Hotness in food either can be related to temperature,
or the hot feeling that it gives, without high temperature!
The Pungency. The Spicy Heat :)
Capsaicin is the reason to that hot burning sensation.
Pungency of chili peppers can be measured using Scoville scale.
SHU (Scoville Heat Units)
But the actual spicy feel depends on various factors.
Therefore the spiciest food can not be determined clearly!
Carolina Reaper, Ghost Pepper (Bhut Jolokia),
Komodo Dragon Chili Pepper, Trinidad Moruga Scorpion, Naga Viper pepper, Infinity Chilli, Naga Morich, Trinidad Scorpion Butch T pepper, Bedfordshire Super Naga, Spanish Naga Chili are some of the spiciest and they fall in the 855000–2200000 SHU range!!!
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