I want to warn everyone to never use garlic powder or garlic salt. It is the #1 cause of indigestion. It's a very popular ingredient in so many prepared foods, I can't even begin to provide you a list. Why avoid this kind of garlic? Garlic oil is extremely volatile and it goes bad very quickly when exposed to air, becoming a stomach irritant instead of a digestive aid. It causes the famous "acid reflux" that many people endure, especially those of us who are older. Having worked in restaurants I can tell you that they buy their garlic powder in huge tins that stay open for as long as a year or more before they're used up. This allows the powder to react and transform into a nightmare food. Imagine the huge industrial food producers who put tons of garlic powder into everything from potato chips to sausages to frozen foods. Most people don't understand why they get acid indigestion, but this simple rule of avoiding powdered garlic will make a huge difference to those who suffer this malady. There's no reason to take all those drugs to suppress acid reflux once you've stopped taking garlic powder in your diet. IT PAYS READ THOSE LABELS! If you don't, well you deserve what you get.
Minced and dehydrated are better, but again it depends on how old it is and how big the pieces are. The more surface area exposed to air for the longer period of time, the more rancid the garlic.
BTW, I've come across a lot of warnings not to make oil with garlic unless you're going to consume it immediately. People don't realize that garlic, once infused in oil will begin producing toxins due to the anerobic condition. Commercially prepared infusions are usually OK cause they have preservatives. But if you make anything with garlic immersed in oil, it should be consumed asap. This has nothing to do with garlic going rancid and causing acid reflux, btw.
This explains why every time I put garlic powder on my pizza at this one pizza shop we go to I burp up the garlic the rest of the night. Can't stand it!