Yes, mayonnaise. It is sold unrefrigerated, and is an essential component of any back-country sandwich. Has anyone reached its limit and had it spoil on them?

(I haven't. Of course, I've never had an opened bottle last more than about week unconsumed...)

To clarify just a bit - I know that the labels and recommendations are to refrigerate, but I think western society waaaay over-refrigerates everything... and that paranoia prevents a lot of great food from making it into the back-country.

So my question is not: what does the label say, but: has anyone ever had it turn south on them?

Edit from a fellow mayonnaise lover: Any developments in packaging in the past 4 years that might help?

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    I'm just wondering why you put mayo on your pb&j Commented Mar 15, 2012 at 16:40
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    Consider buying a crate of single serve mayo packets? Shelf life is a year and a cursory amazon search shows $23 for a box of 200 :)
    – Ryley
    Commented Mar 15, 2012 at 23:19
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    @Ryley - More trash... more hassle... and with the quantity of mayo I consume, ripping all those little packets open is just annoying.
    – Lost
    Commented Mar 16, 2012 at 1:45
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    Use mustard -- it won't go bad. :-)
    – xpda
    Commented Mar 17, 2012 at 15:33
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    Mayo, really!? Some things simply aren't appropriate for backpacking trips. My answer is to bring something else. There are plenty of foods. Is it really that big a deal to not have mayo for a few days? You wouldn't bring cans of diet coke, milk, ice cream, and a lot of other stuff either. What's the big deal? Commented Dec 19, 2012 at 16:30

6 Answers 6


Mayo will generally discourage the growth of bacteria while sealed. It is stored at room temperature in the store, but that's after being packed in a controlled environment that minimizes the risk of contamination. Once you open that container, you'll increase the contamination risk, which is why they say to refrigerate it. Once mayo is contaminated, it is no longer safe.

However, if either type of mayonnaise is cross-contaminated by foods such as raw beef, unclean utensils, or E. coli O157:H7-infected food handlers after commercially processed containers are opened, the pathogen may survive at 58C for several weeks

No one can tell you exactly how long it will last. It will usually last until you contaminate it with bacteria and medium for growth. Then it will spoil pretty rapidly, usually within hours at the wrong temp, days at a good one. The problem is that bacterial or fungal contamination is not guaranteed to leave a detectable smell or taste. So by the time you find out you have poisoned yourself, you've almost guaranteed that your past the point where you can treat the poisoning. Botulism is usually fatal.

If there were a guaranteed way to tell when it's spoiled, you could be safe just by inspecting each time before using. Since there is no guaranteed way to tell, and spoilage is due to contamination, I have to say that from a safety standpoint, it is good until you use it the first time. After that there is always the chance that you have contaminated it and will kill yourself eating it. You could do this for years, or your entire life and not suffer consequences, but you are rolling the dice and I cannot recommend rolling the dice like that.

It's also worth noting, from the study above, that the acidity level is not the primary inhibitor of e.coli:

The tolerance of E. coli O157:H7 inoculated at the two highest test populations was greater in real mayonnaise than in reduced-calorie mayonnaise dressing held at 20 or 308C, temperatures within the range used for normal commercial mayonnaise distribution and storage, despite the lower pH and actual higher acidity in the nonfat phase of real mayonnaise than in that of reduced-calorie mayonnaise dressing. This indicates that factors other than pH and acids may be influencing the viability of cells in the two formulas. The presence of 0.09% sodium benzoate in reduced-calorie mayonnaise dressing would be expected to adversely affect the viability of E. coli O157:H7

Indeed, it has been found that low pH alone does not really slow Ecoli down:

The ability of E. coli O157:H7 to tolerate an acid pH in several types of foods has been documented by other researchers. E. coli O157:H7 can grow at acidities approaching 0.8%

The risk is not so much in the mayonnaise per-se, as in carrying it around in a container that you have to repeatedly open and close in a intrinsically non-sterile setting. Certainly using a squirt bottle would reduce risk of contamination, but individual packets will completely remove the risk.

