Sulfur content and sugar determines which onion you pick. Yellow onions are high in sulfur, which means they is pungent and will keep for months. Sweet onions is low in sulfur and high in sugar, making them mild but unable to keep long. Match the onion to how you’ll cook with it.
Sweet onions are best used when eaten raw (e.g., salads). When purchased, use quickly as they don’t keep very well in the refrigerator. Yellow onions is best cooked. The high heat break down the sharp sulfur compounds and turns them into a rich, savory taste that sweet onions just can’t match. Keep your yellow onion stash in a dry dark location at room temperature where it will last months.
How to Choose the Right Onion
When you cut an onion, what’s happening is that gas (syn-propanethial-S-oxide) gets released from the onion. That’s what makes your eyes tear up and burn. One way to lessen that is to chill the onion before cutting. Place the onion in the refrigerator for half an hour beforehand. That will slow down enzyme responsible for producing the gas.
The Wrong Onion: A lot of people pick the wrong onion for the job. It is mushy. It is probably a sweet onion used for caramelizing. Does it taste harsher? It is probably a pungent onion for salsa. Choose the right onion for the dish.
Mild onion for fresh use. Use strong onion for a soup base or slow cooking. Don’t store various types together. Sweet onions need airflow and cool temps. Onions for storage are best kept in a dry dark place. One type will usually go bad and spoil the others if kept together. Separate to keep longer and retain more flavor.
Once you know the difference, things become simpler both at the store and in the kitchen. It’s not “just onions.” With purpose, you select the onion. Mild for stewing. Mellow for stewing. Sharp for a pickled relish. There’s no onion that belongs there; you’ve got the right onion. All you should of do is figure out which onion it is before you cut into it.