Sourdough starters are popular because they are easy to create and easy to share with friends and co-workers. Unfortunately they can also pose a significant health risk if not handled properly.
Sourdough bread is prized for its tangy, acidic taste that is produced by the spontaneous fermentation caused by the interaction of the yeast and sugar in the starter with the flour and other ingredients in the recipe.
Typical directions for a sourdough starter encourage the baker to leave the starter un-refrigerated for a period of up to 10 days.
According to the United States Department of Agriculture leaving a sourdough starter in an un-refrigerated state for an extended period of time can allow numerous harmful organisms, including salmonella, to grow in the mixture.
Sourdough bread starters can be refrigerated, which will prevent the growth of harmful organisms, but this will also slow down the fermentation process.
Any sourdough starter that begins to give off an odd or offensive smell or appears slimy has gone bad and should be thrown out.