Freezing and Thawing: What Happens to Food at the Molecular Level

Freezing is one of the oldest and most effective forms of food preservation. It’s also one of the most misunderstood. The common assumption is that freezing kills bacteria and makes food safe indefinitely. Both parts of that are wrong. Understanding what freezing actually does at the molecular level explains why it works, where its limits are, and why thawing safely is just as important as freezing safely.

What Freezing Does to Bacteria

Bacteria need liquid water to carry out the chemical reactions of life. Metabolism, reproduction, toxin production: all of it depends on dissolved molecules moving through liquid water in the cell.

When you freeze food, ice crystals form in and around bacterial cells. The available liquid water drops to nearly zero. Bacterial metabolism essentially stops. Growth and reproduction stop. The bacteria enter a kind of suspended animation.

Here’s the critical distinction: freezing stops bacterial activity, but it doesn’t kill bacteria. Think of bacteria in a frozen environment like seeds in winter. The seeds aren’t dead. They’re dormant. Spring comes, conditions become favorable, and they become active again. The same thing happens to most bacteria when food thaws. They pick up where they left off.

Some bacteria are damaged or killed by ice crystal formation, which physically punctures cell membranes. But the more dangerous food pathogens (Salmonella, E. coli O157:H7, Listeria, Staphylococcus aureus) are hardy enough to survive freezing in large numbers. Listeria monocytogenes can even grow (slowly) at 29°F / -2°C, which is barely below freezing.

What this means in practice: if raw chicken had 10,000 Salmonella bacteria per gram before you froze it, those same bacteria are there when you thaw it. If you thaw it improperly and it spends hours in the temperature danger zone, those bacteria multiply. The freezing period bought you time. It didn’t give you a clean slate.

Ice Crystal Formation and Food Quality

While freezing preserves food safely, it affects quality through ice crystal formation. This is where commercial freezing and home freezing diverge significantly.

When water in food freezes, it forms ice crystals. The size of those crystals matters enormously to food texture.

Slow freezing (like placing food in a home freezer) produces large ice crystals. As ice crystals grow, they puncture and expand cell walls. Plant cells have rigid walls that crack. Muscle fibers in meat get disrupted. When the food thaws, those broken cells release their liquid. You see this as the pink fluid (called “drip loss”) in the bottom of a package of thawed meat, or as the soft, waterlogged texture of strawberries thawed from a home freezer.

Fast freezing produces small ice crystals. Small crystals cause less physical damage to cell structures. The food retains more of its original texture. This is the principle behind flash freezing in commercial operations, and why commercially frozen strawberries tend to hold up better than home-frozen ones.

Commercial IQF (individually quick frozen) technology exposes individual pieces of food (peas, corn kernels, shrimp, blueberries) to extremely cold temperatures very rapidly. The result is small ice crystals, minimal cell damage, and much better quality on thawing.

At home, you can approximate fast freezing by spreading food in a single layer on a baking sheet, freezing until solid, and then transferring to a storage bag. This exposes more surface area to cold air and freezes the food faster than a dense block would.

Freezer Burn: What’s Actually Happening

Freezer burn is water leaving food through a process called sublimation. Under the right conditions (cold and dry), ice can convert directly from solid to vapor without going through a liquid stage. This is the same process that makes dry ice disappear. The solid CO₂ goes directly to gas.

In a freezer, ice crystals at the surface of food sublimate into the freezer air. The food loses water directly from its surface. What remains is dried out, discolored, and concentrated in ways that affect flavor. Meat develops a grayish-white crust. Ice cream gets a crunchy icy surface. Fruits and vegetables become leathery.

Freezer burn doesn’t make food unsafe. It’s purely a quality problem. The fix is preventing air contact: wrap food tightly in freezer wrap or heavy-duty foil, use proper freezer bags with the air pressed out, or use a vacuum sealer for long-term storage.

Optimal Freezer Temperature

The standard recommendation for home freezer temperature is 0°F / -18°C. At this temperature, bacterial growth is completely halted and enzyme activity (which causes quality degradation over time) is very slow.

A freezer at 10°F or 15°F isn’t as effective. At 10°F (-12°C), bacterial growth is still halted, but enzymatic changes continue faster, degrading quality more quickly. A freezer that cycles between 0°F and 20°F due to frequent opening or mechanical problems is less effective at quality preservation than a stable 0°F.

The relationship between temperature and food quality in the freezer follows an Arrhenius-type relationship: roughly, every 18°F (10°C) increase in temperature doubles the rate of chemical reactions. Colder freezers preserve quality dramatically longer.

Safe Thawing Methods

Thawing is when the safety issues with frozen food actually occur. As food warms toward 40°F (4°C) and into the temperature danger zone above that, bacteria resume growing.

Refrigerator thawing is the safest method. Food thaws slowly in an environment that stays below 40°F throughout the process. Bacteria resume activity slowly, but because the temperature stays below the danger zone, growth is minimal. Food thawed this way can be safely refrozen. The downside is time. A whole chicken can take 24 to 48 hours to thaw in the refrigerator.

Cold water thawing works faster. Submerge food in its leakproof packaging in cold tap water. Change the water every 30 minutes to maintain cold temperature and continue the thawing process. Small packages (1 pound or less) may thaw in an hour. Larger items take 2 to 3 hours. Cook immediately after cold water thawing. The outer surfaces of the food approach the danger zone briefly during this process, so it’s not suitable for refreezing without cooking first.

Microwave thawing is fast but uneven. Microwaves heat unevenly, and some portions of the food will begin cooking before the center thaws. Cook immediately after microwave thawing. Don’t microwave-thaw chicken breasts and then put them back in the refrigerator.

Counter thawing is not safe. Room temperature is in the middle of the temperature danger zone (40-140°F). The outside of a large piece of food reaches dangerous temperatures while the center is still frozen. A large roast thawing on a counter can have its exterior sitting at 60-70°F for hours while the center thaws. That’s ideal conditions for bacterial growth on those surfaces. The temperature danger zone explains in detail why this specific range accelerates bacterial growth so dramatically.

The Refreezing Question

You can safely refreeze food that was thawed in the refrigerator, even if it was never cooked. The food quality will suffer another round of ice crystal damage, but it’s safe.

You should not refreeze food that was thawed in cold water or the microwave without cooking it first. Those thawing methods allow brief periods of surface warming, and refreezing without cooking first locks in whatever bacterial growth occurred during thawing.

Cooked food can be frozen, thawed, and refrozen without the same safety concerns, because the cooking step eliminated most of the bacterial load. Quality will decrease with each cycle, but safety is maintained as long as thawing is done properly.

The overarching principle is straightforward: bacteria are always present, freezing only pauses them, and every time food enters the temperature danger zone, the clock starts again.