Validating microbiological testing in-house

Hello everyone, been a while and I know where exactly to go if I have questions, neve disappointed!

How does one go about validating their in-house micro testing? 

So we pasteurize and bottle fruit juices, and do testing for TPC and Y&M on finished product. And I have some doubts about the way we plate/read/sample or the process as a whole for our micro sampling, we had fermented juice that was detected organoleptically, but on our plates, there was no growth at all shown... weird. We use "peel plates", where just pipette the juice, dispense it on the film, seal the film and incubate it. 

What is the best approach to begin and implement a validation study to confirm what we do is correct?

I did reach out to a third party lab to coordinate sending them samples and comparing their samples with ours to see if there are any weird results. 

Is this a right approach? or does anyone have any other suggestions?

Looking forward to hearing everyone's expertise on this.

Hi The Food Scientist,

Most people would validate and then periodically compare their “peel plates” with results via traditional methods, for example the Conventional Plate Count Method on an ongoing basis.

In addition there should be periodic confirmation of accuracy of results via a certified third party laboratory including proficiency testing for the site laboratory/staff/methodology.

Kind regards,

Tony

We do the same at our ice cream factory: testing every batch of our products for TC/E. Coli using Petrifilms, and sending a set of samples to the external lab once a month, for the same micro.

You may want to look into the USP for the suitability of testing method for either their dietary supplements or pharmaceuticals.  its a simple, yet requires some work and organisms to verify your method can recover the target organisms.  

Simply add the organisms at low levels (we use organisms with know concentrations that are easily rehydrated and pipetted) and then plate the product as you usually do.  If you recover the correct number of organisms your pass, (they even have a +/- % recovery requirement).  If you don't pass you would need to adjust your method either by dilution, buffer change, chemical additives (there are other ways) and re do the suitability.  

Your film manufacturer would also be a good source for a procedure to ensure the method is working for your product.