Extract the listed ingredients involved in the making of Sourdough bread, in a comma separated list

Context: Sourdough baking requires minimal equipment and simple ingredients – flour, salt, and water – but invites practice. Purism is a part of the appeal. As described by one enthusiast, "If you take flour, water, yeast and salt, and play around with time and temperature, what comes out of the oven is something utterly transformed." Many bakers feed their starters on elaborate schedules, and many name them. Some approach sourdough as science, attempting to optimize flavor and acidity with careful measurements, experimentation, and correspondence with professional microbiologists. Some lineages of starter are freely shared, and others can be purchased, but many prefer to cultivate their own. Some techniques for doing so are fiercely debated, such as the use of commercial yeast to jump-start a culture while capturing wild yeasts, or adding grapes or milk.

Answer: flour, salt, water, yeast or starter