A grape tray with fresh yeast paste should be put into each cage at least once every 24 hours.  If it is absolutely impossible to do this, two trays of food should be put in the cage and the adults should not be left for more than 48 hours before changing the trays. See below for preparation of yeast paste.

1. Prepare a grape plate for each cage of adults by spreading a layer of yeast past down the center of the tray. The amount of yeast paste depends on what the flies are consuming. Begin with a strip that is about 1/8 inch deep by 1inch wide. Adjust the amount of food so that most of it is eaten between tray changes.
2. Remove the tray of food that may already be present in the cage by grasping the end, tipping the tray to a 60° angle and tapping the lower edge on the wall of the cage. Repeat this by tipping the tray in the other direction. This knocks the adult flies off from the tray. Be careful not to tip the tray too much as the layer of grape food may fall from the styrofoam tray. Remove the old grape tray and add the fresh tray. Tie a loose knot in the cloth cover.
3. The embryos on the tray may be used for numerous things or simply discarded. Some embryos will be used to inoculate corn meal tubs to start the next population cycle. For preparation of nuclear extracts, the trays of embryos can be stored in a tub in the cold room for up to 3 days. This allows for several rounds of 12 hour collections to be done over a period of 3 days.  The embryos are then used to prepare a nuclear extract on the third day. For chromatin structure work, the embryos can be frozen at -75°C.
4. We typically recycle the grape trays if they have not become dehydrated. The embryos are washed off and the residual yeast paste is scraped off with a spatula. The trays can be wrapped in plastic and stored in the refrigerator for future use.

In a plastic container mix:

225 ml water
1.36 ml propionic acid

The mixture should have the consistency of thick peanut butter. If it is too thick, add a little water. If it is too syrupy, add a little yeast.

Store the yeast paste in the refrigerator.