It’s actually not (just) the acidity. Pineapples and several other tropical fruits have an enzyme called papain that breaks down peptide bonds in a rather unique way, to the point that the texture of the meat can become quite off-putting if the enzyme is not used properly
Edit: acidic or not, it’s definitely best not to put any sensitive parts of your anatomy in pineapple juice
Not to mention “barley in middle school”
I don’t know about you, but I think that’s pretty good for a cereal crop.