Is Ed Currie truly the king of the red hot chilli peppers?

Ed Currie holds one of his world-record-holding Carolina Reaper peppers by the stem, which looks like the tail of a scorpion.

On the other end is red fruit with a punch of heat nearly as potent as most pepper sprays used by police.

Last month, The Guinness Book of World Records decided Currie's peppers were the hottest on earth, ending more than four years of campaigning to prove no one grows a more scorching chilli. The heat of Currie's peppers was certified by students at Winthrop University who test food as part of their undergraduate classes.

But whether Currie's peppers are truly the world's hottest is a question that one scientist says can never be known. The heat of a pepper depends not just on the its genetics, but also where it is grown, said Paul Bosland, director of the Chile Pepper Institute at New Mexico State University.

The science of hot peppers centres around chemicals compounds called capsaicinoids. The higher concentration the hotter the pepper, said Cliff Calloway, the Winthrop University professor whose students tested Currie's peppers.

The heat of a pepper is measured in Scoville heat units. Zero is bland, and a regular jalapeno pepper registers around 5,000 on the Scoville scale. Currie's world record batch of Carolina Reapers came in at an average of 1,569,300, with one individual pepper measuring 2.2 million. Pepper spray weighs in at about 2 million Scoville units.

Pharmacist Wilbur Scoville devised the scale 100 years ago, taking a solution of sugar and water to dilute an extract made from the pepper. A scientist would then taste the solution and dilute it again and again until the heat was no longer detected. The rating depended on a scientist's tongue, a technique that Calloway is glad is no longer necessary.