Understanding the Freezing Point of Foods in Preservation

Freezing is one of the most reliable methods of food preservation – but it’s far more complex than just lowering a temperature dial. One of the most important concepts to grasp in post-harvest management is that foods do not freeze at the same temperature as pure water. The presence of dissolved substances inside food – sugars, salts, acids, and proteins – means that every food has its own unique freezing point, and understanding this is critical to preserving quality from field to freezer.

Why foods don’t freeze at 0°C 🔗

Pure water freezes at 0°C (32°F). Food, however, is not pure water. It is a complex mixture of water, proteins, carbohydrates, fats, minerals, organic acids, and other dissolved substances. According to the University of Illinois Extension, although water freezes at 32°F, not all foods freeze at that temperature because most food contains a combination of substances – including air, fiber, sugar, and water – that collectively alter the freezing behavior of the product.

The underlying science is known as freezing point depression – a well-established colligative property of solutions. A colligative property is one that depends on the concentration of dissolved particles in a solution, not on what those particles are. When any solute – whether salt, sugar, or acid – is dissolved in water, it interferes with the water molecules’ ability to lock into a crystalline ice structure. This disruption means that more energy must be removed from the solution to freeze it, effectively lowering the temperature at which freezing begins.

Research in biosystems engineering confirms that the initial freezing point of most foods typically falls between −0.5°C and −2.2°C – well below the freezing point of pure water. The reason is that the water inside foods is not pure: as heat is removed and freezing begins, the remaining liquid becomes more concentrated with dissolved solutes, which continues to depress the freezing point further.

The role of organic and inorganic substances 🔗

The dissolved substances in food fall into two broad categories – organic and inorganic – and both play a significant role in depressing the freezing point.

Inorganic substances: salts and minerals 🔗

Inorganic salts such as sodium chloride, potassium chloride, and other mineral salts are among the most effective freezing point depressants. Sodium chloride, for instance, can depress the freezing point of water to approximately −21°C at sufficient concentrations. The reason ionic compounds are especially powerful in this role is that they dissociate into individual ions in solution, effectively doubling or tripling the number of dissolved particles. For example, one molecule of sodium chloride splits into a sodium ion and a chloride ion – creating two particles that both contribute to freezing point depression, compared to the single particle contributed by a sugar molecule.

This is why foods with naturally high salt or mineral content – seafood, cured meats, pickled vegetables – tend to have lower freezing points than fresh fruits or plain vegetables. The mineral composition of food is, therefore, a key variable when designing freezing protocols in post-harvest and food processing operations.

Organic substances: sugars and acids 🔗

Organic compounds – primarily sugars and organic acids – are equally important in determining a food’s freezing point. Sugars such as glucose, fructose, and sucrose are naturally present in fruits and many vegetables. The higher the sugar concentration in a food, the greater the freezing point depression. This is why high-sugar fruits like grapes, mangoes, and cherries freeze at considerably lower temperatures than low-sugar produce like lettuce or cucumbers.

Organic acids – such as citric acid in citrus fruits, malic acid in apples, and lactic acid in dairy products – also contribute to the total solute concentration, further lowering the freezing point. The combined effect of multiple solutes leads to a greater overall freezing point depression, which is why foods with complex chemical compositions (like dairy products, meat, or fruit juices) can be significantly harder to freeze solid compared to simpler, more water-rich foods.

Freezing points of different food categories 🔗

Because each food has a different composition of dissolved substances, each food has a distinct freezing point. Understanding these differences matters for setting correct storage temperatures and choosing the right freezing approach.

Fruits 🔗

Fruits vary widely in their freezing points based on sugar and acid content. High-sugar fruits have markedly lower freezing points than low-sugar varieties. Strawberries, for instance, have an initial freezing point of around −0.78°C. Watermelon, with lower sugar concentration, freezes closer to 0°C, while grapes, being high in sugars, freeze at a noticeably lower temperature. This variation directly impacts how fruits must be handled before and during freezing to preserve their quality.

Vegetables 🔗

Most vegetables have a relatively high water content and low dissolved solid concentration, which places their freezing points close to – but still slightly below – 0°C. Fresh fruits and vegetables have a very high water content held within rigid plant cell walls that provide structure. Leafy greens like lettuce and spinach, with their exceptionally high water content and low solute levels, freeze close to 0°C but are also among the most susceptible to cell damage during the process.

Meat and seafood 🔗

Meats typically freeze at around −1°C to −2°C due to the presence of dissolved proteins and natural salts within muscle tissue. The freezing of water in meat can be summarized into three distinct chronological stages: cooling to its freezing point, the phase transition during which latent heat is removed, and finally reaching the storage temperature. Seafood – particularly saltwater fish and shellfish – often has an even lower freezing point due to higher natural salt content.