  • It's too bad no one has done research into backcountry mayo. I have certainly consumed large quantities of it even after many days out - maybe I should limit that from now on!
    – Greg.Ley
    Commented Mar 16, 2012 at 17:58
  • @Greg.Ley - Well not directly on "backcountry" mayo, but on unrefrigerated mayo. Just posted some citations.
    – Lost
    Commented Mar 18, 2012 at 4:39
  • @LBell -- My answer itself says that the mayo itself is fine as long as you don't contaminate it. The problem is that if you contaminate it, you may not notice, and you may not taste or smell it before consumption. Research does not show that mayo stops bacteria when mixed with other products, just that it slows it. Commented Mar 18, 2012 at 13:13
  • So if the data suggests mayo slows the growth (though the 3rd reference I gave suggests it kills Ecoli) even after contamination, then what evidence do you have for saying it will spoil rapidly and people will die? Given the inhibitive properties, seems like one could almost argue mayo is safer than any other fresh food (which also spoils at some point.)
    – Lost
    Commented Mar 19, 2012 at 1:41
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    @LBell -- The sentence from the research you quoted, which you left out: " However, if either type of mayonnaise is cross-contaminated by foods such as raw beef, unclean utensils, or E. coli O157:H7-infected food handlers after commercially processed containers are opened, the pathogen may survive at 58C for several weeks". Your study is about the viability of mayo on the shelf, still sealed, after professional industrial processing. That's not at all similar to an open container in the woods. Commented Mar 19, 2012 at 1:54

I sidestep the problem by bringing only single serving foodservice packets of mayo. The stuff might not be the best tasting on day one, but on day 6 the packets are going to taste just the same. The fresh stuff... ugh.


About 30 years ago, my husband and I spent a couple years cruising Mexico on our sailboat. I was very careful to keep our mayonnaise in the refrigerator, however, a scientist and his wife on another boat, never refrigerated theirs and told me that if you are careful to always use a clean utensil when dipping into it and never introduce protein into the mayo, it will stay safe and fresh. They, and many Mexicans I might add, kept their mayonnaise this way and even though the weather was hot, they had no problems.


The problem is this is highly dependant on the environment. If I was on a trek in the middle of winter then I'd feel pretty comfortable taking it out for a few days and being cautious, but in the height of summer I doubt it'd safely last more than a few hours at most.

The other main issue is that when you're in the back country it's just not worth risking - if you're brought down with food poisoning it often happens suddenly, and being away from help complicates things massively in this sense. And that's before you get to the psychological worry that you're worried you may have got food poisoning from some gone off mayo!

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    From personal experience, I've carried a squeeze bottle of mayo for a week+ backpacking in the heat of the desert... no issues.
    – Lost
    Commented Mar 16, 2012 at 1:49
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    @LBell -- You got lucky. The odds are largely in your favor with a squeeze bottle, but you're still taking a risk. Even if it lasts a week on one trip, the very next trip you could poison yourself within the first two days. Commented Mar 16, 2012 at 15:06
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    @LBell -- 3 years can still be luck. As I said, the odds are in your favor, so most of the time, you'll be fine. However just like you can go to Vegas and win, even though the odds are against you, you can get a bad draw with your mayo and end up being a S&R Treasure Hunt prize. If it's worth the risk to have the wonderful mayo flavor, that's a choice. Being outdoors at all is a risk and we all make our own risk/reward balances to maximize our enjoyment of the environment. As an example of the risk spectrum, some people smoke and never get cancer... that doesn't make it safe for all... Commented Mar 17, 2012 at 16:24
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    @RussellSteen - It might be a little extreme to compare the occasional consumption of mayo beyond its prime with smoking cigarettes or winning repeatedly in Las Vegas... Just sayin!
    – Greg.Ley
    Commented Mar 18, 2012 at 17:06
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    @Greg.Ley It is in a way, but the point still stands - even though it's a small risk, it's one with potentially devastating results (especially in the wilderness.) Whether you choose to take that risk is up to you, but there's no denying that it's there.
    – berry120
    Commented Mar 18, 2012 at 22:01

Taking the lead from MrWizard, I did a little look for what the boys and girls in white lab-coats say.

In addition to the evidence presented at Snopes.com and the experts at the Ketchup Advisory Board (see MrWizard's answer) a few interesting articles (see below) all seem to point to this conclusion:

Commercially produced mayonnaise made with acetic acid is safe and due to its low pH, as well as the salt, and other additives it contains.

