A banana releases one positron (anti-electron) every 75 minutes. Bananas contain a small amount of potassium-40, a naturally occurring isotope of potassium. Potassium-40 decays and emits a positron in the process. Positron which is antimatter annihilates on contact with matter.
I saw this in a Symmetry Magazine article titled “Ten things you might not know about antimatter.” I highly recommend this very readable and informative article.
Regarding positrons and bananas you should also check out a short note by Philip Tanedo who mentions that
“Discover magazine had an interesting article on radiation exposure a couple of years ago. That article stated: Eating 600 bananas is about the equivalent of having one chest X-ray. The article also lists other sources of radiation—radon, medical, cosmic rays. According to the article, the average American gets a dose of radiation roughly the equivalent of 36 X-rays per year.”