Blast Freezing Time: 7 Critical Factors for Efficiency

Blast Freezing Time is the time required for a product to reach a specified core temperature from its initial temperature. Product thickness, composition, packaging, entry temperature, airflow and the system’s capacity under actual operating conditions must be evaluated together. A reliable time estimate combines engineering calculations with product temperature measurements under representative loading conditions.

Before answering “How many hours does it take to freeze one tonne of product?”, the preparation and loading arrangement must be defined. One tonne of thin portions and one tonne of thick blocks should not be assumed to require the same processing time.

Prepared by: Hamza Ilıman
Last updated: 30 September 2026

Table of Contents

1. Blast Freezing Time and the target temperature
2. Seven critical factors affecting freezing time
3. Calculation method and worked example
4. Daily production capacity planning
5. Recommended and not recommended practices
6. Loaded testing and process records
7. Project evaluation with TunelGroup
8. Frequently asked questions

Blast Freezing Time and the Target Temperature

The endpoint of a freezing cycle should be assessed at the product’s thermal centre. This is the point that remains warmest at the end of freezing; it does not necessarily coincide with the geometric centre under every product and loading condition.

For quick-frozen foods within its scope, Codex CXC 8-1976 specifies a thermal-centre temperature of −18°C or colder after temperature stabilization. Product quality and storage specifications may require a lower target. This temperature must not be confused with the room’s air-temperature setting. www.fao.org

For example, an air temperature of −35°C does not demonstrate that every package has reached −18°C at its core. Evaluating Blast Freezing Time requires records of both air and product temperatures.

7 Critical Factors Affecting Blast Freezing Time

1. Product Thickness

Removing heat from the centre of a thick product takes longer. Doubling the thickness does not necessarily mean that freezing time will only double. Maximum product thickness should therefore be specified alongside product weight.

2. Product Composition

Different formulations behave differently during freezing. The same heat-removal value per kilogram should not automatically be used for meat, fruit purée and a sweetened product.

ASHRAE explains that the thermal properties of foods depend on their composition and temperature. handbook.ashrae.org

3. Initial and Final Temperatures

A warmer incoming product requires more heat to be removed. The entry temperature stated in the quotation should represent the conditions measured during production.

4. Packaging

Cartons, lids and trapped air within packages influence heat transfer. Testing should use the packaging intended for commercial production.

FAO’s freezing-time guidance identifies packaging and product thickness among the principal variables affecting the process. www.fao.org

5. Air Conditions

Air velocity at the fan outlet does not, by itself, describe the velocity experienced by the product. Assessment should include airflow through the loaded racks.

6. Loading Arrangement

Air can bypass the load through surrounding open spaces. Baffles and rack clearances should direct it through the gaps between products.

7. Actual Refrigeration Capacity

Compressor horsepower alone is insufficient. Refrigeration capacity should be compared at stated refrigerant, evaporating and condensing conditions. www.fao.org

How Is Blast Freezing Time Calculated?

The calculation involves two checks: the quantity of heat that must be removed and the rate at which heat can move from the product’s interior to its surface.

Calculate the Product’s Heat-Removal Requirement

The product heat-removal requirement can be calculated using the enthalpy difference between the initial and final conditions:

Heat to remove, kJ = Product mass, kg × Enthalpy difference, kJ/kg

The enthalpy difference should include both temperature reduction and the heat released as water freezes. A simple calculation based only on temperature difference can omit the phase-change requirement.

When the target time is known, the average refrigeration capacity required for the product is:

Average product refrigeration capacity, kW = Heat to remove, kJ ÷ (3,600 × time, hours)

Use enthalpy data appropriate to the product composition and temperature range. ASHRAE’s thermal properties guidance explains the relationship between enthalpy, sensible heat and latent heat. handbook.ashrae.org

Illustrative Calculation

The following values are assumptions used to demonstrate the method. They are not measured properties of a particular food or a TunelGroup performance commitment.

