Question answered by Catherine Riddle, PhD, Radiochemist, Idaho National Laboratory
Both chocolate and butter contain fat (triglycerides) which is why they melt differently than ice. Most chocolates (especially milk-chocolate) tend to melt into this sticky (albeit delicious) mess at slightly above room temperature as does butter. So why does this happen? Since chocolate and butter share similar properties when it comes to melting and fat content, let’s focus on the tastier of the two; chocolate. In order to understand how chocolate melts, we first need to learn the chemistry of how it is made.
Our chocolate bars start with the cocoa tree (Theobroma Cacao) which produces seeds known as cocoa beans. The cocoa bean is where chocolate gets its unique taste and bitterness when unsweetened. The fat from the cocoa bean is used to make a pale-yellow, edible vegetable fat called cocoa butter. Cocoa butter is not only used in the production of chocolate, but it is also used in ointments, skin care products and pharmaceuticals. Cocoa butter contains fats and oils, organic molecules made primarily of three fatty acids, which are linked together with glycerol through chemical bonding. These fatty acids include: Palmitic acid, Stearic acid, and Oleic acid. Oleic acid is an unsaturated fat (meaning it has a double bond on its carbon chain), making it odd shaped and unable to pack well with other molecules. Because of this, a greater portion of oleic acid in the fat results in a lower melting temperature for cocoa butter. Chocolate makers can adjust the amounts of each fatty acid to produce a chocolate that melts only in the mouth, and not on store shelves! Chocolate is a solid mixture which includes cocoa butter as well as cacao powder, sweeteners such as sugar, milk solids, added flavors, modifiers, and preservatives.
Now that we know why chocolate melts, let’s look at how it melts. When chocolate begins to get warm, it will soften and then it will melt and become a liquid. The softening point of chocolate is about 85°F and the melting point is about 93-101°F which is why chocolate melts in your mouth. Like many substances that contain natural vegetable fat, cocoa butter will crystallize to form a solid when it reaches its freezing point which is approximately room temperature. The invisible molecules of cocoa butter fat in the chocolate are arranged in a geometric pattern causing the chocolate to be a solid. When the chocolate reaches its melting point, the geometric pattern (crystal structure) with in the chocolate is disturbed and it loses its rigid structure turning the once square (or numerous other shapes) into a blob of chocolate in your hand or mouth as it melts! At this point you may ask yourself, what happens if I put the chocolate that has melted into the freezer? Unfortunately, fat crystals from the cocoa butter are not very forgiving once they have been disrupted from their original form and although you will still have chocolate when it re-solidifies, it can be grainy, soft or crumbly, and could form white patches on the surface of the chocolate called a fat bloom or chocolate bloom. This is due to the fat crystal structures in the cocoa butter forming random crystal patterns as it re-solidifies.
So why do different chocolates melt faster or slower at room temperature? Once again the answer comes down to fat content and by adding other fats the melting point can be shifted. For instance, milk chocolate contains cocoa butter diluted with milk fat (same as butter or cream), which gives it a lower melting point as well as compromising the fat crystallinity. Dark chocolate has the highest percent of cocoa solids and cocoa butter (70%) making it melt faster than white chocolate at room temperature because white chocolate has no cocoa solids and only a small percent of cocoa butter (35%). Although the chemistry of chocolate can be complicated, gathering experimental data on altering chocolate’s crystal structure can be fun, just pop a piece in your mouth and slowly let it melt!
Chocolate Melting Race
Materials: Two squares of chocolate, aluminum foil
- Break off two squares of a chocolate bar. Wrap each square in foil.
- Place one square in the freezer for a few hours and leave the other at room temperature.
- Unwrap the room temperature square and place it on your tongue. How fast does the chocolate melt? How long does it take to taste the chocolate flavor?
- Take a few sips of water or bites of an apple or saltine cracker to cleanse your palate.
- Unwrap the square from the freezer and place it on your tongue. Is the experience the same as the room-temperature square?
You should taste the room-temperature more quickly than the frozen chocolate because the frozen chocolate must first be warmed up in the mouth before it melts, while the room-temperature chocolate quickly melts in the mouth.
From the American Chemical Society