Determination of background pyruvic acid concentrations in onions,Allium species,and other vegetables

Background pyruvic acid concentration is a critical factor to determine onion pungency but greatly different values have been reported. This study was initiated to measure accurate background pyruvic acid levels in onion cultivars, other Allium species, and non-Allium vegetables using an HPLC method. A microwaving time of 2 min per 100 g was sufficient to deactivate alliinase and further degradation of pyruvic acid was not observed up to 6 min. Five percent trichloro acetic acid was suitable for both fresh and frozen tissues, while 80% ethanol did not stop the alliinase activity completely. The background pyruvic acid concentrations in various onion cultivars were less than 0.34 μmol ml⁻¹, while garlic and onion sprouts were 0.84–0.95 μmol ml⁻¹. Non-Allium vegetable also showed very low levels (0.09–0.34 μmol ml⁻¹) of pyruvic acid in the microwaved tissues. This study confirmed that background pyruvic acid levels in onion were minimal and could be disregarded in a routine analysis. Previous reports of high levels of the background pyruvic acid may need to be reviewed for accuracy.