Though fermented foods like soy sauce, cheese and yoghurt have long been household staples, we have the popularity of the likes of kimchi, miso and sauerkraut to thank for really switching us onto fermentation as a technique. Over the last few years, the process – essentially a chemical change in food caused by enzymes – has gone mainstream, with ferments now a common sight on both restaurant menus and home kitchen shelves. For environmentally-minded restaurants, its role in preserving food – its original purpose – is essential to limit waste, but today it plays an equally important role in deepening and transforming ingredients' flavours. One type that has particularly caught chefs' attention is garum, an umami-heavy seasoning used in Roman cuisine which, in its most traditional form (you can read more about it here), is the liquid that fish expels as it ferments, similar to fish sauce or Worcestershire sauce.

Modern garum has been reinvented, though, and today – spearheaded by Noma's René Redzepi – it's made using everything from vegetable scraps to beef trimmings. That's become possible thanks to one missing link being filled. In traditional garum, the enzymes in the fish guts would have triggered fermentation, so, without that, chefs needed something else to kick off the process. Enter koji, a fungi grown on cooked grains like rice or soybeans which gets fermentation under way, and is responsible for the likes of miso, soy sauce and sake (we have a koji deep dive this way, and a guide to making it here). 'In the modern sense, garum is really just any protein that's been fermented with enzymes,' agrees Ryan Walker, head of fermentation and development at Hackney zero-waste restaurant Silo. Essentially, koji has upped our garum game. Once the protein and koji are introduced, it's a case of adding salt and water, letting science do its thing – and being patient.

Fermentation is vital at Silo, allowing chefs to put potential waste to good use. The restaurant – led by chef and owner Doug McMaster – has dozens of garums and ferments ticking over, transforming everything from crab and venison to Parmesan. So important is the process that Silo this year crowdfunded £10,000 for its own Fermentation Factory, where it will produce commercial levels of koji for not only its own use, but that of other restaurants too. Ryan, who studied at Noma's MAD Academy in Copenhagen, has been overseeing the project, as well as keeping a close eye on current ferments and dreaming up clever new ways of repurposing produce. We asked him to talk us through a few of the garums Silo makes to give a glimpse at what's possible.

The Silo team takes whole cuts of meat and finds uses for everything, saving and freezing parts that can't be cooked immediately to use later in the likes of sausage or kofte. 'One thing that has very little value is sinew,' Ryan says. 'You don't want to mix it into a sausage because it's like silver skin, and you don't want to cook with it, so it has no real value unless you turn it into a garum.' They freeze sinew until they collect enough, then roast it to give it extra flavour, before combining it with koji, water and salt and leaving it for nine months, creating an intensely savoury, umami-filled seasoning. 'When it's ready, we use it in what we call pure expressions, really letting the flavour come through,' Ryan explains. 'We have had a dish of kalettes, for example, and we’ll mix the garum, brown butter and regular butter together and it’s just pure umami. Incredible.’

'We have a lot of fish sauces,' Ryan says. 'We make Thai or Vietnamese-style fish sauces where we basically take the whole fish, with the guts, and salt and mince them together, which increases the surface area and makes a better ferment. Then we age that at forty degrees for two weeks – we’re trying to mimic ageing out in the open with the sun hitting it. So after that two-week period, we strain off the liquid and remove all of the fat. You can age that liquid for six months to a year and it turns very much into a Thai fish sauce, which we use as a seasoning.' Although traditional fish sauces use anchovy, Ryan uses whatever is in season and can be caught with hook and line, including sea bream, pollock and sea bass. 

Science might be the bedrock of fermentation, but in a kitchen it's nothing without creativity. Take Silo's burnt beetroot garum – it came into being when some beetroot – intended for a dish which involved them being charred – were left in an oven for too long, burning them through. Keen to avoid the waste, Ryan started experimenting. 'Originally we tried to turn some into a miso, but we weren't happy with the results, so we minced the beetroot and blended it with koji, rice and water, turning it into a beetroot soy sauce, like a burnt beetroot tamari. We strained it after a year and a half, so a very long age, and we weren't entirely happy with the favours. Then we let the liquid age for two to three weeks more and I remember tasting it and thinking 'this has got to be syrup'. It had changed so much and was incredible.'

When fashioning their own takes on sauces and seasonings, making sustainable swaps is essential for the Silo team – instead of using soybeans, which are not commonly grown in the UK, they use fava beans, and swap cracked wheat for options like spent bread and beer grain. 'They create something that to 99% of peoples' palates is indistinguishable from soy sauce,' Ryan nods. Silo also uses fava beans as the base of misos. 'We’ll take split fava beans, cook and soak them and we’ll mix koji which we’ve grown on rice or barley into them, with some salt. We’ll mince all of that and pack it into a container for six months to a year, and that’s the miso.'

'A new trend is ferments that have a very short turnaround time, or sauces that are fermented with koji,' Ryan says. 'We make shio koji, which means salt koji. Traditionally, that's koji water and salt, and you ferment that at room temperature for a week or two. What we do to make it a more rounded seasoning is, instead of adding water, we add dashi made of kombu seaweed from Cornwall. And that allows us to have a very quick umami sauce seasoning that’s delicious. We also add dried mushrooms, which adds another umami component. We tend to do that at room temperature because it's so quick, but it could be done in a day hotter, at sixty degrees for example.'

Got the bug for all things fermented? See our full recipe collection packed with ferments here.