For beef and pork slaughter and processing, there may not be a more challenging step to implement and maintain than chilling. Chilling often serves as the final step of the slaughter procedure, and it is frequently completed before fabrication/cutting can occur. Few processes impact both food safety and quality factors more than chilling carcasses, and no process is less studied and/or fully understood, especially during this time of steadily increasing beef and pork carcass weights.
Carcass chilling impacts food safety, minimizes shrinkage and maximizes shelf life of the finished products. Chilling also impacts tenderness and quality of products, however, processors place less emphasis on designing programs to optimize these traits compared to those related to safety, shrinkage and shelf life. It may be that those factors are more easily measured than how chilling impacts the eating quality of beef or pork.
The size of today’s livestock often create unique challenges in how best to chill these larger and heavier beef and pork carcasses. “Hot boxes,” the term used to describe the initial chilling coolers, are taxed with having to hold carcasses that require more room, which results in little to no space for sufficient air movement around them for adequate chilling. These heavier carcasses simply contain more heat than smaller carcasses and managing the chilling processes to ensure that carcasses are adequately chilled in a timely manner so that fabrication/cutting can commence can be difficult for some establishments.
The first 24-hours
Chilling is simply not just heat removal. The rate and extent of chilling impacts many factors, including biochemical and structural changes occurring in carcasses that determine the ultimate product. Initially muscle is still working to maintain its necessary biological activities and energy pathways, so it breaks down glycogen through postmortem glycolysis for ATP production, but with a side effect of the buildup of lactic acid as a byproduct. Lactic acid causes the pH of the muscle to decrease from 7.0 to 5.6 under normal conditions (too little or too much lactic acid in the postmortem muscle is what causes the conditions of dark cutting/dark, firm, and dry or pale, soft, and exudative, respectively). As long as there are sufficient supplies of ATP present, the muscles will be in a relaxed state. Once ATP supplies diminish, rigor mortis begins to develop causing the muscles to become inextensible. Both postmortem glycolysis and the rate of rigor mortis are greatly influenced by temperature. Higher temperatures speed up both processes, and lower temperatures slow them down.
During the late 1950s and early 1960s, it was determined that carcasses must be chilled quickly before they could be exported to Europe. Researchers discovered that there was a toughness to the meat associated with this change. The term coined for this phenomenon was “cold shortening,” which meant that the muscles would undergo shortening at the microscopic level due to the exposure to cold temperatures before rigor mortis set in. Scientists began exploring ways to prevent this cold-induced toughening. Many different strategies, such as the application of electrical stimulation, alternative carcass suspension methods and, of course, different ways to chill carcasses were investigated.
For beef, the challenge was how to reduce carcass temperatures rapidly enough to meet export requirements without doing it so quickly that cold shortening would be a problem. One such approach was to simply delay chilling. This allowed some of the postmortem conditions to run their course, thereby minimizing the effects of cold shortening, before the carcasses were moved into colder conditions with more air movement to finish the chilling process. In the United Kingdom, this process is still used today and is called “considerate chilling,” which is a best practice for carcasses from grass-fed beef where the insulating effect of adequate fat cover is not present.
If chilling too rapidly created a problem, especially as related to eating quality, then today’s heavyweight beef carcasses may actually be a benefit, not a liability. This may be the case as evidence of improved tenderness ratings compared to the past are common when data from national surveys are evaluated. Nonetheless, these heavier and slower chilled carcasses still create issues for reaching the correct surface or internal temperatures to allow them to be moved to the next step of the process.
The issue faced by the pork industry is that pork carcasses need to be chilled more rapidly. Slow chilling of pork carcasses compounds many issues related to meat quality, especially the impact that the lower pH and high muscle temperatures has on color development and resultant water-holding capacity. If rapid chilling of pork carcasses is a best practice for quality, increased pork carcass weights has added to the challenge.
Rapid chill systems have been developed for pork. In these systems, carcasses enter an area or tunnel where they are transferred through extreme cold conditions with high-velocity air movement. Because of the pork’s muscle fiber composition, they do not undergo cold shortening, so rapid chilling is an acceptable practice for them. This system results in freezing of some surfaces of the carcasses after several hours through this process. Then, the carcasses can equilibrate in another chiller where additional heat removal takes place. Pork carcasses are cut sometimes at the end of the same day they are slaughtered, and the quick development of rigor mortis (a few hours for pork, more hours for beef) allows them to be ready to do so if the temperatures are correct. One of the quality challenges pork faces is that chilling differential, especially in the hams, may cause the exterior of the muscle to be darker than the interior portion of it creating a two-toned appearance.
Spray chilling is a way to minimize cooler shrinkage and enhance the chilling process through evaporative cooling and has been used widely by the beef industry since the early 1980s. Many of these systems involve initial cycles of spray chilling followed by a drying period before the carcasses are moved out of the hot box to the sales coolers.
Although there are some systems where antimicrobials are used in spray chilling systems, most of these applications occur at the end of the slaughter process and/or immediately before entering fabrication. Pathogen reduction on carcasses is a high priority for both beef and pork processors.
Nothing may be more frustrating to those who manage the chilling process than fighting condensation. Placing freshly slaughtered carcasses into blast chillers where extreme temperatures exist between the two are the perfect storm to create condensation. Properly loading the blast chillers helps minimize condensation.
The challenges associated with chilling are often overlooked. Transitioning from hot to chilled carcasses goes as well as it does because of the employee’s dedication to the process. The individuals who manage this process play an important role in converting livestock into meat.