The pyruvate scale measures pungency in onions and garlic with units of µmol/gfw (micromoles per gram fresh weight). It is named after pyruvic acid, the alpha-keto acid co-product created in the biochemical pathway that forms syn-Propanethial-S-oxide, the main lachrymatory agent in onions.
The standard onion has an eight rating, while "sweet onions" have a two or three rating on the scale. The lower the score or scale the more "sweet" the onions are rated. Anything less than five is considered a sweet onion.
The Vidalia onion variety is considered sweet and must have a score of 5.0 µmol/gfw or less. The Supasweet onion (usually grown in Lincolnshire, England) registers 1.5 to 2 on the scale. A standard brown onion is usually in the range of 6-7 out of 10.
Soil type, rain, and sunlight affect the pungency in onions and garlic and, therefore, their score on the pyruvate scale.