Why does cold brew need hours when hot coffee needs minutes?

Answer: Slow diffusion work

Slow diffusion work ✓ — Right. Temperature is the engine of extraction speed. Hotter water gives molecules more kinetic energy, so soluble compounds leave the grounds quickly; cold water can reach useful extraction only by trading heat for time. That is why cold-brew recipes often sit for 8-24 hours while drip or pour-over coffee finishes in minutes.

Beans must ferment — Not quite. Normal cold brew is not relying on fermentation; it is still an extraction of roasted grounds into water. Fermentation can be part of coffee processing before roasting, but that is a different stage. In the jar, the slow step is diffusion and mass transfer at low temperature.

Caffeine alone lags — Not quite. Caffeine does extract, but it is not the lone reason cold brew needs hours. Cold-brew studies track caffeine, acids, phenolics, aroma compounds, TDS, and sensory attributes together. The hour-scale need is the whole compound mix moving slowly at low temperature.