Pre-Rigor / Hot Boning of Pork Carcasses

Pre-rigor/Hot Boning of Pork Carcasses

April 26, 2002

Lynn Knipe

Associate Professor

The Ohio State University

The fresh, “bloom” color of meat is enhanced with rapid chilling (using CO2) of pre-rigor meat, as soon after hot boning as possible.  This improvement in color can be reflected in a sharper particle definition (less smeared look), as well as a leaner appearance.  While there are other functional advantages to hot boning of meat, currently, the main commercial reason for pre-rigor boning of pork is to extend the shelf life (time until the lean loses color) of the fresh color.  Other advantages to pre-rigor processing include a firmer texture to the final cooked sausage, with less cooking loss. 


Pre-Rigor Processing Techniques

The slaughter process for hot-boning is typically no different from conventional slaughter, except for hide pulling, rather than scalding.  Removal of the hot muscle from the bones/skeleton requires sharp knives, and while it may be easier for new boning crews, it is perceived as being more difficult for boners, experienced with post-rigor meat.  Pre-rigor meat needs to be ground or chopped, in order to get the CO2 well mixed into the meat to rapidly reduce the meat temperature.  Ice may be used, except that fresh sausage is limited to 3% water and that level of ice is not sufficient to adequately reduce the meat temperature.  Grinding is more efficient and produces more consistent sized particles, than chopping, but adjustments may need to be made to the feeding rate with hot meat.  Choppers do allow for immediate mixing of added salt, water and CO2 as chopping is occurring, but the batch size of a chopper may limit the speed of this operation.  A grinder-mixer operation slightly extends the time to get the hot meat in contact with the salt and CO2.  If grinding, it is important to prevent or control “roll back,” which is the churning of meat between the auger and the barrel of the grinder.  Roll back is caused by the auger forcing the meat against the plate, at a faster rate than the cutting knives can handle.  With the normal clearance between the auger and the grinder barrel, meat is rolled back, which causes excess damage to the hot meat, and ultimately more smearing of the fat.  To reduce roll back, either decrease the auger speed and increase the number of cutter blades (which reduce output) or change the pitch and reduce the clearance of the auger flights (to potentially increase output).  To maximize the effect of pre-rigor/hot-boned meat, the salt, water and CO2 need to be mixed with the ground/chopped blend within 90 minutes or less, after the hog was bled on the kill floor.  Additional information that may help explain the rigor process follows (links to other information on the internet are in parenthesis). 


Rigor Mortis

            Rigor mortis is the process in which the muscles of animals are converted to meat.  It is referred to as the stiffness of death.  This stiffness is due to the contraction at death, which proceeds until the energy supply is exhausted (due to loss of blood supply), at which point, the contracted muscles are not able to relax.  This process is essentially irreversible, with the exception that as the carcass ages, with time, inherent enzymes may cause some degradation of the contracted structure.

            Normally, contraction occurs through a complex series of biochemical reactions, in which myosin and actin slide past one another to cause muscle contraction.  The heads, on the myosin filament, extend toward and attach to the actin filament, pulling the actin, which causes the contracting action.  With additional energy, the myosin heads release from the actin and move (or creep) along the actin molecule.  This release of myosin from actin is also associated with the relaxing of muscles, in which the actin and myosin slide apart.  The muscles of people and animals will continually contract and relax except at times during sleep and upon death.

            There is a time delay in the onset of rigor mortis, which is unique to each species (Table 1).  In general, the larger the animal, the longer the delay in the onset of rigor mortis.  This time delay is useful in allowing muscles to be removed from carcasses, while the muscles are still in the pre-rigor state and to be combined with salt to preserve the pre-rigor state of the muscles. While this table shows 15 minutes to 3 hours time delay before onset of rigor mortis, pork muscles need to be removed from the skeleton and mixed with salt, water, CO2, etc., within no longer than about 90 minutes, to maximize the water-holding capacity and color advantages.  As mentioned previously, the pre-rigor form of muscle is the easiest form, from which to extract the highly functional protein, myosin. 


Table 1. Time Delay Before Onset of Rigor Mortis in Muscles

                                                Species                        Hours

                                                Beef                            6-12

                                                Lamb                           6-12

                                                Pork                             1/4 - 3

                                                Turkey                         <1

                                                Chicken                       < 1

                                                Fish                             < 1/2

Source: Principles of Meat Science, p. 102, Judge, M., ed., Second Edition


Properties of Fresh Meat

Water-Holding Capacity

            Water-holding capacity (WHC) is the ability of meat to retain moisture during cutting, grinding, cooking, pressing, etc.  Sometimes water-binding capacity is used particularly with the ability to bind (rather than hold) moisture during cooking. 


Postmortem Effects on Water-Holding Capacity

            Chemically, moisture is held by proteins to different degrees, and the differences in these types of moisture are referred to as bound water, immobilized water and free water.  Bound water is the most tightly bound and is not affected by the addition of salt or changes in pH.  However, bound water is reduced as muscle goes into rigor mortis and when cooked.

            The net charge effect of proteins is a major cause of changes in WHC of muscle during the rigor process.  Proteins have both negative and positive charges on their side chains, but at the time of death, the charges on muscle proteins are predominately negative.  This predominance of negative charges causes the proteins to natural repel one another, much like two negative poles on a magnet.

            As the pH of muscle drops, during the rigor process, due to the accumulation of lactic acid, the positive charges from the acid cancel out the negative charges of the muscle.  Therefore as muscle approaches the post-rigor state, there is nearly an equal number of positive and negative charges on the proteins.  The iso-electric point is the pH of muscle, in which the number of positive charges equals the negative charges on proteins.  In meat, the iso-electric point is approximately pH 5.1-5.3.

            The final postmortem pH of meat is in the range of 5.5 to 6.0, which is near the iso-electric point.  The point of concern is that equal numbers of positive and negative charges cause the protein side chains to be attracted to one another, which physically causes moisture to be forced out between the protein chains and therefore, reduces WHC. 

            The formation of actomyosin physically reduces the space between protein chains and the potential binding sites for binding water, which also reduces WHC.  Finally, the denaturation of muscle tissue reduces the WHC of muscle.  The best example of denaturation occurring during the rigor process is that of PSE pork.  Freezing of meat also reduces the water-holding capacity of meat.