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Survival of Listeria monocytogenes in raw milk treated in a pilot plant size pasteurizer
J F Fernandez Garayzabal1, L Dominguez Rodriguez, J A Vazquez Boland
1Departamento de Microbiología e Inmunología, Facultad de Veterinaria, Universidad Complutense, Madrid, Spain.
Abstract:
The survival of Listeria monocytogenes in raw milk treated in a pilot plant size pasteurizer was investigated. Raw milk was inoculated with different initial concentrations of L. monocytogenes and heated at temperatures ranging from 69 degrees to 73 degrees C. Listerias were not isolated from any of the milk samples immediately after thermal treatment. They were isolated, however, from 46.6% of heated samples (none from samples heated at 73 degrees C) after variable periods at refrigeration temperature. The results suggest that a low number of listerias survive some thermal treatments, but a cold enrichment is necessary to repair the thermally injured cells and detect these organisms in milk. The importance of the isolation technique in the recovery of listerias from pasteurized milk samples is discussed.
Insights
A small number of Listeria monocytogenes can survive pasteurization, especially at lower temperatures. Cold enrichment is crucial for detecting these surviving bacteria in milk after thermal processing.
Area of Science:
- Food Microbiology
- Dairy Science
- Food Safety
Background:
- Listeria monocytogenes is a significant foodborne pathogen.
- Raw milk poses a risk for Listeria contamination.
- Pasteurization is a key control measure in dairy processing.
Purpose of the Study:
- To investigate the survival of Listeria monocytogenes in raw milk after pilot-scale pasteurization.
- To determine the impact of thermal treatment temperature on Listeria survival.
- To evaluate the necessity of post-treatment recovery methods for Listeria detection.
Main Methods:
- Inoculation of raw milk with varying concentrations of Listeria monocytogenes.
- Thermal treatment of milk in a pilot plant pasteurizer at temperatures from 69°C to 73°C.
- Isolation and detection of Listeria monocytogenes immediately after heating and after refrigeration.
Main Results:
- No Listeria monocytogenes were detected immediately after thermal treatment.
- Listeria monocytogenes were recovered from 46.6% of samples post-refrigeration (none from 73°C treatment).
- Thermal injury necessitates cold enrichment for Listeria recovery.
Conclusions:
- Some Listeria monocytogenes can survive pasteurization, particularly at sub-optimal temperatures.
- Cold enrichment is essential for repairing thermally injured Listeria and successful detection.
- Effective isolation techniques are critical for accurate Listeria recovery from pasteurized milk.