Heat Distribution vs Heat Penetration: Questions for a Food Process Trial

Heat distribution and heat penetration answer different questions. Confusing them can leave a manufacturer with an equipment report that does not establish how the food heats, or a product trial whose relationship to the production retort is unclear.

This guide focuses on retort studies for commercially sterile foods in hermetically sealed containers. It explains questions New Zealand food businesses can put to a competent thermal specialist. It is not a cooking schedule, a study method or a claim that every heating operation needs the same work.

Heat distribution vs heat penetration: what is being measured?

The Canadian Food Inspection Agency’s heat-penetration guidance describes these related studies in the retort context. It identifies evaluation of retort temperature and heat-transfer uniformity before a heat-penetration study, whose purpose is to determine the heating and cooling behaviour of a product/package combination in a specific retort system.

That page was modified in June 2018 and presents technical guidance developed through the Institute for Thermal Processing Specialists. It is used here to explain the study concepts, not to impose Canadian requirements on a New Zealand operator.

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Study names are useful, but the report should explain what was actually measured. Ask whether “distribution” describes temperature uniformity, relevant heat-transfer behaviour or another specific assessment. A title alone is not enough to determine what a report proves.

Distribution concerns the retort system and its load

A retort controller reaching its set point is not, by itself, a map of conditions throughout the load. The specialist needs to understand the heating medium, circulation, racking and loading arrangement, and the operating conditions relevant to that system.

These details matter because containers and racks form part of the processing arrangement. The CFIA guidance identifies retort type, container orientation, rack design and loading among factors that can influence heating behaviour. A result for one arrangement should not be silently applied to a different one.

Ask the specialist which conditions were represented and how the study identifies relevant slow-heating locations. The answer helps determine where product test packs should be placed for penetration work. It should also explain whether the proposed commercial load is covered by the existing distribution evidence.

For example, a processor may already have a report for its retort but propose a different rack or container arrangement for your product. The useful question is whether that arrangement remains within the report’s scope. This is a reason for specialist assessment, not an instruction to assume that every change needs an identical new study.

Penetration concerns the food inside the package

A heat-penetration study records the product’s response to the process. The food may heat differently according to its formulation and physical behaviour: a thick liquid, a product with particles and a product that changes consistency while heating need not behave alike.

The CFIA guidance identifies factors such as viscosity, solids and particle characteristics, preparation before filling, fill weight, headspace, package dimensions and initial product temperature. It also explains that the slowest-heating location can depend on the food, container and process. It is not always the geometric centre.

Keep two locations distinct: the relevant slow-heating area within the retort load, and the relevant slow-heating point or component within the product package. Knowing one does not automatically establish the other. The study design should explain how both are addressed.

Sensor placement and measurement quality are part of the evidence. Ask how calibration, the recording system and the location of the sensing element were controlled. If a sensor moves, a test pack leaks or the measurement method changes the product’s heating behaviour, the specialist needs to assess the effect on the data.

This is why a temperature trace should be accompanied by a description of the tested food and pack. A graph labelled only with a product name or a retort number may leave important questions unanswered.

Why one report does not substitute for the other

Good distribution evidence does not show how quickly every formulation or pack reaches the required product treatment. Conversely, a penetration result obtained under one set of retort conditions does not establish uniform conditions for a different system or load. Ask for the technical explanation connecting the equipment evidence, product response and proposed process.

Keep the New Zealand requirements within their actual scope

MPI’s Food Notice: Requirements for Food Control Plans and National Programmes, issued on 24 April 2023 and effective from 30 June 2023, contains specific requirements for FCP operators preparing or manufacturing low-acid canned foods. Clause 3.2.1 covers competency requirements for day-to-day thermal-processing supervision, the principles to be followed and the qualified persons developing or supervising development of thermal processes.

The Notice references principles in the current Codex code for low-acid and acidified low-acid canned foods and specified United States thermal-processing requirements. These are references made by the New Zealand Notice within that defined context; they are not a reason to apply every foreign rule to every New Zealand heating operation.

Confirm the product’s classification, applicable legislation and required specialist role before commissioning the work. Do not assume that a general food-safety course, an equipment supplier’s attendance or a consulting engagement fulfils the specific competency provisions. The relevant requirements for other products, chilled foods or other regulatory regimes need their own assessment.

