Crystallization – A Physical Change
Crystallization is a physical change by which one substance in a mixture separates itself from the mixture and forms solid, crystalline particles with other molecules of the same substance. If you’ve ever seen rock candy, or made it at home, you have seen the result of sugar crystallization. Rock candy is made by dissolving as much sugar as possible into a pot of hot water, then cooling the sugar-water mixture to initiate crystallization. As the sugar-water cools, sugar molecules will separate out from the water and stick to other sugar molecules, forming solid crystals. This happens as the temperature cools because cool water cannot hold as much dissolved sugar as hot water.
Many factors can influence crystallization in food. Controlling the crystallization process can affect whether a particular product is spreadable (smaller crystals), or whether it will feel gritty or smooth in the mouth (larger crystals). In some cases, crystals are something you try to develop; in others, they are something you try to avoid.
Different foods contain crystals with different characteristics and qualities. Butter, margarine, ice cream, sugar, and chocolate all contain different types of crystals, although they all contain fat crystals. For example, ice cream has fat crystals, ice crystals, and sometimes lactose crystals.
The fact that sugar solidifies into crystals is extremely important in candy making. There are basically two categories of candies: crystalline (candies that contain crystals in their finished form, such as rock candy, fudge and fondant); and non-crystalline (candies that do not contain crystals, such as lollipops, taffy, and caramels). Recipe ingredients and procedures for non-crystalline candies are specifically designed to prevent the formation of sugar crystals because they give the resulting candy a grainy texture. One way to prevent the crystallization of sucrose in candy is to make sure that there are other types of sugar—usually fructose and glucose—to get in the way and slow down or inhibit the process. Acids can also be added to “invert” the sugar, and to prevent or slow down crystallization. Fats added to certain confectionary items will have a similar effect.
When boiling sugar for any application, the formation of crystals is generally not desired. These are some of the things that can promote crystal growth:
- Pot and utensils that are not clean
- Sugar with impurities in it (A scoop used in the flour bin, and then used for sugar, may have enough particles on it to promote crystallization.)
- Water with a high mineral content (“hard water”)
- Too much stirring (agitation) during the boiling phase
Crystallization may be prevented by adding an interferent, such as acid (lemon, vinegar, tartaric, etc.) or glucose or corn syrup, during the boiling procedure.
As mentioned above, ice cream can have ice and fat crystals that co-exist along with other structural elements (emulsion, air bubbles, and stabilizers such as locust bean gum) that make up the “body” of the ice cream. Some of these components crystallize either partially or completely. The bottom line is that the nature of the crystalline phase in the food will determine the quality, appearance, texture, feel in the mouth, and stability of the product. The texture of ice cream is derived, in part, from the large number of small ice crystals. These small ice crystals provide a smooth texture with excellent melt-down and cooling properties. When these ice crystals grow larger during storage (recrystallization), the product becomes coarse and less enjoyable. Similar concerns apply to sugar crystals in fondant and frostings, and to fat crystals in chocolate, butter, and margarine.
Crystallization is important in working with chocolate. The tempering process, sometimes called pre-crystallization, is an important step that is used for decorative and moulding purposes, and is a major contributor to the mouth feel and enjoyment of chocolate. Tempering is a process that encourages the cocoa butter in the chocolate to harden into a specific crystalline pattern, which maintains the sheen and texture for a long time.
When chocolate isn’t tempered properly it can have a number of problems. For example, it may not ever set up hard at room temperature; it may become hard, but look dull and blotchy; the internal texture may be spongy rather than crisp; and it can be susceptible to fat bloom, meaning the fats will migrate to the surface and make whitish streaks and blotches.
Attributions
This page is based on “Chemistry 2e” by Paul Flowers, Klaus Theopold, Richard Langley, William R. Robinson, PhD, Openstax which is licensed under CC BY 4.0. Access for free at https://openstax.org/books/chemistry-2e/pages/1-introduction
This page is based on “Chemistry of Cooking” by Sorangel Rodriguez-Velazquez which is licensed under CC BY-NC-SA 4.0. Access for free at http://chemofcooking.openbooks.wpengine.com/
This page is based on “The Basics of General, Organic, and Biological Chemistry” by David W Ball, John W Hill, Rhonda J Scott, Saylor which is licensed under CC BY-NC-SA 4.0. Access for free at http://saylordotorg.github.io/text_the-basics-of-general-organic-and-biological-chemistry/index.html
