As public outrage over food waste grows, almost every British supermarket has responded to consumer pressure and linked up with food redistribution organisations such as FareShare and Foodcycle.

But while good practice is emerging, supermarkets’ work with charities is barely denting the waste problem. Fareshare, for example, estimates it accesses just 2% of supermarkets’ available food surplus.

Sainsbury’s donated nearly 3,000 tonnes of food last year (up from 1,200 tonnes the year before). This sounds impressive but it is only 7% of their surplus. Nine times as much went to anaerobic digestion, encouraged by perverse subsidies that promote turning waste into fuel and fertiliser over actually feeding people.

What’s worse, the food that charities do get hold of can be the food they are least able to use. The overproduction of bread may be the most striking example. Figures from Tesco suggest that [pdf] up to 44% of bread produced in the UK is wasted, and only half of that occurs in homes. This means the likes of supermarket bakeries and distribution depots regularly have far more surplus bread than charities can use.

Surplus figures don’t even touch on the vast quantities of unprocessed, healthy, fresh food currently wasted further up the supply chain on farms.

With almost a million portions of fruits and vegetables rescued by our Gleaning Network last year, we know the quantities are vast. But to get a true picture of the waste we would need the one thing most retailers refuse to provide: full transparency.

In 2013, Tesco released a third-party audited report of food waste throughout its supply chain, but others have failed to follow. While Sainsbury’s has started releasing data on its in-store waste, it has declined to do the same for its supply chain. Morrisons, Asda, Waitrose and other retailers are even less transparent.

One of the reasons retailers are reluctant to publish these figures is that they would lay bare the perverse impacts of big supermarkets’ concentrated power.

Supermarkets are in a position of breathtaking asymmetry with their suppliers, from farmers in the UK and around the world, to food processing companies or butchers. These businesses know that unless they provide the exact amounts requested, at the exact time required and often in the exact shape specified, they’ll lose business. So they overproduce, resulting in huge amounts of waste when forecasters change their minds on how many pork pies they think their shopper will buy this month.

This asymmetry is now mirrored in supermarkets’ relationships with the charities that take surplus food off their hands, creating yet another barrier to efficient use of food.

Several of our colleagues who have visited food banks’ warehouses and kitchens have been taken aback by the high proportion of supplies made up by confectionary. Charities do not feel able to turn down food, but they don’t necessarily get the kinds of food they need.

A truly systematic approach to reducing food waste would see retailers avoiding waste in the first place whenever possible. Where they can’t, this food should be available on a virtual marketplace to redistribution charities, so they can make best use of what they most need, both in terms of logistics and the kinds of foods they supply. Several food waste apps including FoodCloud and Plan Zheroes are helping to make this happen.

Only then should waste that isn’t fit for human consumption be passed down the food chain for animal feed, anaerobic digestion or, as a last resort, landfill.

For such a system to work the supermarkets would need open up their data to food waste social entrepreneurs and others, in order to work out where avoidable waste is occurring and how to link up with charities in the ways that work best for them.

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