Short communication: Effect of storage and preservation on total bacterial counts determined by automated flow cytometry in bulk tank goat milk

This study was designed to evaluate the effects of different storage conditions on total bacterial count (TBC) determinations made in goat bulk tank milk using an automated flow cytometry method. The storage conditions tested were storage temperature (refrigeration at 4 and 10°C or freezing at −20°C), the use of a preservative (no preservative, NP; azidiol, AZ; or bronopol, BR), and the age of the milk samples for each analytical condition (storage times at 4°C: from 0 h to 5 d for NP; and from 0 h to 22 d for AZ and BR; storage times at 10°C: from 24 h to 2 d for NP and from 24 h to 22 for AZ and BR; storage times at −20°C: from 24 h to 22 d for NP, AZ, and BR). Significant effects on individual bacterial count (IBC) variation were shown by the bulk tank milk sample, preservative, storage temperature, interaction preservative × storage temperature, and milk age within the interaction preservative × storage temperature. In preserved samples, the highest IBC were obtained for AZ and the lowest counts were obtained in samples preserved with BR. Because of the variation in IBC recorded in BR-preserved samples, we recommend that BR should not be used for TBC determinations using the automated flow cytometry method. The NP samples stored at 4 and 10°C showed significantly higher IBC at 24 h postcollection, also invalidating these analytical conditions for TBC analyses. The practical implications of our findings are that goat milk samples preserved with AZ and stored at 10 or 4°C are appropriate for TBC by the BactoScan flow cytometry method for up to 24 h and 11 d postcollection, respectively.