Laboratory pasteurization count (LPC) is used to assess raw milk quality; this data is used by some dairy producers to designate raw milk quality premiums paid to farmers and may also be used for troubleshooting bacterial contamination issues.
LPC typically enumerates thermoduric bacteria – the type that can withstand pasteurization – but it’s not known the type of bacteria, e.g. sporeformers or non-sporeformers, can be determined by just by looking at the LPC level.
To test if LPC levels can be an indicator of the type of bacteria present in raw milk, researchers from Cornell University collected 94 organic raw milk samples from across the US, assessed these via LPC, and characterized the bacterial isolates.
They found LPC concentrations that ranged from undetectable to 4.07 log10 cfu/mL, and 380 bacterial isolates were analyzed, with the thermoduric bacteria isolated from the samples being split in 3 groups: gram-positive sporeformers (52%), gram-positive non-sporeformers (44%) and gram-negative bacteria (2.4%).
In terms of the LPC levels, the majority of samples (66) had a LPC ≤100 cfu/mL; 6 samples were with a LPC between 100 and 200 cfu/mL, and 10 samples - with a LPC ≥200 cfu/mL. Additional analysis showed that there was an inverse trend between the proportion of gram-positive sporeformers and the LPC concentration – samples with higher LPC had lower proportions of gram-positive sporeformers. But that trend was not significant, according to the research team.
“LPC level alone provides no insight into the makeup of the thermoduric population in raw milk and further characterization is needed to elucidate the bacterial drivers of elevated LPC in raw milk,” the researchers concluded.
They leveraged a microbial identification tool called MALDI-TOF MS, which is said to be more readily available to dairy producers than 16S rDNA PCR and sequencing.
“While our data indicated agreement between 16S and MALDI-TOF MS for 66.6% of isolates at the genus level, 24.2% and 9.2% could not be reliably identified or were mischaracterized using MALDI-TOF, respectively,” the researchers said.
“This suggests that further optimization of this method is needed to allow for accurate characterization of thermoduric organisms commonly found in raw milk.
“Ultimately, our study provides a contemporary perspective on thermoduric bacteria selected by the LPC method and establishes that the LPC alone is not sufficient for identifying the bacterial drivers of LPC levels. Further development of rapid characterization methods that are accessible to producers, cooperatives, and processors will support milk quality troubleshooting efforts and ultimately improve outcomes for dairy industry community members.”
Troubleshooting high laboratory pasteurization counts in organic raw milk requires characterization of dominant thermoduric bacteria, which includes non-sporeformers as well as sporeformers
Authors: Lee, T. R., et al
Published: Journal of Dairy Science, 20 January 2024