Doing Standard Plate Counts- On the Farm

Aerobic plate count Petrifilm Plate, red dots are colony forming units, black dots have been counted using a Sharpie pen

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 our own, in house, milk quality tests will help our small, licensed dairy to stay on top of cleaning regimens and milk quality. Even though our results will not be official (you have to be a certified lab to have official results) they will still assist us and even help inspectors know that our food safety program is more complete. So just what is a “plate count and how do you do it yourself?

Plate counts were traditionally preformed by taking a small sample of a substance and pouring or swabbing it onto a glass petri dish that held had a gelled growth medium. The plate was then kept warm for a certain number of hours after which a lab technician would literally count the number of “dots” on the plate. The dots were each a cluster of bacteria called a “colony forming unit” (cfu for short).  The most common plate test is the “standard or aerobic plate count” (SPC or APC).

Fortunately for us, 3M makes a wonderful, simplified product called Petrifilm Plates. These plates are ready to use, needing no added growth medium. They are also inexpensive, costing about .70 cents each (for the aerobic count) and come in a box of 100. You will also need an incubator and luckily, a compact, low tech unit (costing about 70.00) is sold by the same company from which we already buy a lot of our supplies, Nelson Jameson ( They also sell the 3M Petrifilm plates and other needed

Compact Incubator from Nelson Jameson

supplies. In addition to the aerobic count plates, it is a good idea to also buy coliform plates (a box of 50 is 38.00). You will need a count plate spreader (a little plastic disc made especially for spreading the sample onto the Petrifilm plate) and, if you want to do swab tests on dry surfaces, 3M Quick Swabs work great. The Quick Swabs are a bit more expensive, about 1.50 each and come in a box of 50.

The SPC grows all kinds of bacteria from milk or swabs of surfaces- even the good bacteria. For example, if you took a sample of milk during cheesemaking, the plate count numbers would be through the roof, but that is what you would want. Milk fresh from the udder, however, should have very low counts, preferable less than 1,000 cfu per milliliter.


Step by Step Instructions for Plating a Milk Sample

  1. Obtain a 1 ml sample of milk using a sterilized 1ml syringe or a pipette.
  2. Lift the film on the room temperature Petrifilm plate and place the sample in the center.
  3. Lower the film gently.
  4. Center the plate spreader, smooth side up, over the sample, lower onto film and press firmly to spread the sample in an even circle.
  5. Place the Petrifilm in the incubator at 90 F (note: the compact incubator from Nelson Jameson states that the shelf temperature is 10 degrees lower than the thermometer readout, so adjust your temperature accordingly) and incubate for 48 hours.
  6. After 45-50 hours (48 is ideal) remove the plate from the incubator.
  7. 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 cfu’s).
  8. If the plate has very few red dots, then count the entire plate. If there are quite a few, you can count one square and multiply the result by 20.  Do this with several squares so that you get an accurate average. (Each square represents 1 square centimeter and the plate area is 20 square centimeters, thus the multiplication by 20)


If an undiluted sample grows too many cfu’s it is impossible to get a good count, since the plate will be over crowded with overlapping colonies.  You can carefully dilute the sample with sterile water by 50% and then multiply the resulting count by two. (For example, say I diluted the ml of milk with half sterile water and then count between 200 and 250 cfu’s per square. I would then multiply that number by two for 400-500, and then multiply that by 20 for 4,000-5,000 cfu/ml.)

Another useful Petrifilm plate is called the coliform count plate. These have a growth medium that will only allow for coliforms (harmless and bad) to grow. So if you want to know how many of those cfu’s on your standard count are coliforms, this test is a great follow up. Coliforms are the most common problem bacteria in milk and in a cheese plant (and sometimes the deadliest). So low coliform counts from work surfaces and equipment, as well as in milk and brine, are a great confirmation of good processes. Coliform counts should be much lower than SPC’s, a reading of less than 10 cfu/ml is ideal.

Petrifilm plates should be stored in a cool, dry area. Be sure to tightly seal the individual film packets. They are so sensitive that they can simply be exposed (with the cover film pealed back) to the air and culture contaminants via that route. So you don’t want to expose them until ready to inoculate.

