My sister, last time she came over, brought with her a gallon sized ziploc bag of sourdough starter. It was given to her by a friend, who told her the starter has been in her family for some 75 years. Whether or not that's true, it is certainly true that there are starters that old and even older which have lost none of their vigor and flavor.
Sourdough starter culture
This will take a few days, but is not at all difficult. It's quite amazing that with the repeated additions of only wheat and water, you will develop an active and living sourdough starter!
Mix together 1 cup of whole wheat or rye flour with ¾ cup of water. Make sure that all the dough is wet into a ball. It will be stiff, but don't worry about it. Keep in a clean container covered with plastic wrap at room temperature.
Mix together 1 cup bread flour with ½ cup of water. Add this mixture to the mixture from yesterday, and mix it all together. Yesterday's dough will likely be a little bit softer than it was, but there will not likely have occurred any rise. Cover with plastic wrap as before, and leave at room temperature.
Mix together 1 cup of bread flour with ½ cup of water. Take the dough from the day before, and discard half of it. Mix the new and old dough's together. It will be getting wetter, and there will probably be some rise by now. Cover with plastic wrap, and leave at room temperature.
Repeat the procedure exactly as from Day 3. A few hours after you have mixed the dough's together, your starter should have doubled in size. It is now ready for use.
Take 1 cup of your active sourdough starter, and, mix with 3 ½ cups of bread flour and 2 cups of water. Mix together well and cover with plastic wrap. After about 6 hours, the dough should have doubled in size, and become quite bubbly. It is now ready to use in a sourdough bread recipe!
You can use this starter right away, or it can be held in the fridge until you are ready to use it. I keep it in a clean large covered Tupperware container in the fridge, and take it as needed.