Medium roast keeps more of the bean's original character. When beans are roasted darker, you lose the delicate notes—fruity, floral, nutty—and replace them with roast flavor. Medium preserves enough of that origin character to make cold brew interesting, without going so light that it turns sour.

Cold brew naturally smooths out acidity. The long steep time extracts differently than hot brewing, so even light roasts can taste mild. But medium roasts bring a rounder body and a hint of sweetness that makes the finished drink feel more complete. You get chocolate or caramel notes without the burnt aftertaste that dark roasts sometimes leave.

Another practical point: medium roast is forgiving. If your brew ratio is slightly off or you steep a bit too long, medium roast still tastes decent. Dark roasts can turn bitter when over-extracted; light roasts can turn sour. Medium sits in the safe zone.