Baby food

*not interested in blw*

I’ve recently started purées with my 6 month old and we’ve done green beans peas and sweet potatoes so far.

I see a lot of purées that are mixed like raspberries avocado or kale apple spinach. Stuff like that. But I don’t see any that are just straight up spinach or just avocado or just raspberries.

I thought I was supposed to introduce one thing at a time so I know if there’s any reactions? If I do a kale apple spinach and he hasn’t had straight kale or straight spinach how will I know which thing it is if there’s a reaction??

Also what am I looking for besides serious reactions like rashes and swelling?