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Ambient yogurt microbial testing

quality control ambient yogurt longlife yogurt microbial testing dairy yogurt reconstituted milk microbiology

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Posted 31 August 2018 - 08:57 AM

Hi guys


I was hoping one of you could give me some advice. We are busy developing an ambient yogurt - heat treated after fermentation with a shelf life of 3 months. We make the yogurt from Full Cream milk Powder (reconstituted milk) and not fresh milk.


Regarding the microbial testing of the product, I would suggest testing the final product for the following: 


Total aerobic MOS

Yeasts and Moulds

Bacillus cereus



Would you say that these are the absolute minimum that I should test for? Or am I being over concerned? 

Should every single batch be tested for all of these? Including the milk powder when received? 


This is the first time I am working with dairy and I am not sure if I am going in the correct direction. 


Hope someone can assist :)


Thanks in advance - Anna


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Posted 31 August 2018 - 03:45 PM

Hi Anna,


Since you will be heating your product after fermentation your biggest concern may be recontamination in the packing line.

In my experience it is not necesary to analyse more than Yeasts and Moulds and Enterobacteriaceae in the finished product.

Since the yoghurt is acidified only the Yeasts and Moulds should be a problem for the product stability. They can grow in the acidic environment

The Enterobacteria won´t multiply in the product, but are a good indicator for the production hygiene.

I don´t know about the Namibian regulations but in the EU there has to be done testing (at least in monitoring) for listeria.

For analysing the incoming powder, it depends if you have a heating step after the reconstitution (befor fermenting). If "YES", check for spore forming count. Any other bacteria will not impact your product.

If "NO", I would advise to analyse the total plate count at the temperatur you use for fermentation. By that you can eliminate some unpleasant surprises like wrong acidification (not sure if the Englisch term is right, in German you cal it "Fehlsäuerung"). Also make sure, the powder is realy inhibitor free. In that regard don´t rely on the information from the supplier certificate.

I hope some of the information cab help you


Best regards



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Posted 03 September 2018 - 07:03 AM



Thank you so much for the response, I appreciate the advice. 


Why would you say is it not necessary to test for B.cereus? Wont the heating of the reconstituted milk activate the spores? Which will then thrive if the product is kep at ambient temperatures (in Namibia we are going into summer and the temperatures range between 30-40 deg C). Do you think it is necessary to test the incoming milk powder we use for the yogurt for B.cereus


Namibia doesn't have any formal food regulations, and manufacturers mostly base their products on the South African regulations, so I will have a look at that and then look into testing for Listeria. 


We do have a heating step (30 min 97 deg C) after mixing the reconstituted milk and before the mixture is feremented. Should I then test the end product for spore formers, or the recosntituted milk before it is fermented? 


What is meant by saying the milk powder is inhibitor free? Are there certain organisms I should test for specifically with the incoming milk powder?


Thank you once again for your help! As you can see I am quite confused with what I should test for and what is not necessary, so your advice has helped a lot. 


Have a lovely day



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Posted 03 September 2018 - 07:40 AM

Hi Anna,


You may find these previous yoghurt threads of some interest -





Kind Regards,



Also tagged with one or more of these keywords: quality control, ambient yogurt, longlife yogurt, microbial testing, dairy, yogurt, reconstituted milk, microbiology

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