Wash potatoes with tap water, peel and cut into 1 cm cubes. Rinse with tap water quickly and boil 200 g of potato cubes with 1 L of distilled water for 20 min. Mash and squeeze through a muslin bag. Add agar and boil till melted. Add yeast extract and glucose, and stir till dissolved. Make up to 1 L. Adjust pH. Autoclave to sterilize.