Effects of Carcass Cooling Rates on Meat Quality

After an animal is slaughtered, the clock starts ticking. What happens to the carcass over the next 24 to 48 hours – especially how quickly or slowly it is cooled – has a direct and lasting impact on the quality of the meat that reaches your plate. Cooling too fast, too slow, or freezing at the wrong time can all cause serious texture problems. For meat processors and food science students, understanding the practical implications of different carcass cooling rates is essential to producing consistently tender, high-quality meat.

What happens inside muscle after slaughter? 🔗

Once an animal is slaughtered and blood circulation stops, the muscles can no longer receive oxygen or remove waste products. The muscle shifts from aerobic to anaerobic metabolism, producing lactate as a byproduct. This lactate builds up and causes the pH of the tissue to drop – from around 7.4 in living muscle to approximately 5.5-5.6 in the finished meat, typically over 24 hours.

During this time, the muscle goes through rigor mortis, a process where the proteins actin and myosin form permanent cross-bridges, causing the muscle to stiffen. The rate at which pH drops and the temperature at which rigor mortis sets in are the two most critical factors that determine meat tenderness, colour, and water-holding capacity. And both of these are directly influenced by how the carcass is cooled.

Cold shortening: when cooling is too fast 🔗

Cold shortening is one of the most well-known quality defects caused by improper carcass cooling. It occurs when muscle temperature drops below 10°C before the pH has fallen below 6.0 – meaning the muscle hasn’t yet completed rigor mortis.

Here’s what happens at the cellular level: the rapid drop in temperature destabilises the sarcoplasmic reticulum, which is the structure inside muscle cells responsible for storing calcium. When it loses its ability to hold calcium, the calcium floods into the cell. If there is still enough ATP (adenosine triphosphate – the cell’s energy currency) available, this calcium triggers a powerful and irreversible muscle contraction. The muscle fibres shorten dramatically, becoming extremely dense and tough.

The result? Meat that is noticeably tougher and chewier, no matter how well it is cooked later. The fibres are packed so tightly that they become very difficult to cut through, whether by knife or by tooth.

Which muscles are most affected? 🔗

Not all muscles are equally susceptible to cold shortening. Smaller muscles closer to the carcass surface cool faster and are therefore at greater risk. Research from North Dakota State University has shown that lighter, leaner carcasses with less subcutaneous fat cool more rapidly than heavier, fatter ones. Fat acts as an insulating barrier, slowing the rate of temperature decline in the deeper muscles and reducing the risk of cold shortening.

For beef and lamb, published research in meat science recommends that muscle temperature should not drop below 10°C before the pH reaches 6.2 to prevent cold shortening. This is the critical threshold that processors must manage carefully.

Heat shortening: when cooling is too slow 🔗

Heat shortening is, in many ways, the opposite problem. It occurs when carcasses are cooled too slowly and the deep muscle temperature remains elevated – above approximately 19-20°C – while rigor mortis is progressing.

Studies on beef muscle have shown that minimum muscle shortening occurs at temperatures between 14°C and 19°C. Below this range, cold shortening kicks in. Above it, heat shortening takes over. In the heat shortening zone, the elevated temperature accelerates metabolic processes and allows muscles to contract more intensely during rigor without adequate relaxation.

In heavy, well-insulated carcasses – particularly those with thick fat cover – the deep muscles can stay warm for a long time. This is especially problematic in warmer climates or in facilities with inadequate refrigeration. According to Meat & Livestock Australia, if cooling of very heavy, fat carcasses is delayed, pale and watery regions can develop in the deep muscles, and these areas tend to brown faster during retail display.

Heat shortening is particularly relevant for pork processing. If the pH drops very rapidly while the muscle is still at near-body temperature (above 35°C), it can cause a condition called PSE – pale, soft, and exudative meat. PSE pork has extensive protein denaturation, poor colour, and severely reduced water-holding capacity.

Thaw rigor: when meat is frozen before rigor mortis 🔗

Thaw rigor is a distinct quality defect that occurs when meat is frozen before it has completed rigor mortis – and is then thawed. According to Britannica, when meat frozen in a pre-rigor state is thawed, the remaining glycogen in the muscle allows for rapid and forceful contraction, making the meat extremely tough.

