For a creamier kefir, use whole milk. Adjust the amount of strawberries to your liking, depending on your desired level of sweetness. Kefir grains can be reused indefinitely to make future batches of kefir. Store them in a small amount of milk in the refrigerator between uses. If you don't have fresh strawberries, feel free to use other frozen fruits such as blueberries, raspberries, or mangoes.