Food Freezing Techniques: Theory and Application in Food Preservation

Food freezing is one of the most effective and widely used methods of food preservation. By lowering the temperature of a food product well below its freezing point, we can significantly slow down microbial growth, enzymatic activity, and chemical reactions that cause spoilage. Whether it’s frozen peas from the supermarket or flash-frozen seafood shipped across continents, the science behind freezing is what makes safe, long-term food storage possible.

How food freezing works: the basic science 🔗

At its core, food freezing involves removing heat energy from a product until the water inside it crystallizes into ice. But it’s not as simple as just making things cold. The process relies on two key thermodynamic concepts: sensible heat and latent heat.

Sensible heat is the energy removed to bring the food’s temperature down to its freezing point. You can “sense” this change because the temperature drops. Once the food reaches its freezing point, something different happens – the water begins to change phase from liquid to solid. This phase change requires the removal of latent heat, which is the energy needed to convert water into ice without any change in temperature. For pure water, the latent heat of fusion is approximately 334 kJ/kg. In food products, this value depends on the moisture content of the specific food item.

After most of the freezable water has turned into ice, sensible heat removal resumes to bring the product down to its final storage temperature, typically −18°C or lower.

Freezing point depression in foods 🔗

Unlike pure water, which freezes at 0°C, most foods begin to freeze at slightly lower temperatures – usually between −0.5°C and −2.2°C. This happens because foods contain dissolved sugars, salts, proteins, and other solutes that depress the freezing point. As ice crystals form, the remaining unfrozen water becomes more concentrated with solutes, continuously pushing the freezing point even lower. This is why the freezing process in foods happens over a range of temperatures rather than at a single fixed point.

Why ice crystal size matters for food quality 🔗

The size and distribution of ice crystals formed during freezing have a major impact on the final quality of the frozen food. This is one of the most critical aspects of food freezing technology.

When freezing happens slowly, fewer nucleation sites form within the food. Water molecules migrate to these limited sites, resulting in large ice crystals. These big crystals can rupture cell walls and damage the cellular structure of the food. When such food is thawed, it often turns mushy, loses excessive moisture (called drip loss), and has a degraded texture.

Rapid freezing, on the other hand, creates many nucleation sites simultaneously. Water molecules spread across all these sites, forming numerous small ice crystals that cause minimal damage to cell structures. This is why rapidly frozen foods tend to retain better texture, colour, flavour, and nutritional value after thawing. As noted by food science researchers, rapid cooling causes many nucleation sites to appear, preventing large crystal build-up at any single point.

The critical temperature zone of 0°C to −5°C is where most ice crystal formation occurs. Passing through this zone as quickly as possible is the goal of all commercial freezing systems.

Freezing systems: indirect vs direct contact methods 🔗

Commercial freezing equipment can be broadly classified into two categories based on how the food interacts with the cooling medium: indirect contact systems and direct contact systems.

Indirect contact freezing 🔗

In indirect contact freezing, the food product and the refrigerant are separated by a physical barrier – typically a metal plate or wall. The food never touches the refrigerant directly. According to the University of British Columbia’s food science curriculum, food is placed on belts or trays while a refrigerant circulates through a wall or plate beside it, cooling the food through conduction.

Common types of indirect contact freezers include:

Plate freezers – These consist of flat, hollow metal plates through which a refrigerant (often ammonia) circulates. Food packages are placed between the plates, and pressure is applied to ensure good thermal contact. Plate freezers are highly efficient for flat, uniformly shaped products like fish fillets, meat patties, and block-shaped packaged goods. They provide fast heat transfer but are limited to regular-shaped products.

Air-blast freezers – These use fans to circulate cold air (typically −30°C to −40°C) at high velocities over food products. The moving air strips away heat from the food surface through convection. Air-blast systems come in various configurations, including tunnel freezers (where food moves through on a conveyor belt) and spiral freezers (which use a spiralling belt to maximise capacity in a compact space). They are highly versatile and can handle a wide range of products, from meat carcasses to packaged meals.

Scraped surface freezers (slush freezers) – These are used exclusively for liquid or semi-liquid food products. The classic example is the ice cream maker, where a rotating scraper keeps the product moving against a cold surface, promoting rapid, uniform freezing and small crystal formation.

Direct contact freezing 🔗

In direct contact systems, the food comes into direct contact with the cooling medium, resulting in much faster heat transfer rates.

Immersion freezing – The food is submerged in a cold liquid medium such as brine, glycol solution, or a salt-sugar mixture. Because the entire food surface is in contact with the coolant, heat transfer is extremely fast and uniform. This method works well for irregularly shaped items like poultry pieces. One challenge is the potential dilution or contamination of the freezing solution by the food product, which can alter the process parameters.

Cryogenic freezing – This involves spraying or immersing food in liquefied gases, most commonly liquid nitrogen (boiling point: −196°C) or carbon dioxide (−78°C). The FAO notes that cryogenic freezing is a relatively newer approach where food is exposed to temperatures below −60°C through direct contact with liquefied gases. The extreme cold produces ultra-rapid freezing, resulting in very small ice crystals and excellent quality retention. Cryogenic freezers are commonly used for high-value products like premium seafood, delicate fruits, and prepared meals.

Individual quick freezing (IQF): a game changer 🔗

Among all commercial freezing techniques, Individual Quick Freezing (IQF) stands out as one of the most important innovations in modern food processing. As the name suggests, IQF involves freezing individual pieces of food separately and rapidly, so they remain loose and free-flowing rather than clumping into a solid block.