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

There are several other Petrifilm plates that I will probably try out on our farm, including staph aureus for udder health, and yeasts and molds, for cheese quality. I’ll be sure to share whatever I learn with all of you.  But for now, this is enough to work on. Oh, as a way of logging our results, I plan on take a photograph of each counted plate and keeping those on file.


Your Happy Lab Geek, Gianaclis


Doing your own Plate Counts

I finally plugged in the little petri-film incubator we purchased from Nelson Jameson and I am, at the moment, cooking our first anaerobic plate counts.  It took me a long time to get around to this, but I think it will go a long way toward making sure our milk is super clean, as well as our process.

The films must incubate for 48 hours, so I don’t have any exciting things to share with you guys, other than I am pleased that I finally tried it!  My friend at Rogue Creamery, Shawn Fells, showed me how to do these simple, on-site quality tests for milk and environment, but I was still intimidated, I have to admit! Turns out it is as easy as squirting 1ml of milk on a plate and sticking it in to cook (much simpler than making dinner, right?).

I’ll write a full description of how to do it (maybe a YouTube video for you all too?) once I figure it out and have a better idea on how to implement it as a part of our quality assurance program.

Oh, the little incubator was under 100.00 and the plates are about 7.00 each. Still cheaper than shipping samples out for testing (or having your inspector let you know your milk is not as clean as hoped).

So pictures and updates to come, unless I botched the entire process….

Keeping your Home Milk Supply Clean

Here is another article I wrote about a year ago.  I did a little study using several local volunteer farms who promised to not change anything about their milking techniques and allowed me to take samples of their milk, which I then sent to a certified laboratory for testing.

How Clean is Your Home Milk Supply-Really?

By Gianaclis Caldwell

First Printed in United Caprine News, September 2010

Like most people who have a few dairy goats you probably believe the milk you drink and feed to your family (and maybe even sell) is clean and safe.  But you might want to think again.

We run a small, licensed cheese dairy inOregon.  We too thought our Grade A milk produced in our inspected, licensed facility was as pure as it could be.  After all, the state conducts routine tests on our milk to verify its safety, but when we first got started I didn’t know how to correctly read the results from these tests. I assumed that if the numbers were less than stellar, that someone would tell me, but in reality, only when the bacteria counts reach the official violation level is the dairy owner usually informed.

Without our noticing, the bacteria counts began a slow and steady climb that suddenly shot sky high.  While the milk was still technically safe to use for cheese, it was far below the standards that are ideal.  This was a pivotal moment for us.  It did three things: It caused me to study and learn how to read and interpret milk lab tests, inspired me to pay close attention to the lab results when they arrived, and completely changed our approach to sanitation in our milk processing areas.

After experiencing this from the standpoint of a licensed producer that believes in the right to drink raw milk, I started thinking about the home producer and their milk quality.  If a licensed dairy performing rigorous cleaning rituals could find their milk not clean, what about the home producer?  Here inOregon, raw milk can be sold legally direct from the farm, as long as only a specific herd size is not exceeded and other criteria are met.  So not only could the home producer be serving tainted milk to their families, but they could be selling it to any number of people for whom bacterial contamination might cause health concerns.  Even beyond the health issues, there is the flavor concern.  A common cause of poor flavor in milk (especially goat milk) is the breakdown of fats by bacterial enzymes. So, for quality milk from all standpoints, cleanliness is paramount.


The Study


I decided to recruit some volunteer farms that would allow me to collect samples of their milk to send to a lab for testing, as well as fill out a questionnaire about their milk sanitation practices. Each producer promised to keep their cleaning and collecting routine the same.  The results of the lab tests were amazing; from milk that was extremely clean, to one that was over the usual violation level. The good news was that even the “dirtiest milk” did not have high levels of the most common pathogenic (illness causing) bacteria e.coli. (The samples were not tested for another bad bug, listeria)

I collected two milk samples from five farms.  One sample was fresh, one just a few days old. Two of the farms hand milked and three used milking machines.  The samples were packed on ice and overnight shipped to Agrimark Central Laboratory inMassachusetts. (I chose Agrimark for their easy to read and order “raw milk profile”)  Before we go into the results, lets go over a little background on just what kind of tests can be done on milk and what these tests can tell you.


