Locating Cold Spots in Food Containers for Accurate Heat Processing

Every time you open a can of food, you trust that its contents are safe to eat. That safety depends on one critical factor during processing: making sure every part of the food inside the container has received enough heat to destroy harmful microorganisms. The point inside the container that heats up the slowest – known as the cold point or cold spot – is the key to that assurance. If the cold point reaches the required temperature for the right duration, every other point in the container has already been adequately treated. Get it wrong, and you risk under-processing, which can lead to dangerous pathogens like Clostridium botulinum surviving inside a sealed can.

What exactly is the cold point? 🔗

The cold point is the location inside a food container that takes the longest to reach the target processing temperature. During thermal processing, heat travels from the outside of the container – where steam or hot water makes contact – inward through the container wall and then through the food. Some regions heat up quickly, while others lag behind. The last spot to reach the required temperature-time combination is the cold point. All thermal process calculations are based on the temperature history recorded at this slowest-heating location, because if adequate lethality is achieved there, the rest of the container is certainly safe.

Why the cold point matters for food safety 🔗

Commercial sterilisation processes are designed around the concept of F-value – the equivalent time (in minutes) at a reference temperature of 121.1 °C needed to achieve the desired level of microbial destruction. Regulatory standards, such as those in 21 CFR Part 113 by the U.S. FDA, require that low-acid canned foods (pH above 4.6) receive a minimum thermal process sufficient to destroy spores of C. botulinum. The standard benchmark is an F₀ of at least 3 minutes, meaning the cold point must accumulate that much sterilising effect. If the cold point is incorrectly identified – even by a small margin – the actual lethality delivered at the true slowest-heating zone could fall below the required threshold, resulting in a potentially hazardous product.

Overestimating the required process time to “play it safe” is also problematic. Areas of food closest to the container wall receive considerably more heat than the cold point. Excessive processing leads to nutrient degradation, texture breakdown, colour changes, and loss of overall product quality. As Britannica notes, regions farthest from the cold spot receive more severe heat treatment that can impair the overall quality of the product. Accurate cold point identification therefore strikes a balance between safety and quality.

Where is the cold point located? 🔗

The cold point location depends primarily on how heat moves through the food, which is dictated by the food’s physical properties and consistency.

Conduction-heated foods 🔗

Solid or semi-solid foods – think canned corned beef, thick pastes, or meat loaves – transfer heat almost entirely by conduction, where thermal energy passes molecule to molecule through the material. This is a slow mechanism. In a standard cylindrical can, heat enters equally from all sides (top, bottom, and the cylindrical wall), so the point farthest from every surface is the geometric centre of the container. That is where the cold point sits for conduction-heated products. Research from the University of British Columbia confirms this: conduction-heated foods in cylindrical containers have their cold point at the geometric centre.

Convection-heated foods 🔗

Liquid or thin semi-liquid foods – such as broth, fruit juices, or evaporated milk – heat primarily by convection. When the liquid near the container wall warms up, it becomes less dense and rises. Cooler liquid sinks to replace it, setting up circulation currents. These currents are weakest near the bottom of the container, where cooler liquid tends to settle. As a result, the cold point in convection-heated foods is typically found about one-third of the container height up from the bottom, along the central vertical axis. This is a well-established principle confirmed across USDA thermal processing training materials.

Mixed and broken-heating foods 🔗

Many real-world foods don’t fall neatly into one category. A can of vegetable soup with chunks of potato in broth heats by a combination of conduction (through the solid pieces) and convection (through the liquid). Some products exhibit what’s called broken heating – they start heating by convection, but as starch gelatinises or other physical changes occur, the product thickens and shifts to conduction heating partway through the process. In these cases, the cold point may actually migrate during processing as the dominant heat-transfer mechanism changes. Products like starchy soups or noodle preparations are classic examples of this behaviour.

How the cold point is determined experimentally 🔗

Identifying the cold point is not a theoretical exercise alone – it requires careful experimental measurement. The Canadian Food Inspection Agency (CFIA) provides detailed guidelines on conducting heat-penetration studies, and their protocol is representative of international best practices.

