When coffee compounds meet milk proteins, the glow they produce gets stronger if the milk proteins have more of a specific building block called proline — so proline seems to help them stick together better.

When you add milk to coffee, the proteins in the milk might help keep the healthy compounds in coffee from breaking down too quickly — but scientists weren’t sure exactly how this works.

Coffee compounds called polyphenols dissolve better in environments full of a substance called proline than in plain water, which suggests they like to stick together with proline.

A natural compound in coffee and fruits called caffeic acid sticks to a specific part of milk proteins because they both like to avoid water — kind of like oil clinging to oil.