11 replies to this topic

[_creeks_]

Hello everyone,

I work as a lab technician in a goat cheese manufacturing facility, where we perform routine microbiological controls on both intermediate and finished products. We use 3M Petrifilm Coliform Count plates for in-house testing.

This week, we obtained a coliform count of approximately 1700 cfu/g on a finished, MAP-packed goat cheese product made from pasteurized milk. To rule out errors, I re-tested a counter sample the next day and got a similar result. Confident in our process, we sent a retained sample to an accredited external lab, which reported <10 cfu/g.

This contradiction have left me puzzled. All handling and incubation conditions were properly controlled, and the samples came from the same production lot.

I’d like to ask:

Has anyone else faced such discrepancies between Petrifilm and external lab results?

What factors could explain such a large difference in coliform counts?

How do you usually handle this kind of situation when it comes to internal quality records or non-conformities?

I'd say this happens quite often...
I’d really appreciate your insights or shared experiences.

Thanks!!

[GMO]

Hi, I have never worked in a micro lab but I've audited them.  I've never found an issue specifically between petrifilm and traditional plates.  To my understanding they're both validated processes.

 

I'd say that microbial loading on cheese can be patchy even within the same batch.  Do you have any situations where the cheese is handled, cut or packaged before you test?  If so, my money would be on both results being genuine.  You can get real differences through handling.

 

But the best way to test this would be to do proficiency testing (yourselves and the lab) and check how good your results are.

 

My little caveat to all this though is a bit of a story.  I have directly and indirectly been involved in two lab incidents where faking of results was happening.  One was an external accredited lab where they were sending back fake negatives.  The other was an in house lab where an employee was not testing samples but entering results as <10.  We caught that one because swabs were in the bin with the same volume of nutrient medium still present.

 

It happens and the reason it happens is because lab work is badly paid and high volume.  Perhaps a way to "test" this will be to actually split some samples in your production area (the same cheese) and send half for testing internally and have externally.  Of course there could and will be some differences but you'd expect them to be similar.  The other thing would be to audit your lab and if they have CCTV, see if you can go back to when your specific samples were tested.  

 

Sorry to be suspicious, it's just happened in the UK a few times.  

[kingstudruler1]

Is the product fully fermented when you tested it internally and what was the ph?

 

If the ph in product drops over time,  it could kill off the more ph sensitive coliform.   

Perhaps your retest eliminates that as a posibility?

[GMO]

Is the product fully fermented when you tested it internally and what was the ph?

 

If the ph in product drops over time,  it could kill off the more ph sensitive coliform.   

Perhaps your retest eliminates that as a posibility?

 

A good question.  I was assuming a rinded goats cheese which would increase pH over maturation.  It would be good to know as it is an assumption.

[Scampi]

You're certain your petri plates have been stored properly and are not past the expiration date? Are you using the plate reader as well?  When was it calibrated last

[sanidadexterior]

Hello, so far we think the error lies with the internal laboratory.

The pH value would be relevant for a cheese made with raw milk that requires aging to eliminate the possible presence of unwanted bacteria.

 If the cheese is made with pasteurized milk, the pH factor might be less important.

 I think it's easier to get a false negative than a false positive.

 Perhaps the cheese is contaminated, and the error lies with the external laboratory.

Perhaps the pasteurization was insufficient or there was subsequent recontamination.

[_creeks_]

Is the product fully fermented when you tested it internally and what was the ph?

 

If the ph in product drops over time,  it could kill off the more ph sensitive coliform.   

Perhaps your retest eliminates that as a posibility?

 

We don't measure the pH of the intermediate product when performing microbiological testing. I'm not entirely sure, but the pH seems to decrease throughout the stages of the process — from the milk in the tank, through the maturation/fermentation stage, to the cheese packed in modified atmosphere

 

A good question.  I was assuming a rinded goats cheese which would increase pH over maturation.  It would be good to know as it is an assumption.

 

 

You're certain your petri plates have been stored properly and are not past the expiration date? Are you using the plate reader as well?  When was it calibrated last

Yes, my Petrifilms are not expired. We don't use any Petrifilm reader either.

 

Good news, everyone! I had previously analyzed three points in the production process (intermediate product), and the external lab has confirmed the presence of total coliforms — except in the finished product. They report no coliforms detected there. Could this perhaps be due to the modified atmosphere? I tested the cheese log immediately after packaging (so maybe the atmosphere hadn’t had time to reduce the total coliforms), while the external lab takes longer to analyze (around 18 hours delay).

 

I have a question:

Do you consider the presence of total coliforms in pasteurized goat cheese to be a safety issue? In Europe, or at least according to Spanish regulations, this microorganism is not listed as a criterion for batch release. However, it is considered a hygiene indicator, so high levels are concerning.