Milk Quality Tests


Everyone who milks their goats, cows, or sheep should know a little bit about milk quality tests.  The first thing you should know is that anyone can have their milk tested.  It is not expensive (the tests run in this study cost about 20.00 for each sample) and can be very helpful in you quest for producing good, safe food for your family and neighbors.

The first, and most common, test done on milk is the raw standard plate count (SPC) (some times called simply “raw count”). The SPC counts the TOTAL amount (measured in colony forming units) of bacteria in the milk.  Both good and bad.  It is a starting point for looking at the milk quality.  For example, if the SPC is extremely low, you can bet that the bad bacteria count is also low.  But if the SPC is high, it doesn’t necessarily mean that the milk is “bad”.  If you are a cheesemaker, then you know that by adding culture, you are adding bacteria to your milk.  Sometimes the home milk supply is “pre-cultured” with some pretty good bacteria from your animals.

The next test is the pasteurized count.  For this test, the milk is pasteurized and then the bacteria units are counted.  Remember, normally pasteurized milk (in other words NOT ultra-pasteurized) is not sterile, a certain number of bacteria do survive.  These are the heat loving bacteria (thermophiles).  For most of us who are drinking and using our milk raw, the pasteurized count is of little significance.

The test that I find most interesting is the preliminary incubation count (PI or PIC).  For the PI, milk is held at 50 degrees F for a period of time and then the bacteria colonies are counted.  Bacteria that like the cold (psychrohiles) will grow well under these conditions.  They are of great importance for issues regarding milk flavor when the milk is stored for periods of time.  Once the milk is chilled, the cold loving bacteria will continue to grow (very slowly if the milk is about 38 degrees F).  Some bad bacteria are psychrophiles too, such as listeria (a potentially deadly pathogen).  So knowing what your milk’s PI is after it has been stored for a while, can be very useful information.

Then there are specific tests for certain bad bacteria such as e. coli, staph aureus, and listeria.  First a count is done for the total number of coliforms (of which e. coli is the worst).  If that number is low, then the e.coli number will be of little value, but if it is high, then it is important to know just how much of those coliform colonies are e. coli.

Staph. aureus is a very common cause of clinical and subclinical mastitits in lactation animals.  It is also of increasing concern for human health.


The Study Results

First lets take a look at the lab results as a chart as well as the acceptable ranges, then I’ll break it down to analyze each farm and their procedures, test results, and some possible remedies where needed. Remember, I took a sample of milk that was 5 days old when tested (numbered “1”) and a sample that was 2 days old when tested (numbered “2”).  Then I will go over the suggested cleaning regimen for milking equipment.  But, as you will see from the volunteer farms, many of them perform cleaning procedures that vary greatly from those suggested and are evidently working quite well! ( If you are unsure of how to determine measurements of products and proper use, contact the supplier, they should be able to help you.)


Bacterial Testing Results


Farm Sample


Past. Count


Total Coli

E. coli


































































Milk Test Result Ranges (From Dairy Practices Council)





Common Industry*

Regulatory Limit

Raw Count





300,000/commingled tanker




20,000/ml (pasteurized milk)

PIC (should be <3-4x SPC)



No legal limit




<10/ml (pasteurized milk)


*Related to premium payments within industry

 Farm A

This farm’s milk was pretty clean. They hand milk into a hooded pail in a fairly typical setting- milk stand is in the barn with the animals. Teats are not dipped after milking (note, the farm with the cleanest milk in the study did not post dip either). The milk is chilled by placing in a refrigerator next to the vent from the freezer compartment.  They were unsure of how long it took for the milk to cool completely.