Prepare a brief that separates product, pack and equipment assumptions

Give the specialist enough information to determine the study needed. The following is a recommended enquiry checklist, not an official study protocol or a prescribed set of test conditions:

• Product: controlled recipe, preparation steps, solids and liquid components, relevant composition information, consistency and expected production variation.
• Package and fill: material, dimensions, closure, fill and net weights, headspace, orientation and any relevant restrictions on pack thickness.
• Retort and load: system identification, heating medium, rack arrangement, load configuration, circulation and any agitation.
• Initial and subsequent conditions: product condition before heating, relevant delays, heating and cooling arrangements, and intended storage.
• Available evidence: existing distribution and penetration reports, process specifications, monitoring records and previous study limitations.
• Proposed change or decision: which product, pack, equipment or operating condition is new, and what the specialist is being asked to establish.

Identify assumptions explicitly. If the production pack differs from a development pack, supply both specifications. If a filled pouch can vary in thickness, ask how the study will represent the relevant commercial condition rather than describing the product only by its nominal weight.

Packaging suitability also needs attention beyond the temperature trace. MPI’s food packaging guidance says packaging should suit the food, processing and conditions of use, and protect food during its intended life. Ask what package-performance evidence accompanies the thermal assessment; a heating result does not replace that separate question.

For the broader commercial relationship, the food contract manufacturer selection guide covers responsibilities, specifications and trial arrangements. This article addresses the narrower distinction between the two kinds of thermal evidence.

Ask for a report that defines what the findings cover

A useful report should let a competent reader identify the system and product versions tested, the conditions represented, the measurement approach, the results and the interpretation. The CFIA guidance sets out extensive examples of pre-test, test and post-test documentation. Ask the specialist which are relevant to this study.

Before relying on the conclusion, resolve these questions:

• How was the relevant commercial worst case selected, and what variability was considered?
• Which distribution evidence supports the location of the penetration test packs?
• How was the relevant product cold point or component established?
• Were the product, package and process conditions consistent with the proposed commercial operation?
• What measurement problems, deviations or exclusions occurred, and how were they handled?
• What process conditions and critical factors does the specialist’s conclusion cover, and what changes require further review?

The specialist should explain the relationship between the observed time-temperature behaviour and the intended food-safety outcome. Neither an equipment set point nor one attractive curve should be treated as a complete scheduled process without that assessment.

Retain the report with its approved specifications and supporting records. If the recipe, package, fill, rack or retort arrangement later changes, provide the differences for review rather than assuming the old conclusion transfers. Not every difference has the same effect; the assessment should explain what remains applicable.

Common mistakes when requesting thermal evidence

• Asking for “a validation” without identifying whether the unanswered question concerns the equipment, product response or both.
• Treating the retort set point as the temperature achieved throughout every pack.
• Calling the centre of every container the cold point without product-specific justification.
• Using a development pack or load without explaining the commercial differences.
• Reading a passing result without checking the report’s assumptions and exclusions.
• Presenting a consultant’s review as formal process development or approval by a qualified thermal specialist.

How AFVA can help prepare the supporting product file

AFVA’s new product development service includes product and process information review, food safety risk review, packaging and storage review, specifications and launch-readiness support. Its HACCP consulting service also covers review of control measures, validation evidence and monitoring records.

Ask AFVA to scope that supporting work and identify the information and unresolved questions to take to the appropriate thermal specialist. This does not imply that AFVA designs retorts, conducts specialist heat studies or holds a qualified process-authority credential. Those capabilities and any regulated role must be verified and agreed separately.

Thermal evidence is also one part of the wider product assessment. Shelf-life planning support can help organise the separate storage and product-life questions. Laboratory testing, where required, is performed by appropriate accredited third-party laboratories. Consulting does not grant product approval or guarantee a verification outcome.

Frequently asked questions about heat distribution and heat penetration

Define the unanswered thermal question first

Bring the retort information, product specification, pack details and existing reports together. A clear distinction between the equipment question and the product question helps you commission the work that addresses the actual decision.

General information only: requirements depend on the complete food activities, product, process, packaging, market and applicable legislation. This article does not specify a thermal treatment, study protocol, commercial-sterility outcome or shelf life. Obtain product-specific advice from the appropriately qualified specialist.