Kitchen Mysteries is a weekly exploration of oddities surrounding cooking and food. They could be recipes that fail when they shouldn’t, conflicting advice from different sources, or just plain weirdness. If it happens in a kitchen, and you’re not sure why, send atweet to The Food Geekoremail对brian@thefoodgeek.com to find out what’s happening.

On January 13th, Kellie Lyn asked:

Hi, Kellie,

First of all, there’s no reason to feel dumb about this. Almost every type of food these days proclaims that it’s best to eat or drink it before a certain date. This does two things. The first is to make people wonder how bad it is if it’s just not at its best. The second is to stop people from thinking about why food goes bad and how and instead to just look at the date. The second consequence was probably not intentional, but it happened all the same.

Now, there are those I know who will throw things out when the food is near or past that date, and there are those I know who will use something waaaaaay past the date because, really, it’s not that bad, right? Sometimes that results in a muffin that doesn’t rise very much, and sometimes that results in a trip to the hospital.

In general, knowing how to tell if your food is good or bad is a better idea than trusting a date on the side of the package. Although sometimes it’s a matter of what will make you sick, many times it’s a matter of whether the food will taste good. And, depending on whether your goal is to make something taste as good as it can or just to make something that will be filling and not make you sick, you will have different interpretations of bad.

Fortunately, eggs are a relatively forgiving on the “going bad” front. People are afraid of eggs because of the whole “salmonella” thing, but the things about salmonella are that it lives on the outside of the egg and cooks away nicely. You see, the egg has this handy shell, plus a couple of layers of membranes that are designed to keep things like bacteria and viruses out of the egg. After all, an egg isn’t about protecting your food, it’s about protecting a chicken-to-be, so it’s quite serious about what it does.

As an egg ages, it loses water out of the shell, so it becomes less dense. The white becomes less white and more clear, and the yolk starts to become watered down, so an older egg isn’t going to be as tasty as a fresh egg, but it won’t kill you. If an egg goes rotten, it’s going to smell like sulphur (or, as many would say, it will smell like rotten eggs). This is unmistakable, and if your main goal in life is to avoid that smell, then you aren’t going to want to open eggs that you suspect are rotten.

So, aside from the date on the carton, what can you do about knowing if an egg is aged or not? The easiest and most popular test is to put the egg into water. Remember how I told you that an egg loses water as it ages? Since the egg shell stays the same size, but the mass decreases, this means that it becomes less dense. A fresh egg is a little more dense than water, so it sinks. An old egg is less dense than water, so it floats.