Challenge testing in food: webinar questions answered by our Technical Services experts

If you’re stepping into the world of food microbiology, food law, and food safety, challenge testing is one of those topics you’ll hear again and again – and for good reason.

That’s why in our inaugural Food Insights blog post, we introduced you to the role of challenge testing – particularly considering the regulations impacting food manufacturers who market products in the EU.

Challenge testing sits at the intersection of science, regulation and real-world food production, helping answer some of the most important questions in the industry, like:

• Is a ready-to-eat product truly safe throughout its shelf life?
• Will a product support pathogen growth?
• How can food businesses back up shelf-life decisions with solid evidence?

Our professional challenge testing services put food safety, quality and shelf-life to the test with scientific knowledge. This enables the confidence of our clients in their products and helps support their compliance with food safety regulations.

At our recent global webinar, “Challenge test in RTE food. Supporting EU Listeria regulatory compliance,” we answered these and many other great questions from attendees. If you were among them, thank you for joining us. It was great to engage with so many professionals across the food sector – from experienced specialists to those just starting their journey in food microbiology and safety.

The number and quality of questions we received showed just how relevant this topic is for today’s food industry.

What the challenge testing webinar covered

During the session, our experts looked at the technical and practical role of challenge testing in supporting food safety and regulatory compliance, especially for ready-to-eat (RTE) foods.

Key topics included:

• What challenge testing is and when it is needed
• How it supports shelf-life validation
• Why it matters for assessing the growth potential of pathogens such as Listeria monocytogenes

The webinar also highlighted an important point: challenge testing is not just about meeting a requirement. It is about generating meaningful evidence that can support better decision-making for product safety, quality, labelling and consumer protection.

A full file of the entire Q&A from the webinar is available on request if you’d like to explore the discussion in detail.

For this edition of Food Insights, our Technical Services experts chose to highlight two of the questions that we think you’ll find informative.

Question one

We are a fudge manufacturer (shelf life: 12 months; aw = 0.60; pH = 5.0–6.0). Our products are used as ingredients without further processing; they are simply added to ice cream by our customers. Our customers have asked us to conduct Listeria testing on the products we supply to them. Is this our responsibility, or is it the responsibility of the final manufacturer?

Answer:

With an a_w of 0.60 and pH 5.0–6.0, your fudge falls well below the minimum water activity required for Listeria monocytogenes growth.

Lm requires a minimum a_w of ~0.92 to grow. At a_w = 0.60, your product is microbiologically stable and Lm should not grow, and will in fact decline over time.

This means your fudge is Category 1.3 under Regulation (EC) No 2073/2005 – a ready-to-eat food unable to support the growth of L. monocytogenes.

A challenge test or shelf-life study for Lm is therefore not required for your product as a standalone product.

The ice cream manufacturer is the final FBO (food business operator); they are responsible for ensuring their finished product complies with the Lm criterion of 100 cfu/g throughout its shelf-life.

When your fudge is incorporated into ice cream without further processing, it becomes part of a composite RTE product. The ice cream manufacturer must assess whether the final composite product can support Lm growth, and that assessment is their responsibility, not yours.

However, you have a practical and commercial obligation to supply a correctly specified, hygienically produced ingredient, following these steps:

• Set up robust HACCP and prerequisite programs.
• Provide accurate physico-chemical specifications (a_w, pH) to your customer.
• Demonstrate that your ingredient is not a source of Lm contamination.
• Provide evidence that your product cannot support Lm growth (your a_w = 0.60 is a very strong argument).
• Ensure your GMP and hygiene controls are adequate to prevent contamination during production.

You should therefore provide your customer with a comprehensive product specification document including confirmed a_w, pH, and evidence of your hygiene controls. This will be far more useful to them than a challenge test on an ingredient that cannot support Lm growth, but it can be reasonable to undertake Listeria testing as part of your management plan, to meet the microbiological specification agreed with your customer.

Question two

In the case of frozen products that are thawed in-store, which route should I take?

Answer:

The moment your product is thawed in-store and presented for sale as fresh, it transitions from a frozen product to an RTE chilled product and must be managed accordingly, with the shelf-life clock continuing from the moment of thawing. Freezing preserves the microbiological status of the product at the time of freezing. It does not reset it. Any Lm present before freezing will likely survive and resume growth upon thawing.

The answer which route you should take depends on what your product is and its intrinsic characteristics after thawing:

Route A. Category 1.3 (cannot support growth)

Your product qualifies if, after thawing, it has:

Evidence required:

• Physico-chemical measurements on thawed product, not frozen
• Measurements at multiple timepoints during post-thaw shelf-life
• Predictive modelling confirming no growth (should the physico-chemical properties lie within the growth range of Lm).
• Confirmation that parameters remain stable throughout post-thaw shelf-life.

Critical point: Parameters must be measured on the thawed product. Thawing can change texture, pH distribution and moisture availability, particularly for products containing ice crystals or frozen matrices.

Route B. Category 1.2 (supports growth)

Your product falls in this category if, after thawing, pH and a_w are both above their respective minima for Lm growth. This is the case for most thawed products such as:

This is the case for most thawed products such as:

• Raw or cooked fish and seafood
• Cooked meat products
• Ready meals
• Frozen, pH neutral, fruits and vegetables intended for RTE consumption

In-store thawing creates a new food safety scenario that must be assessed independently of frozen shelf-life. The key questions are: does your product support Lm growth once thawed, and who, manufacturer or retailer, is responsible for validating the post-thaw shelf-life? In most cases, a challenge test performed on the thawed product under realistic retail and consumer temperature conditions will be the most defensible route to demonstrating compliance with Regulation (EC) No 2073/2005. The retailer thawing a product in-store becomes the FBO responsible for the safety of the thawed product. The retailer must define, validate and manage the post-thaw shelf-life, ideally supported by data and guidance from the original manufacturer.

The retailer inherits the food safety risk at the moment of thawing, so your only real protection is the quality of information and guarantees you obtain from the manufacturer BEFORE the product enters the supply chain.

The core issue: If a frozen ready-to-eat product arrives at the retailer already contaminated with Listeria monocytogenes, the retailer has no practical means of eliminating that contamination before thawing and selling it.

There is no kill step available to the retailer, no decontamination treatment available post-thaw, no reformulation option available at the retail stage, and no meaningful corrective action available once the product is in the store.

The retailer's only window of control is at these stages:

1. Before purchasing: supplier approval and specification requirements
2. At goods-in: incoming product testing and verification
3. During thawing: hygiene controls to prevent additional contamination
4. Post-thaw shelf-life management: temperature control and date management

Only the first of these 4 stages gives the retailer any meaningful leverage over the contamination risk they will inherit.

Thanks for all the great questions from the webinar – they show how challenging it is to understand the regulations – and reveal the importance of staying informed about the practical application of microbiology, food law and product responsibility across the supply chain.

Get access to the full webinar Q&A or learn more about challenge testing and shelf-life support from ALS.