The mechanism is related to what happens during cold shortening, but the severity is often worse. During freezing, the ice crystals can disrupt the sarcoplasmic reticulum within muscle cells. When the meat thaws and the ice melts, calcium floods out uncontrollably. Combined with residual ATP, this triggers violent muscle contraction. The meat also suffers from significant moisture loss (drip loss) as water that was trapped inside damaged cells leaks out during thawing.

Why thaw rigor matters in commercial processing 🔗

Thaw rigor is a practical concern in situations where carcasses or primal cuts are frozen soon after slaughter – for example, in export operations where meat must be blast-frozen quickly for transport, or in small-scale operations that lack sufficient chilling room space. In some countries, hot carcasses are immediately placed into rapid freezing tunnels at temperatures below −18°C before rigor sets in, which directly leads to this problem upon thawing.

The key prevention strategy is straightforward: never freeze meat before rigor mortis has been completed. If freezing must occur early, processors should use electrical stimulation to accelerate rigor onset before the meat enters the freezer.

Managing cooling rates: techniques and best practices 🔗

Given that cooling too fast causes cold shortening and cooling too slow causes heat shortening, processors must strike a careful balance. Several well-established techniques help manage this process effectively.

Electrical stimulation 🔗

Electrical stimulation (ES) is one of the most widely used post-slaughter interventions for improving meat quality. It involves passing an electric current through the carcass shortly after slaughter. This causes rapid, repeated muscle contractions that accelerate postmortem glycolysis – essentially fast-tracking the pH decline and the onset of rigor mortis.

By ensuring that rigor sets in sooner, ES allows processors to begin chilling the carcass more aggressively without the risk of cold shortening. Since the muscle has already completed its biochemical transition, the rapid drop in temperature won’t trigger the calcium-release mechanism described earlier.

There are two main types of electrical stimulation used in the industry. Low-voltage ES (LVES) typically operates below 100 volts and is applied soon after bleeding, usually within 10 minutes. It’s cheaper and safer but less consistent in its effects. High-voltage ES (HVES) uses 300-1,000 volts and can be applied up to 60 minutes after slaughter. It produces more reliable improvements in tenderness and meat colour, though it requires greater safety precautions and capital investment.

Beyond preventing cold shortening, ES may also benefit tenderness by causing physical disruption of muscle fibres and by releasing enzymes (lysosomal cathepsins) that further break down structural proteins during aging.

Carcass hanging methods 🔗

How a carcass is hung during chilling directly affects which muscles are stretched and which are allowed to contract during rigor. There are two primary approaches:

Achilles tendon suspension is the traditional and most common commercial method. The carcass is hung by the hind legs through the Achilles tendon. While practical and space-efficient, this method allows many of the major muscles – especially in the loin and hindquarter – to contract freely during rigor, potentially increasing toughness.

Tenderstretch (pelvic suspension) involves hanging the carcass from the pelvic bone instead. This positions the legs at roughly a 90-degree angle from the body, which physically stretches the loin and hindquarter muscles and prevents them from contracting during rigor. Research from Australia’s CSIRO found that tenderstretching improved tenderness scores for rump, striploin, topside, and cube roll compared to Achilles-hung carcasses.

A notable practical advantage of tenderstretching is that it reduces or eliminates the need for electrical stimulation. However, it requires more chiller space per carcass, which adds to operational costs.

Controlled chilling protocols 🔗

The ideal chilling regime varies by species. For beef and lamb, the goal is to avoid dropping muscle temperature below 10°C while the pH is still above 6.2. For pork, the concern is different – processors need to cool the carcass rapidly enough to prevent PSE while avoiding cold shortening. A recommended protocol for pork loins suggests a temperature of 35-37°C at one hour postmortem, declining gradually to about 5-6°C by 20 hours.

Spray chilling is a technique where chilled water is intermittently sprayed onto carcasses during the early stages of cooling. This improves cooling rates through evaporative cooling and helps reduce carcass shrinkage (weight loss due to moisture evaporation). It also promotes more uniform temperature distribution across the carcass surface.

