Cooking Alters the Metabolites of Onions and Their Ability to Protect Nerve Cells from Lead Damage

Onions (Allium cepa L.) are nutritious vegetables; however, variations in processing methods can influence their chemical composition and functional properties. Raw processing and cooking are the two main food-processing methods for onions, but it is not clear what kind of changes these two methods cause. In the present study, ultrahigh-resolution liquid chromatography-mass spectrometry (UHPLC-MS) was utilized to observe the changes in onion composition during cooking and to investigate the protective effects of raw and cooked onion extracts against lead damage in vitro and at the cellular level. Many compounds were identified, including amino acids, nucleosides, flavonoids, and organosulfur compounds. Cooking causes changes in the content of numerous amino acids (e.g., DL-glutamine) in onions and increases nucleoside content (e.g., 5'-S-methyl-5'-thioadenosine, adenine). Both raw and cooked onion extracts can reduce neuronal cell damage caused by lead exposure, but cooking increased the free radical scavenging (e.g., DPPH, ABTS, hydroxyl radicals) and chelating of lead ions (up to about 25%) of the onion extracts. In conclusion, cooking can cause changes in the chemical composition of onions and increase their antioxidant and lead chelating capacity.