Sandro Demaio has just returned from a weekend at his family’s farm in Victoria's east. Along with a team of volunteers, he’s created a huge, portable veggie patch that will be transported to the exhibition centre for the festival. He is clearly heavily – and physically – invested in the project. Soil still under his fingernails, he talks avidly about festival21 for almost an hour. There’s a sense that he’s nervous at the prospect of it all coming off a little preachy.

Demaio, 30, is the global health expert behind festival21, a free, one-day celebration of fresh food and a sustainable future at the Melbourne Convention and Exhibition Centre on December 11.

He started festival21 from the ground up, after the Festival of Ideas was cancelled earlier this year. “I want to stress that this is actually a festival,” he says. “It’s not going to be this big talk-fest. There’ll be music, comedians, art, short films – and lots of food. We’re starting a conversation about sustainable food that we want people to actually want to get involved in.

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“Climate change, the biggest conversation, has also become the biggest conversation killer,” he explains. “Five years ago, if you wanted to get someone to leave your dinner party, you’d offer coffee. Now you just bring up climate change and everyone’s running out the door.”

With three degrees in medicine and public and global health (from Monash University, the University of Copenhagen and Harvard Medical School), it’s safe to say that Demaio knows his stuff. He’s not righteous, or pretentious – he’s passionate. He understands that the people who can really make a difference to what he describes as our “broken” food system aren’t going to attend a serious environmental conference. It’s Melbourne. They’ll attend a food festival.

He needn’t worry about it coming off as preachy. For every serious part of festival21, there’s something a bit strange to engage participants. Yes, there’ll be inspirational talks from people such as Elliot Costello (YGAP), Joost Bakker and Simon Griffiths (Who Gives a Crap). But they’ll be followed by a sit-down chat between human-rights activist Julian Burnside and beloved chef and food campaigner, Stephanie Alexander. Photography, film and art exhibitions (including a huge healthy-eating graffiti mural) will appear around the building. National Geographic and Jamie Oliver’s Ministry of Food will hold masterclasses at a 45-metre-long communal dining table. In the background a choir of Italian nonnas will sing about food.

“We wanted to create something fun, but that still addresses the biggest issues of our time,” Demaio says. “We want to encourage people to make better choices – but we’re spreading the message through a celebration of food.”

So what actually is the message? What needs to be fixed?

“Melbourne’s three biggest challenges are climate change, obesity and chronic disease,” says Demaio. “Two thirds of Australia is now classified as overweight or obese. I truly believe it’s a reflection of a broken system – not laziness, or poor choices. And it’s something we can change as a collective.”

Festival21 is about teaching people about the impact of what we eat, how much we eat and the quality of what we eat, on our health and on the environment. It explains how we can fix bad effects.

“You can make small changes to how you eat, how you cook and where you get your food from,” Demaio says. “Look at what’s seasonal – it’s usually what’s freshest and cheapest anyway.”

Demaio’s overall tip is to eat fresh, and eat less. He’ll teach you what vegetables to buy, when to buy them and how to cook them. The communal dining table that sits in the middle of the exhibition centre will serve food from Fonda and Feast of Merit throughout the day. Fresh herbs will grow down the middle of the table. Demaio’s giant veggie patch will be there – which you’re allowed to pick from.

He can’t promise anything, but says: “You’re probably not going to hear the words ‘climate change’ on the night. That’s not what it’s about. We’re talking about empowering people to realise that the food on their plate is one of the most powerful tools they have for change.

“Food is a truly unifying factor across all schools of thought – politics, religion and race. Everyone loves food and everyone loves to eat. I don’t think it will be hard to get people on board.”

Eat and learn, Melbourne.

festival21 takes place on Friday December 11 at the Melbourne Convention and Exhibition centre from 10am to 11pm. Entry is free but bookings are recommended.