Yet despite the scale of the industry, most of us never see how it works, so I visited the pilot site of circular mushroom cultivation in Rotterdam.

“When you see greenhouses, you see the plants behind it, but with mushrooms, it's these big containers so you have no idea what's going on,” said Florian de Deugd, operations manager at Rotterzwam, the Dutch mushroom cultivation site I visited. He's also the co-founder of the mushroom production company SPORO.

Mushrooms grow in highly controlled environments in substrates – a specifically prepared base material – made of agricultural waste, sawdust or straw, in well-balanced humidity and light. The common way is growing them on layered shelves in dark rooms, as the popular white button mushrooms don’t require light at all.

But first, coffee

At Rotterzwam, they do things differently with their substrate. “We use coffee waste because in nature mushrooms also grow on organic matter that's decaying, it's their food,” de Deugd said.

Using coffee grounds is not new, but collecting enough of them to support large-scale production requires a dedicated infrastructure. “This enormous mountain of waste that we pick up is actually very good for the cultivation of food,” he explained.

They collect around 70,000kg of organic waste annually in Rotterdam and other Dutch cities. Companies pay them to take their coffee grounds to reduce their waste and score corporate sustainability points.

De Deugd showed me the bags full of the optimal mixture of substrate. They first need to be sterilised, to prevent competing fungi from taking over. Then, they add the so-called spawn, a carrier of the spores which then grow through the coffee substrate.

From this small brick-like piece of spawn that contains mycelium, the fungi can grow in thousands of bags. Depending on the type of mushroom, the growth takes anywhere from two weeks to two months.

Inside the containers

We walk into what looks like a row of shipping containers. That's the cultivation site where the magic happens. They are all insulated, with tops covered in solar panels.

One of the warm and humid “summer phase” containers is full of shelves with bags with thin white lines expanding through the soil. “Next week we'll put it into the next container, and we'll start growing mushrooms,” de Deugd said.

The next container is a lot cooler, still quite humid. “In nature, you have one day in October when the temperature drops very low. And that's basically the signal for most mushrooms to think, now I have to grow before it’s freezing,” he explained.

For de Deugd, it's important to invite people to see the side of growing mushrooms that most customers never see.

“Sometimes people come here, and say okay, this is fun, but I don't actually eat them. There’s still not a lot known by the general public, what mushrooms are, or how to cook them properly, we’re trying to change that by also showing them food they can enjoy,” said de Deugd.

Jurian Wijnheijmer, a founder of the MOGU Mushroom Lab, an educational-experimental cultivation lab and restaurant in Amsterdam, agreed when we talked in their fully vegetarian restaurant, full of fresh soup smell. Lion's mane, a type of mushroom that is often associated with supplements claiming to support brain function, is a good example.

“If people know lion's mane, it's mostly in a medicinal context,” he told me. “We try to give people an easy access to see it in a culinary context. So here we try to show that these mushrooms look amazing and they taste amazing.”

Fungal intelligence

Can you just grow mushrooms into…anything?

In short: sort of. Mycelium, the underground network of microscopic thread-like structures that makes up most of a fungus, is surprisingly adaptable, de Deugd explained.

“It's called fungal intelligence. This fungus contains information on how it's growing and how it can adapt to a specific substrate. So basically you can also train them to grow in coffee, or you can train them to break down plastics,” he said.

But there are limits to this flexibility. Making a local high-quality substrate, however, is not easy. For example, much of substrate used to grow shiitake mushrooms, the most popular mushroom type in East Asia, is imported from China, while the wood and straw are sourced from elsewhere in Europe.

But even such circular production faces a challenge: if it’s more expensive or lower quality than the non-circular product, it’s difficult to argue with customers or even farmers to adopt new methods of growing, according to de Deugd. With already low margins, the process needs to be thought-through.