That is because the pineapple is acidic and can wear away or ‘erode’ the ridged bits of your skin that make up your finger prints.. they same is true with your teeth – if you drink too much fruit juice .. your teeth can start to erode!
To add to Jack’s point – the acid softens the tips of your fingers (and your teeth) until the balance (pH) is neutralised – that’s why you shouldn’t brush your teeth within an hour of eating because the brushing action on the softened enamel can do more harm.
In fact, pineapple juice is very rich with an enzyme called a “protease” – quite literally meaning that it attacks proteins, breaking them down to their constituent amino acids. When you get the juice on your fingers the proteases “digest” the protein in your skin (it is mainly a tough protein called keratin). The first bit of skin to be attacked is the tops of your fingerprints so these are digested first. The pineapple is eating you rather than the other way round!!!!! Of course, when you swallow it, it doesn’t stay in the same place for long and is soon digested by your own body – in particular the acid in your stomach destroys the protease, so pineapples are quite safe to eat (as long as you don’t try to eat a whole one in one go!!!!).
In the food industry, pineapple juice is used as a meat tenderiser.
The question is, seeing as an enzyme is also a protein, why doesn’t the protease attack another molecule of protease?? The answer is that it has been designed to recognise molecules like itself and not attack them. Nature is very clever.
By the way, your fingerprints always grow back. In the past, some criminals tried to destroy their fingerprints with acid but they needn’t have gone through all the pain – their fingerprints still grew back.