My starter is slow, what should I do?
A recurring problem many sourdough fans face is that their starter slows down, or it loses it characteristic flavor. This is relatively uncommon in commercial bakeries because they use their starter on a daily basis, and because the starter is used so frequently that the acidity of the starter stays under control.
A sourdough starter is a symbiotic relationship between one of 3 strains of yeast and one of 5 strains of lactobacillus bacteria. The lactobacillus bacteria produce a number of antibiotic compounds that protect the starter from invasion by other organisms. The bacteria also increase the acidity of the starter, so most types of yeast cannot survive the acidity of the starter.
If the acidity is too high, the starter will dissolve the gluten in flour, which causes problems in making wheat based breads. If the acidity gets a bit higher, the yeast will die.
Bakers yeast cannot survive more than a few refreshments in a healthy starter, which is why we strongly encourage you to not use bakers yeast to start your starters, or with them to make bread. We talk about it in the "Starting a starter" FAQ.
When your starter slows down, or loses its characteristic taste, you need to reduce its acidity and revitalize it.
In the section on starting a starter we encouraged you to use organic, stone ground, unbleached whole wheat or rye flour to start a starter because these flours contain more of the organisms we want to encourage. However, in this case we are trying to revive a starter, not create one. If you use a flour with lots of healthy organisms on it, you might wind up with an active starter - just not the one you started with and wanted to revive.
Some people worry about the possibility of starting a culture instead of reviving one that they sterilize the flour they use to feed the slow starter. Unless you have a starter that has been ignored a long time and looks dead, I wouldn't worry about this possibilty. However, if you are very worried about losing the starter, you may want to think about sterilizing the flour and water you are using to revive the starter. You can do a reasonable job of sterilizing the flour by boiling the water you will use and pouring it over the flour and stirring the mixture. You can also bake the flour in the oven at 250F or so until the flour is hotter than boiling water (let it cool before you use it, of course). Some people use a microwave oven to heat the flour, but microwave ovens tend to heat unevenly so I am not a fan of this approach.
After twelve hours or so, your starter should be fairly lively. But the revival isn't over yet - whisk in 1 cup of water and 2 cups of additional flour. This time, the starter should take off much more quickly. Check it in 3 hours or so.
If your starter still isn't fast enough, repeat this procedure, at the paragraph above that starts, "I do this by putting 2 TBSP...". It is a rare starter that won't revive after one or two cycles through this procedure. I would not go through more than 4 cycles. Uncommon as it is, at some point you have to decide that starting a new starter would be easier than trying to raise the dead.
If, like me, you hate throwing away starter, you can use the rest of the starter to make pancakes, waffles, biscuits, or pizza shells. More often than not, a really slow (or dead) starter is very acidic and will work well with baking soda.