Hi Heidi, it's possible that your starter wasn't fully mature when you put it in the refrigerator, which would leave it more vulnerable to outside invaders. I also think that even mature starters that spend most of their time in the refrigerator can become more vulnerable over time if they aren't allowed adequate time at room temperature to begin fermenting after a feeding and before refrigeration. In other words, if you tend to feed your starter and then stick it right in the refrigerator, you may end up maintaining a starter that is mostly unfermented flour and water. Such a starter isn't getting the full benefit of fermentation, and isn't likely developing enough acidity to ward off  the bad bacteria. The remedy is to offer your starter more time at room temperture (at least until it begins bubbling) before putting it in the refrigerator, and optimally incorporating a day or two of twice a day feeding at room temperature when you take it out of the refrigerator and are planning to bake bread. If you haven't seen a return of the pink streak after a few days of twice a day feedings at room temperature you may have nipped this one in the bud and could continue with your old starter, but we do usually recommend discarding. I hope this helps! Let us know how it goes. 

Barb

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