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What is the Shelf-Life of Mayonnaise?

 What is the shelf life of mayonnaise, once opened and stored in the refrigerator?

 A while. Actually quite a while.

Commercially produced mayonnaise is a lot tougher than most of us think. From the standpoint of safety, you don't even have to refrigerate an opened jar of mayonnaise because it contains enough vinegar and/or lemon juice to provide an environment hostile to the development of harmful bacteria. Manufacturers universally tell you to refrigerate mayonnaise once opened, though, because it helps preserve the flavor. Warmth causes mayonnaise to break down after a while, and it gets to the point where it just tastes oily.

Mayonnaise manufacturers stamp a "best-use" date on their jars and tubes and, in their cavalier way of thinking, they don't care whether you open it the day you get it home from the store or the very last day before the date stamped on the jar. According to the manufacturers, the mayonnaise will be yummy up to that date, but they just can't promise anything beyond that. Of course, if you don't open the jar until that date, you surely will have a little time to eat it before you notice a decline in flavor. The same must be true if you opened it months ago and haven't quite finished it up.

In a completely unscientific sampling of the mayonnaise in the Ochef test kitchen, we found some that were already past their best-use dates and others that were still more than two years in the future. Some manufacturers use calcium disodium (EDTA) as a preservative (which helps account for the two-plus years of pending yumminess). Some manufacturers do not use preservatives (Cains in New England, at least), and their best-use date is measured in months rather than years.

There is one issue to watch out for, though, and it is entirely in your hands. If your child, spouse, or houseguests dip a knife into your mayonnaise, spread it on bread, and dip it back into the mayonnaise, they are quite likely to introduce bread crumbs into the jar. The same is true for the making of tuna salad or any other preparation, really. Do not return a contaminated utensil to the mayonnaise jar; it will almost certainly cause spoilage or the growth of mold. Keep your mayonnaise pure and it will last for a long time — and you will be able to tell when it is past its prime, because you will think, "hey, this isn't all that yummy anymore. Must be time to open a new jar."

Homemade mayonnaise, of course, is a different matter. With its use of raw eggs, it is highly perishable and should be consumed immediately or refrigerated and finished off very shortly.

And thank you, by the way, for reminding us to clear out our old mayonnaise....

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