SHU stands for Scoville Heat Units. The Scoville Heat Unit is named after American Pharmacist Wilbur Lincoln Scoville who created the "Scoville Organoleptic Heat Test". Scoville invented this test back in 1912 to measure the pungency/spiciness of chillies.

Back in the day, there were no computers, so this involved a panel of tasters who were subjected to dilutions of capsaicin extracts mixed with sugar water. The Scoville Unit would be established through the dilution ratio required to nullify the sense of burn. In essence, a Jalapeño would require 5,000 - 8,000 drops of sugar water to nullify the burn.

Fast forward and we now have machines to test for SHU. High Performance Liquid Chromatography (HPLC) tests are conducted by laboratories to establish the capsaicin concentration in any given chilli or liquid for that matter.

Thanks to Mr Scoville and his heat unit, we today have the Scoville Scale which in essence is a pungency/spiciness scale that illustrates the relative spiciness of a chilli or chilli product (and other non chilli based compounds). The scale ranges from "0 - 16,000,000" for Capsaicin (compound causing pungency in chillies).  But the Scoville Scale also measures other compounds of various origins that measure up to "16,000,000,000" for Resiniferatoxin - the worlds most pungent compound.