Shelf-Life Determination: Real-Time, Accelerated and Challenge Studies
How to set and defend a shelf life: real-time studies, accelerated tests, challenge tests and predictive models
Key takeaways
- Shelf life is a safety and quality decision, and the food business is responsible for justifying it.
- The limiting factor may be microbiological safety, microbiological spoilage, chemical change (such as oxidation) or sensory quality.
- Real-time studies under realistic and abusive conditions are the reference; accelerated tests suit some shelf-stable products only.
- Challenge tests show whether a pathogen such as Listeria monocytogenes can grow in the product; ISO 20976-1 describes how to perform them.
- Shelf life must be reviewed when recipe, process, packaging or storage conditions change.
1. What determines shelf life
| Limiting factor | Typical products | What to measure |
|---|---|---|
| Pathogen growth | Chilled ready-to-eat foods, smoked fish, soft cheese, cooked meats | Growth potential of Listeria monocytogenes and other relevant pathogens |
| Spoilage organisms | Fresh juices, dairy, bakery | Yeasts, moulds, lactic acid bacteria, total counts |
| Chemical change | Oils, nuts, snacks, infant formula | Peroxide value, free fatty acids, vitamins, browning |
| Physical change | Biscuits, powders, confectionery | Moisture, texture, caking |
| Sensory quality | Most products | Panel tests for appearance, odour, taste and texture |
The shortest of these determines the shelf life, with a safety margin.
2. Intrinsic and extrinsic factors
Shelf life depends on the product and its environment:
- Intrinsic: pH, water activity, salt, preservatives, fat content, initial microbial load.
- Extrinsic: storage temperature, packaging atmosphere, oxygen and light barrier, distribution conditions, consumer handling after opening.
A change in any of these, for example lower salt for health reasons, can shorten the safe shelf life.
3. Study types
| Study | Description | Best used for |
|---|---|---|
| Real-time storage study | Product stored under intended conditions and tested at intervals until failure | All products; the reference method |
| Real-time with temperature abuse | Part of the storage at higher temperature to reflect retail and consumer conditions | Chilled foods |
| Accelerated shelf-life test | Higher temperature or humidity to speed up chemical change, with a model (e.g. Q10) to estimate shelf life | Shelf-stable products where the failure mechanism is understood |
| Challenge test | Product inoculated with the target pathogen and stored to measure growth potential | Ready-to-eat foods that may support pathogen growth |
| Durability study | Naturally contaminated product tested at the end of shelf life | Verification of Listeria levels at end of life |
| Predictive modelling | Mathematical models estimate growth from pH, water activity, temperature | Screening and supporting decisions, not replacing data |
4. Challenge tests in practice
A challenge test, performed according to ISO 20976-1 or equivalent guidance, typically includes:
- Selecting relevant strains of the target organism.
- Inoculating several batches at a realistic level.
- Storing under reasonably foreseeable conditions, including temperature abuse.
- Counting at defined intervals until the end of the intended shelf life.
- Calculating growth potential: the difference between the final and initial log counts.
For Listeria monocytogenes, a growth potential above an accepted threshold (commonly 0.5 log) indicates that the food can support growth, which affects the criteria that apply and the shelf life that can be justified.
5. Building the study plan
| Element | Points to decide |
|---|---|
| Batches | At least three production batches to account for variability |
| Conditions | Intended storage plus realistic abuse (e.g. part of the time at a higher chilled temperature) |
| Sampling points | Including beyond the proposed shelf life to find the failure point |
| Tests | Microbiological, chemical, physical and sensory as relevant |
| Acceptance criteria | Defined before testing: legal limits, customer specifications, internal limits |
| Safety margin | Shelf life set shorter than the observed failure point |
6. Linking shelf life to the food safety system
- Shelf life appears in the product description of the HACCP study.
- FSSC 22000 Version 7 includes production and shelf-life trials in product design and development (2.5.13).
- BRCGS Food Safety expects shelf-life trials as part of product development, with documented results.
- Stock rotation and shelf-life control are part of storage requirements.
7. Common weaknesses
- Shelf life copied from a competitor or a similar product.
- Studies done only at ideal storage temperature.
- Sensory evaluation by one person without criteria.
- No review after a recipe or packaging change.
- “Use by” and “best before” chosen without considering whether the limit is safety or quality.
Conclusion
A defensible shelf life combines knowledge of the product, realistic storage conditions, the right study type and a safety margin. It protects consumers, reduces waste from overly short dates, and gives auditors and customers the evidence they expect.
Choose your most sensitive chilled product and ask: is its shelf life supported by our own study, under realistic conditions, on more than one batch?