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Putting bread in the refrigerator can slow mold growth, but it can also accelerate the chemical changes responsible for staling.
Starch molecules reorganize during storage, making the crumb firmer and less pleasant to eat.
Freezing is generally better for preserving bread for longer periods because it slows these changes much more effectively. This creates a strange storage trade-off: the refrigerator can help with one problem while making another worse. (USDA)
© USDA Food Safety and Inspection Service