Zest Regards

Retrogradation and Staling

TL;DR: Don’t refrigerate bread, because while it increases the shelf life, it also causes the staling process (retrogradation) to happen at the fastest speeds possible.

What does it mean for bread to stale? It seems somewhat intuitive - bread dries out. So staling is essentially the dehydration of bread. The loss of moisture firms the starch networks and gluten matrix.

But dehydration is not the main process in staling. Mainly, staling is driven by retrogradation.

Retrogradation is the process of gelatinized starches crystallizing back into an insoluble form, which occurs over time and is sped up in a specific temperature range. This, in turn, results in further dehydration, as the water is no longer captured in the starches. Even when sealed airtight, bread will stale over time due to this process.

This process happens the fastest in the temperature range of 39°F to 45°F, which makes refrigeration the worst choice for prolonging a bread’s shelf life. But wait, why would crystallization happen faster as it gets cold, but then slow down again as it gets even colder? Recrystallization happens with two interoperating processes:

  1. Nucleation: short segments of starch (amylopectin) chains align to form tiny crystal “seeds.” This process does require molecular motion (though not much), and thus can happen at low temperatures - fastest at 32°F.
  2. Propagation: starch (amylopectin) chains dock onto the nucleation “seeds” to grow the ordered regions. This process requires molecular motion, and thus happens at higher temperatures - faster as temperature increases (up to 120°F)

Thus recrystallization happens fastest in this temperature range where both processes overlap most strongly. Well, technically it happens faster as temperature drops due to nucleation, but the frozen water and water-starch matrix prevents the formation of the crystals due to kinetics. As in, the crystal formation requires kinetic movement, which is slowed down and prevented by the ice crystals.

In plain language, this all means that refrigeration temperatures are the worst for bread.

Getting bread past this “temperature danger zone” of refrigeration as fast as possible reduces the amount of retrogradation, so slicing bread and aligning the slices in an airtight container with some spacing in between and freezing them will prove to be the best option for longer term storage. The smaller sizes of slices compared to the whole loaf and proper spacing means diffusion of heat happens faster, and thus the bread will reach a frozen temperature as quickly as possible.

A small caveat is that residential freezer temperatures will never quite reach glass transition temperature (below -4°F) which would truly lock the molecules in place to stop aging, staling, and crystal growth. So we are typically operating within a range of “acceptability.”

Bread storage will always have a tradeoff. Storing bread in an airtight container can reduce dehydration, but this is not always ideal for the crust. Storing bread in an airtight container in the refrigerator can increase the shelf life, but reduce the quality during that shelf life.

The best scenario is to store a bread in a flour sack towel at room temperature, and to use it within 2 days of baking.

Reheating stale bread will redistribute remaining water and melt the starch crystals, and is best between the temperatures of 160°F to 180°F. This will only work once because the heating process results in: