Testing raw milk quality

Raw milk has many benefits but also poses a higher risk of bacterial contamination. Many farmers do their best to keep things sanitary but we have a few tips for you to be sure.

Smell and taste

Raw milk when fresh should have no specific “off” odor or taste. In the fridge it should keep for about 3 days at the very least, given that your bottles were clean before filling. If your milk develops an off taste or smell after 3 days it’s an indicator that there might be bacterial contamination.

Testing the bacteria culture load

The most common test for raw milk is a Methylene Blue Reaction Test (MBRT). This is used by many farmers as well since it’s a cheap and effective way to get a measure of the bacterial load in your raw milk.

How does it work?

Raw milk naturally contains bacteria. The test checks for the total bacteria load, which in non-contaminated milk should not be very high. Pathogenic bacteria multiply rapidly, so if the test shows a high load of bacteria in fresh raw milk it’s an indicator of sanitary issues in the farm or the milk bottling/dispensing equipment.

Doing your own MBRT

For this test you need a thermometer, a water bath, 1ml of 0.05% methylene blue solution (you can buy it online for cheap or make your own from bulk powder), your fresh raw milk sample and a sterilized vial or closable container to hold your milk during the testing.

  1. Sterilize your vial/closable container.
  2. Heat up the water bath to 37° C and keep it there throughout the entire test.
  3. Fill 10ml of fresh raw milk into your sterile vial/closable container.
  4. Add 1ml of 0.05% methylene blue solution to the milk.
  5. Seal the vial/container.
  6. Very lightly shake or turn the vial/container a few times to disperse everything.
  7. Add the vial to your water bath.

Ensure your water stays at 37° C if possible since that is the best temperature for bacteria to grow. If the temperature drops too low or too high the interpretation results at the end won’t be as accurate.

Next up, check back on your test every 30 minutes. You are looking for color changes. The methylene blue in your milk will gradually disappear as the bacteria multiplies. As soon as the blue liquid has turned back to pure white or translucent color, refer to the reference ranges below:

  • < 30 minutes: Very poor milk quality (should not be consumed)
  • 30 minutes to 2 hours: Poor milk quality (avoid consumption)
  • 2–4 hours: Fair milk quality (ok to consume)
  • > 4 hours: Excellent milk quality (gold standard)

Keep in mind that your raw milk sample should be fresh. Test it immediately after you’ve acquired it. Do not bother testing old milk.