Prepping produce ahead is a cooking convenience, but it also has shortcomings. Most prep involves cutting or slicing in some way, which can expose produce to oxygen or trigger other chemical reactions that change flavor, texture and appearance (consider how quickly avocados and apples turn brown).
On Milk Street Radio, caller Eric Moeller, of Louisville, Kentucky, wondered just how far ahead he can chop onions before using them.
In the case of onions—similar to garlic—cutting increases their pungency. That’s because it allows enzymes called alliinases to react with sulfur-containing chemical compounds. The longer cut onions sit, the longer the reaction and the more pronounced their pungency.
We’d heard you can counteract this by rinsing cut onions just before cooking them. To find out if this worked, we diced and refrigerated onions overnight. We then rinsed them just before cooking. As a control, we also tried dicing and cooking an onion immediately with no rinsing. No surprise, we found that freshly diced onions that were cooked immediately had the sweetest, mellowest flavor. Rinsing the onions we had prepped ahead did tame their bite a bit, but they still had a pronounced pungency.
For this reason, we recommend against prepping onions in advance when they are a main ingredient in a dish. When they are more of a background note, as in soups and stews, prepping them ahead is fine, but we suggest rinsing them just before cooking.
See below for more onion tips: