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Putting together a milk lab

I've had a milk lab on my goals list for years now! As a scientist, I wanted to have the ability to test my milk quickly and efficiently (and in a cost-effective manner). However, I've had a lot on my hands over the past 3 years, building the herdshare program and building the licensed dairy, in addition to my full time job where I was in review for tenure. Once I had the dairy up and licensed and our RAWMI listing active, I was able to prioritize putting together a milk lab.

Let's set a baseline here on what milk testing I have been sending out:

For the past 4 years, I have sent out monthly samples for Aerobic Plate Count (APC or PIC, depending on the lab you send it to) and Coliform testing. APC is a measure not only of the bacterial loads in your animals' milk, but also a measure of your sanitation procedures, as your milk lines and bucket can hold residual bacteria that will show up in this test if not cleaned properly. Coliform testing is a measure of your udder prep and animal housing cleanliness - if your animals' udders are lying in feces, and/or not prepared for milking properly, you'll see positive Coliform samples.

We also participate in DHI monthly milk testing, so our milk is also tested monthly for Somatic Cell Count (SCC). Finally, I test each doe's milk with the California Mastitis Test (CMT) weekly, and any time that I have an even slightly positive CMT test, I send that milk off to the state lab for mastitis culture. Note that this is indicative of a slightly elevated SCC - there are NO clinical signs of mastitis.

Why was it important to me to set up a lab? I have SO many reasons.

  1. With the herdshare members receiving raw milk, it's vital that the milk be super clean. Particularly with goat's milk, my herdshare members tend to be infants and immunocompromised people, so I'm very aware and conscious of the quality of milk that I send out. More frequent testing can help me have confidence that the milk is clean, rather than just once monthly testing. My raw milk pretty regularly lasts 6 weeks with no change in taste, so I've definitely got the sanitation side of things down. That said, I've had two instances over the past 5 years where the does' CMT tests were all negative but the milk started to taste a little funny after 5 days or so. Culturing every doe in the herd revealed a single subclinical mastitis case - so I'm a big proponent of testing.

  2. Mastitis Culture: I also do not use the milk until I have the results back. This means that I lose up to 7 days worth of milk, and I'm out about $30 for shipping and culture fees, for upwards of $105 in losses for each sample I send out. Additionally, 90% of the samples that I send out are negative for mastitis, so I have lost that milk and money in vain.

  3. As a licensed dairy, we are tested monthly for the Aerobic Plate Count (APC or PIC) - but the state does not test for coliform. The Raw Milk Institute requires coliform testing for their labs, and so I would still need to ship a second sample out monthly.

  4. Cost: If you calculate the costs of sanitation plate testing as well as mastitis testing, the laboratory can very quickly pay for itself.

I do want to give thanks to the Family Cow, an organic raw milk farm in Pennsylvania. Another dairy farm sent me their protocol for lab testing, so I was able to get a start and build from there.

What is needed for the lab?

  • Incubator - this is what I purchased for $160. You don't have to have a scientific quality incubator and it doesn't need to be one with humidity control or a fan to distribute the temperature. What you do need is something that will hold temperature at 32 degrees Celsius, plus or minus 1 degree. This is another model (analog 3055-80) that's often recommended, though a little pricier.

  • thermometer for incubator - any will do. I've got a ThermoWorks fridge thermometer in mine, but really anything will work. Just make sure that it is on the same shelf as your samples, so that you know the temperature is correct. Read the temperature before you put samples in each time.

  • A 100-1000 uL pipette & tips (the more precise option - just make sure you watch YouTube on how to pipette), or single use pipettes

  • Culture plates (APC and Coliform) or Petrifilms (APC and Coliform) - both of these brands are approved for dairy use. Your choice! I went with the Charm plates because they're about half the price and were with the same vendor I purchased my other supplies. You can also store them at room temperature. Manuals for use will come with each and also be listed in the 'Documents' section of the item pages.

