Why was chocolate gritty until Lindt left a machine running?

Early chocolate bars were gritty. Before conching, chocolate has an uneven and gritty texture, with particles your tongue can feel. The cure came in 1879, when the Swiss chocolate maker Rodolphe Lindt invented the conche in Bern. It produced chocolate with superior aroma and melting characteristics compared to other processes at that time.

The company story is a nice one. The Lindt chocolate company says that Lindt, perhaps mistakenly, let a mixer full of chocolate run over a weekend. When he came back, he found the chocolate had a smoother texture and greater shine than normal chocolate of his time. Other versions say overnight, and the story may be apocryphal.

What does conching do? A conche is a surface-scraping mixer and agitator. It spreads cocoa butter evenly through the chocolate and polishes the particles until the cocoa and sugar are smaller than the tongue can detect, typically around 20 micrometres. It also develops flavour, through frictional heat, the release of volatiles and acids, and oxidation. Done for hours, sometimes up to 78, it can give a mild, rich taste. Even a little moisture would make the chocolate much thicker, so the machinery is cleaned with cocoa butter instead of water.

The change was big. Conching generalised the use of added cocoa butter, whereas moulded chocolate had often been just cocoa mass and sugar. The smoother chocolate was moulded into thinner tablets and called chocolat fondant, or melting chocolate. Lindt guarded the process, and in 1899 he sold his factory and the secret of conching to Chocolat Sprüngli.