According to available industry documentation, IQF is widely used for smaller food items such as berries, diced vegetables, shrimp, small fish, pasta, cheese, and grains. The short freezing time prevents the formation of large ice crystals, helping the product retain its shape, colour, taste, and nutritional value far better than block freezing.

How IQF works 🔗

IQF systems generally work by transporting food items into a freezer on a processing line belt or infeed shaker. Inside the freezer, the product passes through a freezing zone where cold air or cryogenic gases rapidly lower its temperature.

There are two main types of IQF technology:

Mechanical IQF freezers – These operate on the principle of cold air circulation. Air flows from underneath the bed plate or conveyor belt, passing through and around the individual food pieces. Fluidised bed freezers are a key example, where high-velocity cold air actually suspends the food particles, ensuring each piece is frozen individually while preventing sticking. This method is ideal for products prone to clumping, such as peas, diced carrots, french fries, and small seafood.

Cryogenic IQF freezers – These immerse or spray the product with liquid nitrogen at extremely low temperatures. While they offer excellent freezing results, operating costs can be higher per unit weight due to the cost of the liquid nitrogen consumed.

Benefits of IQF 🔗

The advantages of IQF technology go well beyond just faster freezing. Because each piece is frozen separately, consumers and food service operators can thaw only the exact quantity they need – reducing food waste significantly. The rapid freezing process also preserves cellular integrity, meaning the product looks, tastes, and feels closer to fresh after thawing. This is particularly valuable for fruits and berries, where texture is easily compromised by conventional freezing methods.

IQF products are also commonly used as a pre-treatment before freeze-drying, since both methods prioritise preserving the size, taste, and cell structure of the food.

Factors affecting freezing time and quality 🔗

Several factors determine how long the freezing process takes and how well the product quality is maintained:

Product thickness and shape – Thinner products and those with greater surface area (like spheres) freeze faster than thick, flat slabs. A thinner fish fillet, for instance, will reach its target temperature much quicker than a whole fish carcass.

Freezing medium temperature – The colder the freezing environment, the faster the heat can be removed. Air-blast freezers typically operate between −30°C and −40°C, while cryogenic systems can go as low as −196°C.

Convective heat transfer coefficient – This measures how efficiently heat moves from the food surface to the surrounding medium. Higher air velocities, direct liquid contact, and cryogenic media all increase this coefficient, leading to faster freezing. As Engineering LibreTexts explains, the convective heat transfer coefficient is a property of the freezing equipment rather than of the food itself – plate and immersion freezers achieve much higher values than still-air systems.

Packaging – Packaging materials add resistance to heat transfer. Cardboard cartons, for example, significantly slow down freezing compared to thin plastic wraps. Engineers must factor in packaging thermal conductivity when designing freezing processes.

Initial product temperature – Starting with a pre-cooled product reduces the total energy that must be removed during freezing.

Frozen storage and maintaining the cold chain 🔗

Freezing the product properly is only half the battle. Maintaining quality during storage and distribution is equally important. The recommended storage temperature for most frozen foods is −18°C or below. Temperature fluctuations during storage are particularly damaging – they cause small ice crystals to melt partially and then refreeze into larger crystals, progressively degrading product texture.

Freezer burn is another common storage issue. It occurs when food is improperly packaged or exposed to air, leading to surface dehydration and oxidation. The result is dry, discoloured patches that affect both texture and flavour. Using airtight, moisture-resistant packaging is the best defence against freezer burn, as Ohio State University Extension recommends.

The cold chain – the entire temperature-controlled supply chain from processing plant to consumer – must remain unbroken to ensure frozen food safety and quality. Any break in the cold chain accelerates quality deterioration and can create food safety risks.

Emerging freezing technologies 🔗

Research continues to advance freezing technology beyond traditional methods. Pressure-shift freezing uses high pressure to supercool water throughout the food uniformly before releasing the pressure to trigger instantaneous, uniform nucleation. This produces very small, evenly distributed ice crystals.

Ultrasound-assisted freezing uses acoustic energy to control ice nucleation and crystal growth, potentially reducing freezing time while improving crystal uniformity.

One of the more novel approaches is isochoric freezing, developed by USDA researchers. In this method, food is placed inside a sealed rigid chamber filled with an aqueous solution, which is then cooled. As the USDA Agricultural Research Service describes, ice forms only in the solution outside the food – not within it. The food is preserved at subfreezing temperatures without any internal ice crystal formation, potentially eliminating texture damage entirely. This technology shows special promise for delicate products like fresh berries and certain fish products that traditionally suffer quality loss from conventional freezing.

What do you think? Given that rapid freezing consistently produces better product quality, why do you think slow-freezing methods are still commonly used in many parts of the world? And as emerging technologies like isochoric freezing become more practical, how might they reshape the frozen food industry – especially for perishable fruits and vegetables in tropical regions?

References

1. https://eng.libretexts.org/Bookshelves/Biological_Engineering/Introduction_to_Biosystems_Engineering_(Holden_et_al.)/06:_Processing_Systems/6.01:_Freezing_of_Food
2. https://www.fao.org/4/y5979e/y5979e03.htm
3. https://foodsciencetoolbox.com/cold-preservation-of-foods-freezing/
4. https://wiki.ubc.ca/Course:FNH200/Lessons/Lesson_07/Page_07.3
5. https://en.wikipedia.org/wiki/Individual_quick_freezing
6. https://www.messer-us.com/blog/mechanical-vs-cryogenic-iqf-guide
7. https://ohioline.osu.edu/factsheet/hyg-5341
8. https://tellus.ars.usda.gov/stories/articles/isochoric-freezing-future-food-preservation