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Butter, milk, and cheese should all be avoided when it comes to dehydrating. The high-fat content of cheese and butter makes it a lot of work to dehydrate, only to go rancid very quickly.
If you really wanted to, you could dehydrate olives, but the taste isn’t worth it. You wouldn’t necessarily get food poisoning or anything, but the process removes a lot of nutrients and turns the olives into a bizarre-tasting mush.
Since people dehydrate almost every fruit and vegetable, you might think avocados work too. Unfortunately, they don’t. The high content of good fats that make avocados a superfood also makes them useless to dehydrate.