Suppression of pathogens in properly refrigerated raw milk

Authors: COLEMAN, MARGARET - Coleman Scientific Consulting; Oscar, Thomas; NEGLEY, TIM - Tig Environmental; STEPHENSON, MICHELE - Syracuse University

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/27/2023
Publication Date: 12/12/2023
Citation: Coleman, M.E., Oscar, T.P., Negley, T.L., Stephenson, M.M. 2023. Suppression of pathogens in properly refrigerated raw milk. PLOS ONE. 18(12):e0289249. https://doi.org/10.1371/journal.pone.0289249.
DOI: https://doi.org/10.1371/journal.pone.0289249

Interpretive Summary: Carefully produced hygienic raw milk for direct human consumption is rarely associated with food-borne disease outbreaks. In addition, raw milk has health benefits that are lost when it is pasteurized. Pasteurization reduces levels of beneficial microorganisms that suppress growth of milk-borne human pathogens like Campylobacter, Listeria monocytogenes, E. coli O157:H7, and Salmonella. However, risk assessments that provide the scientific basis for risk management strategies have overestimated growth of milk-borne human pathogens and risk of milk-borne illness because they have relied on data collected with pasteurized milk. In the present study, data collected with hygienically produced raw milk showed that at the recommended storage temperature of 39.9F, Campylobacter, E. coli O157:H7, and Salmonella did not grow but died during 14 days of storage, whereas Listeria monocytogenes grew but only after 6 or 9 days of storage. Thus, hygienically produced raw milk does not support the rapid growth of pathogens when properly stored after purchase.

Technical Abstract: Conflicting claims exist regarding pathogen growth in raw milk. A small pilot study was designed to provide definitive data on trends for pathogen growth and decline in raw bovine milk hygienically produced for direct human consumption. An independent laboratory conducted the study, monitoring growth and decline of pathogens inoculated into raw milk. Raw milk samples were inoculated with foodborne pathogens (Campylobacter, E. coli O157:H7, Listeria monocytogenes, or Salmonella) at lower (<162 colony forming units (CFU) per mL) and higher levels (<8,300 CFU/mL). Samples were stored at 4.4°C and quantified over time after inoculation (days 0, 3, 6, 9, 12, and 14) by standard culture-based methods. Statistical analysis of trends using the Mann-Kendall Trend Test and Analysis of Variance were conducted for 48 time series observations. Evidence of pathogen growth was documented for L. monocytogenes in 8 of 12 replicates (P=0.001 to P=0.028). Analysis of variance confirmed significant increases for L. monocytogenes at both initial levels in week 2. No evidence of growth was documented over 14 days for the three pathogens predominantly associated with raw milk outbreaks in the US (Campylobacter, E. coli O157:H7, and Salmonella). Further research is needed to characterize parameters for pathogen growth and decline to support re-assessment of risks that were based on incorrect assumptions about interactions of pathogens with the raw milk microbiota.