7 7 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 foods 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, in order 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 average, only 40 microns across. Over time (if the ice cream in your house ever lasts long enough in the freezer) the ice crystals grow, and the result is a grainy texture.
When a crystal forms, molecules of a particular 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. Allowing something to crystallize is a way of purifying it.
Pure substances melt at a definite temperature. Solids that are not pure crystals, but instead a mixture of different crystals, gradually soften over a range of temperatures instead of melting all at once. By watching the temperature of a substance as it is heated, you can tell whether it is a pure substance or a mixture. If there is a sharp melting point, you know it is a pure substance.
Pure cocoa butter melts in a very narrow range between 97°F and 99°F (36°C and 37°C), right at the temperature in your mouth. In your hands, which are colder than the inside of your mouth, dark chocolate (where the fat is pure cocoa butter) does not melt. Contrast this with milk chocolate, which contains a mixture of different fats (including butterfat) and softens over a wider range, so you get sticky fingers.
Cocoa butter contains only three main types of fat (in this case, monounsaturated 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 for which 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 cocoa butter easier to spread on toast or a graham cracker.
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, because large crystals have less surface area to hold the water than 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.
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, you must 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 ice 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. 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 affects the crystal size the most, making ice cream with liquid nitrogen is an excellent way to keep 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 tbs 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. The rate of crystal formation is controlled by the rate of cooling. Fast cooling makes small crystals. Slow cooling allows larger crystals to form. 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” you 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.
Sometimes, however, you want big crystals. The many varieties of salt on the market are indistinguishable when they are dissolved in water. The main difference is the size of the crystals and how the crystals are stuck together. Rock candy is just huge sugar crystals (sometimes with color and flavorings added, which lodge in the imperfections of the crystals).