Volume 13: Industry Processes and Controls
6. Rendering
Rendering and inactivation of BSE
Homogeneity of MBM

6.45 A critical question in relation to the emergence of BSE is whether or not the widespread change from batch processing to continuous processing, or the decline in the use of solvent extraction, could have resulted in the BSE agent surviving the rendering process for the first time, and therefore being allowed to enter the animal food chain.

6.46 This is considered further in vol. 2: Science. As explained there, the timing of the phasing out of the more traditional processes casts doubt on the connection. In addition, laboratory experiments that attempted to replicate the conditions to which scrapie-infected material would be subjected during each of these processes were inconclusive. Essentially, the results indicated that the more heat-resistant strains of scrapie were not completely inactivated by conditions less rigorous than 133°C for a minimum of 20 minutes, or by the use of solvent extraction at 100°C for 30 minutes, although all rendering processes investigated resulted in some degree of inactivation.

6.47 This suggests that, in most cases, neither the older-style batch atmospheric systems nor the newer continuous systems, nor solvent extraction, were or are capable of completely destroying either the scrapie or BSE agents. The results of the experiments do not lend themselves, either, to a definitive conclusion on whether the change to continuous systems produced a significant change in deactivation. Of the 11 experiments used to replicate different continuous systems, all produced MBM that tested positive for scrapie infectivity, but levels of infectivity were calculated for only six of these. For these six, the levels of infectivity were slightly lower than that of the MBM produced by the experiment designed to replicate batch atmospheric processing. Therefore, it cannot be concluded from the results that either the change from batch to continuous processing, or the decline in the use of solvent processing, was to blame for the origin of the BSE epidemic, or for a significant increase in infectivity of rendered material.

Homogeneity of MBM

6.48 One of the more puzzling aspects of the BSE story is why as few as one or two cattle in a herd might become infected with BSE, when the whole herd was fed the same compound feed. This gave rise to the 'packet theory' of infectivity, discussed in vol. 2: Science. The theory was that the rendering process might produce MBM that was not perfectly mixed and homogeneous: a small amount of BSE-infective material might end up confined to a packet or clump of MBM consumed by one or two cows, having not been broken down sufficiently during the rendering process to cause the BSE agent to spread evenly throughout the batch. Thus one cow might receive an infective dose while the remainder would not.

6.49 Mr Stephen Woodgate of PDM told the Inquiry that, in order to maintain the homogeneity and consistency demanded by its customers, PDM was careful to keep its raw material at a consistent ratio of bone and offal. Before entering the heating phase of rendering, the raw material would be crushed into particles ranging in size from 10 mm to 50 mm in diameter (the diameter in continuous systems typically being 10 mm to 30 mm). There would be a reasonable amount of mixing by turbulence in the heating medium (usually fat) particularly in systems where extra fat was added as part of the heating process. Added fat might produce more turbulence. In natural or low-fat systems (that is, those with no added fat) the material might have moved through the system in clumps, more so than in other systems. Upon leaving the heating phase, the greaves were pressed, cooled and ground into a powder or meal with a maximum particle size of 2 or 3 mm. This would be mixed during the process of storage and packaging of the MBM. In the opinion of Mr Woodgate, the MBM would be homogeneous both in terms of the mix of protein, fat and ash, and in terms of its original ingredients. However, he did agree that a 1 gram packet (about the size of a pea) might be conveyed from the raw material to the end consumer intact. ¹

6.50 The UK Agricultural Supply Trade Association (UKASTA) likewise provided the Inquiry with an explanation of the mixing process involved in compound feed production, and concurred with Mr Woodgate's comment that MBM delivered to feed mills was 'a reasonably homogeneous mixture'. UKASTA noted that assessments of the performance of feedstuffs manufacturers occurred at six-monthly intervals, and typical results showed 'that a homogeneous distribution of trace ingredients . . . is achieved by conventional feed milling plant and equipment'. ²