Cold Preservation Of Meat Products

Cold preservation of meat products

Meat itself is not a living organism but it is subject to endogenic enzymatic activity, or proteolysis, which causes muscle tissue to mature, become tender and develop a typical taste. This process is retarded by cold.

Due to its chemical composition which is rich in proteins, lipids and water, meat is a particularly favourable substrate for the growth of microorganisms. The lipidic content also makes it very sensitive to oxidation.

Healthy animals, hygienically slaughtered after resting and fasting, provide a practically aseptic meat. However, following slaughter the evisceration and dressing operations inevitably produce microbial contamination in depth and especially on the surface, through contact with equipment, tools, hands and clothes, despite all precautions.

Again, micro-organism growth is a temperature-dependent process. To avoid it, it is absolutely essential to reduce the temperature of the meat, especially on the surface, immediately after dressing. Cooling must therefore be carried out in the slaughterhouse itself. This operation is known as primary chilling.

Meat loses weight through surface evaporation. This process depends on differences in temperature and relative humidity between the meat and the environment.

Slaughter operations and carcass dressing separate the parts of the animal which have distinct histological properties and are intended for different uses. The carcass itself incorporates mainly muscles, bones, fat and connective tissue. The offal includes some edible organs, while some glands are used in pharmaceutical preparations. These different parts must be subjected to varying cooling conditions according to their susceptibility to microbial growth, to temperature effects and to the risk of surface dehydration.

CHILLING

To prevent or even to reduce the deterioration process, particularly microorganism development, chilling has to be carried out quickly after carcass dousing at the end of the slaughter process and the chilled state has to be maintained until the meat is processed for consumption.

Chilling can be defined as the fundamental operation in applying cold to meat to reduce its temperature quickly. This is done in a cold chamber with intensive air draught or movement. Rapid cooling of the meat surface not only slows and nearly stops the development of surface micro-organisms but also reduces weight loss and discoloration of the surface owing to haemoglobin oxidation. Different systems of primary chilling are in use (including immersion in iced water, especially for poultry) but air chilling is the most common.

The cold chambers where chilling takes place must have a low air temperature, a high air speed, a high relative humidity and a high refrigerating capacity.

Air temperature must be in the region of 0°C, with no decrease below -1°C, which could freeze the meat surface and impair its appearance.

Air speed can range from 0.25 to 3.0 m/s. However, for economical reasons the most common speeds in use are from 0.75 to 1.5 m/s in the empty section of the cold chamber.

Air speed over the carcasses will be much higher because of the reduction in air circulation. Increased air speed reduces the cooling period but it has a limit as there is a threshold above which fan-power consumption increases more than the chilling rate, resulting in an increase in operational costs. Also, the higher the air speed the greater the weight loss.

Relative humidity during the chilling operation should be kept fairly high to prevent excessive weight loss. The recommended rate is between 90 and 95 percent, though this is the most difficult factor to control.

Primary chilling is completed when the warmest point of the carcass has reached a temperature of about 7°C (3°C for edible offal). With current technology these temperatures can be arrived at in 16–24 hours in small carcasses and in less than 48 hours in large carcasses (centre of the hind leg). Average and surface temperatures are obviously much lower, reaching 0°C on the surface within four hours; this is important to slow microbial proliferation.

Quick chilling has its problems, cold shortening being the most common. Cold shortening can often be seen in beef and mutton, when the meat, still in its pre-rigor phase, reaches temperatures of 10°C or lower. These conditions cause irreversible contractions of the muscle tissue which toughen the meat even after prolonged ripening.

Quick primary chilling also signifies an increase in investment and higher operational costs. The chilling period can be reduced by lowering the air temperature (surface freezing risks) or increasing air speed (higher operational costs) or both. Occasionally cold chambers are refrigerated in advance to reach lower temperatures than those in operation (-5°C/-6°C for beef; -10°C/-12°C for pork), taking advantage of thermal inertia to offset the effect of warm meat loads.

Quick primary chilling can be performed in small chambers or in cooling tunnels. In cold chambers it is carried out in two or three phases. During the first phase the air temperature is maintained at about 0°C, carefully controlling the risk of superficial freezing while air movement is maintained at a high level. For large carcasses, after 10–12 hours the air circulation inside the store is reduced, maintaining temperature and humidity conditions; this second phase lasts another six to 10 hours. After this period the meat is transferred to cold storage chambers where the carcass temperature is stabilized, concluding the third phase.

Small cold chambers used for chilling must be designed so their capacity can be filled in two hours at the slaughterhouse's normal work rate. The number of chambers should be sufficient for a peak working day. Particular care should be taken that warm humid carcasses are placed behind those already chilled or in the process of being chilled so that the air, which is still cold, reaches them and there is no risk of superficial condensation.

Cooling tunnels used for chilling meat are usually of the continuous type. Here again meat is subjected to a two-phase process, with conditions similar to the cold chamber. However the temperature can be as low as -5°C for a short time. Beef carcasses can reach an average temperature of about 15°C in a four-hour period, while pork and mutton reach the same temperature in two to two and a half hours. Surface temperature decreases to 4–5 °C. During the second phase, conditions are less exacting, and an average temperature of about 4°C is stabilized after 15–16 hours in a secondary refrigerating chamber. This method is used in high-capacity slaughterhouses particularly for pig carcasses; for beef