Who would have thought having your own on-farm lab would be so easy, and affordable? I am kicking myself for not trying this sooner. Doing in-house tests help our small licensed dairy stay on top of cleaning regimens and monitor milk quality. The results are private and allow us to change our protocols BEFORE any problems occur. Though only a certified lab can provide official results for inspectors, DIY tests assist us in knowing that our food safety program is complete. Here's how we do it:

Aerobic Plate Counts

The most basic lab test for counting bacteria is called the aerobic plate count, or APC (also known as a standard plate count or SPC.)

APCs are preformed by taking a small sample of the substance to be tested and swabbing it onto a petri dish, the "plate," which is coated with a growth medium. The plate is incubated for a certain number of hours to allow bacteria to multipy, and then a count of the number of microbial dots is performed. These dots are clusters of bacteria, called "colony forming units" or cfu for short.

Note that an APC itself is not a specific test for "bad" bacteria. A plate will grow all kinds of aerobic (oxygen-needing) bacteria, from undesirable illness-causing bugs to vital cheesemaking microbes. So if you took a sample of milk after culturing in the vat, the plate count would be through the roof; but that's what you'd want. Milk fresh from the udder, however, should have very low levels of all bacteria.

The Dairy Practices Council lists an ideal APC number as < 1000 cfu/ml,  and an acceptable number as < 5,000 cfu/ml. The regulatory limit (the number at which the milk would be considered in violation) is quite high: 100,000 cfu/ml for a single producer and 300,000 cfu/ml from a comingled sample (such as from a milk tanker truck). Obviously, for raw milk products the lowest possible level should be the goal, despite what the regulations allow!

Equipment

3M makes simple paper petri dishes called Petrifilm plates, which are perfect for our purposes. These thin paper films are ready to use, needing no added growth medium. They are also inexpensive, costing about 70 cents each, and come in a box of 100. Each packet of plates also comes with a plate spreader, a little plastic disc made especially for spreading the sample onto the plate.

For sampling liquids, you'll also want a sterile no-needle syringe or pipette with milliliter gradations. You can buy disposible syringes from suppliers like Nelson Jameson for as little as 15 cents each, or reusable, sterilizable syringes from your local pharmacy or feed store.

Finally, you will need an incubator. A compact, low-tech unit costing about $70 is available from Nelson Jameson. They also sell Petrifilm plates and other supplies.

(If you want to do environmental tests, such as checking equipment and surfaces, you might want to do swab tests. For that you will need the 3M Quick Swabs which cost about $1.50 each and come in a box of 50. Petrifilm plates can also be used to sample the air and surfaces directly, without using swabs. I’ll cover these procedures in the next installment.)

A fine-point Sharpie or other felt-tiped permanent marker for counting cfus.

Of course, you will also want a log book to record your information.

Procedure

When collecting and plating samples, it is important to practice careful technique. Your plating area should be as clean as possible and free from dust and airborne contaminants. When collecting a milk or brine sample, make certain to mix the bulk fluid well with a sanitized utensil. Be sure that the sample container is sanitized and that your hands, gloved or not, do not contaminate the utensil or collection container.

1. Retrieve a plate from foil pouch and reseal tightly. Label the plate with date and sample source.
2. Collect a sample from thoroughly mixed solution.
3. Remove a 1 ml sample using a sterilized syringe or a pipette.
4. Save the collected sample until results are complete (in case dilution is needed—see below)
5. Lift the film on the Petrifilm plate and place the sample in the center.
6. Lower the film gently.
7. Center the plate spreader, smooth side up, over the sample, lower onto film and slowly press down, from the center out, to spread the sample in an even circle.
8. Let plate set for one minute to allow growth medium to gel.
9. Place the Petrifilm in the incubator at 90℉ (note: the compact incubator from Nelson Jameson states that the shelf temperature is 10 degrees lower than the thermometer readout, so the thermometer should read 100℉) and incubate for 48 hours. The instructions say the temperature can be + or – 1 degree ℉.
10. After 45-50 hours (48 is ideal) remove the plate from the incubator.
11. Using a fine-tipped Sharpie pen, count each red dot, no matter how small, using the pen to mark as you count (so that you don’t double count any cfus).
12. If the plate has very few red dots, then count the entire plate. If there are quite a few, you can count several squares, obtain an average, and multiply the result by 20. (Each square represents 1 square centimeter and the plate area is 20 square centimeters, thus the multiplication by 20)
13. As a part of a good food safety program, document each result in a log book with date, source, etc. for later reference. I also take a quick digital photo of each plate.

Overcrowding and dilution: If a sample is packed with overlapping colonies, it can be impossible to count. To solve this problem, carefully dilute your reserved sample with sterile water by 50% and then multiply the resulting count by two.

For example, say I have an overcrowded plate. I dilute 1 ml of milk with 1 ml sterile water, and add only 1 ml of the resulting mixture to the plate. The new count reads 250 cfu’s per square. I would then multiply that number by 2 for 500, and then multiply that by 20 for 10,000 cfu/ml. Please note, this is not the official way to do a dilution! But it should supply sufficient information for use.

Remember: You may not run tests for anyone other than yourself. You can let people run their own tests using your incubator, but you may not run a test and provide a count result for others—that is only for certified professionals. It doesn’t mean you can’t have a “plating party” and show other folks how to take their own counts!  

Frequency

Our current protocol here at our farm is to test every vat of cheesemaking milk and test our brine monthly. Brine is only used twice a week and so far every test shows zero growth. If we had any growth, we would implement a more frequent testing schedule.

Storage

Petrifilm plates should be stored in a cool, dry area. Be sure to tightly re-seal the individual film packets using tape. Remember they can be inoculated by exposure to the air, so sealing tightly is important. Aerobic films can be stored in a freezer or refrigerator. In order to be considered accurate, the product literature says that they should be used within a month of opening the foil packet, but for unofficial, in-house use, they seem to last far longer.   They come in a box of 100, there are two packets of 50 each. So only one packet of 50 is open at a time. It is quite easy, if you like, to split the order if you know another cheesemaker or dairyperson that is also interested in testing. This will help ensure the plates remain “fresh”.

Disposal

Official guidelines call for the treatment of inoculated plates as biohazards and disposal in an autoclave, however, my sources tell me that for simple APCs, disposal by double bagging and placing in the trash is very common. If you have safe facilities for the incineration of waste, you can deal with them in that fashion, too. Because you are not growing anything other than a mix of what is already growing on your farm, it is not a serious concern. The same is not true for listeria or other pathogen-specific cultures!

Finally

Pre-packaged supplies offer the small cheesemaker an affordable way to test for bacteria and keep track of cleanliness. I have used the tests to help motivate our daughters to collect the milk more cleanly—we had a little contest to see whose milk grew the fewest cfus. Lots of fun and a very good home science lesson to boot!

Although the results are unofficial, testing and logging your results will establish a record showing proper intent and good practices when your facilities are inspected.

I am excited about the potential these test hold for improving our process and proving that our practices here on our small, on-farm creamery are producing high quality milk and cheese- raw milk and raw milk cheese.

Resources

Alternate instructions for performing aerobic plate counts from 3M
Other dairy industry testing products from 3M

Wehr, H. Michael, Standard Methods of Examination of Dairy Products, 2004, American Public Health Association.

Special thanks to my lab mentors: Shawn Fels from our neighbors at Rogue Creamery, and Sonny Simonian of Cypress Grove Chevre, for helping me to play lab geek!