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Have questions about testing for yeast and mold in food processing? We have answers. (Part 4 of 5)

Have you seen results on your 3M™ Petrifilm™ Plates that you aren’t sure how to interpret? Here are some questions asked by food safety labs that have been answered by 3M global and U.S. technical specialists. Additional information is available for you as well on both the 3M™ Petrifilm™ Rapid Plates Portfolio as well as the 3M™ Petrifilm™ Standard Plate Portfolio.

Question: If I’m sampling for yeast and mold and answers are too numerous to count (TNTC), you suggested further diluting the sample. Should I take a sample from my tested sample and dilute with under 1ml of solution again? Do I need to leave incubating for another 48 hours?

Answer: If the results for yeast and mold are TNTC, we suggest taking the original sample and conducting several serial dilutions (for example 1:10, 1:100, 1:1000, and 1:10 000). The number of dilutions is up to you and is dependent on how many colonies are on the original plating. Additionally, if your spec doesn’t allow for any counts beyond a certain dilution, it may be wasteful to dilute any further as the sample already fails the specification.

After diluting the sample, 1 mL of each dilution can be plated and incubated for the time indicated by the method.

Question: What dilution media is best for mold and yeast?

Answer: FDA Bacteriological Analytical Manual (BAM) and ISO 21527-1 guidance is to use 0.1% peptone water. Standard Methods for the Examination of Dairy Products is Butterfield’s Phosphate Buffered Dilution Water. 3M Petrifilm Plates can be used with either in addition to other appropriate sterile diluents (see package insert for more information).

Question: Sometimes it’s hard to differentiate between mold and yeast on the 3M Petrifilm Plate, even using the descriptions of each. What are some other ways to tell them apart?

Answer: In addition to the descriptions provided in our interpretation guide and the training webinar, we find that practice and the use of microscopy are the best ways to gain confidence in differentiating between yeast and mold colonies. We can also provide additional training to help. It is important to remember that even the most skilled microbiologists cannot successfully differentiate between colonies on plates 100% of the time regardless if the medium is agar or 3M Petrifilm Plates.

Question: Do yeast and mold colonies also grow on Aerobic count plates ?

Answer: Maybe. Yeast may or may not react with the indicator in the 3M™ Petrifilm™ Aerobic Count Plate or 3M™ Petrifilm™ Rapid Aerobic Count Plate and may appear as colorless colonies. While the likelihood of mold growing at elevated temperatures (such as 32°C or 35° C) within 24 or 48 hours is low, we have seen it happen. For the reasons above, to get the most accurate yeast and mold count we suggest using a yeast and mold enumeration method in addition to total plate count methods.

Question: What is the pH range of the 3M™ Petrifilm™ Rapid Yeast and Mold Count Plate and the 3M™ Petrifilm™ Yeast and Mold Count Plate? I have used both types of plates to inoculate a very acid product and the rapid plate turned blue, while the standard plate did not.

Answer: 3M Petrifilm Rapid Yeast and Mold Count Plate and 3M Petrifilm  Yeast and Mold Count Plate do not have recommended pH ranges as most fungi are acid tolerant. While verifying either method for products that are very acidic, you may find that adjusting the pH of the sample will help recover any fungi that cannot tolerate very low pH environments. 3M Petrifilm Rapid Yeast and Mold Count Plate uses a different indicator system than 3M Petrifilm Yeast and Mold Count Plate. Therefore, it is possible that there is something in the sample reacting with the indicator in the 3M Petrifilm Rapid Yeast and Mold Count Plate that is not reacting with the indicator in the 3M Petrifilm Yeast and Mold Count Plate.

Question: On Rapid Yeast and Mold plates, for estimation by averaging, do you multiply by 20 or 30 like the regular Yeast and Mold plates?

Answer: Both the 3M Petrifilm Rapid Yeast and Mold Count Plate and 3M Petrifilm Yeast and Mold Count Plate have an area of 30 cm2 which should be used when estimating counts.

Question : Will yeast grow on the Lactic Acid Bacteria Count Plate ?

Answer: The 3M™ Petrifilm™ Lactic  Acid Bacteria Count Plate contains a selective agent to inhibit yeast growth. Like most media, high concentrations of yeast can overcome the selective agent. In the case of the 3M Petrifilm Lactic Acid Bacteria Count Plate this may appear as excess gas production on the initial dilution. The secondary dilution will have no colonies.

Want more information on how we can help with total 3M Food Safety testing solutions? Simply reach out and we’ll be happy to help.