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Eliminating the risk of exposure to infection with bovine spongiform encephalopathy (BSE) continues to be a public health and veterinarian concern. Although a variety of precautionary firewalls have been implemented in the United States to prevent the occurrence and spread of infection to humans and animals, a second line of defense based on processing steps that kill the causative agent would serve as another layer of prevention.
Ultra-high pressure cooking of tissues has been shown to be an effective and practical means to eliminate the presence of the infectious agent that causes BSE. Beginning in 2001, researchers conducted an initial set of experiments to determine whether ultra-high pressure at various temperatures and exposure times would inactivate transiform encephalopathies (TSEs).
The objective of this study was to build upon the previously conducted research and find more commercially viable methods of applying this technology.
Special rendering runs of a known composition were obtained from a commercial beef processing facility and included materials designated as specified risk materials (SRMs) by the U.S. Department of Agriculture (e.g. brains, eyes, tonsils, spinal cords and distal ileum).
The researchers used the same experimental design that was used in their previous research. Samples were prepared with appropriate agent strains of TSE (hamster-adapted 263K and mouse- adapted 301vBSE) and exposed to various conditions of pressure and temperature. The reductions in prion concentration were analyzed using Western blot procedures. Bioassay performance was conducted using ten-fold serial dilutions and end-point infectivity titers were collected.
Effect of heat/pressure on spiked SRM
The results from this project were consistent with previous research conducted on spiked meat products. Progressively higher temperatures produced greater inactivation, and single pulsing was as effective as multiple pulsing. Results were reproduced in multiple test runs. In contrast to the previous projects, exposure to 690 MPa appeared to be as effective as exposure to 1,000 MPa. End-point titrations on these samples could not be performed due to a lack of funding.
Except for a TSE comparison study, these projects were conducted on hamster-adapted 263K scrapie that was either homogenized in saline, or spiked into a hot dog homogenate.
Reproducibility of TSE reduction
Nine independent runs of duplicate samples of 263K scrapie hamster brain spiked hot dog pastes were exposed to 690 MPa for five minutes at pre-pressure temperatures of 93 to 98ºC. The pressurized tempertures were approximately 125 to 135ºC. Western blot analyses showed that TSE reductions were identical in paired samples and did not vary more than two-fold in independent runs, which demonstrated excellent repeatability.
Low dose infectivity reduction
The researchers also verified that the degree of infectivity reduction was comparable to the results obtained in studies using low-input dosing, or samples that were more similar to what might be encountered in a real-life situation.
Infectivity reductions of different TSE strains
Previous research demonstrated that there might be differences in reduction rates among different strains of TSEs, however this project demonstrated that three different strains of TSE exposed to 690 MPa for five minutes at temperatures of 130ºC had reductions greater than or equal to four log LD50 per gram of tissue (³ 4 log LD50/g).
Low temperature, longer time pressure exposures
The researchers attempted to reproduce results of a German study in which excellent prion reductions were obtained with lower temperatures (60°C) and longer exposures (120 minutes) to pressure (³ MPa). In three separate attempts, the researchers were not able to duplicate those results.
Application of high temperature treatments to bovine carcass tissues that are banned from any use (specified risk materials, or SRMs) was shown to be a commercially viable alternative to their currently mandated destruction.