the most luxurious expressions of soy sauce.

After it’s had a slow, invigorating spin, the soybean-koji-flour mix is sent to those fibreglass silos to relax in a brine bath for several months. Enzymes in the koji go to work on the soybeans and wheat, breaking proteins down into various amino acids, including monosodium glutamate (MSG), the savoury molecule that makes so many processed snack foods addictively delicious. Starches are likewise converted to simple sugars, which in turn feed wild yeasts and lactobacillus, the same bacteria in yoghurt, to produce tangy lactic acid, 1–2 per cent alcohol and other tasty secondary compounds.

All this waiting isn’t strictly necessary. Chemical or non-fermented soy sauce can be produced in a few days using acid hydrolysis, a widespread industrial process pioneered by Julius Maggi (of Maggi noodle fame) in 1886. In lieu of aspergillus enzymes, the soybeans are cooked in sulfuric or hydrochloric acid to create hydrolysed vegetable protein (HVP), a product long renowned for its meaty, savoury flavour, again due to the presence of MSG. Used to flavour stocks, soups, potato chips, frozen meals and more, HVP is ubiquitous in supermarkets, and the soy sauce shelf is no exception. For my taste, though, this method produces fairly one-dimensional sauces that lack the elegance and balance of traditional soy sauce.

Later, we get a taste of this difference. We’re out by the fermentation tanks. Our guide turns a tap and we watch fresh soy sauce gush into a stainless steel bucket like water. It’s still somewhat rough and ready, but this is an example of the prized “first draw”, which commands higher prices with its delicate, complex body. The slurry may be drawn three or four more times to produce soy sauces of lesser grades, but they’ll all be filtered twice, and heat-pasteurised, prior to bottling. Xinhui’s 2800 staff make about 50 different varieties of soy sauce – including light, dark, gluten-free and mushroom-flavoured – at a peak rate of 48,000 bottles an hour.

At this scale, even a factory using relatively traditional processes is bound to affect the surrounding soil and water with runoff. Soy sauce isn’t just salty – it’s also full of nitrogen. So much so that both the Chinese and Japanese governments grade soy sauce based on nitrogen concentration, with higher concentrations most prized. While nitrogen is a key ingredient in fertiliser, large quantities have a disastrous impact on waterways, sapping free oxygen, killing fish and encouraging algae blooms.

Our last stop on the tour is an artificial wetland, completed in 2021, right on the bank of the Tanjiang River. Its winding paths, neatly manicured ponds and utilitarian fountains remind me of 1996 movie Bio-Dome, and the association isn’t a bad one. The ponds are filled with specially chosen nitrogen-fixing water plants that the company says can treat some four million litres of water a day. That’s in addition to a bevy of hidden pumps and filters that recycle other greywater.

This stop, perhaps more than any other, makes comparing Xinhui to a city feel even more accurate in my mind. When you’re this large, this vertically integrated, with 137 years of history at your back, the operation is less a stereotypical factory where raw materials come in and leave as finished products. It’s an entire industrial ecosystem, built in the service of growing microbes.

The writer travelled to Hong Kong and mainland China as a guest of Lee Kum Kee.