Commercial mayonnaise is not just eggs and oil mixed together. It is:
- Highly acidic (thanks to vinegar or lemon juice)
- Made with pasteurized eggs
- Formulated to resist bacterial growth
- Often contains stabilizers and preservatives
The acidity is the key.
Most harmful bacteria, including Salmonella, do not thrive in a strongly acidic environment. Commercial mayonnaise is intentionally formulated to be shelf-stable before opening.
That’s why you see jars sitting on grocery store shelves, not in refrigerated cases.
But once opened, the story changes slightly.
How Long Can Opened Mayo Sit Out?
According to food safety guidelines:
- If opened mayonnaise sits at room temperature for more than 8 hours, it should be discarded.
- If the room is warmer than 90°F (32°C), the limit drops to 1 hour.
Why?
Because once opened:
- You introduce air.
- You introduce possible bacteria from utensils.
- You disrupt the sealed environment.
Even though the mayo itself is acidic, contamination risk increases.
Now here’s where nana wisdom sometimes clashes with modern food safety rules.
If she leaves it out for a meal or an afternoon and then puts it back in the fridge — that’s usually fine.
If she leaves it out for days continuously?
That becomes risky.
Why It’s Less Dangerous Than People Think
Interestingly, plain mayonnaise alone is rarely the culprit in food poisoning.
Food safety researchers have found that mayo’s acidity actually helps kill bacteria rather than support them.
The real danger often comes from:
- Chicken salad
- Potato salad
- Tuna salad
Once protein or vegetables are mixed in, the protective acidity is diluted.
That’s when bacterial growth becomes easier.
Homemade Mayonnaise Is a Different Story
If nana makes mayonnaise from scratch using raw eggs, that’s a separate situation.
Homemade mayo:
- Lacks preservatives
- May use unpasteurized eggs
- Is less acidic unless carefully measured
Homemade mayo should:
- Be refrigerated immediately
- Be used within 3–4 days
- Never sit out longer than 1–2 hours
Raw egg risk changes everything.
The “Few Days on the Counter” Scenario
If the jar has been:
- Opened
- Sitting at room temperature
- For multiple days continuously
The safest answer is:
Don’t eat it.
Not because mayonnaise is automatically toxic — but because once opened and exposed, bacteria can multiply invisibly.
Food poisoning bacteria:
- Have no smell
- Have no visible warning
- Do not change taste
You cannot “test” safety by sniffing.
Why Older Generations Sometimes Do This
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