Why Pineapple Irritates the Tongue

If you like eating pineapple, then you’re well aware of the stinging feeling you get in your mouth after eating it. This uncomfortable sensation can last for hours and there isn’t really anything we can do but wait for it to subside. Eating a lot of pineapple often results in particularly the tongue becoming sore and textured and can sometimes even cause it to bleed, but the main question is what is causing this response?

Pineapples are tropical fruits that grow from the ground in hot climates and usually slightly acidic soil.1 The fruit and the stem of the pineapple contains an enzyme extract called bromelain, a mixture of two separate enzymes: stem bromelain and fruit bromelain. The structures of these enzymes differ slightly in terms of their primary amino acid sequence, but their functionality is very similar.

In biology there are twenty natural amino acids, each identified by their differing R-groups. These groups ultimately control whether the amino acid itself is polar, non-polar or charged. All amino acids contain both amino and carboxylic acid groups which can condense together, generating a water molecule whilst simultaneously forming a peptide bond. The result of these condensation reactions is the formation of a polypeptide, a linear chain of amino acids which can favourably fold into itself to form a three-dimensional protein structure.

These peptide bonds are relatively weak and enzymes like bromelain are proteolytic, meaning they break down protein structures by breaking the peptide bonds that hold them together. When you eat pineapple the bromelain from the fruit begins to break down the proteins in your tongue, breaking up the collagen and causing the muscles in your mouth to lose shape and firmness, and this is what causes the stinging pain.3

In more recent years many have invested time into finding ways to eat pineapple without hurting their tongue, and the most effective has been to cook the pineapple beforehand. The elevated temperatures cause the bromelain to denature, breaking down its enzymatic structure and stopping it from being able to digest the proteins in your mouth. Another commonly quoted trick is to soak the pineapple in salt water beforehand and, although this method has not been explained scientifically, many people now swear by it.4

Bromelain is a very interesting enzyme because, although it causes the oral irritation we are all familiar with, it also has many well-known health benefits. Bromelain has been shown to reduce swelling and pain in patients with fractured bones, as well decreasing pain and inflammation after dental surgery.5 Interestingly, bromelain has also shown promise as a cancer agent and induced cell death when breast cancer cells were treated with the enzyme.6 This is a huge advancement as natural, biological compounds are sought after for cancer research compared with more synthetic drugs.

References

  1. F. Hossain, African Journal of Food, Agriculture, Nutrition and Development, 2016, 16(4), 11443-11456.
  2. T. Murachi et al, Structure-Function Relationships of Proteolytic Enzymes, 1970, 298-309.
  3. A. Ionescu et al, The Annals of the University Dunarea de Jos of Galati Fascicle VI – Food Technology, 2008, II(XXXI), 9-16.
  4. S. W. H, 2012, The Flesh-Eating Pineapple, The University of Mebourne, Scientific Scribbles, Available at: https://blogs.unimelb.edu.au/sciencecommunication/2012/10/07/the-flesh-eating-pineapple/
  5. Z. Muhammad et al, The Aga Khan University, Department of Surgery, 2017, 67(1), 121-125.
  6. S. Dhandayuthapani, Journal of Medicinal Food, 2012, 15(4), 344-349

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