The pyruvate scale measures pungency in onions and garlic with units of μmol/gfw (micromoles per gram fresh weight).[1] 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.[citation needed] 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.
