why are some foods spicyer than others?
http://www.wired.com/wiredscience/2010/09/why-does-spicy-food-taste-hot/
Some foods are spicier than others due to the presence of certain compounds called capsaicinoids. Capsaicinoids are responsible for the sensation of spiciness or "heat" in foods. The most well-known capsaicinoid is called capsaicin.
When you consume spicy foods, capsaicin interacts with the sensory receptors on your tongue and mouth, particularly a receptor called TRPV1. This receptor is normally triggered by high temperatures, but capsaicin can also activate it, giving you that burning sensation. The level of spiciness depends on the amount of capsaicin present in the food.
The concentration of capsaicinoids can vary among different types of peppers or spices, which explains why some foods are spicier than others. For example, chili peppers and other hot spices like jalapenos or cayenne pepper tend to have higher levels of capsaicin, resulting in a more intense heat.
If you want to determine the spiciness of a particular food, you can typically look at the Scoville scale, which measures the concentration of capsaicinoids in a food or pepper. The higher the Scoville rating, the spicier the food. This scale ranges from mild peppers like bell peppers (0 Scoville) to extremely hot peppers like the Carolina Reaper (over 2 million Scoville).
So, ultimately, the spiciness of a food is directly related to the amount of capsaicin or capsaicinoids present, which stimulates the heat receptors in your mouth.