Food Microbiology Criteria NZ: What n, c, m and M Mean

A laboratory result only becomes useful when it is compared with the right criterion. A limit copied from another food, an old specification or the wrong stage of production can lead to an unreliable conclusion, even when the analysis itself is correct.

This guide is for owners and quality teams reviewing finished-food microbiology evidence. It develops the criteria question in the guide to choosing food testing in New Zealand; it is not a universal test panel, a shelf-life protocol or an instruction to release product.

Identify the purpose of the criterion

FSANZ’s explanation of microbiological limits distinguishes food safety criteria from process hygiene and guideline criteria. The title of a spreadsheet column such as “pass limit” does not establish which type you are using. Record where the criterion comes from and the decision it was designed to support.

On a phone, swipe sideways to read all columns. Keyboard users can focus the table region to scroll horizontally.

A customer specification and a legal criterion may both matter, but they are not interchangeable. Keep their source, purpose and conclusions separate in the review. A customer acceptance statement should not silently become a claim that every applicable food safety requirement has been met.

Check whether it applies to this food

Standard 1.6.1 sets out the assessment rules; Schedule 27 lists particular food–microorganism combinations and their limits. Do not assume its table gives a numeric limit for every organism in every food. Equally, the absence of a particular table entry is not a demonstration that a food is safe.

Before comparing numbers, write down the product identity, processing state, intended use and assessment point. Ask the technical reviewer to resolve any uncertain category. Descriptions such as ready to eat, raw, heat treated or packaged can affect which entry is relevant. A product name alone may not answer that question.

Use the current source and retain its version with your assessment. When a limit depends on product characteristics, establish the evidence for those characteristics rather than selecting the more convenient limit. For example, Schedule 27 distinguishes ready-to-eat foods according to whether Listeria monocytogenes can grow; Standard 1.6.1 contains the associated provisions. A result alone does not establish the product’s growth classification.

The July 2025 FSANZ Compendium of Microbiological Criteria for Food provides guidance as well as identifying applicable Code criteria. Its ready-to-eat guideline limits are not lot acceptance/rejection sampling plans, and that section is not intended to replace food safety criteria in the Code. Read the scope before transferring any limit into your specification.

Read n, c, m and M together

For a Schedule 27 assessment, read across the complete row and then apply Standard 1.6.1. The symbols describe different parts of the sampling and acceptance rules; they are not four alternative limits from which to choose.

• n — sample units: the number of units in the specified sampling plan. Do not assume that a report listing several tests means several independent sample units were tested.
• c — permitted count: the maximum number of sample units allowed to exceed m. It is a count of units, not the concentration of bacteria in the food.
• m — microbiological limit: the value used when counting how many sample units exceed the specified limit. It may be expressed as a count or a presence/absence requirement, depending on the criterion.
• M — upper limit, when specified: a result above this limit in any sample unit makes the lot unacceptable under the Code assessment. An empty M cell does not create permission to exceed m without applying c.

The Compendium explains two-class and three-class sampling plans. A two-class plan separates acceptable and unacceptable outcomes; a three-class plan also provides an intermediate range between m and M. Use the actual plan and its wording. Do not import another food’s n or c, or invent an M because the report software has space for one.

Interpret the complete set of results

A practical review sequence is to identify the applicable row, confirm the sample-unit set, match the reported organism and units, and compare each result against the correct limits. Keep the individual results visible. Averaging them can conceal a unit above a limit and is not the acceptance rule in Standard 1.6.1.

Under that standard, a listed food lot is unacceptable where the number of units above m exceeds c, or any unit exceeds M where M is specified, when assessed in accordance with its provisions. A result below M therefore does not automatically pass: the number above m still matters. Conversely, interpret an intermediate result through the permitted count rather than inventing a rule that every result above m has the same consequence.

Check the reporting expression too. A concentration such as colony-forming units per gram and a qualitative result such as “not detected” in a stated test quantity answer different analytical questions. Ask the laboratory to clarify the method, test quantity and reporting limit if the result cannot be compared directly with the criterion. Do not convert a non-detection into an absolute zero.

Understand a single-sample result

Start by asking what was actually sampled. One pack, one test portion and one sample unit are not necessarily equivalent descriptions. If the applicable plan requires several units, a result from one submitted unit does not by itself demonstrate completion of that plan. Confirm the sampling record rather than relying on the number of lines on the certificate.

