Resolution of Rigor: How Meat Softens After Rigor Mortis

After an animal is slaughtered, its muscles go through a dramatic transformation. They first stiffen during rigor mortis, becoming extremely tough and essentially inedible. But that stiffness doesn’t last forever. Over time, the muscles gradually soften and become tender again – a process known as the resolution of rigor. This natural phase is arguably the most important stage in the journey from muscle to meat, because it directly determines tenderness, texture, and overall eating quality. For anyone involved in meat processing or food science, understanding how and why this softening happens is essential.

What happens during rigor mortis (a quick recap) 🔗

To understand resolution, you first need to know what causes the stiffness. When an animal dies, its muscles lose their supply of adenosine triphosphate (ATP) – the molecule responsible for powering muscle contraction and relaxation. Without ATP, two key muscle proteins, actin and myosin, lock together permanently, forming stiff bonds called cross-bridges. This is what causes rigor mortis – the muscles contract and become rigid.

Simultaneously, the muscles switch from aerobic to anaerobic metabolism, breaking down glycogen into lactic acid. This causes the muscle pH to drop from around 7.0 (neutral) to approximately 5.4-5.8. The timing of rigor mortis varies by species: beef and lamb typically take 6-12 hours, pork may reach full rigor in as little as 15 minutes to 3 hours, and poultry can stiffen in under an hour.

At the peak of rigor – called rigor maximum – the meat is at its toughest. The muscle fibres are fully shortened, and the cross-bridges between actin and myosin are firmly locked. Meat in this state requires significant force to cut through, and cooking it would result in an extremely tough product.

What is the resolution of rigor? 🔗

The resolution of rigor (also called “rigor off”) is the phase where stiff, rigid muscles begin to soften and regain some of their original extensibility. It typically begins 1-3 days after slaughter, depending on species, temperature, and other factors. During this stage, the locked muscle fibres gradually extend back toward their original length. As this extension occurs, the permanent cross-bridges between actin and myosin create a tearing effect, which starts breaking down the rigid muscle structure.

However, this mechanical tearing is only part of the story. The real driving force behind resolution is enzymatic proteolysis – the breakdown of structural proteins within the muscle by the muscle’s own enzymes. This enzymatic activity weakens the internal scaffolding of muscle fibres, causing them to fragment, separate, and ultimately become tender.

The enzymes behind meat tenderisation 🔗

The softening that occurs during resolution is primarily driven by endogenous proteolytic enzymes – enzymes already present inside the muscle cells. Three major enzyme systems are involved, and understanding their roles helps explain why some meat tenderises faster or better than others.

Calpains: the primary tenderisers 🔗

The calpain system is widely regarded as the most important enzyme system in postmortem meat tenderisation. Calpains are calcium-dependent cysteine proteases found inside muscle cells. The two main types relevant to meat science are μ-calpain (which requires micromolar concentrations of calcium for activation) and m-calpain (which requires millimolar concentrations). Research published in Food Science of Animal Resources confirms that postmortem tenderisation is primarily caused by the action of μ-calpain, with m-calpain playing a supporting role during extended aging.

What makes calpains so effective is their target specificity. Rather than breaking down every protein in sight, they selectively degrade key structural proteins – particularly those that hold the muscle fibre together, such as titin, nebulin, desmin, and the Z-line proteins. The Z-line acts as an anchor point within each sarcomere (the basic contractile unit of muscle), and its degradation causes muscle fibres to fragment into shorter segments. This fragmentation is directly linked to increased tenderness.

There is, however, a natural brake on this process: calpastatin, the specific endogenous inhibitor of calpains. The balance between calpain activity and calpastatin levels in the muscle significantly determines how much tenderisation occurs. Animals with genetically higher calpastatin levels tend to produce tougher meat, while those with lower calpastatin allow faster and more complete resolution.

Cathepsins: the lysosomal contributors 🔗

The second group of enzymes involved are the cathepsins, which reside inside lysosomes – small compartments within the cell that act as a recycling system. During and after rigor, as the cellular environment becomes more acidic and cell membranes deteriorate, these lysosomes break open and release cathepsins into the surrounding muscle tissue.

The main cathepsins involved in muscle aging include cathepsins B, D, L, and H. These enzymes are most active at the slightly acidic pH (5.0-6.0) found in postmortem muscle, which makes them well-suited to contribute to proteolysis during aging. According to research compiled by Chéret et al., calpains and cathepsins work synergistically on key myofibrillar proteins to bring about tenderisation.

