Freezing Water
Another common way to calibrate thermometers at home is to use an ice bath. The temperature of the ice bath should be 0°C under atmospheric pressure. This leads to an observation: water can remain liquid at 0°C.
When water forms ice, it starts at nucleation sites where a few water molecules cluster together to form a small crystal. Over time, more and more water molecules join, and the ice crystals grow in size. If the temperature is below 0°C but there are no nucleation sites, ice crystals will not form, and water remains liquid. The nucleation sites can be impurities or physical disturbances. For undisturbed pure water, ice does not form until -39°C, when a phenomenon called homogeneous nucleation happens. Modernist chefs have taken advantage of this property of water and created interesting dishes. Here is an example: https://www.youtube.com/watch?v=N1cZfOyqC78. Controlling the growth of ice crystals during the freezing process has a profound impact on the texture of frozen, and previously frozen, food.
Impurities get in the way of water molecules joining the ice crystals, which makes it harder for water to freeze. The result is freezing point depression. Ice cream is not one big hard block of ice because there is so much sugar dissolved in it that the ice crystals cannot grow very big. On the other hand, when the water molecules do join together and form crystals, they squeeze out the impurities. Freezing can purify water or extract dissolved substances, depending on your goal.
Most of our food is water. About 70% of beef and chicken is water. 93% of spinach is water. When we freeze food, we are freezing water. Water in food exists in two different places: inside cells and outside cells. The water outside cells is relatively pure and freezes at around -1°C. The water inside cells has more solutes like proteins, sugars, and salt, so it doesn’t freeze until under -20°C. Before the temperature drops below -20°C, more and more water molecules migrate out of cell walls and join the ice crystals outside cells. This makes the remaining liquid even more concentrated, and its freezing point further depressed. The growing crystals and the dehydrated cells lead to damaged texture and ruptures at the cell level.
The worst way to freeze food is to freeze it slowly. To freeze food, you need to transfer heat from inside the food to outside the food. This heat transfer path consists of two parts: inside the food, the heat is conducted. Outside the food, the heat is transferred by convection. The conductive heat transfer coefficient is a physical characteristic of the food; that is to say, you can’t change it without changing the food. So the only way to speed up the freezing is to accelerate the convective heat flow out of the surface of the food. According to Newton’s law of cooling, there are two ways to do this: increase the convection heat transfer coefficient or increase the temperature difference. You could change the former by increasing the airflow rate. Restaurants use something called the blast chiller. It’s like an anti-air fryer. Instead of hot air, it forcefully circulates very cold air around the food.
A large difference in temperature between the food and its environment can be accomplished by cryogenic freezing, which involves immersion or spraying the food with liquid nitrogen. Liquid nitrogen’s boiling point is about -196°C under atmospheric pressure. For home cooks without special equipment, a practical method is to cut the food into small pieces to both shorten the conduction path and increase the surface area for convection.
The key to great mouthfeel of ice cream is keeping the ice crystals small. There is a lot of dissolved sugar in ice cream, which interferes with the growth of ice crystals. Also, the churning of ice cream makers helps keep the crystals small by both physical disturbance and incorporating air. (Did you know the FDA requires that ice cream cannot have more than 50% air?) Commercial ice cream makers add ingredients that bond with free water in the mixture so the water molecules are not free to join ice crystals. Some animals have antifreeze proteins in their bodies. These proteins bind to small ice crystals to inhibit their growth. That’s why hibernating bears don’t freeze solid.
But there is a more interesting way to freeze water quickly: manipulating the pressure.