Thermocouples: the primary measurement tool 🔗

The most widely used devices for cold point determination are thermocouples. A thermocouple consists of two dissimilar metal wires (commonly copper and constantan, known as Type T) joined at one end. When this junction is exposed to a temperature different from the other end of the wires, a small voltage is generated proportional to the temperature difference. This voltage is recorded by a data logger connected to the other end.

To find the cold point, thermocouples are inserted at multiple locations within the food container – at the geometric centre, at one-third from the bottom, and at various other heights. Each thermocouple continuously records temperature throughout the entire retort cycle, including come-up time, holding time, and cooling. The location that consistently shows the slowest temperature rise is identified as the cold point.

Step-by-step cold point study 🔗

A systematic cold point determination study typically follows these steps:

Preparation: Food is prepared and filled into containers exactly as it would be in commercial production, because factors like fill weight, headspace, product consistency, and fill temperature all influence heating behaviour. The U.S. FDA’s inspection guide for low-acid canned foods emphasises that heat penetration work must cover all variables expected during commercial operations.

Thermocouple insertion: Multiple thermocouples are carefully inserted into each test container at predetermined positions. The insertion must create an airtight, watertight seal, and the sensing junction should be positioned precisely within the food. For solid products, the thermocouple may need to be impaled directly into the food material. Care is taken to avoid disturbing the product’s natural packing arrangement, as this could alter heat-transfer characteristics.

Processing and data collection: Containers are loaded into a retort at positions identified as the slowest-heating locations (determined through prior temperature-distribution studies). The retort is operated under conditions that simulate commercial production. Temperature data from every thermocouple – and from sensors measuring retort temperature and cooling water temperature – are recorded at frequent intervals, often every 15 seconds for viscous products.

Data analysis: The recorded time-temperature profiles are plotted and analysed. The thermocouple showing the slowest heating rate identifies the cold point location. According to CFIA guidelines, at least 10 working thermocouples should be evaluated per test run, and a minimum of two full replications of the test are recommended. If results show significant variation, additional runs are needed.

Important considerations during testing 🔗

Several factors can introduce errors or complicate cold point identification:

Headspace effects: The air or gas pocket at the top of a sealed container can act as an insulating layer. If headspace is large relative to the container, it can shift the cold point closer to the top of the can, especially in containers where the height-to-diameter ratio is small. Entrapped gases in flexible pouches and semi-rigid containers are particularly important to control.

Container orientation: Whether a container sits vertically or horizontally in the retort can change convection patterns and therefore shift the cold point. This must be studied for each specific product-package combination.

Product variability: Batch-to-batch differences in ingredient size, viscosity, fill weight, and starch content can all alter heating behaviour. A product that heats by convection with one batch of ingredients might exhibit broken heating with another if starch levels vary slightly.

Multiple thermocouples per container: While inserting several sensors in one container provides detailed spatial data, multiple thermocouples can interfere with heat transfer in smaller containers, potentially giving misleading results.

Factors that influence cold point location 🔗

Understanding the variables that affect where the cold point forms helps food engineers anticipate and control thermal processing outcomes.

Food properties 🔗

The thermal conductivity, density, viscosity, and specific heat of the food all determine how fast heat penetrates. High-water-content foods conduct heat more effectively than fatty or oily products. Lab Manager reports that the degree of lethality achieved depends on product type, thermophysical properties, package size and shape, and container thermal characteristics. Starchy products present a unique challenge: as starch gelatinises during heating, the product thickens, convection currents diminish, and the cold point can shift from its initial location to a new position, sometimes quite significantly.

Container geometry and material 🔗

Taller containers require heat to travel greater vertical distances, generally resulting in longer processing times. Short, flat containers (like retort pouches) heat more rapidly because the maximum distance from any point to the nearest heated surface is small. Container wall material also matters – metal conducts heat far more efficiently than glass or plastic, meaning the food inside a metal can begins heating sooner than the same food in a glass jar.

