ARS | Publication request: Pilot-scale crossflow-microfiltration and pasturization to remove spores of Bacillus anthracis (Sterne) from milk

Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 18, 2011
Publication Date: September 1, 2011
Citation: Tomasula, P.M., Mukhopadhyay, S., Datta, N., Porto Fett, A.C., Call, J.E., Luchansky, J.B., Renye Jr, J.A., Tunick, M.H. 2011. Pilot-scale crossflow-microfiltration and pasturization to remove spores of Bacillus anthracis (Sterne) from milk. Journal of Dairy Science. 94:4277-4291.

Interpretive Summary: Fluid milk is preserved by a heating process known as high temperature short time (HTST) pasteurization. However, pasteurization is not effective against specialized survival cells of bacteria known as spores, which may be a part of the natural microflora of milk or which may have been intentionally added to milk. Some bacterial spores are of concern to public health. We evaluated the use of microfiltration, a filtration process that allows milk to pass through the membrane while retaining the spores, for its usefulness in removing spores of a non-illness causing Bacillus anthracis that was added to milk. Our experiments demonstrate that microfiltration of milk followed by pasteurization for preservation can remove most of the spores and serve as a step for improving the safety and security of the milk supply.

Technical Abstract: HTST pasteurization of milk is generally ineffective against spore-forming bacteria such as Bacillus anthracis (BA) but is lethal to its vegetative cells. Crossflow microfiltration (MF), using ceramic membranes with a pore diameter of 1.4 um, has been shown to physically remove somatic cells, vegetative bacteria cells, and some bacterial spores from skim milk and, in combination with HTST pasteurization, has been shown to extend the shelf-life of milk. MF using a membrane with a smaller pore diameter should remove most bacterial spores and if used prior to pasteurization may provide an effective means of protecting the milk supply in the event of intentional contamination of the milk supply by bacterial spores. In this study, crossflow MF was evaluated for its efficiency in removing spores of the attenuated Sterne strain of Bacillus anthracis (BA) in milk and to determine if this step followed by pasteurization of the resulting permeates at 72 deg C for 16 sec and cooled to 4 deg C would remove or prevent the growth of BA in milk. 30 gal of raw milk was skimmed and then inoculated with approximately 4 log 10 BA spores/mL milk or 6 log10 BA spores/mL. It was then microfiltered in total recycle mode at 50 deg C using a ceramic GP membrane with nominal pore size of 0.8 'm at a CFV of 6 m/s and TMP of 124.1 kPa, conditions selected to lessen membrane fouling and facilitate the selectivity of the membrane. For the trials using milk inoculated with 104 BA/mL of milk, no spores were detected in the permeate. An average of 1 log10 BA spores/mL milk was detected in the permeate of milk inoculated with 106 BA spores/mL of milk obtained during 2 h of MF. Pasteurization of the permeates obtained after 10 min (no spores detected), 30 min (1log10 spores/mL detected) and 120 min (1 log10 spores/mL detected) of MF, tended to induce growth of BA spores by at least 1 log10. Storage of the pasteurized milk samples at 4 deg C sampled every 7-d for 28 d showed a die-off of the spores while storage at 10 deg C showed a decline in spore levels up to 12 d. Storage at 25 deg C showed no increase in growth but a decline in milk quality after 1 d. This study confirms that MF of milk using a 0.8 um membrane as a step prior to HTST pasteurization can remove > 99.9999% of BA spores and may serve as a step for improving the quality and safety of the fluid milk supply.