How milk gets into chocolate

Chocolate is a fat-continuous suspension, and even small amounts of water make sugar particles stick together, seizing the mass. Liquid milk is about 87% water, so it cannot go in.

Every milk chocolate process is therefore a way of getting milk solids in while leaving the water out.

The straightforward method: spray-dried milk powder is added with the sugar before refining. It is simple, controllable and dominant in continental European production.

The milk powder's own manufacture matters more than it sounds. Roller-dried powder has a cooked, caramelised character from more severe heat treatment; spray-dried powder is cleaner and milder. Some makers choose roller-dried specifically for that cooked note.

The alternative, characteristic of British and some American production: liquid milk, sugar and cocoa mass are combined and vacuum-dried together into a solid crumb, which is then ground into chocolate.

Because the milk and sugar are cooked together with the cocoa in the presence of water, Maillard reactions run further than they can in the powder route. The result is a distinctly cooked, caramel, slightly cheesy character — and it is the reason British milk chocolate tastes recognisably different from Swiss.

Milk fat behaves differently from cocoa butter: it is softer and it interferes with cocoa butter crystallisation. Milk chocolate is therefore softer than dark at the same temperature and tempers at lower temperatures.

It also means milk chocolate is more prone to bloom and less tolerant of warm storage. The softness people notice is a fat chemistry consequence, not a sugar one.