The certificate of analysis lands in your inbox at three on a Friday. Aerobic plate count 4.2 x 104 cfu/g, Enterobacteriaceae <10, Salmonella not detected in 25 g. The production manager wants to know if the batch can go, and you are looking at eleven rows of numbers with no clear idea which of them matters. Nobody taught you to read a food microbiology lab report. They just started forwarding them to you.
In short
- cfu/g is an estimate of living cells per gram. Read it in logs: 10 000 cfu/g is 4 log, and differences under about half a log rarely mean anything.
- “<10 cfu/g” means nothing grew at the lowest dilution the lab plated. That is the method’s detection limit, not zero.
- “Not detected in 25 g” applies to that 25 g portion only. It cannot prove a batch is free of Salmonella or Listeria.
- Judge each result against the right yardstick: a legal limit, a customer specification or a guideline.
- One report is a snapshot. Trend your results, and meet a failure with a hold, an investigation and a root cause.
A wrong call is expensive in either direction. Interpretation of laboratory results is a lesson in module 4 of the Advanced Food Microbiology Course from ASC Food Safety Training: 8 hours, online, self paced, R1 750.
What does cfu/g mean on a food microbiology lab report?
cfu/g stands for colony forming units per gram. The lab dilutes your sample, plates it, incubates it, counts the colonies and multiplies back through the dilutions to one gram. It estimates the organisms able to grow on that medium at that temperature, never every cell.
Counts span a huge range, so microbiologists work in log10 units.
2 log
100 cfu/g
4 log
10 000 cfu/g
6 log
1 000 000 cfu/g
The 4.2 x 104 on your certificate is 42 000 cfu/g, or about 4.6 log. In Excel it is =LOG10(A2).
Logs stop you panicking over noise. A count that doubles from 20 000 to 40 000 has moved from 4.3 to 4.6 log, and a second analyst plating the same sample could produce that gap. A full log, tenfold, is a real shift. See measurement uncertainty in food microbiology results.
What does “<10 cfu/g” mean, and is it the same as zero?
“<10 cfu/g” means no colonies grew on the plate at the lowest dilution tested. The lab typically blends 10 g of sample into 90 ml of diluent and plates 1 ml, so a single colony would equal 10 cfu/g. No colonies means fewer than 10. The organism may be absent, or present at 3 cfu/g.
The limit describes the method. If 0.1 ml is spread on the plate instead, the same empty plate is reported as <100 cfu/g. So when last month’s results read <10 and this month’s read <100, the lab has changed something. Phone and ask what.
In your trend sheet, never enter a “less than” result as 0. Many sites enter the limit itself and flag it.
Why is “not detected in 25 g” not the same as pathogen free?
“Not detected in 25 g” is a presence or absence result on one 25 g portion. The lab enriches that portion to grow a single cell up to detectable numbers. If it finds nothing, that portion was clean. The rest of the batch was not tested, and pathogens are rarely spread evenly.
If one pack in a hundred carries Salmonella and you test five, you pass the batch about 95 times in 100. At one pack in ten, five samples still pass it more than half the time. Most plants test one.
A certificate never made a batch safe. The cook step and the high care barrier did that.
A negative is still useful verification, alongside cook records and your environmental monitoring programme for Listeria. A positive is different: strong evidence, and it stands.
Releasing product on a negative you cannot explain is uncomfortable. Module 4 of the advanced microbiology training for QCs and food technologists takes you through aseptic sampling, interpretation of laboratory results and the corrective actions that follow.
Start the Advanced Food Microbiology Course, R1 750
What is the difference between a specification, a guideline and a legal limit?
A legal limit is set in regulation under the Foodstuffs, Cosmetics and Disinfectants Act, and product that exceeds it may not be sold. A specification is a limit agreed with a customer or set internally, so failing it is a contractual or internal non-conformance. A guideline is published advice with no force of its own.
Your response depends on which of the three you have breached, so mark each line of the specification, and check that it names the method and units. “Coliforms <10” with no units and no method cannot be enforced by either party.
What do n, c, m and M mean in a sampling plan?
n is the number of sample units tested. m is the count that separates good from marginal. M is the count no unit may exceed. c is how many of the n units are allowed to fall between m and M.
Take n = 5, c = 2, m = 10 000, M = 100 000 for an aerobic plate count. The lot passes if none of the five packs exceeds 100 000 cfu/g and no more than two sit between 10 000 and 100 000. That is a three class plan. Pathogens use a two class plan such as n = 5, c = 0, not detected in 25 g: one positive fails the lot. A single sample per batch is n = 1. That is fine as verification, provided nobody on site mistakes it for lot acceptance.
Which tests appear on a typical report, and what does each one tell you?
