Crystallization is important in many aspects of food preparation and storage, as well as in creating and refining the ingredients for foods and household products.

Crystallization is used to purify sugars and fats.  It is used to change the texture of things such as ice cream, fondant, fudge, and chocolate.  Controlling crystallization is important when freezing foods.

The difference in the way dark chocolate snaps when you break it from the way milk chocolate breaks is due to differences in the way the cocoa butter is crystallized in the two.  Chocolate is “tempered” by controlling the heat while it cools, to make a lot of very small crystals.  The white bloom that sometimes forms on chocolate that has been stored improperly is a result of changes in the crystals of the cocoa butter from one type of crystal to another.

Ice cream is smooth because its billion or so crystals of ice per liter are on the average only 40 microns across.  Over time (if the ice cream in your house ever lasts long enough in the freezer) the crystals grow, and the result is a grainy texture.

When a crystal forms, molecules of a compound arrange themselves in a close periodic order.  Molecules of different compounds are either too large or too small, or do not bind to the others in the same way, and are excluded from the crystal.  Crystals are thus pure substances, and allowing something to crystallize is a way of purifying it.

Pure substances melt at a definite temperature, as we have discussed earlier.  Solids that are not pure crystals, but instead a mixture of different crystals, gradually soften over a range of temperatures instead of melting.  By watching the temperature as a substance is heated, we can tell whether it is a pure substance or a mixture, just by watching for a sharp melting point.  When a solid like ice melts, the temperature stays constant (at the melting point) until all of it is no longer solid.

Pure cocoa butter melts in a very narrow range between 97° F and 99°F, right at the temperature in your mouth.  In your hands, which are colder than the inside of your mouth, dark chocolate does not melt.  Contrast this with milk chocolate, which contains a mixture of different fats, and softens over a wider range, so you get sticky fingers.

Cocoa butter contains only three main types of fat (in this case mono-unsaturated triglycerides), while butterfat from milk contains over 400 types.  Cocoa butter thus melts at a much sharper temperature point than butterfat does.  Knowledge of this can help you when designing recipes where you might want to control the melting point or spreadability of your creation.  Cocoa butter is not spreadable, but butterfat is, so adding butterfat will make the coca butter easier to spread on toast or a graham cracker.

Sugar crystals

Sugar is refined from cane syrup by growing large crystals.  The mix of crystals and remaining liquid is separated by spinning the liquid away in a centrifuge, leaving only the crystals.  Large crystals do not trap the liquid between them like small crystals do.

Your tongue and palate can detect graininess of sugar crystals as small as 15 microns.  For a fondant, caramel, or fudge to feel smooth and creamy, the sugar crystals must be at least that small.

Controlling the size of crystals

In many foods such as ice cream, chocolate, caramel, fondant, fudge, and even butter and margarine, the size of the crystals is very important to the texture and consistency of the result.  Whether the crystals are fats, ice, or sugar, controlling their growth is the key to the recipe.

With ice cream, we stir the mixture constantly as it freezes.  The ice crystals form at the edge of the container, and are scraped off and mixed into the more fluid center.  The growth of the crystals is affected by the rate of cooling, and the proteins in the milk, which compete with water molecules to surround the growing crystal.  In a similar fashion, the lactose crystals in the ice cream are also limited by proteins that coat them and prevent them from joining other nearby lactose crystals.  And the crystals of sucrose and lactose and butterfat also help keep the ice crystals from joining together into larger crystals detectable by the tongue.

Since the speed of the freezing process is what affects the crystal size the most, making ice cream with liquid nitrogen is an excellent way of keeping the crystals small.  One of my favorite recipes (because it is so simple) is short enough to fit into a Twitter post: One gallon half and half, 2 cups sugar, 4 tbsps vanilla. Add 1 gallon liquid nitrogen slowly while stirring. Serves 32.  I stir it with a portable electric drill with a paint mixing attachment.  The result is very smooth and creamy.  (My first attempt used heavy whipping cream and was so disgustingly rich a single scoop was almost too much.)

In chocolate making, the process of tempering the chocolate creates small crystals, as we have mentioned before.  The rate of crystal formation is controlled by the rate of cooling.  If the crystals are too large, the chocolate is dull and grainy, without the smooth shine of a nice dark chocolate bar.  The tempering also controls whether the crystals are in one form (the β(V) form) or in another (the β(VI) form).  The first form produces the desired shiny surface, and the second form causes the whitish “fat bloom” we see on chocolate bars that have been stored at too high a temperature.  Fat bloom in some recipes is prevented by adding butterfat, which prevents the recrystallization by adding many different types of triglycerides to the mix.