Every time I make fajitas, I cut into bell and chili peppers and notice how hollow they are. I always think to myself: what is the gas inside this pepper, and how does it get there?

Perhaps Capsicum peppers are porous. I did a little kitchen experiment to try to find out. (I'm afraid I don't know much about biology, so this "experiment" is likely to be flawed in at least one basic way, but I figured I should at least try to figure it out myself before asking.) Here's what I did:

I filled a small container with water and submerged an uncut Anaheim pepper. It was quite buoyant! I put the lid on the container so it would stay submerged. It didn't seem to become any less buoyant, and its interior didn't seem to fill up with water. No bubbles emerged from the pepper, as least as far as I could tell.

So air doesn't seem to easily pass in or out of the pepper. And of course they don't grow around air, so how does the gas get in?

Here are my guesses:

  1. The fruit is porous to air, but the holes are small enough that, in my experiment, they were sealed by the surface tension of the water.
  2. The fruit isn't porous to air when fully grown, but it is porous to air while it's still developing.
  3. The fruit does some amount of photosynthesis, and the air is waste oxygen.
  • 1
    $\begingroup$ Nice one for the experiment :). Bear in mind that there are many many processes happening in living thing that can produce gasses. Breathing, photosynthesis, fermentation etc etc. $\endgroup$
    – terdon
    Apr 1, 2013 at 20:06
  • $\begingroup$ Yeah! This very issue puzzled me a year ago, and I asked several people, but received no reliable explaination. I also guessed, that air could it not be, as the fruit is growing from inside out of... well... "nothing", so the air must get into it, as the fruit doesn't grow it around, as the author here has written too. Then, I thought, maybe some part of oxygen, the product of photosynthesis ongoing in the plant's leaves, in some natural way, gets into the fruit's inside, thus filling up the empty space, or... a kind of "blowing it up" (hmmm, well, well...). $\endgroup$
    – vege-Romek
    Jun 8, 2020 at 17:16

1 Answer 1


To answer your question of what the gas is, how about the glowing splint test for oxygen! I guess it might work with a recently snuffed out match too. Cut a plug in the the pepper but leave it in place until you have the glowing match ready, then quickly open the plug and hold the match into the opening. It should relight if it is oxygen and you may hear a popping noise.

As for whether the surface tension is preventing the flow of air, you could try adding some detergent (washing up liquid or soap) to the water to break the tension and see what happens.

However, I have found an article suggesting the environment is, compared to the standard atmospheric concentration anyway, low in O2 and high in CO2 in two different pepper cultivars.1

Samples withdrawn from the fruit locule over time show that CO2 ranges between 0.5% and 3% (5000–30,000μmol·mol–1) (Fig. 2, top) while O2 ranges between 18% and 20% (Fig. 2, center), compared to the composition of standard atmosphere (400μmol·mol–1CO2; 21% O2)

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1 Blasiak, J., Kuang, A., Farhangi, C.S., and Musgrave, M.E. Roles of Intra-fruit Oxygen and Carbon Dioxide in Controlling Pepper (Capsicum annuum L.) Seed Development and Storage Reserve Deposition. J. AMER. SOC. HORT. SCI. (2006) 131(1):164–173.

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    $\begingroup$ If we ever get a community ad on another site, I suggest "Biology: World changing research one pepper at a time". Good suggestions, I'm tempted to try them! $\endgroup$
    – Rory M
    Apr 1, 2013 at 23:15

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