Retort system and agitation 🔗

In stationary (still) retorts, the food does not move, and natural convection is the only mechanism mixing liquid products. Agitating retorts – which rotate or oscillate the containers – create forced mixing that can substantially increase the heating rate and may shift the cold point location. However, for viscous products in rotary systems, the cold point can move relative to a fixed thermocouple during rotation, producing unreliable data if not accounted for properly.

From cold point data to process schedule 🔗

Once the cold point is identified and its time-temperature history is thoroughly documented, this data becomes the foundation for calculating the scheduled process – the official time and temperature combination required for commercial production.

Two main methods are used for process schedule calculation. The General (Graphical) Method calculates lethality directly by numerically integrating the lethal rate over the entire heating and cooling cycle at the cold point. The Formula Method, originally developed by Ball, uses mathematical models based on heat-penetration parameters to determine the required process time. Both approaches rely entirely on accurate cold point temperature data as their input.

The calculated process is then often validated through inoculated pack studies, where containers are seeded with a known number of heat-resistant spores, processed at the calculated conditions, and then examined for surviving organisms. This provides biological confirmation that the process delivers adequate lethality at the cold point.

Emerging technologies in cold point detection 🔗

Traditional thermocouple methods, while reliable, have limitations. They are invasive (requiring penetration of the container), labour-intensive, and provide data only at discrete points. Newer approaches are addressing these constraints.

Self-contained data loggers – small wireless temperature recorders that can be placed inside containers – are especially useful for continuous retort systems where wired thermocouples are impractical. These devices travel through the retort with the product and record temperature data internally for later retrieval.

Computational fluid dynamics (CFD) modelling allows engineers to simulate heat transfer within containers digitally, predicting cold point locations and temperature histories before running physical experiments. While not a replacement for experimental validation, CFD can dramatically reduce the number of trial runs needed.

Microwave-assisted thermal sterilisation (MATS) represents a fundamentally different approach. Unlike conventional retorts that heat from outside in, microwave systems heat food volumetrically, generating thermal energy directly inside the product. This changes the nature of cold spots entirely – they become a function of electromagnetic field distribution rather than simple distance from the container wall. Ensuring a stable, predictable heating pattern is critical for these systems to maintain food safety.

Practical significance for the food industry 🔗

Cold point determination is not just a research exercise; it sits at the centre of regulatory compliance and consumer safety. Every commercially sterilised product on the market has a process schedule developed from cold point data. The U.S. Code of Federal Regulations requires that scheduled processes be established by a competent processing authority based on adequate scientific data – and that data starts with identifying where the cold point is.

Any change in product formulation, container size, container material, retort system, or even ingredient supplier can potentially alter cold point behaviour. When such changes occur, new heat-penetration studies may be necessary to verify that the existing process schedule remains adequate. This is why food processing facilities invest significantly in maintaining accurate records and repeating studies whenever process parameters shift.

What do you think? If the cold point in a can of chunky soup can shift during processing as the product thickens, how should food engineers account for this when designing a single fixed process time? And with newer technologies like microwave sterilisation changing how heat distributes in packaged food, will traditional thermocouple-based cold point studies eventually become obsolete?

References

1. https://www.britannica.com/topic/food-preservation/Sterilization
2. https://eng.libretexts.org/Bookshelves/Biological_Engineering/Introduction_to_Biosystems_Engineering_(Holden_et_al.)/06:_Processing_Systems/6.02:_Principles_of_Thermal_Processing_of_Packaged_Foods
3. https://www.ecfr.gov/current/title-21/chapter-I/subchapter-B/part-113
4. https://wiki.ubc.ca/Course:FNH200/Lessons/Lesson_06
5. https://www.fsis.usda.gov/sites/default/files/media_file/2021-04/6-Principles-of-Thermal-Processing.pdf
6. https://inspection.canada.ca/en/food-safety-industry/packing-food/heat-penetration-studies
7. https://www.fda.gov/guide-inspections-low-acid-canned-food-22
8. https://www.labmanager.com/thermal-processing-pasteurization-sterilization-and-blanching-34291
9. https://pubs.rsc.org/en/content/articlehtml/2024/fb/d3fb00176h