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Bacteria are amazing little critters. They can do so much good, such as fermenting the feedstuffs in cows’ rumens to provide them with the nutrients to live, grow and produce milk. They can also do so much bad, for example E. coli and Salmonella can cause severe diarrhoea and sometimes death in calves.

As you can see in this cool video, bacteria grow by splitting – so each generation is twice the size of the one before. For example, if you start off with one bacterium in one millilitre (mL) of milk, in one generation there’s 2, then 4, then 8, then 16 and so on. In 10 generations there’s more than a thousand bacteria in that mL, and after 20 generations there’s a whopping one million bacteria.

How quickly bacteria grow (the generation time), depends on the conditions that they’re living in, such as the temperature, pH and nutrient source.

First-milking colostrum is full of antibodies and white blood cells that help newborn calves fight disease. It’s also got more fat and protein than normal milk to give calves nutritious first feed (see table below). The trouble is – it’s delicious for bacteria as well! If colostrum is stored at ambient temperature, E. coli and Salmonella can double every 15 –50 minutes, whereas under 5°C growth is practically nothing. To put it in perspective, if you have freshly collected colostrum that is sitting at ambient temperature with just one E. coli bacteria in it, after 5 hours there will more than a million of these nasty critters.

Key point: even small numbers of bacteria at the start can result in high levels of contamination given time.


Component First-milking colostrum Milk
Total solids % 23.9 12.9
Fat % 6.7 4.0
Protein % 14 3.1
Casein % 4.8 2.5

From Colostrum Management for Dairy Calves, Sandra Godden. Vet Clin Food Anim 24 (2008) 19-39.

 The Vet Group Microbe Maths Graph

This diagram from https://dairyprocessinghandbook.com/chapter/microbiology illustrates the effect of temperature on bacterial growth.


What can we do about bacterial growth?

Adding a preservative like potassium sorbate to colostrum can reduce bacterial growth.

Total plate counts are a way of measuring how many total bacteria (good and bad types) are present in milk and colostrum. A result of 1500 cfu/mL means that there were 1500 bacteria that were able to grow when the sample was plated on the agar.

Below are some total plate counts of first-milking colostrum that was divided into 4 and stored for 30 hours: at room temperature +/- potassium sorbate and in the fridge +/- potassium sorbate.

When refrigerated, the number of bacteria was 3x less with potassium sorbate. But when stored at room temperature the difference with potassium sorbate was a whopping 2000x less (for the record, the target for colostrum is a total plate count of less than 100,000 cfu/ml).

The take home from this? If you don’t have a fridge for storing colostrum, or if you are trying to cool large volumes of colostrum at a time, add potassium sorbate to slow down bacterial growth.

Potassium sorbate and instructions on how to use it are available from our clinics.

The Vet Group Microbe Maths