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Emprical determination of CNS and DRG contamination of carcasses and its reduction using alternative procedures

Project Code: M03026

01/02/2006

University of Bristol
Fisher, A ;
Silsoe Research Institute
Whyte, R; Tinker, D;
Bristol University Veterinary School, Department of Clinical Veterinary Science, University of Bristol
Helps, C;
Meat and Livestock Commission
Owen, M;
Norwegian Meat Co-operative, Norway
Rotterud, O

This project has examined the fate of the spinal cord and associated dorsal root ganglia (DRG) in beef carcasses during slaughter, dressing, and boning. These two components of the central nervous system (CNS) are known to house the infectious form of prion in BSE-infected animals and at a relatively high concentration compared with other (non-CNS) infective tissues.

A study, involving dissection of the vertebral region of beef carcasses and determination of the fate of DRG after boning the vertebral primal joints, showed that when vertebral bones are diligently removed from a beef carcass with little adherent tissue to meet the objective of maximising meat yield, some DRG can be removed with the meat. Such boning methods therefore introduce a risk of DRG entering the human food chain The distance of a ganglion from the outer surface of its associated vertebra is probably an important factor in determining whether it is likely to be removed but the size of the foramen (opening) in the vertebra, through which the ganglion-bearing lateral nerve runs, also appears to be a factor.

A subsequent study quantified the cost to industry of three boning methods that have different levels of risk of ganglia being included in edible meat. Methods were compared in respect of meat yield, weight of removed vertebral column (which would be designated as Specified Risk Material in some classes of cattle) and labour (time). Costs would be higher if amounts of saleable meat were reduced and weights of vertebral column SRM were increased, and if the time taken to execute boning were longer. The three methods were (a) Traditional boning – often used in retail butchers; removes bones cleanly with little adherent meat; (b) Sheet boning - carried out mainly in large-throughput cutting plants where maximising yield by removing meat cleanly from the bone is secondary to speed/throughput. Bones of the vertebral column are removed from each primal joint in one piece, the operator cutting flat, along a plane, without delving in skeletal crevices; (c ) ‘DRG Special’ – a modification of the Sheet boning method devised by the Meat & Livestock Commission to help reduce the likelihood of DRG entering the food chain. It is not currently used commercially. The process follows very closely the sheet boning method but leaves more meat attached to the vertebral column, thus reducing the likelihood that DRG will be removed with the meat. In addition, the same measurements were obtained for carcass sides whose main bulk of vertebral column was removed at the end of carcass dressing by the oval saw, designed originally to remove the column from intact carcases. Because some DRG can be removed with the fillet and with the channel fat, an Enhanced method was applied to the removal of these items and compared with a Standard method. Briefly, the Enhanced involved releasing and pulling the fillet, rather than cutting it free along its length (Standard), and leaving the portion of the channel fat closest to the sacral vertebrae attached to those bones, rather than completely removing it as an item deemed fit for human consumption (Standard).

The ranking of the boning methods, in order of greatest yield of saleable meat and smallest weight of vertebral bone, was Traditional > Sheet > DRG Special > Oval Saw. The difference in these yields (per side) between Sheet and Traditional was about 1.6kg; between Sheet and DRG Special about 1kg; and between Sheet and Oval Saw about 2kg. The time taken to bone the vertebral joints was much greater in the Traditional method at around 30 minutes compared with approximately six minutes for the Sheet method, five minutes for the DRG Special and seven minutes for the Oval Saw. The main difference in yield of saleable meat between the boning methods originated from the weight of lean trimmings. These would normally be used to produce mince and hence have a relatively low value. As an example, the likely cost difference between the Sheet and DRG Special methods was 1kg saleable meat (mince) more in the Sheet method at £3/kg = £3, less the 1 minute reduced boning time for the DRG Special at £15/hour = £0.25, giving a net difference of £2.75 per side in favour of the Sheet method.

The weight of the fillet removed according to the Standard method was greater (mean 0.28kg) than in the Enhanced method. The weight of channel fat left attached to the vertebral column close to the sacral bones, deemed to be the least amount remaining to ensure that DRG remain with the SRM, was 0.26kg. Channel fat was valued at approximately £30/tonne compared with a disposal cost of £55/tonne if it were classed as SRM.

A special saw, with an oval shaped guide and a thin and moderately flexible toothed blade, has been developed to remove a large part of the vertebral column from whole (un-split) carcasses, with the spinal cord encased within. This saw was used to remove the bisected vertebral columns, complete with DRG, from carcass halves after conventional bandsaw splitting. Each half column was visually examined for depth of cut and the possibility of DRG being left in the carcass. Sections of some half columns were dissected to confirm the presence of DRG. Although tissues at every vertebral position in every half column were not revealed through dissection, it was concluded that all DRG were removed by the saw.

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