I love hot chocolate. It’s the perfect winter drink, with endless possibilities of festive flavor. Due to its lack of caffeine, it has honestly become one of my year-round favorites. As the dreary Ithaca winter days start to sink in, what better way to honor hot chocolate than by discussing the science behind it!
Hot chocolate is made from only a couple main ingredients: chocolate and a liquid like milk or water. In many cases, the “chocolate” comes from cocoa powder. Interestingly, cocoa powder actually has hydrophobic (water-repelling) properties due to its fat content. This means that if you dip a spoonful of cocoa powder into water and lift it up, the cocoa powder will remain dry.
Because of this, cocoa powder cannot fully dissolve in milk or water. Instead, they form a colloid. A colloid is a mixture in which microscopic particles of one substance are thoroughly suspended throughout another substance. In this case, the colloid forms as cocoa powder is suspended throughout milk or water. In order for the cocoa powder to dissolve in milk or water as best as possible, a hot liquid is needed. At higher temperatures, the increased kinetic energy of the liquid molecules will collide with the cocoa powder particles more frequently and with greater force, helping the cocoa solids disperse into liquid, hence the name hot chocolate.
Additionally, many hot chocolates begin with alkalized cocoa powders, where cocoa beans are treated with an alkaline solution, such as baking soda, to neutralize acids. This helps alleviate the tartness that naturally occurs in cocoa beans, making for a darker and more chocolate-y hot chocolate.
Another crucial component of many hot chocolates is the milk. Milk in itself is a very interesting compound, as milk is an emulsion. First, we need to understand that milk is water-based, yet contains fat compounds as well. Water and fat don’t mix, as is highlighted by the density tower experiments that we did in middle school. In order for milk to exist as we know it, the water and fat molecules need to be able to mix. This unity is able to occur because of compounds called emulsifiers, which are both water and fat soluble. Proteins like casein in the milk act as emulsifiers, allowing milk to be the perfect rich liquid to use for hot chocolate.
I would argue that marshmallows are the best hot chocolate topping, so why not explore the science behind them too? Marshmallows are made by aerating a mixture of sugar and gelatin, allowing them to form a foam that will stabilize into the sticky squishy sweet once the gelatin sets. Gelatin is a product derived from collagen, which is a major structural protein that is found in an animal’s muscles and bones. When purified collagen is treated with an acid or alkaline, it becomes more elastic, with the fibers being able to twine together to form a gel under cold temperatures, and unravel and move fluidly under warm temperatures. This is now gelatin. When air is whipped into a mixture of sugar and gelatin, the air bubbles become trapped in the gel structure formed by gelatin, creating a fluffy marshmallow that can sit right on top of the perfect mug of hot chocolate.
Every time you drink a nice warm mug of hot chocolate this winter, I hope you take a moment to consider the science underlying the tasty drink.