There are always those few mystery bottles of condiments tucked away in the back, the ones you forget about until you're rummaging through for something else.

Should you keep them, or pitch them?

While there's a huge disagreement over whether an opened bottle of ketchup should be kept on the shelf or in the fridge, the answer according to Heinz is that it's the fridge, all the way.

The acidity technically makes it shelf-stable, but once you open it, it can impact the balance of what's inside.

For comparison, your refrigerated bottle will be good for between nine to 12 months while that same bottle will only last about a month in the pantry.

What about ketchup-based cocktail sauce?

That's not going to last quite as long, but should be good for between six and nine months.

In other words, use the bottle you opened for New Year's Eve by June, and you're fine.

According to French's, it's not technically necessary to refrigerate your regular mustard, but they say popping it in the fridge will help it hold flavor.

Horseradish-based mustards and Dijon should definitely be kept in the fridge, because while there's nothing in either that will spoil, they will lose flavor if they're not kept cold.

Mustard in a glass jar will last longer than the stuff in the squeeze bottle, but the general rule is that mustard will still be good for two to three months beyond the "Best By" date, which they stress isn't the same as an expiration date.

You might not have to throw this one away after all.

"But you've mistakenly wandered off with my Grey Poupon!"

"Ow!"

"The scoundrel!"

While Worcestershire sauce has been a staple in kitchens for decades and while it's always been fine just sitting on the shelf, our homes are generally warmer than they were a hundred years ago.

That means you're prolonging the life of the product by keeping it refrigerated, and it can last up to three years.

While we're talking about long-lasting condiments, let's talk about soy sauce.

Like Worcestershire sauce, soy sauce is fermented and that helps make it last longer, but proper refrigeration makes a huge difference here, too.

At room temperature, your bottle will last about six months.

In the fridge, it'll be good for around two years.

Chiles and vinegar are main ingredients in many hot sauces, and these preserving agents help make them last.

As long as you properly refrigerate your sauce, an opened bottle could be good for up to three years… but, there's a "but."

Dr. Doug Archer from the University of Florida says you shouldn't keep that open bottle in the fridge for more than a year.

That's because of hot sauce's tendency to develop flecks of mold that can react with the ingredients to start producing gases that will eventually, if forgotten about for long enough, burst the container they're in.

No condiment is as polarizing as mayonnaise, and since there's probably at least one person in your house that doesn't like it, it might sit on the door of the fridge for a long, long time.

How long should it stay there?

Not long, because a jar of mayo is only good for about two months.

In other words, finish that jar of mayo you opened for your summertime cookouts by the time school is in full swing.

The same goes for mayo- and cream-based dressings.

They're good for a maximum of two months, but you're better off using it within a month.

If it's a vinaigrette dressing, you have a little more time, because those open bottles tend to be good for about three months in the fridge.

Since a lot of standard Barbecue sauce ingredients, like salt and vinegar, act as preservatives, it'll last longer than you might think.

Just be sure to keep your sauce in the fridge, all the time, which means not letting it sit out, say, alongside the grill while you're cooking.

That's going to shorten the life of the sauce immensely.

That said, let's assume you've kept it in the fridge most of the time it's been open.

There's still a huge variance here because there's so many different types of barbecue sauce.

Sweet Baby Ray's says an open bottle of their sauce won't be good past the 4-month mark, but some other sauces can last up to a year in the fridge.

Ultimately, use your judgement and throw away anything that looks dark or moldy.

If you've ever ordered takeout or stopped at a fast food joint, you probably have a collection of those little packets of condiments.

But, do they go bad?

Yes.

Here's the scoop.

Ketchup packets are good for around seven months, while mayo (regular and fat-free), tartar sauce, horseradish, and Tabasco packets are good for eight months.

Then, you've got relish, barbecue sauce, mustard, steak sauce, and cocktail sauce packets, which will last around nine months.

And here's the weird thing.

While mild taco sauce packets will last up to nine months, the same hot taco sauce is only going to be good for around four.

Now you know!