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I have heard that onions and potatoes shouldn't be stored together. Why will this make the potatoes spoil faster?

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As you can see at the Chapter 7: Storage of horticultural crops of the book PDF: Small-Scale Postharvest Handling Practices: A Manual for Horticultural Crops (4th Edition):

  • If you look p.166 (p.173 on the PDF), you can see that the onions need to be stored in this conditions:
    • between 0-5°C with a relative humidity (R.H.) of 65-70%
  • If you look p.168 (p.175 on the PDF), you can see that the potatoes have different needs:
    • Fresh market: between 4-7°C
    • Processing: between 8-12°C
    • seed potatoes: between 0-2°C
    • But must of all, potatoes need very highest relative humidity (R.H.): 95-98% versus 65-70% for onions

So, if you store them together, either one or the other will be in bad storage conditions.

Further, as you can see at Section 7: Storage of horticultural crops of Small-scale postharvest handling practices - A manual for horticultural crops - 3rd edition:

  • Dry onions (Group 3) are very sensitive to moisture, which makes their storage incompatible with the high R.H. needed for potatoes.
  • New potatoes (Group 6) may also produce ethylene, which will badly damage green onions (Group 2) if stored together.

Anyway, storage of horticultural crops cannot just be resumed by "don't put this or that together", as you may have seen:
it is important to consider R.H., temperature, and many other factors.

If the subject interests you, all this (and many other things) is pretty well explained in the following references:

Ethylene is also a "critical factor", and this topic is more developped in the following resources:

  • 2
    This answer makes me think that, if RH and temperature are fixed, there should be no significant effect on potato spoilage (due to the presence/absence of onions). That contradicts the other answer, but maybe this older thinking is a myth; is there any published experiment to check this? It sounds like a good time lapse science experiment candidate. – bobuhito Feb 16 '16 at 17:03
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When stored together, onions release gases that will cause the potatoes to spoil faster and become mushy, if you will.

  • This is the simple and correct answer to the OP's question. – ElmerCat Feb 8 '16 at 21:47
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If you store your potatoes with your onions, your potatoes will begin to grow eyes (stems). I like to put these two together only to start my potatoes off before I plant them. It is a much faster process, instead of starting from seeds. Hope this helps!

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