On a recent exploration of our refrigerator, my spouse uncovered a jar of mayonnaise past its expiration date. Although it still looked okay, we decided to chuck it. Upon reflection, it did seem unlikely to me that food could have an on/off-good/bad switch, so I decided to find out what expiration dates really mean.
The possibility that there might be Salmonella bacteria lurking in raw eggs, gives mayonnaise a dodgy reputation. But vinegar or lemon juice acidifies the mixture enough to kill off Salmonella. So commercial mayonnaise should be fine, though it lacks the love of homemade. But what about the expiry date?
Foods that you can still recognize as part of the food chain, like meats and eggs, tend to be more perishable and have a sell-by date. They are more susceptible to chemical reactions and microbial activity that can change them over time. Heat speeds activity up and cold slows it down. Freezing can stop some microbes, but damages certain kinds of food.
For most processed foods, expiration or best before dates are voluntary indications of how long manufacturers think the food will be at its best. As a side note on these date codes, JN is for June and JA is for January. Manufacturers have R&D people who test samples of new products under various conditions to see if they will last.
But the best before date is not a safety guide. If it were, I would probably forget and then get sick. A Harvard study found that 90% of Americans think they need to throw out food unnecessarily, because of this misconception. The food industry just sells more stuff through this misunderstanding.
An unopened container of mayonnaise could be good indefinitely. This is a little disconcerting, actually. However, once opened, the mayo could be contaminated and all bets are off. But if the jar gets tightly sealed and refrigerated, mayonnaise can last two or three months after opening—even after the best before date, according to Still Tasty and Shelf Life Advice. It might get a little yellowy or lumpy, but should still be okay. However, Eat By Date only gives an unopened jar a week past its expiry date and an a month after it had been opened. If food starts to look or smell off, then chuck it. Health Canada says, “When in doubt, throw it out.”
We should probably pitch out processed foods for all kinds of reasons, but the expiration date does not seem to be one of them.