In the test kitchen, we’ve established that in applications featuring raw onions, it is best to chop them immediately before you need them.
That’s because when an onion is cut, the ruptured cells release enyzmes called alliinases. The alliinases interact with sulfur-containing amino acids in the vegetable, forming thiosulfinates, which are the source of that intensely sulfurous “old onion” smell that can unpleasantly dominate a dish. The more time the alliinases have to mingle with the proteins, the more smelly thiosulfinates are formed. But we wondered if we had to impose such strict time limitations on onions we intended to cook. To find out, we chopped and stored onions on four successive days, and on the fourth day we used them in two different cooked applications.
After sampling batches of onions seasoned lightly with salt and cooked gently in butter, most tasters were able to distinguish the fresher onions—described as “sweet” and “pleasantly pungent”—from the older, “sour,” “bland” sample. However, once they were incorporated into a more complexly flavored dish—braised green beans with tomatoes—very few tasters could identify the older onions.
Lesson learned: If onions are a mere accent in your dish, feel free to prep them in advance. If they’re a focal point, chop them right before using.