This quick guide explains how natural inhibitors can affect the test result and what actions need to be taken when testing milk samples that contain them.

Natural inhibitors

'Natural inhibitors' appear in milk samples mostly due to sick animals with high somatic cell counts and/ or colostrum milk (after calving). This type of milk is not meant to be added by the farmer to the bulk tank. The yoghurt and or cheese production can be interfered, also when there are ‘natural inhibitors’ in the milk.

Delvotest is a microbial inhibition test, meaning that a positive result indicates a true positive for growth inhibition. It is true that this kind of POSITIVE result is not due to antibiotic residues, as an result of incorrect use of antibiotics (such as wrong withdrawal times or administrative errors).

Natural inhibitors (such as lacto-ferine, lacto-peroxidase) are not very frequent found in milk.

Our advice is when a POSITIVE is found repeat the milk sample after a heat treatment:

• Treatment option 1: Heat the sample as 'au bain marie'* for 10 min at 80 °C

• Treatment option 2: Heat the sample as 'au bain marie'* for 5 min at 100 °C

* for a 2ml milk sample in a glass tube, create a 'heated bath' submerge the milk sample and hold for the indicated time ensuring the temperature remains stable.

Colostrum milk

Colostrum deviates from “normal” milk and shall not be added to the farm bulk tank. Unless approved for special operations / productions, were as the receiver of this colostrum bulk tank milk is able and intended to process.

In the colostrum milk composition changes in concentration over time for example given; antibodies IgG, fat, protein and lactose.

Due to the higher concentration of natural inhibitors (example given; lacto-ferrin, lacto per-oxidase) Delvotest will give positive/purple results due to the presence of these natural inhibitors, when comparing with typical raw milk samples.

Fig. 1. Spectrum of colostrum colors in dairy cows compared with mature milk (right) [1].

[1] Gross, J., Kessler, E., & Bruckmaier, R. (2014). Colour measurement of colostrum for estimation of colostral IgG and colostrum composition in dairy cows. Journal of Dairy Research, 81(4), 440-444. doi:10.1017/S0022029914000466

In principle, the whiter samples of colostrum milk (not the brown ones shown on Figure 1) could be tested with Delvotest only when following a validation with the following attention points:

• Use negative samples of colostrum milk to set the required incubation time. (Control Time).

• Use positive samples, spiked samples of colostrum milk, to confirm the test sensitivity.

The user is responsible of the local validation made as described above.

? Got any questions? Need help? Get in touch support@delvotest.com