What Are The Fresh Garlic and Onion Prep Rules to Follow?
Onions are a kitchen hero—whether you’re slowly caramelising them for a rich stew, soup, or sauce, shaving them for a salad, or tossing them into a stir-fry. But there’s a little onion secret you should know; cutting them too far in advance can make them taste strange. Very strange.
When you slice an onion, you expose it to the air, and a chemical reaction starts happening almost immediately. Onions contain sulphur-containing amino acids that are usually kept in check inside the onion’s cells. But when you cut through them, the cells break open, and a reaction begins. This releases sulphur compounds, particularly syn-Propanethial-S-oxide, which is responsible for making you tear up when you chop onions. As the onion continues to interact with oxygen in the air, these compounds can break down into new ones, some of which contribute to a bitter, off-tasting flavour.
The problem with all of this? After about 20 minutes of exposure to air, the onion's flavour shifts from sweet and mild to sharp and weirdly ‘off’. The longer the onion sits after being chopped, the worse the taste gets. This is why freshly chopped onions give you that lovely, crisp, sharp bite that you’re after, but once they hang around a while, they can ruin your dish with an overly pungent, sour kind of taste.
You can peel your onions ahead of time, then just chop or slice them right before cooking. That way, you can still semi prep ahead, and avoid that strange taste .
What About Garlic?
Garlic is a bit tricky too. When garlic is chopped or minced, it releases a compound called allicin, which gives garlic its bold punch. Once garlic is chopped and exposed to air for a while, the allicin begins to break down and lose its potency. That’s why pre-chopped garlic from the supermarket can taste a little weird—it’s been sitting around for a while, losing its fresh flavour and turning a bit flat or even bitter. Plus, it’s often pasteurised for shelf stability, which lessens the zing and fresh edge. Not ideal. So seriously—for the love of Mike, please don’t use garlic out of a jar. In fact, don’t even use a garlic crusher, unless you want a full-on garlic hit that can also be a bit bitter.
Yep, that’s right. Put down the garlic crusher. Many chefs won’t even use one. Why? Because crushing garlic smashes the cells more thoroughly, releasing a lot of allicin, resulting in a more intense, often bitter taste. Crushing is fine if you're after a really strong flavour when you can use it quickly, but it can overpower the dish if you don’t want the garlic to steal the spotlight.
On the other hand, mincing or slicing garlic gives a sweeter, more subtle flavour because you’re not releasing as much allicin. The flavour still infuses into the dish, but not as aggressively. It’s a more controlled way of incorporating garlic, especially if you want it to complement other ingredients rather than dominate.
So, here's the deal: Always chop your garlic fresh. It doesn’t take long, and the flavour will be ten times better. If you’ve got leftover garlic after chopping, store it in a jar covered with olive oil and use it for gentle pan-frying, or for making a dressing.