Blast chilling uses high-velocity cold air to rapidly cool carcasses. While effective at controlling microbial growth and reducing processing time, it must be managed carefully to avoid cold shortening in pre-rigor muscle, particularly in leaner, lighter carcasses.

The role of carcass size and fat cover 🔗

Carcass weight and fatness significantly influence cooling rates. Research published in Meat and Muscle Biology found that variability in carcass size affected temperature decline and postmortem metabolism, and recommended that postmortem management practices should account for carcass weight to optimise meat quality. Heavier carcasses with thicker fat cover cool more slowly and are at greater risk of heat shortening, while leaner, lighter carcasses cool faster and face a higher risk of cold shortening.

This means that in a commercial setting, where carcasses of varying sizes often go through the same chiller under identical conditions, some carcasses may be cooled appropriately while others are not. Sorting carcasses by weight class or adjusting chiller settings can help address this variability.

Effects on meat colour and water-holding capacity 🔗

Cooling rates don’t just affect tenderness – they also influence colour and moisture retention. Slow cooling tends to produce lighter-coloured meat due to greater protein denaturation at elevated temperatures. In severe cases, it leads to PSE-like conditions with excessive drip loss.

Rapid cooling, on the other hand, generally preserves a desirable darker red colour and better water-holding capacity – as long as cold shortening is avoided. Proper electrical stimulation combined with controlled chilling also tends to produce a brighter, more appealing lean colour because the faster pH decline at warm temperatures promotes the formation of the desirable oxymyoglobin pigment.

Aging as a complementary strategy 🔗

Even with imperfect cooling, some degree of toughness can be mitigated by postmortem aging. Aging allows natural enzymes (primarily calpains and cathepsins) to gradually break down structural proteins within the muscle fibres, improving tenderness over time. For conventionally hung carcasses, aging for 14 to 21 days at refrigeration temperatures (0-4°C) can significantly improve eating quality. Tenderstretched carcasses often reach acceptable tenderness in as little as two days – a major commercial advantage.

However, aging cannot fully reverse the toughness caused by severe cold shortening or thaw rigor. Prevention through proper cooling management remains the most effective strategy.

Bringing it all together 🔗

Managing carcass cooling rates is a balancing act. The ideal chilling protocol must account for species, carcass size, fat cover, ambient conditions, and the availability of technologies like electrical stimulation and alternative hanging methods. The following table summarises the three main cooling-related defects:

By combining electrical stimulation (to accelerate rigor onset), appropriate hanging methods (to prevent contraction), and carefully calibrated chilling protocols (to hit the right temperature-pH window), meat processors can consistently deliver tender, high-quality meat while minimising waste and maximising efficiency.

What do you think? If you had to prioritise one intervention – electrical stimulation, tenderstretch hanging, or aging – which do you think would have the greatest practical impact on meat tenderness in a commercial setting? And how might the rising trend of heavier carcass weights force the industry to rethink its standard chilling protocols?

References

1. https://porkgateway.org/resource/the-role-of-carcass-chilling-in-the-development-of-pork-quality/
2. https://pmc.ncbi.nlm.nih.gov/articles/PMC4093271/
3. https://www.ndsu.edu/agriculture/extension/publications/influence-carcass-weight-and-external-fat-thickness-chilling-rate-commercial
4. https://pubmed.ncbi.nlm.nih.gov/22063744/
5. https://www.sciencedirect.com/science/article/abs/pii/S0309174000000796
6. https://www.mla.com.au/contentassets/90641826a813482d99188760bb863f7f/ref41_carcass_chilling.pdf
7. https://www.britannica.com/science/thaw-rigor
8. https://pmc.ncbi.nlm.nih.gov/articles/PMC11394318/
9. https://www.researchgate.net/publication/318215482_THE_USE_OF_ELECTRICAL_STIMULATION_IN_MEAT_PRODUCTION
10. https://nosetotailapp.com/tenderstretch-vs-achilles-tendon
11. https://www.iastatedigitalpress.com/mmb/article/id/13893/