The freezing process: from initial temperature to storage 🔗

Freezing a food product is not a single-step event. It is a thermal process that involves progressively removing heat from the product across multiple stages. The freezing process entails three phases: cooling to the freezing point, extracting latent heat to form ice crystals, and continued cooling below freezing for optimal storage.

In the first stage – the precooling stage – the product is cooled from its initial ambient or harvest temperature down to its initial freezing point. No ice is forming yet; the food is simply losing sensible heat. In the second stage – the phase transition stage – water begins converting to ice and latent heat is released. This is the most energy-intensive phase because large amounts of heat must be removed to convert liquid water to ice without a significant further drop in temperature. Finally, in the subcooling stage, once the bulk of the water has frozen, the temperature of the food continues to drop until it reaches the intended storage temperature, typically between −17°C and −30°C for commercial frozen storage.

Freezing time is defined as the time required to lower a product’s temperature from its initial temperature to a given temperature at its thermal center, with the geometrical center of the product used as the reference point. The time it takes to complete this process depends on the product’s dimensions, its thermal properties, the initial and final temperatures, and the efficiency of the freezing equipment.

What happens inside food cells during freezing 🔗

When heat is removed from food during freezing, the water inside begins to crystallize – but not uniformly. The extracellular fluid, being less concentrated than the cell contents, starts to freeze first. As extracellular water converts to ice, the remaining liquid becomes more concentrated. This creates an osmotic gradient: water inside the cells is drawn outward through the cell membrane, where it too eventually freezes.

The rate of freezing has a direct bearing on the size of ice crystals formed and, therefore, on the integrity of the food’s cellular structure. Rapid freezing produces smaller ice crystals that cause less damage to cell walls, while slow freezing produces larger crystals that puncture cell membranes and result in a softer, waterlogged texture after thawing. Large ice crystals associated with slow freezing tend to rupture the cells, causing an undesirable texture change, which is why fast freezing is strongly preferred in both commercial and home preservation settings.

Fluctuating storage temperatures compound this problem further. When frozen food warms slightly and then refreezes, small ice crystals merge into larger ones – a process called recrystallization – causing additional cellular damage over time. Each food will freeze differently because of the substances present within it – its water content, sugar levels, protein structure, and mineral concentration all interact to determine both when freezing begins and how the process unfolds inside the tissue.

Practical implications for post-harvest management 🔗

Understanding the freezing point of foods has real consequences for how agricultural produce is handled after harvest. Freezing preservation retains the quality of agricultural products over long storage periods and is generally regarded as superior to canning and dehydration with respect to retention of sensory attributes and nutritive value. However, this advantage is only realized when the product is frozen correctly – starting with high-quality raw material, freezing rapidly, and maintaining a consistent, sufficiently low storage temperature.

Because different foods have different freezing points, storage facilities must be designed or calibrated to ensure that temperatures are held low enough to keep the product fully frozen without unnecessary energy expenditure. Freezing keeps food safe almost indefinitely, but quality is still affected by storage temperature, time, and packaging – making it critical to understand the chemistry of each food being preserved rather than applying a one-size-fits-all approach.

What do you think? Given that every food has a slightly different freezing point based on its composition, how should post-harvest managers adjust their freezing protocols for different crops or product types? And how might the sugar or salt content of a food at the time of harvest – influenced by variety, maturity, or growing conditions – affect the effectiveness of freezing as a preservation method?

References

1. https://extension.illinois.edu/food-preservation/freezing
2. https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/Introductory_Chemistry_(LibreTexts)/13:_Solutions/13.09:_Freezing_Point_Depression_and_Boiling_Point_Elevation
3. https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/16:_Solutions/16.13:_Freezing_Point_Depression
4. https://eng.libretexts.org/Bookshelves/Biological_Engineering/Introduction_to_Biosystems_Engineering_(Holden_et_al.)/06:_Processing_Systems/6.01:_Freezing_of_Food
5. https://en.wikipedia.org/wiki/Freezing-point_depression
6. https://foodsafety.institute/food-fundamentals-chemistry/cooling-freezing-techniques-food-preservation/
7. https://www.numberanalytics.com/blog/ultimate-guide-to-freezing-point-depression
8. https://extension.umn.edu/preserving-and-preparing/science-freezing-foods
9. https://pmc.ncbi.nlm.nih.gov/articles/PMC8620417/
10. https://broudat-kar.com/lang/en/freezing-types/
11. https://www.fao.org/4/y5979e/y5979e03.htm
12. https://www.sciencedirect.com/topics/engineering/frozen-storage
13. https://extension.usu.edu/preserve-the-harvest/freezing-basics
14. https://www.ndsu.edu/agriculture/extension/publications/food-freezing-guide
15. https://extension.psu.edu/understanding-the-process-of-freezing
16. https://www.fsis.usda.gov/food-safety/safe-food-handling-and-preparation/food-safety-basics/freezing-and-food-safety