In 2003 Experiment, High-Pressure Treatment Eliminates BSE in Meat
By Rick Mitchell
PARIS—Last year an international team of scientists reported that high-pressure treatment combined with heat could be a cost-effective way to protect human food against infection with mad cow-disease particles. However, when the first U.S. case of mad cow was discovered in a Holstein in December, the experiment had still not attracted much attention.
In a report published last May in the Proceedings of the National Academy of Sciences, the team of U.S. and European researchers described how they deliberately infected hot-dog meat with BSE, then treated it with varying levels of pressure and temperature. The report, “Ultra-high-pressure inactivation of prion infectivity in processed meat: A practical method to prevent human infection,” says that past a certain threshold, infectivity levels in the meat declined, at steeper rates at higher pressures.
Mad cow’s scientific name, bovine spongiform encephalopathy (BSE), evokes the disease’s tendency to turn the brain into a perforated, spongy mass. Most scientists say it is caused by a diseased form of prions—normally harmless protein particles found in mammals. These cause normal prions to twist into lethal forms that travel slowly through the central nervous system, gumming it up with plaque, to devastating effect. The human form of BSE is variant Creutzfeldt-Jakob disease (vCJD). Some 153 cases have been documented worldwide, with 143 in the U.K., where the disease was first found in 1996. Six cases have been documented in France, and one each in Italy, Ireland, Canada and the U.S.
In December, U.S. agriculture officials announced drastic measures to avoid a spread of BSE, and new vCJD cases. In Europe, where BSE cases number about a million, governments have acted to stem its spread among cattle by ordering the destruction of entire herds in which cases were discovered. To keep infectious meat out of the human food chain, European Union laws require removal of the brain and vertebral-column nervous tissue during slaughter. This material is most likely to contain prions. However, it’s still possible for infected tissue to spatter onto meat bound for someone’s plate. According to the report, vCJD victims probably got infected from eating processed-meat products, such as hot dogs, which contain mechanically recovered meat including the high-risk tissue.
The report recounts that high pressure’s effect on food-borne diseases was studied in the early 20th century, but not seriously employed until reliable equipment entered the market a few decades ago. Bacterial endospores—the most hardy conventional pathogens—withstand pressures that kill bacteria or fungi but succumb to heat combined with pressures above 600 MPa (about 95,000 PSI). Prions are even more resistant to standard disinfection methods. Cooking doesn’t faze them, so even boiling a hot dog for 20 minutes is not enough to protect from vCJD. In the experiment, led by Paul Brown of the National Institutes of Health, researchers made a paste out of hot dogs mixed with brain tissue from hamsters infected with scrapie, a disease related to BSE. They put 2 g quantities of the paste into polyethylene pouches, heat-sealed, then wrapped them again in nylon packing, aluminum foil and polypropylene, followed by another heat seal.
They immersed the samples in hot castor oil, and put the container in a high-pressure apparatus with an external heater. They blasted the meat with short pulses of pressure ranging from 100,000–174,000 PSI (690–1,200 MPa) with 60-second intervals of decompression. Adiabatic heating during pressure raised the temperature as high as 135°C. Laboratories in Rome performed Western blot and bioassay tests to assess infectivity. They injected the paste into hamsters’ brains and observed the animals for 9 months for signs of the disease, examining the brains of dying animals for clinical signs.
The untreated, spiked hot dogs had an infectivity level of approximately 9 log10 mean lethal doses (LD50) per g, the report said. A pressure of 690 MPa, combined with heat, reduced that by 2-3 log10 LD50 per g. At 1,000 MPa, infectivity fell by 4 log10 LD50 per g., and at 1,200 MPa it fell 6 log10 LD50 per gram. In other words, the 690 MPa pressure reduced infectivity 1,000-fold and the 1,200 MPa pressure reduced it 1-million-fold.
Although steam autoclaving, a simpler, cheaper method, drastically reduces or eliminates infectivity without pressure, it also destroys the flavor and texture of the food. According to the authors, ultra-high pressure leaves meat texture and flavor intact. So, although you might not want to blast your prime porterhouse cut with this method, it should be OK for hot dogs, beef bologna, and other processed meats.
Richard T. Ellison, 3rd, commenting on the experiment in the July 11, 2003, issue of Journal Watch Infectious Diseases, pointed out that at the peak of the U.K.’s BSE epidemic, infection was probably less than 0.5 to 5.0 LD50 per g of meat. “Thus, a food processing method that reduces prion infectivity by 3- to 4-log-fold could greatly improve food safety.” He added, “This approach has commercial potential, although extensive studies using other foods and other temperature and pressure combinations are needed before it can be considered for use.”
Contacted by e-mail in February, Brown said that, “Studies in progress have shown similar results for several different strains of TSE, including BSE. Government agencies have not been approached, but some food processors have expressed interest, and we will be making a formal presentation to the National Food Processors Association at the end of March.”
Read the full scientific report, or an abstract
