Coffee

Even if you're a fan of cold brew, there's nothing tempting about a cup of hot coffee gone cold. But hectic mornings, unreliable travel mugs, and chilly temperatures can mix to leave your morning cuppa joe tepid and unappetizing. If you drink coffee with cream, you might be tempted to put off adding that cream until the last minute to keep it hot for as long as possible. According to physics, though, that's the opposite of what you want to do. Coffee with cream stays hotter longer.

Hold Me Now, Warm My Heart

It seems counterintuitive: if you want coffee to stay hot, why would you add cold liquid? It comes down to several principles in physics.

Anyone who's felt the temperature difference between the road and the sidewalk on a hot summer day knows the first principle: dark colors absorb more heat than light colors. That means they emit more heat, too. The velvety darkness of black coffee, then, is better at emitting heat — and thereby loses heat faster — than the chestnut brown of café au lait. One point for cream in your coffee.

The next principles aren't quite as intuitive. Hotter things lose heat faster, according to something called the Stefan-Boltzmann law. Also, the bigger the difference in temperature between two objects in contact with each other, like the coffee and the air, the faster the hotter one will lose its heat to the cooler one, according to Newton's Law of Cooling. At first, this might seem like coffee with cream is just getting off on a technicality: it cools down more slowly because it starts off colder, sure, but wouldn't the head start extra-hot black coffee has help keep it hotter than its creamy counterpart?

No, especially if your plan is to add cream eventually. Say Jessie and AC are both holding cups of coffee. Jessie's impatient, so she adds cream to her coffee immediately, reducing the temperature right then and there. AC, on the other hand, wants his coffee to stay as hot as possible until he's ready to drink it, so he waits five minutes to add the cream. But because AC's coffee starts off hotter, it loses heat so quickly that once five minutes has passed, it's about the same temperature as Jessie's — and he still needs to add the cream, which will lower the temperature even more.

Turn Up The Volume

Finally, there's the fact that adding cream to coffee increases its volume and its viscosity. More volume means it takes more time to cool off — just like a bathtub cools more slowly than a pot of warm water, you have to remove more heat from a bigger cup of coffee than a smaller one in order to lower their temperatures by the same amount. Substances with more viscosity evaporate more slowly, and evaporation takes heat with it. That's the same reason that the melted butter at your dinner table cools off faster than the gravy does.

The choice is clear: if you want your coffee to stay hot, add cream as soon as possible.

Bonus fact: Have you ever noticed that droplets of cold cream will sometimes sit on the coffee's surface for a few seconds before dissolving? MIT scientists just determined why that happens. The temperature difference between the cold cream and the hot coffee creates circulating currents in the air that cradle the droplet as it sits on the coffee's surface, preventing it from sinking in. The greater the temperature difference, the longer the cream will sit there.

For this and other scientific answers to food and drink questions, check out "What Einstein Kept Under His Hat: Secrets of Science in the Kitchen" by Robert L. Wolke. The audiobook is free with a 30-day trial of Audible. We handpick reading recommendations we think you may like. If you choose to make a purchase through that link, Curiosity will get a share of the sale.

– SciFri

Key Facts In This Video

1. Drip your coffee into separate cups to isolate the different characteristics of your brew. 00:30

2. The first cup will showcase the coffee's acidity and should taste sour, while the next cup will be less acidic and have more aroma. 00:51

3. Once the aroma dissipates, you will get the body of the coffee, where the plant carbohydrates coming out that give the brew it's presence. 01:03

Ashley Hamer