The chemical that makes peppers hot is capsaicin. While it hasn’t been settled scientifically, it appears that peppers evolved to have capsaisin to avoid being eaten by rodents who register pain upon consuming capsaisin, whereas birds do not have pain sensors that register consumption of capsaisin. Why is being eaten by birds better? Rodents grind the pepper seeds with their teeth when they eat them and then the seeds can’t germinate upon passing through the animal. The seeds consumed by the birds weren’t ground and were able to be spread and germinate.
In 1912 Wilbur Scoville released his eponymous Scoville Scale. It measures the “hotness” of a chili pepper or any food whose hotness derives from a chili pepper. “As originally devised, a solution of the pepper extract is diluted in sugar water until the ‘heat’ is no longer detectable to a panel of (usually five) tasters; the degree of dilution gives its measure on the Scoville scale.” Thus, the Scoville Scale is subjective and the hotness of chili peppers based on the scale is not precise.
As a reference, here’s where a number of popular hot sauces fall on the Scoville Scale:
- Original Tabasco: 2,500
- Sriracha 2,200
- Frank’s Red Hot 450
- Cholula 3,600
- Marie Sharp’s Fiery Hot 6,775
Source for quote above: https://www.chilliworld.com/factfile/scoville-scale#TheScovilleScale
Marie Sharp, patron saint of hot sauce:
They need to add the Carolina Reaper pepper to the scale. It is hotter than the Ghost Pepper.