This is great video for providing background information about using red cabbage juice as an acid/base indicator.
1. An easier way to extract the juice from the red cabbage is to cut a head of red cabbage in half. Put the two pieces in freezer bags, and then place the bags in a freezer until the cabbage is totally frozen–over night will work.
2. Remove the cabbage from the freezer and allow to thaw.
3. You should be able to use your hands to squeeze the juice out of the cabbage.
4. Store the juice in a sealed jar and in the refrigerator to prevent souring.