Select firm, clean heads with either pale, creamy-white or bright green leaves (this will depend on whether they have been forced in the dark or not). Avoid any that are dirty or wilting.

Trim off the green leaves and the root. Cut stalks into even-sized pieces.

Blanch in boiling water for 3 minutes. Drain. Plunge immediately into a bowl of iced water to cool. Drain again and dry on kitchen paper. Pack in convenient quantities in polythene bags, remove the air, seal, label and freeze. Storage time: 9 months.