If you will notice, their raw counts are not too bad, but the PI is quite high on the older milk sample.  This indicates that there has been a significant growth of cold loving bacteria while the milk was stored, usually indicating that the milk did not reach ideal storage temperatures rapidly enough, or that it warmed up a bit during storage.  Interestingly, the fresher sample had a higher bacteria count and the staph count was elevated, but the PI was low.  The fresh sample was warm when I collected it from the farm.  I cooled it rapidly in my collection ice, this probably led to the much reduced PI count.

Suggestions: Cool the milk more rapidly and check the milking does for subclinical mastitis with an on-farm somatic cell (SCC) test such as the California Mastitis Test (CMT).


Farm B

This farm’s milking procedure, according to their questionnaire, sounds pretty thorough- so it was initially baffling to see the very high bacteria counts.  Farm B uses a milking machine, washes the udders with hot water and a wash cloth, pre-strips, post dips, cools the milk in a refrigerator, and cleans the milking equipment with chemicals and cleaners designed for the job.

The older milk sample has a raw count far beyond the violation level and an extremely high PI. The fresher sample has a much lower raw count, but still has an out of proportionally high PI.  It also has a coliform count 10 times the ideal.  So what could the problem be?

Suggestions: The first place troubleshooting should be checking the milking equipment for cleanliness. Using a magnifying glass and strong flashlight, each component should be disassembled and inspected. A common area for buildup is in the typical goat claw used on many milking machines. In addition rubber gaskets and hoses might need replacing.  Remember, microscopic nicks and grooves can easily harbor colonies of bacteria that your fresh milk will wash over during each milking- providing food for these colonies and contamination for your milk. After addressing these issues, a ramped up cleaning should be preformed using double the amount of detergent, utilizing 140 degree water (140 degrees at the start of cleaning will ensure it is still hot enough at the end of the cycle), and performed for 10-15 minutes (or until the water drops below 120 degrees).  This should be followed with a 5 minute acid wash. After this “super cleaning”, they will want to ensure that their water is the right temperature (usually 120-140 degrees) during daily cleaning.  When wash water drops below the ideal, it re-deposits proteins and fats on the equipment. The other factor that can limit the effectiveness of cleaning is the pH and hardness of the water source.  Farm B also does not pre-sanitize their milking hoses, this step should be considered as well.  The last thing that Farm B should do is chill the milk more rapidly, this will help prevent that elevated PI.


Farm C

This farm hand milks into open pails, does not post dip teats, cleans the milking equipment in an automatic dishwasher and routinely uses an acid wash.  They milk two goats and then filter that milk and begin its cooling before proceeding with the next goats, this is probably contributing to their lower PI counts.

The older sample has a remarkable low raw count and an acceptable PI, the newer sample has a bit higher raw count and an elevated staph count (probably leading to the higher raw count).

About the only suggestion I have for this farm is routinely checking does with a somatic cell detecting test such as the California Mastitis Test (CMT), as staph is often present in mastitic milk, even in subclinical cases (where no clumping, stringing, or udder heat is present).  Good job!


Farm D

This farm uses a milking machine, uses single use wash clothes for cleaning udders, and does not post dip teats.  The milk is chilled within 10 minutes of milking by filtering into a container which is then set into an ice water filled pail. They use antibacterial dish soap to manually clean their milking hoses and equipment.

While I spilled the fresher of the two milk samples when preparing them for shipping, Farm D’s older milk sample is amazingly clean.  This farm claimed that their milk would maintain its flavor for up to two weeks, and I believe it!

Suggestions: Keep doing what you are doing!  Bravo!


Farm E

This farm uses a milking machine, cleans udders with a quaternary udder wash, and post dips teats. The milk is filtered into a clean container within 30 minutes of beginning to milk and placed in a refrigerator. The milking equipment is cleaned by pumping rinse, wash, and acid rinse solutions through the hoses.  155-180 degree water (although water temperature does not need to be higher than 140, especially when low temp detergent is used) is used with lo-temp automatic dishwashing detergent with chlorine added.

Both samples have raw counts above the acceptable level and PI counts out of proportion to the raw count (remember, the PI is typically 3-4x as high as the raw count). Total coliforms on the older sample are quite high as well.

