? Testing milk with preservatives (e.g., azidiol)
In some countries Azidiol and/or other reagents are used to preserve the milk sample from the collection to the milk payment testing laboratory. This is used when the milk sample cannot be kept refrigerated during the transport from the farmhouse to the payment laboratory or the dairy industry. Due to higher transporting temperatures and longer transport times, a bacterial contamination can affect the milk sample influencing the analysis for the milk quality payment.
The Delvotest® microbial inhibition tests are slightly affected by such preserving reagents causing the incubation time to be longer.
Multiple systems and dosages of Azidiol and or other preserving reagents can be found worldwide. In most cases also a colorant is added to visualize the milk sample is ‘preserved’.
Together with the preserving reagent the milk sample volume has an influence on the preserving reagent concentration in the milk. The milk container is filled with a variable amount of milk while the amount of preserving reagents is fixed. This can bring a big variability in the preserving reagent concentration in milk.
For example:
0.2 mL of concentrated preserving solution is added to an empty milk container of 100 mL.
Milk sample is then added to the container. Its amount can range from for example: 50 to 90 mL.
Making the final preservative concentration as approximation ranging from 0.2 mL / 90 mL to 0.2 mL / 50 mL milk.
Set the correct incubation time.
An antibiotic free milk sample with the corresponding preserving reagents must be used to set the correct so-called ‘control time’ for reading.
Minimize the variations of the milk sample volume causing variation of the preserving reagents concentration in the milk sample.
Checking the volume of the milk in the sample container and/or visually checking the milk color intensity and discard samples too “coloured” and/or with lower milk volumes.
In case these milk samples need to be tested, evaluate the possibility to standardize the milk sample volume by adding antibiotic free milk to uniform the preserving reagents concentrations.
Example of color changes in Delvotest® microbial inhibition tests.
The preservatives that contain colorants (e.g. Azidiol) will change the color of the preserved milk sample
This coloured milk can alter the intensity of the colours observed in Delvotest® after incubation.
The concentration and type of the colorant in the preservative mixture can vary worldwide and therefore the shade of colours observed at the end of the incubation might differ.
An example of how colorant can change the observed Delvotest® colours after incubation is shown in Figure 1. In this case, when blue colorant is contained in the preserved milk, the result of the negative control milk will appear more green-yellow than a milk sample that does not contain colorant.
Figure 1: Example of color variations in Delvotest® microbial tests with preserved milk.