Most routine microbiology certificates of analysis carry two kinds of test. Counts of indicator groups, such as aerobic plate count, Enterobacteriaceae, coliforms, yeasts and moulds, describe hygiene, process control and spoilage potential. Presence or absence tests for Salmonella and Listeria monocytogenes address safety directly. E. coli sits between the two.
| Test | What it tells you | Typical action on a poor result |
|---|---|---|
| Aerobic plate count (TVC) | General microbial load: raw material quality, temperature history, age. Meaningless on fermented foods. | A rise of a log over baseline: check chill chain, raw materials and cleaning. |
| Coliforms | Hygiene indicator for heat treated foods and water. Naturally present on raw produce. | In cooked product, look for contamination after the heat step. |
| Enterobacteriaceae | The wider family, including Salmonella. A process hygiene indicator after a kill step. | Check cook records first, then post process handling and cleaning. |
| E. coli | Points to faecal contamination and possible enteric pathogens. | In ready to eat product: hold, then check water, raw and cooked separation and hand hygiene. |
| Yeasts and moulds | Spoilage potential, mainly in acidic, dried, baked and dairy products. Reflects air quality. | Check air handling, packaging hygiene and preservative dosing. |
| Salmonella in 25 g | Presence or absence of a pathogen in the portion tested. | Detected: hold, raise an incident, assess recall. Never retest into a pass. |
| Listeria monocytogenes in 25 g | Presence or absence. The main concern in chilled ready to eat foods. | Detected: hold, raise an incident, review swab history and swab that line. |
On when these groups mislead, see indicator organisms versus index organisms in food testing.
Why should you trend results instead of judging one report?
A single microbiology result describes one sample on one day. Twenty or thirty results show what your process normally delivers, and that baseline shows drift weeks before anything fails.
Keep one sheet per product and sampling point, results in logs, a chart, and an alert level set from your own history. At Mzansi Fresh Meals, the fictional ready meal plant ASC uses as a training case, the plate count on cooked rice climbs from 2.5 to 3.8 log over six weeks. Every result passes the 5 log specification. On a chart the climb is obvious, and it leads the team to a blast chiller that is cooling more slowly every week.
What should you do with an out of spec microbiology result?
When a microbiology result is out of specification, put the affected product on hold, confirm the result is valid, assess the risk, find the cause and verify the fix. A clean retest does not cancel the first result, least of all for a pathogen, because contamination is patchy.
What then happens to held product is decided by your site’s procedure, your customer’s requirements and the law, never by the dispatch schedule.
1
Hold. The batch, plus anything made on that line between the same two cleans.
2
Check validity. Ask the lab about sample temperature on receipt and control results.
3
Assess risk. Which organism, what level, which consumer. Pathogens go straight to the incident procedure.
4
Find the cause. Process and cleaning records, raw material results, environmental swabs.
5
Fix and verify. Corrective action, then extra sampling until results return to baseline.
Step four tends to stall at “retrained the operator”. See corrective action and root cause analysis.
What does the Advanced Food Microbiology Course cover, and what does it cost?
The Advanced Food Microbiology Course from ASC Food Safety Training is an 8 hour online course that costs R1 750. It has five modules: micro-organisms in a food handling environment, growth and reproduction, indicator organisms, sampling, enumeration and testing, and food quality and safety in microbiology.
Course facts
| Course | Advanced Food Microbiology Course. Provider: ASC Food Safety Training. Instructor: Mthokozisi Nkosi |
|---|---|
| Level | Advanced, the third step in the ASC microbiology ladder |
| Time | 8 hours |
| Price | R1 750 |
| Format | Online, self paced, lifetime access |
| Assessment | An assessment for each of the five modules, then a final assessment |
| Certificate | Certificate of completion |
| What you will be able to do | Interpret laboratory results, choose the corrective action, and apply microbiology to safety, quality and shelf life decisions |
| Who it is for | QCs, QA managers, food technologists and HACCP team members who must act on lab results |
Enrol in the Advanced Food Microbiology Course, R1 750
Questions people ask before they enrol
How long does the Advanced Food Microbiology Course take?
The course is 8 hours, online and self paced, so you can spread it over a few weeks of evenings. Access is for life, which means you can return to the laboratory results lesson whenever a strange certificate arrives.
What does it cost, and how do I pay?
The price is R1 750. That covers all five modules, the assessments, lifetime access and the certificate of completion. Payment is by card or EFT through PayFast when you enrol on the course page.
Do I need the introductory and intermediate courses first?
It is the third step after Introduction to Food Microbiology (R1 195) and Intermediate Food Microbiology: Principles and Application (R1 450). If you already handle lab results and know the main organism groups, you can start here.
How is the course assessed, and do I get a certificate?
Each of the five modules has an assessment, and a final assessment closes the course. You receive a certificate of completion, which you can file as training evidence in your food safety team records.
Can I enrol from outside South Africa?
Yes. The course is delivered fully online. The price is in rand, and the course page shows an indicative USD, EUR and GBP equivalent. Payment settles in rand, and your bank handles the conversion.
Another certificate will arrive next week. One sound batch scrapped, or one doubtful batch released, will cost your site far more than R1 750. Enrol in the ASC Advanced Food Microbiology Course now, spend eight hours on it, and become the person on site who can read the report and act on it. For the full ladder, see the food microbiology training hub.