Total heat to remove = 1,000 × 250 = 250,000 kJ

Average product refrigeration capacity = 250,000 ÷ (3,600 × 4) = 17.36 kW

This result represents only the average product load. Heat entering through the room envelope, air infiltration, fans, packaging, trays and other loads must be evaluated separately.

If the other average loads, calculated over the same time period, are assumed to total 8 kW, the combined requirement becomes 25.36 kW. This is not sufficient on its own for final equipment selection: changing loads throughout the cycle and the actual operating conditions also require assessment.

These values describe thermal refrigeration capacity, not electrical input power.

Check Heat Transfer Through the Product

Having sufficient total capacity does not prove that the centre of a thick package will reach its target within four hours. A separate time assessment using product geometry and thermal properties is necessary.

Plank’s equation and more advanced freezing models can support preliminary estimates. Their assumptions, packaging effects and irregular product shapes influence the result. The calculated Blast Freezing Time should be verified through product trials. www.fao.org

For further technical background, see FAO’s guide to freezing fruits and vegetables.

How Does Blast Freezing Time Affect Daily Capacity?

Daily production planning must include loading, unloading and other required downtime in addition to freezing. The following schedule is an illustrative planning example, not a record from an operating facility.

Under these assumptions, a 20-hour available production window allows four completed cycles. If each cycle processes 1,000 kg, the scheduled capacity is 4,000 kg/day.

Quoting 5,000 kg/day based only on the four-hour freezing period leaves no time for the other activities. If the actual defrost and cleaning schedule requires more time, the daily target must be revised.

Recommended and Not Recommended Practices

Recommended

• Compare quotations using the same product and packaging specifications.
• State batch capacity as net kilograms of product.
• Identify the thickest product format separately.
• Include measurement locations and target temperatures in the acceptance criteria.
• Reassess the process when formulations or packaging change.

Not Recommended

• Using unconditional statements such as “every product freezes in four hours.”
• Treating an empty-room pull-down test as proof of product performance.
• Monitoring only the control-panel display instead of product temperature.
• Applying one trial result directly to different products.
• Quoting daily capacity without allowing for downtime.

This comparison provides a practical checklist for preparing project acceptance specifications.

How Is Blast Freezing Time Verified Under Load?

A commissioning record can include the following information:

Probe locations should represent products and load positions expected to cool most slowly. Recording several locations during initial trials helps establish the monitoring plan for routine operation.

The applicable food safety plan also needs separate consideration: freezing should not be treated as a process that eliminates all microorganisms. www.fao.org

Project Evaluation with TunelGroup

TunelGroup’s shock-type evaporators and refrigeration units are product groups that can be evaluated together when planning a system. Tunelgroup Cooling Systems

A quotation request should go beyond describing a “one-tonne blast freezer.” A more useful specification is:

We require a blast freezing system for … kg of net product per batch, entering at … °C, with a maximum product thickness of … mm, using … packaging, and a target process time of … hours to reach the specified core temperature.

Add the room dimensions, number of daily batches, project location and available electrical supply.

For application information, explore TunelGroup’s blast freezer for meat and blast freezer for seafood pages, or contact TunelGroup to discuss the project.

Frequently Asked Questions

Is Blast Freezing Time the Same for Every Product?

No. A freezing time is meaningful only when the product and operating conditions are defined together. Different thicknesses, packaging or entry temperatures require reassessment.

Is Compressor Power Enough to Specify a One-Tonne Freezer?

No. The product’s heat-removal requirement matters, but so does the rate at which heat can be removed from its centre.

Can Freezing Time Be Guaranteed if the Calculated Capacity Is Sufficient?

A capacity calculation alone should not be used to guarantee a freezing time. Any commitment should define the product, packaging, loading arrangement, operating conditions and acceptance test.

Where Should Products Be Held After Blast Freezing?

Products should be transferred promptly to frozen storage that meets the specified holding conditions. Transfer and storage arrangements should also be included in the production plan.