Caspases and the proteasome system 🔗

More recently, scientists have identified a third group of enzymes – caspases – that may play a role in the early stages of postmortem muscle degradation. Caspases are enzymes typically associated with apoptosis (programmed cell death), and there is growing evidence that the initial conversion of muscle to meat involves apoptotic pathways. According to a review published by Food Production, Processing and Nutrition, apoptotic enzymes participate in the early stages of muscle aging by degrading proteins like titin and nebulin, and they also help regulate the calcium-activated calpain system.

The proteasome system (particularly the 20S proteasome) is another proteolytic mechanism present in muscle cells, although its exact contribution to postmortem tenderisation is still being studied.

Structural changes during resolution 🔗

As these enzymes go to work, the internal architecture of muscle fibres undergoes visible and measurable changes. The most important structural changes include:

Z-line degradation: The Z-lines, which hold sarcomeres together, become weakened and eventually fragment. This is often considered the hallmark of successful postmortem tenderisation, and it can be observed under an electron microscope.

Myofibril fragmentation: As the Z-lines break down and structural proteins are degraded, the long, continuous myofibrils break apart into shorter segments. This is measured using the myofibril fragmentation index (MFI) – a higher MFI value indicates more fragmentation and, consequently, more tender meat.

Weakening of the cytoskeleton: Proteins like desmin and vinculin, which form the internal scaffolding connecting muscle fibres to each other and to the cell membrane, are degraded. This loosens the overall structure and allows muscle fibres to separate more easily when chewed.

Studies on lamb meat quality have shown that the Warner-Bratzler shear force (WBSF) – a standard measure of meat toughness – increases during rigor mortis but then decreases substantially during resolution. For example, in ovine longissimus muscle, shear force values were found to rise during rigor and then drop significantly by 72 hours postmortem.

Factors that influence the resolution of rigor 🔗

The speed and extent of rigor resolution are not fixed. Several biological and environmental factors determine how effectively the process unfolds.

Temperature 🔗

Temperature is one of the most critical factors. Resolution occurs most effectively at refrigeration temperatures (0-4°C), where beneficial enzymes remain active while bacterial growth is kept in check. According to research on beef dry aging, achieving the same level of tenderness at −0.5°C takes about four weeks, compared to just two weeks at 5°C. Higher pre-rigor temperatures (above 35°C) can activate calpains prematurely but may exhaust them quickly, ultimately reducing the total aging potential and resulting in tougher meat.

On the other hand, if meat is chilled too rapidly before rigor is complete, it can lead to cold shortening – a severe and permanent muscle contraction that makes the meat extremely tough and resistant to resolution. This is why controlled, gradual cooling protocols are essential in commercial meat processing.

pH 🔗

The postmortem pH of meat significantly affects enzyme activity during resolution. A normal ultimate pH of 5.4-5.7 creates favourable conditions for calpain and cathepsin activity. However, meat from stressed animals – which may have depleted their glycogen reserves before slaughter – often fails to reach this pH range. Such meat, known as DFD (dark, firm, dry) meat, tends to have a high ultimate pH (above 6.0), which impairs enzyme activity and leads to poor resolution.

Conversely, an unusually rapid pH decline at high muscle temperatures can produce PSE (pale, soft, exudative) meat, particularly in pork. In PSE conditions, proteins denature prematurely, and proteolytic enzymes lose their effectiveness.

Species and animal age 🔗

Different species show different rates of rigor resolution. Poultry tenderises relatively quickly – often within 24 hours – while beef typically requires 7-14 days (or longer) for optimal tenderness. Younger animals generally resolve rigor faster because their muscle fibres contain fewer and less stable cross-links in the connective tissue, and their enzyme systems tend to be more active.

Muscle type 🔗

Not all muscles within the same animal tenderise at the same rate. Muscles that are more metabolically active or have higher calpain-to-calpastatin ratios tend to resolve rigor more quickly. Locomotion muscles like those in the limbs, which contain more connective tissue, may age slower and remain tougher compared to loin or rib muscles.

Pre-slaughter stress 🔗

The condition of the animal before slaughter has a profound impact on resolution. Animals that experience significant stress, exhaustion, or dehydration before slaughter often arrive at the processing facility with depleted glycogen stores. As a result, insufficient lactic acid is produced, the pH does not decline adequately, and the carcass may remain stuck at rigor maximum – never fully resolving, even with extended aging.

How the meat industry manages resolution 🔗

Modern meat processing facilities use several strategies to manage and optimise the resolution of rigor.

