The trust's Carbon Footprints in the Supply Chain looked at how PepsiCo sources and processes potatoes for Walkers crisps. It concluded the manufacturer could save £1.2m a year by buying potatoes with less moisture in them. That's because frying drives water out of the potato and replaces it with oil, so the more water in the potato, the more work the fryers have to do.
Reducing the water content of the potatoes could save 9,200t of carbon emissions from fryers.
But growers have little incentive to keep the moisture content of potatoes down, the report said. And moisture is added to the tubers by using humidifiers during storage.
"We need to make sure growers are adding water to meet quality specs, but not so much that it costs everyone more money," said report author Euan Murray, strategy manager at the Carbon Trust. "Some temperature and moisture control is necessary, otherwise quality suffers but there are tweaks you can make to save energy."
The trust suggested growers should be rewarded for producing potatoes with a high dry matter content.
But that view has been blasted by farmers. "It's ludicrous to say that farmers add water to potatoes," said Richard Hirst, horticulture board chairman at the NFU. "They're suggesting that farmers almost inject water into tubers. You can't add water; they'd pop."
Keeping humid air blasting through potatoes in store is vital to maintaining quality during the months between lifting and use, he added.
PepsiCo is considering the Carbon Trust's report, but considers it an opportunity for savings.