Have you encountered similar situations in your companies, or are you aware of comparable cases?

[kingstudruler1]

Ive made other dairy product including some cultured product (not cheese, but yogurt, sour cream, buttermilk, etc.)   The joke for when we found coliform was to wait a day and retest as the ph would kill it off.   It could be due to MAP as well.   If you still have the sample have your people test it again and see if it now "normal"   

 

Yes, I didnt want to say it, but should.   1700 cfu / g coliform ( I prefer enterobacter) in pasteurized product would have terrified me.   You definitely have something to look into.    

 

Some bacteria produce heat stable endotoxins when they die (e.coli, salmonella, shigella, etc).   The endotoxins can cause serious illness as well.   

[GMO]

Absolutely the pH change is important if that's the kind of cheese you're talking about (and I think it is as you've said "log").  What I think you'll find is the vat is relatively low pH but as the mould starts to grow, you will have gradual break down of the proteins and increasing pH.  You may then have a risk not from bacteria present in the vat but pathogens or indicators being introduced in your maturation, cutting and packing process (it happens).  So that's one possibility.  And for some pathogens, Listeria monocytogenes particularly, that pH change might be enough to risk survival or even growth.

 

But that's possibly not what's happening with your coliforms.  I would wager that they were present in the vat or early in maturation and might, if stored, be out competed by your cultures, moulds etc.  It's not certain though.  

 

Is it a safety issue?  Not directly but it does indicate some hygiene issues in your plant which need investigation.  Eventually if you have enough of these failures, you'll get an E Coli and then eventually an STEC E Coli.

 

Also dependent on whether the contamination occurred in the vat, on maturation etc (or in brine if these are brined cheeses), will depend on how likely one cheese will come up with a positive and another cheese from the same vat will.  It's not always homogeneous contamination which occurs with micro.  So a retest may indicate it's been outcompeted.  Or it may indicate only the other log was contaminated...

[PrplomSolved]

Hello everyone,

I work as a lab technician in a goat cheese manufacturing facility, where we perform routine microbiological controls on both intermediate and finished products. We use 3M Petrifilm Coliform Count plates for in-house testing.

This week, we obtained a coliform count of approximately 1700 cfu/g on a finished, MAP-packed goat cheese product made from pasteurized milk. To rule out errors, I re-tested a counter sample the next day and got a similar result. Confident in our process, we sent a retained sample to an accredited external lab, which reported <10 cfu/g.

This contradiction have left me puzzled. All handling and incubation conditions were properly controlled, and the samples came from the same production lot.

I’d like to ask:

Has anyone else faced such discrepancies between Petrifilm and external lab results?

What factors could explain such a large difference in coliform counts?

How do you usually handle this kind of situation when it comes to internal quality records or non-conformities?

I'd say this happens quite often...
I’d really appreciate your insights or shared experiences.

Thanks!!

 

Currently working in a food micro lab... how far away is your external lab? Do you ship out of state? These can sometimes be compounding factors.

 

The pH would be my next guess. I would also ask to ensure you're both preparing/running the sample similarly, same dilution factor and such. 

[Tony-C]

I have a question:

Do you consider the presence of total coliforms in pasteurized goat cheese to be a safety issue? In Europe, or at least according to Spanish regulations, this microorganism is not listed as a criterion for batch release. However, it is considered a hygiene indicator, so high levels are concerning.

Have you encountered similar situations in your companies, or are you aware of comparable cases?

 

Hi _creeks_,

 

Positive coliform tests of the order of 1,700/g need to be investigated as this is an indicator of poor hygiene post-pasteurisation.

 

I would also be following up with E.coli tests on the product. Coliforms are used as an indicator that other, potentially harmful, bacteria could be present, it is a good idea to further analyse coliform positive samples to determine if E. coli are present. E. coli is a more specific indicator of faecal contamination and a positive would indicate potentially more harmful pathogens being present.

 

Kind regards,

 

Tony

[_creeks_]

Hi _creeks_,

 

Positive coliform tests of the order of 1,700/g need to be investigated as this is an indicator of poor hygiene post-pasteurisation.

 

I would also be following up with E.coli tests on the product. Coliforms are used as an indicator that other, potentially harmful, bacteria could be present, it is a good idea to further analyse coliform positive samples to determine if E. coli are present. E. coli is a more specific indicator of faecal contamination and a positive would indicate potentially more harmful pathogens being present.

 

Kind regards,

 

Tony

 

We always analyze total coliforms together with E.Coli analysis, to see if there is a relationship; and this is not the case.