Hi Nikarta, assuming you're still feeding your starter fairly consistently, I doubt the bitter flavor has anything to do with unwanted bacteria. Is it possible your whole wheat flour is out of date? Whole wheat flour can go rancid over time and this could contribute to an off flavor in your starter or bread. Once you've checked that your flour is fresh, I would resume twice a day feedings at room temperature and see if this improves the aroma and flavor of your starter and bread. Barb

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