How pineapple breaks down proteins on your tongue

HostYou know that weird feeling you get after eating a few chunks of fresh pineapple? It starts as a little tingle on your tongue, and then it kind of turns into a dull ache or a fuzzy sting. I always thought I was just eating too much sugar, or maybe I was a little bit allergic, but it turns out there's something way more active going on in my mouth. Why does this fruit seem to be fighting back while we eat it?

GuestIt's not in your head, and you're likely not allergic. What you're feeling is a very specific type of attack. Pineapple is one of the only things we eat that contains a group of tiny, sharp tools called bromelain. Think of these like a billion microscopic pairs of scissors. Most fruit is just sitting there waiting to be fuel for your body, but the pineapple has these tools that are built to take things apart. Specifically, they're built to break down proteins. Since your tongue and the inside of your mouth are made of protein, the fruit is quite literally starting to break down the surface of your mouth while you're chewing it. It sounds a bit scary when you put it that way, but you're basically being digested by your snack.

HostWait, that's a bit of a jump. When I hear the word digest, I think of what my stomach does to a sandwich. Are you saying the fruit is trying to turn my tongue into mush? That feels like it should be a much bigger deal than just a little sting.

GuestWell, it's the same basic idea as what happens in your gut, just on a much smaller scale. These tools, which scientists call enzymes, have one job. They look for the long chains that make up a protein and they snip the links. In your mouth, they're targeting the soft, thin skin on your tongue and the roof of your mouth. The reason it doesn't turn into a horror movie is that your body is very good at fixing itself. As soon as you stop eating and the juices are washed away, your tongue starts to heal. But for those few minutes, those tiny scissors are working through the protective layer of your mouth. They're softening the tissue, which is why your tongue feels tender or even a bit raw if you eat a lot of it at once.

HostIf it's that good at snipping through meat, I'm surprised it doesn't hurt my hands when I pick it up. I can hold a slice of pineapple all day and my fingers don't start stinging. Is the skin on my hands just too tough for these tiny scissors to work?

GuestThat's exactly it. The skin on your hands is thick and has a heavy layer of dead cells on top that acts like a shield. Those little tools in the fruit can't really get through that wall. But your tongue is different. It's much softer, it's wet, and the living cells are right there on the surface. It's the perfect place for those scissors to get to work. It's also why you might notice your lips getting a bit sore too. They're much thinner than the skin on your hands. This is also why people use pineapple juice to make tough meat soft before they cook it. If you soak a steak in pineapple juice, those tools go to work on the tough bits of the meat and break them down so the steak is easier to chew. If you leave it in there too long, though, the steak will literally turn into gray mush because the scissors never stop snipping.

HostThat makes me think about my stomach. If these scissors are still working when I swallow the fruit, why do they not just start eating my throat or the inside of my belly? It seems like a bad idea to put a bunch of tiny meat-eaters into my system.

GuestYour body has a few tricks to handle that. First, your stomach is full of very strong acid. This acid is much harsher than anything in a pineapple. When those tiny fruit tools hit that wall of stomach acid, they get bent out of shape. Once they lose their shape, they can't snip anything anymore. They just become another piece of food for you to use. Plus, your whole throat and stomach are lined with a thick layer of slime called mucus. This slime acts like a coat of armor. The pineapple tools get stuck in the slime before they can ever touch the living cells of your body. So, the only place they can really cause trouble is in your mouth, where the shield is thin and the acid hasn't reached them yet.

HostIt feels like the plant is being kind of mean. Is there a reason a pineapple would want to hurt the things that eat it? Usually, fruits want to be eaten so their seeds get spread around.

GuestNature is full of these little trade-offs. While the fruit wants to be eaten by the right animals at the right time, it also needs to protect itself from bugs and tiny pests that might try to eat the plant before the fruit is ready. These protein-cutting tools are a great way to keep certain insects away. If a bug tries to bite into the plant, these tools go to work on the bug. It's a defense system. We just happen to be big enough that it only feels like a tingle to us, whereas for a tiny pest, it could be a real problem. But there's a way to stop the scissors if you don't like the sting. If you heat the pineapple up, like when you grill it or buy it in a can, the heat breaks the tools.

HostSo a warm pineapple is a defeated pineapple because the heat ruins the scissors before they can get to my tongue.

GuestThe living parts of the fruit that do the cutting are so fragile that even a quick trip to the stove turns them into plain old sugar and fiber.

HostThat fresh sting is just the price we pay for eating a fruit that tries to take a bite out of us first.

GuestThe pineapple is the only food that chews you back while you're chewing it.

HostMy next bowl of fruit is going to feel a lot more like a fair fight now that I know the scissors are coming for me.