If you make a mesophilic starter culture from your buttermilk first you will only need 1 large cube per gallon. I made my cheese this way for about 30 years before buying comercial cutlures using Junket rennet.
Making a Mesophilic Starter Culture
1. Start with 2 cups of FRESH or store bought Cultured Buttermilk.
2. Let the 2 Cups of buttermilk reach room temp. (70 F/ 21 C).
3. Then allow the buttermilk to ripen for about 6-8 hrs. (Store bought buttermilk does not have a high enough concentration of bacteria to serve as a starter culture without ripening.)
4. The resulting buttermilk will be much thicker and sour then what you started with. It should have the consistency of fresh yogurt, if it doesn't let it sit a few more hours.
5. Pour this culture into a full sized CLEAN ice cube tray and put into your FREEZER. As with all steps of cheese making, cleanliness is next to godliness.
6. Once frozen, remove the cubes and put into a CLEAN sealed container or plastic freezer bags. It is a good idea to label the container to distinguish it from your thermophilic culture.
7. The resulting ice cubes are each 1 oz of mesophilic starter.
8. Add these cubes (thawed) to your recipes as required. The cubes will keep for about one month.
To make more starter simply thaw one cube and add into 2 cups of fresh milk. Mix thoroughly with a fork or a whisk. Allow the milk/culture to stand at room temperature (70 F/ 21 C) for 16-24 hours or until the consistency of fresh yogurt. Then follow from step 5.