Coloured scanning electron micrograph (SEM) of a blue vein in a piece of Stilton cheese. The colour and flavour of the cheese is produced by the fungus Penicillium roquefortii, the spores (blue beads) of which cover the surface of the cheese vein. In the commercial production of cheese, the curd is pressed so as to create cracks. These cracks contain enough oxygen to support colonies of the fungi, which are introduced in pure form. Magnification: x400.

© DR JEREMY BURGESS / SCIENCE PHOTO LIBRARY