Dried agar powder for use as a medium in yeast culturing. To prepare a malt-agar solution for plates (petri dishes) and slants (test tubes), you will need 4 oz of dry malt extract and 10 to 20 grams of agar (the more agar, the firmer the media will be) per liter of water. This is enough for approximately 20 plates and 2 dozen slants.