Why Food Goes Stale: Water Activity, Starch Retrogradation, and Moisture Migration

Stale bread and soft crackers are opposites — one has lost something it needed, the other has gained something it didn’t want. But they’re both the same problem at the molecular level: water moving from where it is to where it isn’t.

Water activity is the concept that ties it all together, and it’s more useful than moisture content for predicting exactly what will happen to any food over time.

Water Activity: The Number That Predicts Shelf Life

Water activity (Aw) is measured on a scale from 0 to 1. Pure water is exactly 1.0. Completely anhydrous (zero water) food would be 0. Most foods fall somewhere between 0.6 and 0.99.

The number isn’t about how much water is present. It’s about how available that water is to move and participate in reactions. Dissolved sugars and salts bind water molecules, reducing Aw below what the total moisture content would suggest. Honey has roughly 17-20% moisture content, but its Aw is around 0.6 because the high sugar concentration ties up most of the water. At Aw below 0.6, most bacteria can’t grow. Below 0.85, most molds are suppressed. Below 0.7, even xerophilic (drought-tolerant) fungi struggle.

Think of Aw like the humidity in a microclimate around the food. High Aw food creates a humid local environment. Low Aw food creates a dry one. Water always moves from humid to dry — from high Aw to low Aw — until equilibrium is reached.

How Bread Stales: Two Things Happening at Once

Most people assume bread stales because it dries out. That’s partly true, but it misses the more important mechanism.

Starch retrogradation is the main driver of staling. When bread bakes, starch granules gelatinize: they absorb water and swell, rupturing their crystalline structure into a soft, amorphous gel. The bread’s soft crumb texture is largely this gelatinized starch network. But starch isn’t stable in that state. Over time, the amylose and amylopectin chains that make up starch slowly reorganize and recrystallize into more ordered structures. This is retrogradation.

Retrograded starch is firmer and drier in texture than freshly gelatinized starch. The recrystallized regions don’t hold water the way the gelatinized regions did, so they also release moisture as they form. The crumb becomes firmer and crumbly — the texture of stale bread — not because moisture has escaped to the environment, but because the starch structure has changed internally.

Moisture loss to the environment is the second factor. The crumb’s Aw is higher than dry room air, so moisture evaporates from the cut surface and through the crust over time. This drying worsens the texture, but even bread kept in a sealed bag stales because retrogradation happens regardless of whether moisture escapes.

Why the Fridge Is the Worst Place for Bread

Retrogradation has a temperature sweet spot. It’s fastest at temperatures just above freezing — roughly 35-40°F. That’s fridge territory.

At room temperature (68-72°F), amylose and amylopectin chains have enough thermal energy to resist tight reorganization. Retrogradation still happens, but more slowly. At refrigerator temperature, the chains are cold and mobile enough to pack tightly but not so cold that they’re locked in place — the worst of both worlds for staling rate.

Freezing is different. Below 0°F, molecular mobility drops to near zero and retrogradation essentially stops. Frozen bread preserved with good wrapping tastes nearly fresh when thawed because the starch chains never had a chance to recrystallize. The thawing process briefly rehydrates the starch structure, restoring softness.

The practical rule: refrigerator for the benefit of suppressing mold, but at the cost of accelerated staling. Room temperature for better texture over 2-3 days. Freezer for anything beyond that.

Why Crackers and Chips Go Soft

Crackers, chips, and crispy snacks have low water activity. They’re manufactured to have Aw typically below 0.4. At this level, they’re shelf-stable for months because bacteria and molds can’t grow.

The problem is the other direction: moisture absorption from the environment.

When the ambient humidity creates an effective Aw in the surrounding air that’s higher than the cracker’s Aw, water molecules migrate into the cracker. As moisture accumulates in the starch matrix, the glass-transition temperature of the starch drops. In a dry state, starch is glassy — rigid and brittle, which is what gives a cracker its snap. Add enough moisture and the starch transitions from the glassy state to a rubbery state. The texture changes from crisp to soggy.

This is why an open bag of chips on a humid summer day goes soft faster than the same chips on a dry winter day. The driving force for moisture absorption is the difference between the chip’s Aw and the ambient humidity. High ambient humidity means faster moisture migration into low-Aw snack foods.

An airtight container dramatically slows this by removing the moisture source. It doesn’t eliminate the process (the container’s internal air still has some humidity) but it reduces it to a timescale of days instead of hours.

Cookies, Bread in a Jar, and the Equilibration Problem

Here’s the practical application of Aw equilibration that catches people off guard.

A soft chocolate chip cookie has Aw around 0.85-0.9. A dry cracker or biscotti has Aw around 0.3-0.5. Fresh bread crumb has Aw around 0.95-0.97.

Store a fresh cookie next to bread in a sealed container and both reach equilibrium eventually — the cookie absorbs moisture from the bread until both are at the same Aw. The cookie doesn’t gain moisture from nowhere; it pulls it from the higher-Aw bread. The bread loses some moisture to the cookie, speeding its own staling.

The same thing happens in reverse if you store a dry cookie with something dry. The cookie loses moisture faster to a dry environment than to a neutral one.

Fruit (very high Aw, close to 0.99) accelerates cookie and cracker softening dramatically if stored in the same container. A slice of apple placed in a cookie jar is an old trick for keeping cookies soft — it works because the apple is continuously supplying moisture to the dry cookies through the enclosed air.

The useful flip side of all this: a chunk of stale bread placed in a sealed container with fresh cookies will accelerate the cookies’ softening, not help them. Mixing different-Aw foods in the same enclosed space always drives them toward an equilibrium neither started at.