Suggestions: The cleaning procedure sounds good on first examination, but obviously something is missing. One thing to remember is that each cleaning step has an ideal amount of chemical, temperature of water, and length of time. Any gap in these steps can lead to problems. It is possible that the length of cleaning time is inadequate, the amount of detergent is not sufficient, or even the water type is negatively effecting the performance of the chemicals.  As with Farm B, the equipment should be disassembled and thoroughly inspected and a super cleaning should be performed.  The milk chilling time and temperature goals should also be improved, rarely is chilling by setting the container in a refrigerator sufficient.


Proper Daily Cleaning of Milking Equipment




Common Errors


Rinse all equipment with 100-110 degree water. Too hot of water causing proteins to bind to surfaces or too cold causing fats to form a greasy film.


Use a chlorinated alkaline detergent mixed in the right amount for your water type (hard or soft and pH) in water that is 120-140 degrees F.

For hand washing, soak all parts in this solution for five minutes and then clean with brushes.

If pumping through the lines, clean for 5-10 minutes, making sure water does not cool below 120 degrees.

Drain wash water.

Water too cold or cooled to below 120 by end of cleaning, causing the re-depositing of fats and proteins.

Not cleaning for long enough.

Not disassembling and manually cleaning parts not cleaned well by pumping or brushes.

Acid Rinse

Mix acid rinse in lukewarm or cool water in amount directed. Soak or circulate solution for 2-3 minutes.

Drain and hang equipment to air dry.

Not using an acid rinse at the proper pH can leave a residue of the alkaline detergent, allow buildup of minerals, and leave the equipment more vulnerable to bacterial contamination between uses.


Just before use, sanitize all equipment with a chlorine or other type sanitizer mixed as directed in cool water for 30 sec min.

Drain well.

Too strong of a solution will pit and erode stainless steel and leave a residue on hoses. Too weak will be ineffective.

Putting it all Together

As they say, milk was never meant to see the light of day- it was meant to go directly from the mother into the mouth of her offspring.  Once we interrupt that cycle and attempt to collect and keep a product that was never intended to be handled in that fashion, we increase the odds of reducing its quality and safety.

While using a milking machine is often thought of as the cleanest method available for collecting milk, this only holds true when a very strict regimen of cleaning is adhered to.  Milking hoses and paraphernalia have many nooks and crannies which make thorough cleaning a challenge. Other than cleaning of equipment, proper chilling of the milk seems to be the most common area that needs improving.  I think once the milk is in our refrigerators, we kind of let our guards down, it seems safe in there! But if you try to picture the milk as a growth medium, for not only your family, but for the hoards of bacteria, both good and bad, that are on the animals, in the air, and possibly on our equipment, then you more likely establish procedures for ensuring the safest and tastiest home milk supply possible.

GC’s Ten Tips for Collecting Clean Milk and Keeping it Clean

  1. Give your doe a dairy clip- clip hair from udder, belly, legs and around her tail for the latest in trendy, dairy fashion!
  2. Milk with dry, clean hands- never wet!- or even better, use a pair of disposable gloves while milking. They say that dirty gloves are probably cleaner than clean hands!
  3. Milk in to a hooded pail or place your stainless strainer (with a fresh, clean, single use filter in it) on top of the pail during milking!
  4. Make sure your milk jars are clean and sanitized before adding warm milk. Remember, any bacterial residue in the container will love the warm milk!
  5. Keep it on ice- right after milking, set the filtered milk into a pail of ice water and stir.  If you can get the temperature to 40 degrees within 2 hours of milking, you are chilling perfectly.
  6. Clean up immediately! Never put off cleaning your milking equipment.
  7. Use cleaning detergents and sanitizing chemical in exacting amounts and with the right temperature water.
  8. Let equipment dry between use- bacteria need moisture to surviv- a wet surface is likely a contaminated surface!
  9. Don’t forget to replace any rubber, plastic, or silicone parts of your set-up as recommended!
  10. And last, use your milk within a week, it might still taste good, but any cold loving bacteria will be developing a population by then.