The impact of processing temperatures on fresh meat quality
Spoilage and pathogenic microorganisms are ubiquitous in livestock production, and colder temperatures limit their growth. In commercial slaughtering, a chilling step is designed primarily to limit microbial growth.
However, through years of extensive research, a secondary benefit has emerged: Meat quality attributes such as color, water-holding capacity and texture may be maintained or improved. Therefore, chilling carcasses after slaughter regulates fresh meat quality attributes by controlling the conversion of muscle to meat.
Conversion of muscle to meat
After an animal is harvested for meat, the muscles go through a complex series of biochemical reactions in an attempt to maintain energy levels (ATP). To maintain energy, enzymes within the muscle metabolizes stored carbohydrate (glycogen) into lactate.
Eventually the energy is completely utilized in a reaction that produces a hydrogen ion (H+), which drops the pH of the meat making it more acidic. The complete exhaustion of energy is known as rigor mortis. This drop in pH is fundamental in the development of meat quality attributes.
In general, a lower pH results in denaturation in which the functional properties of proteins within a muscle are altered. Denaturation is further enhanced when temperature is high. Beef, lamb and poultry are all susceptible to this problem. However, it is most apparent in pork, since porcine muscle exhibits a rapid postmortem pH decline. If muscle pH declines rapidly at high temperatures, the pork exhibits a pale color, soft texture and is exudative, resulting in the PSE condition.
The development of these inferior meat quality attributes associated with high temperature and low pH can be mitigated by carcass chilling. Enzymes are susceptible to ambient temperatures. In general, enzymes function best at normal body temperature and lose activity as temperature decreases. By lowering the muscle temperature, enzyme activity is decreased, resulting in a slower pH decline and preservation of the meat quality attributes. So chilling carcasses maintains quality during the conversion of muscle to meat.
The basics of chilling
In the simplest terms, carcass chilling requires a refrigeration system designed to reduce the temperature of a warm carcass. Most livestock animals have a body temperature between 100ºF-105.8ºF, and it is the goal of meat processors to reduce this temperature to approximately 39.2ºF for refrigerated storage.
Chilling typically begins after the animal is eviscerated and inspected for wholesomeness. Carcasses are oftentimes chilled in two steps: a chilling cooler followed by a holding cooler. The chilling cooler is designed to rapidly reduce the carcass temperature and is typically colder than the holding cooler with temperatures ranging from 32ºF to -31ºF. The holding cooler temperatures usually range from 32ºF to 39.2ºF to allow for a further reduction in carcass temperature and the resolution of rigor mortis before further processing of the carcass. Chilling systems vary between different species, but in general there are two types of chilling in the meat industry: air and liquid chilling.
Air chilling encompasses a number of different cooling methods. This type is sometimes known as rapid, blast or extreme chilling, in which carcasses are chilled in coolers at temperatures from 32ºF to -31ºF with rapid airflow. This cooling type speeds up the time to dissipate heat in carcasses by reducing chilling time by 25 percent to 35 percent.
Typically, beef, lamb and pork carcasses are chilled using air chilling. During air chilling, the humidity of the chilling and holding coolers are kept high to prevent cooler shrinkage. Cooler shrinkage is the reduction in weight between the hot carcass weight shortly after slaughter and the final carcass weight after chilling. These losses are typically caused by water evaporation resulting in a 1 percent to 3 percent loss in total weight of the carcass. Preventing these losses is a constant concern for processors utilizing air chilling.
Liquid chilling is a technique in which carcasses are generally sprayed or immersed in cold water to reduce temperature. Additionally, antimicrobial agents are added to the water as a means to combat microorganism growth.
One advantage of liquid chilling is the mitigation of cooler shrinkage. The water immersion or spray onto the carcass reduces or replaces the water lost through evaporation and reduces the total percentage of water lost during chilling. While beneficial, it is important to regulate the total spray or immersion time and quantity of water to ensure carcasses do not gain weight and require an alteration in product labeling. Both poultry and fish are predominantly cooled through immersion, while beef, pork, lamb and veal carcasses can be cooled through spray chilling.
Both chilling options are viewed as a beneficial step in the harvesting and processing of meat carcasses. In general, each meat animal species has somewhat different chilling regimens to optimize meat quality. For instance, beef typically follows a chilling protocol in which muscle temperatures should not fall below 50ºF prior to the muscle pH reaching 6.2, which is usually around 10-12 hours postmortem. Conversely, since pork is chilled more rapidly due to a more rapid pH decline, it is optimal to chill carcasses below 50ºF by 12 hours postmortem. These recommendations are based on previous research in which both meat safety and quality are optimized.
Quality defects from insufficient chilling
Apart from the fact that insufficient chilling is unsafe from a microbiological perspective, insufficient chilling also can result in inferior meat quality. After an animal is harvested for meat, the pH decline begins shortly thereafter. When high muscle temperature is combined with a low pH, protein denaturation begins.
The problem of protein denaturation from insufficient chilling occurs in beef, lamb, pork, chicken and turkey meat and results in meat that exhibits increased paleness, decreased redness, increased purge, increased toughness, increased cook loss and a coarse texture. All of these characteristics are associated with poor appeal. Although most processing facilities do not actively heat carcasses, ambient plant temperatures can be at or above the body temperature of the animals, making it difficult for heat to dissipate.
Additionally, energy metabolism within the muscle is an exothermic process causing the muscle to increase in temperature shortly after slaughter. Thus, if temperature of the muscle is not reduced while pH is dropping, the meat may be susceptible to the development of negative quality attributes.
Based on the idea that insufficient chilling is detrimental to meat quality, it is logical to assume that rapid and excessive chilling is the solution to prevent the possibility of temperature induced negative quality attributes. Although that may be true, excessive chilling can be just as problematic. Excessive chilling postmortem can lead to cold-shortening or in extreme cases thaw-rigor. Cold-shortening occurs when the muscle temperature drops below 57.2ºF – 66.2ºF before the onset of rigor mortis.
When this occurs, the meat typically becomes tougher through excessive contraction and shortening of the muscle. Typically, red meat species are more susceptible to this quality defect. The other defect that arises from excessive chilling is thaw-rigor. Thaw rigor develops when muscles are frozen before the resolution of rigor mortis. When the meat is thawed, the muscle contracts from residual energy and becomes especially tough and can release a disproportionate amount of purge.
Chilling carcasses is a complicated science that extends beyond food safety concerns. Chilling directly affects the biochemical processes that directly impact meat quality attributes during the transformation of muscle to meat.
Therefore, when identifying and optimizing new or current chilling protocols, evaluating both food safety and meat quality attributes should be important criteria.
– Eric England is an assistant professor of meat science at The Ohio State University