That, combined with the fact that none of the outdoor experts here have ever had mayo go bad on them... well, draw your own conclusions.

References (added emphasis mine):

Smittle, R.B. “Microbiological Safety of Mayonnaise, Salad Dressings, and Sauces Produced in the United States: A Review.” Journal of Food Protection 63, no. 8 (2000): 1144–1153.

The absence of reports of foodborne illness associated directly with the consumption of commercially prepared acidic dressings and sauces is evidence of their safety. Salmonella, E. coli O157: H7, E. coli, L. monocytogenes, Staphylococcus aureus, and Yersinia enterocolitica die when inoculated into mayonnaise and dressings.


The reported highest manufacturing target pH for dressings and sauces is 4.4, which is below the 4.75 pKa of acetic acid and below the reported inhibitory pH of 4.5 for foodborne pathogens in the presence of acetic acid.

Collins, M. A. “Effect of pH and Acidulant Type on the Survival of Some Food Poisoning Bacteria in Mayonnaise.” Microbiologie - Aliments - Nutrition 3, no. 3 (n.d.): 215–221.

All vegetative bacterial cells were eliminated in acetic acid mayonnaise pH <4.4. S. aureus was not eliminated from citric acid mayonnaise; in addition, at pH >5.0 both S. muenster and C. perfringens survived in this product.

Suggests that the type of acid is important (acetic better than citric) and PH is a factor (as noted in the first article). I believe all commercial mayo is still produced using acetic acid.

Weagant, Stephen D., James L. Bryant, and Don H. Bark. “Survival of Escherichia Coli O157:H7 in Mayonnaise and Mayonnaise-Based Sauces at Room and Refrigerated Temperatures.” Journal of Food Protection 57, no. 7 (1994): 629–631.

Escherichia coli O157:H7 strains, when inoculated and mixed into mayonnaise and stored at 25°C, became undetectable after 72 h storage when assayed by direct plating or by selective enrichment. The same strains inoculated into mayonnaise and stored at 7°C were detectable up to 35 days when assayed by direct plating or by selective enrichment.

Interestingly seems to suggest that at warmer temperatures, Mayo is more effective at killing E.Coli - likely (I imagine) to the general faster pace of biological processes at higher temps.

  • Nice selection of references. +1
    – Greg.Ley
    Commented Mar 18, 2012 at 17:07

From what I understand, it doesn't last in the backcountry. The recommended shelf-life for commercial mayonnaise, opened and inside the refrigerator, is two months. The USDA recommends only storing mayonnaise in the refrigerator.

From the same page:

Perishable food should not be left out more than 2 hours at room temperature (1 hour when the temperature is above 90 °F, 32.2 °C).

Mayo is perishable once it's been opened. Looking at these guidelines, I'm not even sure if it's safe for me to take pre-made tuna salad on a backpacking trip. It might be best to take some sealed take-out sized (single-serve) packets and keep them sealed until needed if you want to keep making those pb&j&mayo sammiches. ;) Those little packets will keep for up to eight months after purchase, according to Heinz.

I live in the Southeastern US, and have never felt safe taking mayo out with me into the backcountry, and I don't care for warm mayo-- so I've never had the opportunity to test out the USDA's recommendations in the field to see if experience would match up with what's on paper.

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    So why does it taste so good on the sixth day?! :-P
    – Greg.Ley
    Commented Mar 15, 2012 at 20:08
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    @Greg.Ley: Everything tastes better on the trail the sixth day! ;) Commented Mar 15, 2012 at 21:00
  • I would agree with the pre-made tuna-salad, for the reason Greg.Ley commented on the question (introducing other contaminants), but I think the USDA is over-cautious...
    – Lost
    Commented Mar 16, 2012 at 1:47
  • @LBell: I agree, they're probably a bit over-cautious-- if we're talking about household items. I'm not sure if I'd want to take my chances in the field, though, heh. Problem is, contaminants get into it as soon as it's opened-- and heat in a nice moist container is a perfect breeding ground for bacteria. Commented Mar 16, 2012 at 1:52

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