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Stanley Prusiner first introduced the idea of prions in 1982 when he identified them as small infectious proteinaceous particles. Prions are the causative agents behind fatal neurodegenerative diseases, which all belong to a single family of Transmissible Spongiform Encephalopathies (TSE). Following Prusiner’s discovery it was observed that prion diseases occur when the three dimensional structure of normal prion proteins (PrPC) is modified through a unknown post-translational process to form abnormal and infectious prion proteins (PrP), like Scrapie, which is identified as PrPSc (Borchelt et al., 1990; Borchelt et al., 1992). Abnormal prions are characterized by an abnormally high β-sheet content, and protease resistance, which prevents its natural degradation and recycling by cellular enzymes that leads to excessive extracellular deposition and neurotoxic aggregation in brain tissue resulting in the manifestation of the disease (Novakofski, et al., 2005; Unterberger et al., 2005). The aggregation of infectious prions in brain tissue leads to the formation of neuronal vacuoles and the over production of astrocytes, which are pathological hallmarks of prion diseases (Prusiner, 1982). McKinley et al. (1983) reported that both PrP and prion infectivity were resistant to proteinase K digestion for up to two hrs under non-denaturing conditions, which may help explain the abnormal accumulation of infectious prions in Central Nervous System (CNS) tissue. Protease resistance is the basis for most current PrPSc detection methods.
Bovine Spongiform Encephalopathy (BSE) was first diagnosed in the United Kingdom (UK) in 1987 as a progressive neural degenerative disease (Novakofski et al., 2005). Since that time, the geographical distribution of BSE has increased to 25 countries, including the United States (World Organization for Animal Health, 2007). BSE is considered a “common source” epidemic, meaning that animals contract the disease from a common element, most likely the consumption of feed containing rendered Meat and Bone Meal (MBM) contaminated with brain and spinal cord from infected cattle (Novakofski et al., 2005). The emergence of BSE from rendered MBM was attributed to the shift from batch rendering to continuous process rendering and the reduction in peak rendering temperatures (Novakofski et al., 2005). Appel et al. (2001) reported that the presence of bovine bone fat increased the heat stability of prion rods to autoclaving, especially at lower temperatures; however, the protective effects of lipids decreased with increasing temperature. Prion rods heated in pure bovine bone fat did not undergo as extensive hydrolysis as water or lipid-water treatments, and formed irreversible aggregates (Appel et al., 2001). Taylor et al. (1997) conducted a study that examined the effect of different rendering procedures on the Scrapie agent. Mimicking batch rendering, various continuous rendering processes, solvent extraction, and steam-under-pressure treatment it was discovered that all MBM samples contained detectable Scrapie infectivity, except for the treatments exposed to steam-under-pressure (Taylor et al., 1997).
The UK initiated a specified bovine offals ban in 1989, which restricted renderers from including these materials in ruminant feed, in an effort to stop the transmission of BSE. The UK however, continued to export MBM until 1996, the same year the UK government required complete exclusion of MBM from all farm animal feeds, and is most likely responsible for propagating the disease worldwide (Taylor and Woodgate, 2003; Novakofski et al., 2005).
The United States followed up shortly thereafter in 1997, with a feed-ban that prohibited the feeding of mammalian-derived protein sources to ruminants. Prior to the 1997 feed-ban, the practice of converting non-consumable animal “waste” (diseased, dead, dying and disabled animals, slaughter offal, supermarket waste and restaurant grease) into functional meat and bone meal (MBM) and tallow (i.e. rendering) consumed approximately 40 billion pounds of raw material annually (FDA, 1997).
The present study investigated the efficacy of alkaline hydrolysis and rendering to destroy infectious prion material. Prior to initiation of the bioassay, validation work was performed to demonstrate the ability of hydrolysis process to destroy high-loads of thermophilic spore-forming agents and vegetative agents. A bioassay was performed to detect prion infectivity as this continues to be the most sensitive method for detecting abnormal prions (PrPSc), and although time consuming and labor intensive, they are able to detect very low levels of infectivity (Novakofski et al., 2005). In addition, the safety and economics of alkaline hydrolysis were investigated to examine its competitiveness to other accepted carcass and SRM disposal technologies. The final objective of this study identified potential applications for the effluent or proteinaceous end-product in an effort to recover value from the process and turn a liability into a commodity of some relative value.
The stated objectives for this work were:
Demonstrate the efficacy, safety, and economics of converting Bovine SRM’s into safe non-ruminant animal feed products or biofuels.
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