Ever opened your carton of ice cream after an absence – say, a short holiday – and found it full of disgusting ice chunks? You've been the victim of Ostwald ripening.
Ostwald ripening is a well-established phenomenon that is even less appetizing than it sounds. Wilhelm Ostwald discovered it in 1896, but it's been playing a hand in ruined food as long as people kept food around to ruin. Ostwald was a chemist who noticed that crystals seemed to form around little imperfections and flaws in a medium. These crystals tended to be small, but over time, the number of small crystals dropped as large crystals took over. It seemed odd. If a medium, like ice cream, supports the formation of small crystals, why should they become rare and large crystals more common?
Although small crystals are formed easily, they have a hard time keeping themselves together. They are thermodynamically less stable than their larger counterparts. Although heat generally breaks crystals into smaller pieces, in this case it tends to build them up. Heat is the movement of atoms and molecules, and not all the molecules in a crystal move the same way. The molecules inside the crystal are locked down, bonded in every available way with the other molecules around them. The molecules on the edges of a crystal, however, have one side exposed to the wind. They can wiggle, and get whisked away, more easily.
Although large molecules have a larger surface area than small ones, the proportion of the number of molecules on their surface to the number in their interior gets smaller and smaller as they grow. A small crystal will have almost all of its molecules exposed in one way or another. All of those exposed molecules can be ripped off with relatively little energy. A large crystal will have a lot of molecules lining its outside, but most will be locked together in the interior of the crystal. Breaking it up takes more work and more energy. So once a crystal starts getting some bulk, it will be tough to break apart. As small crystals break up, their molecules join the larger crystals. This is why ice cream that's perfectly creamy one week, with all its iced water crystals tiny and separate, will eventually turn to a block of ice as those water crystals group together over time. In the food industry it's called "particle coarsening," and it's the reason you need to eat all of your ice cream quickly.
[Via BMC, Particle Coarsening, Live Science]