Jack Convery pops into a London branch of Italian eatery Coco di Mama to grab a cut-price lunch ordered on his smartphone's food-sharing app Karma.

The 27-year-old Amazon employee - with an eye for a bargain and for helping the environment - uses a mobile phone app that sells surplus food from hundreds of British restaurants at discounted prices.

"Anything I can do to help the environment and look after the budget as well is beneficial, so yeah, it's a win-win," Convery says.

"It ends up working out much cheaper than to go to [British supermarket] Tesco and buy a meal for one," he adds, noting that he can usually source his daily dinner for about £3.50 (S$5) via the app.

Decomposing food waste is a key source of greenhouse gases - and a staggering one-third of all food is thrown away, according to industry estimates.

Amid growing public outcry over the climate emergency, consumers are switching on to the environmental danger of carbon dioxide and methane emissions.

Britain's cafes and restaurants are also embracing apps that generate some revenue from rescued food that would otherwise be destined for landfill (though some restaurants happily give unsold food to homeless charities on a daily basis).

Swedish start-up Karma was founded in Stockholm in 2016 and its app now helps serve 1 million users in Britain, France and home market Sweden. In the UK alone, it offers food from 2,000 outlets.

UK-based rival app Too Good To Go also offers cut-price meals with a similar approach.

Meanwhile Olio, also from the UK, enables its 1.7 million customers worldwide to give their own unused food items free to other users, via collection from their homes.

"We do not plan for the day to end with zero waste," says Coco di Mama head of brand Sara McCraight, "because at the end of every day we want to have some stock on our shelves [for customers not using the apps].

"When we give to charity partners, that's great. But with Karma we get some money recovered; [it] doesn't cover costs but it helps."

Joao Campari, food expert at environmental campaigners WWF International, says that apps are an important way to cut waste.

"With food loss and waste creating at least 8 per cent of all emissions, it's vital that as many people as possible reduce their personal food waste," Campari says.

"To do this, we need tools that are easy to use, and also help raise awareness of the scale of the problem.

"Most people have a mobile device so apps are an obvious and important way of educating and enabling action."

These are good for raising awareness but [food-sharing apps] are not in themselves solutions, because there's still food surplus or food waste being produced

Professor Martin Caraher, from City University London's Centre for Food Policy

Wastage occurs throughout the food-supply chain, from farms and during transport to retail stores like supermarkets.

"In truth, the best way to cut emissions from food loss and waste is to prevent food being lost on the farm and in the supply chain before it reaches end consumers," Campari adds.

Yet in developed nations, most food waste occurs in restaurants and private homes - at a time when an estimated 820 million people in the world do not have enough to eat, according to the World Health Organisation.

"We throw away one-third of food we produce each year," says Tessa Clarke, co-founder of Olio.

"What that means is that roughly 10 per cent of annual greenhouse gas emissions come from food waste alone, which I like to point out to people is four to five times greater than the carbon emissions that come from the global aviation industry."

She adds: "I like to say that our main competitor is the rubbish bin."

While these apps are clearly benefiting consumers and food retailers, experts say they do not address the core problem.

"These are good for raising awareness but they're not in themselves solutions, because there's still food surplus or food waste being produced," says professor Martin Caraher at City University London's Centre for Food Policy.

"With 10 billion meals a year still ending up in the bin, the UK has not closed the door on the food-waste problem."