Why would a cook keep dried onion and garlic powders in the spice rack when fresh onions and garlic are kitchen staples, wonders nojunk. Chowhounds agree that fresh is preferable for most purposes, but say there are specific cases where the powdered versions work better.
Onion and garlic powder are standard ingredients in dry rubs for smoked meats, and hounds also use them to season flour or breadcrumbs for dredging meat or fish. Unlike their fresh counterparts, they don't add unwanted moisture to powdery mixtures, and work well in coatings "because they don't scorch easily," says Karl S. Some hounds also find them superior for flavoring creamy dips because they blend easily and flavor the dip uniformly.
Not all dried onion and garlic products are created equal. Some hounds prefer granulated garlic and dehydrated onion flakes to the powdered versions because the spices are in larger pieces. Karl S finds dehydrated onion "extremely useful," and says it is "noticeably even-toned in flavor." ctfoodguy recommends avoiding garlic and onion salts. "That gives you control on the amount of salt you're using," he says.
For a fun project using these spices, check out CHOW's DIY Flavored Nuts.