Photo: scottfidd (flickr)
If you think about it, there seems to be no good reason for packaging eggs with their wide end up as opposed to plunking them into the cartons wide-end down: either way will keep the egg from rolling off. Yet cardboard cartons and the plastic egg-tray in refrigerators are made to accommodate a fat-end-up egg. Is there something more here than just coincidence?
In fact, there is: the reason it’s smarter to store your eggs with the fat end up is that the egg itself does not completely fill the interior of the shell. If you crack open a hard-boiled egg carefully at the fat end, you will see that the white part of the egg, called the albumen, does not quite reach the shell — there’s a pocket of air in-between the two. That isn’t the case for the narrow end of the egg, which fits snugly.
That pocket of air allows for the presence, and reproduction, of bacteria. This is not to say there’s something wrong with your egg: any chicken egg will have the air pocket and some measure of bacteria inside it. The trick, then, is to keep the bacteria as far as possible from the yolk, which is much more susceptible to bacterial infection. Albumen contains bacteria-killing enzymes while yolk does not. In other words, the yolk is more perishable than the white.
If you hold the egg fat end down, that air pocket has a tendency to rise — not completely through the egg, but enough to reach the yolk. By storing eggs fat-end up, the pocket of air stays away from the yolk, and the egg stays fresh longer.