Control your own destiny by understanding one of the most important tools of your business, your milkline and vat and how to keep it clean and bug free.

THERE IS NOTHING worse than receiving a dreaded milk quality grade. Your income takes a hit and you now know you have a lot of extra hours of work to find and rectify the grade. To add insult to injury, you lose your grade-free status!

The key to investigating and rectifying milk grades is to:

  • Work out the most likely causes of the grade.
  • Find and identify the grade and its cause.
  • Fix the grade.

We are only considering the bacterial grades, not the somatic cell grades, or the freezing point grades, or the inhibitory substance grades and so on. Those are the non-bacterial grades, and their cause is normally self-evident.

This article is just about the grades involving bugs, what most people think of as “hygiene” grades.


First, there’s plant hygiene: the cleanliness of the milking machine and the vat. Any milk residues left behind in your plant can support bacteria, which will then contaminate the milk as it passes through. Plant hygiene is the most common cause of bacterial grades.

Next, there are grades caused by cooling and refrigeration problems. Even perfectly good milk contains a lot of natural bacteria. Left unrefrigerated, or even if cooled too slowly, the bacteria numbers will increase rapidly, resulting in a grade. Laboratory experiments show that unrefrigerated milk will almost always grade within 12 to 24 hours.

Another cause of bacterial grades can be mastitis. The Strep uberis infections which are so common in the first few months of the season are a very common cause of bacterial grades, even when they don’t cause a big increase in the bulk somatic cell count.

And finally, there are bacterial grades caused by environmental contamination. That is, bacteria collected off the outside of the teats, and which originated in mud, silage, and manure.


The first is BactoScan which measures the total numbers of bacteria in a sample. The BactoScan test determines how many total bacteria, of all types, are in the sample. It includes both thermodurics and coliforms, as well as all the other more normal types of bacteria. BactoScan tells us how many bacteria are present, but not what they are. The thermoduric and coliform tests zero in on specific types of bacteria, and don’t include any of the other types.

The second type of bacterial grade is thermodurics. These are the bacteria which can survive heat. Thermodurics are most common during the warmer months but can occur anytime. The thermoduric test takes 72 hours to get a result, which usually translates into four days from milk collection.

And then there’s coliforms, which are a specific group of bacteria that are normally associated with unhygienic conditions. Coliforms are especially undesirable because they cause rapid and dangerous spoilage in food. The coliform test takes 24 hours to get a result.

Now when you have identified the cause of your grade, namely rotten milk, in the form of soft chunky single source locations or hard baked protein in many locations in your plant and vat, you now have to get rid of it. You do that by stripping the plant bit by bit. You need to do this thoroughly and check every seal, nut and bolt, the milk line, milk filter, vat inlets, vat outlets, stirrer, rubberware, clusters, rubber elbows and so on.

If these deposits are milk soil related you remove these by using strong alkaline detergent solutions and specific vat cleaning products. You soak, scrub, wash, and recycle these strong alkaline solutions through the plant and vat, until the offending grading deposit has been removed. This strong solution has historically been called a “bomb”.

If these deposits are milk stone, which is a mineral deposit, you need to do exactly the same process with a very strong acid detergent solution. These days milk stone is relatively uncommon due to routine acid wash cycles. Alkalis remove fat and protein deposits, and acids remove mineral deposits.

You are in the food business. Your task is to supply magnificent bug-free milk to your dairy company. I cannot stress enough that it is your responsibility to understand how your milk plant and vat work and all the foibles and quirks that your plant and milking platform may have. The industry has created a rod for its own back by promoting detergent salespeople as plant and vat cleaning specialists. This is not their speciality. So don’t call for your local sales rep to come and solve your milk quality problems, you are passing the buck. Control your own destiny by understanding one of the most important tools of your business, your milkline and vat and how to keep it clean and bug free.

All detergents are registered in this industry and all will do the job that is required. Remember chemicals don’t cause grades, bacteria do.

When all else fails and you are unable to find the grading problem, you can call QCONZ, who are the milk quality specialists and dairy inspectors. They will come out and investigate your plant and vat and do a milk quality traceback to identify the cause of the grade and offer a solution of how to rectify the grade.

So, the moral of the story is you are in charge of your own destiny, you control your plant and its cleanliness, and therefore you control your milk quality and your grading status.

For further detail on how to find and solve these individual grades, or the plant inspection step-by-step process, Agmax Industries Limited has an abundance of milk quality handbooks and tools available to assist in this process.

Article in Getting the Basics Right 2023, written by Paul Whitley and Bernard Kimble, Agmax Industries Limited


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