  • Mastitis Quad plates - these plates test for general bacteria, e. coli, Staph, Strep, and gram negative bacteria all in one plate. You can find the manual (very useful!) here.

    • If you get the mastitis plates, you also need an inoculating loop. I don't like disposable things, so I got the old school type that is flame-sterilized. We have a gas stove, so that's easy to do in our home.

  • Colony Counter (I don't think I'll really need this if my milk is clean and I've got my dilution correct, but oh well - I thought I did.)

  • (optional) plate magnifier (I purchased the counter and the magnifier, but I'm not convinced the magnifier was necessary - it only magnifies 1.7x)

  • Gloves

  • Sterile tubes to collect your milk samples - I like these

  • Buffered water to dilute your milk sample in. Do not use tap water! Well water could contaminate your results and city water has bleach in it and will cause false negatives. I purchased the 99 mL Butterfield's buffered water, but in future will order the 9 mL Butterfield's buffered water (discussed below).

  • (optional but handy) pitch counter

  • A small dedicated space - my incubator is on top of a pie chest in my mudroom in the house. The plate reader sits right next to it - that's really all the space you need!

Lab Cost

All in, here is what the lab cost:

Capital Expenses:

Incubator - $160

Pipette - $37

Colony Counter and magnifier - $235

Inoculating Loop - $8

Pitch Counter - $4 (had on hand)

Thermometer - $19 (had on hand)

Total: $463

Consumable Expenses:

Shipping (colony counter and plates): $56

Coliform plates (100) - $67.50

APC plates (100) - $67.50

Quad mastitis plates (20) - $70

Conical Tubes - $15

Pipette Tips - $10

Gloves - $10

Buffered water - $44

Total: $340

If I amortize my capital expenses over 5 years, and estimate that I run 20 mastitis plates per year and 24 coliform & APC plates per year (twice a month), I estimate that a mastitis plate costs me $7.50 to run and a set of sanitation plates costs me $27 per month (2 of each APC and coliform). For reference, it costs me about $25 to ship and test 1 set of APC and coliform plates each month at the state lab. So for just slightly more, I have my own lab and can test twice as frequently. I could even test twice a week to use all of my plates in the same year (their expiration is 1 year) for $27 per month - I just don't see myself doing that timewise.

The procedures were really quite straightforward - the instructions come with the plates and are simple to follow. Just make sure that you are working in a clean space and using sterile procedure (don't reuse anything, pipette correctly, etc.) I prepared my plates and placed them in the incubator for 24 hours (coliform) - 48 hours (APC) and read the plates on the colony counter.


Reading the plates (video to come, but you can see from below that it's super easy)

aerobic plate milk culture
My first Aerobic Plate!

My findings & hints

  1. Use a thermometer ON the shelf that you will be incubating plates on. Don't trust the outside digital temperature. Test it for a few days before putting plates in to make sure that you have consistent temperature at 32C, plus or minus 1 degree.

  2. My milk was too clean for 1:100 dilution of buffered water, and too many colonies for neat (~150). I've settled on diluting the milk 1:10 for APC instead of 1:100, so that I can get a more accurate reading.

  3. Coliform was negative, as it has been for my samples at the lab, so I report it as <1 - you can never really say Zero.

  4. Quad mastitis plates - I haven't had a doe with a positive CMT test this season, so I can't yet report on it, but will update this post after I have.


As a scientist, I want to make sure that the results I get are the same as I would get in the lab. Since I have monthly samples collected by our inspector, I have that built in. My inspector came yesterday and collected milk samples for testing. Before he took them, I took a portion of the same batch and have them in the fridge until tomorrow morning, to mimic travel to the lab. I will plate and compare results when they come back from the state lab. I plan to do this every month to make sure that my results are accurate and reliable.

If you decide to build your lab, I would strongly encourage running a few validation samples through an established laboratory and your home lab simultaneously, so that you are confident in your results.

Happy Sciencing with your milk!


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