Why should I test liquid culture on agar before using it?

Plating a few drops of liquid culture on agar shows contamination that is hard to see in a cloudy jar. Bacteria and mould appear as off-colour or fuzzy growth against the clean white mycelium, so you catch problems before they reach a whole batch of grain. Many growers keep this step for any culture they did not make themselves. It costs one plate and a few days.