Standard 1.6.1 contains a specific provision allowing an authorised officer to obtain fewer units or smaller samples for a consumer complaint or suspected food poisoning incident. That exception is not a general permission for a business to reduce its routine plan. Nor should an incomplete routine set be used to dismiss a concerning positive result.

A single sample may still provide useful information for a defined investigation or guideline assessment. Record that narrower purpose and its limitations. A conclusion about the submitted sample is different from a conclusion about the whole lot, future batches, the production environment or the food’s entire shelf life.

If the question is how results change over a proposed storage period, use a suitable study rather than stretching a release result into a date-marking claim. AFVA’s shelf-life study planning and result-review support describes that separate service.

Prepare a useful criteria review file

The following is an AFVA briefing recommendation, not a compulsory statutory form. Its purpose is to make the assessment traceable enough for a competent reviewer to follow without reconstructing the reasoning from emails.

1. Decision and scope: identify the food, lot, assessment point and question being answered. State what the review does not cover.
2. Criterion identity: retain the exact source, version, food category, organism, n, c, m and any M. Label legal, guideline and customer requirements separately.
3. Applicability evidence: explain why the chosen category fits, with relevant product, process or shelf-life information and any unresolved assumptions.
4. Sampling evidence: connect the units to the lot, sampling plan and submission record. Explain departures or missing information rather than hiding them.
5. Result comparison: retain the original reports and a unit-by-unit assessment, including units, test quantity and any laboratory clarification.
6. Conclusion and authority: identify who assessed the evidence, the decision reached, its limitations and who is responsible for the business decision.
7. Follow-up: record questions, actions and owners. If more evidence is needed, state what decision remains unresolved and what will resolve it.

Where a result raises a possible food safety concern, deal promptly with affected product and obtain competent advice. Establish the affected scope and whether notification or recall obligations apply; do not wait for a convenient repeat result. A criteria review supports that response but does not replace the business’s procedures or applicable regulatory requirements.

Avoid common interpretation mistakes

• Using a guideline table as though it were a universal legal release specification.
• Choosing a food category because its limit is easier to meet.
• Averaging sample results instead of applying the specified unit-count rules.
• Treating “not detected” as proof of absence throughout the lot or shelf life.
• Calling a sample set complete without checking n and the sampling record.
• Changing the criterion after an unexpected result without a justified, documented reassessment.

Frequently asked questions

Are all microbiological limits in guidance legally mandatory?

No. Identify the source and status of each criterion. Standard 1.6.1 and Schedule 27 contain applicable Code requirements. Guidance and commercial specifications can serve different purposes. A guideline may also reproduce a Code criterion, so read its accompanying explanation rather than classifying it by document title alone.

Does c mean a permitted bacterial concentration?

No. In the Schedule 27 assessment, c concerns the number of sample units allowed above m. The concentration or presence/absence limit is expressed separately. Confusing the unit count with a microbial count can reverse the interpretation of an otherwise accurate result.

Does every criterion have an M value?

No. Use the complete applicable row and assessment rules. Some rows have no separate M. Do not fill the blank with an assumed value or ignore the n, c and m requirements because an upper-limit column is empty.

Can one negative result clear a whole batch?

Not automatically. Confirm what was sampled and whether the applicable sampling and assessment requirements were met. A result from one sample cannot simply be relabelled as a completed multi-unit assessment. A negative result also does not answer every other relevant hazard or process question.

Does failing a process hygiene criterion always mean a recall?

That conclusion cannot be made from the label alone. Review the criterion’s purpose, results, affected product and potential consumer risk. Investigate the loss of control and establish the appropriate response. Do not assume either an automatic recall or an automatic absence of food safety implications.

Can AFVA carry out the laboratory analysis?

AFVA provides investigation support, sampling-plan support, laboratory coordination and result review. Its shelf-life service states that required laboratory testing is performed by accredited third-party laboratories. Agree the review scope and use a suitable laboratory; consulting support is not a laboratory certificate or regulatory release approval.

Need help resolving a criteria question?

Bring the product description, relevant specification, sampling record and full laboratory report, together with the decision you need to make. AFVA can help organise the evidence, review the issue and coordinate further technical or laboratory input where appropriate. Contact AFVA to discuss a defined microbiology review before turning an unclear comparison into a product claim.