Aging: wet and dry 🔗

Wet aging is the most common commercial method, where vacuum-packed primal cuts are stored under refrigeration for days to weeks. The sealed environment prevents contamination while allowing enzymatic tenderisation to proceed. Dry aging, used for premium beef products, involves storing unpackaged cuts in controlled environments (0-4°C, 75-85% relative humidity) for 28-55 days. Dry aging combines enzymatic resolution with moisture evaporation, which concentrates flavours and produces a distinctive taste profile.

Electrical stimulation 🔗

Electrical stimulation of carcasses shortly after slaughter is widely used – especially for beef and lamb – to accelerate glycolysis and pH decline. This helps deplete ATP faster, reducing the risk of cold shortening and promoting earlier onset of calpain activation. According to Kaur et al. (2021), electrical stimulation has been shown to accelerate proteolysis and improve tenderness, though the results can vary depending on voltage, timing, and muscle type.

Carcass suspension methods 🔗

The way a carcass is hung during cooling also affects resolution. Pelvic (hip) suspension, as opposed to traditional Achilles tendon hanging, stretches certain muscles (particularly the loin and hindquarter cuts), preventing excessive shortening during rigor and improving their final tenderness.

Emerging technologies 🔗

Newer processing technologies like high-pressure processing (HPP), pulsed electric field (PEF), and ultrasound treatment are being explored as ways to enhance enzymatic activity during resolution. These methods can disrupt muscle cell membranes, release calcium ions, and activate proteolytic enzymes earlier – all of which can accelerate tenderisation and reduce the aging time required.

Impact of resolution on meat quality 🔗

The resolution of rigor influences much more than just tenderness. It has far-reaching effects on several quality traits that matter to consumers and processors alike.

Tenderness 🔗

This is the most direct and obvious outcome. Properly resolved meat can show dramatic improvements in tenderness – measured as a significant reduction in shear force values. Meat that required very high cutting force during rigor may need substantially less force after adequate aging.

Water-holding capacity 🔗

As proteolysis progresses during resolution, the breakdown of proteins into smaller units increases osmotic pressure within muscle fibres, which helps the meat reabsorb and retain water. This recovery in water-holding capacity leads to juicier meat with less drip loss. However, this recovery is only partial – the structural changes that occur during rigor permanently alter the muscle, so water retention never fully returns to pre-mortem levels.

Flavour development 🔗

Enzymatic breakdown of proteins during resolution produces free amino acids and small peptides that contribute to the savoury, umami-rich flavour of properly aged meat. For example, amino acids like tyrosine, phenylalanine, threonine, and tryptophan – which are nearly absent in fresh meat – become detectable in aged meat as a result of cathepsin-mediated protein degradation.

Colour and appearance 🔗

The postmortem pH decline and subsequent protein changes during resolution also affect meat colour. Properly resolved meat with a normal ultimate pH (5.4-5.7) develops the bright, cherry-red colour that consumers associate with freshness. Abnormal resolution – whether from stress-related high pH or excessively rapid pH decline – can result in undesirable dark or pale colouration.

When resolution goes wrong 🔗

Not every carcass achieves ideal resolution. Several conditions can impair or prevent proper tenderisation:

Cold shortening: Rapid chilling before rigor is complete causes severe muscle contraction. The resulting toughness is largely irreversible – even extended aging cannot fully compensate for it.

Heat toughening: When muscles are exposed to high temperatures (above 35°C) during the early postmortem period with a rapid pH drop, proteolytic enzymes are activated and exhausted prematurely. This reduces the meat’s aging potential, leading to persistent toughness.

DFD meat: High ultimate pH from pre-slaughter stress impairs enzymatic function and produces meat with poor texture, abnormal colour, and reduced shelf life.

Thaw rigor: If meat is frozen before rigor is complete and then thawed, the remaining ATP can cause a sudden and severe muscle contraction, resulting in very tough meat. This is why proper timing of freezing relative to the rigor process is critical.

Practical takeaways 🔗

The resolution of rigor is where the real magic of meat quality happens. It transforms stiff, inedible muscle into tender, flavourful, juicy meat. The key principles for ensuring good resolution are straightforward: minimise pre-slaughter stress to preserve glycogen stores, control cooling rates to avoid cold shortening, maintain consistent refrigeration temperatures during aging, and allow adequate time for enzymatic tenderisation based on species and cut. Whether through traditional aging, electrical stimulation, or emerging technologies, the goal remains the same – giving those endogenous enzymes the right conditions to do their work effectively.

What do you think? How might consumer demand for faster-to-market meat products influence the adoption of newer tenderisation technologies like high-pressure processing or pulsed electric fields? And could these technologies ever truly replicate the flavour complexity achieved through traditional long-term aging?

References

1. https://opentextbc.ca/meatcutting/chapter/chemical-changes-associated-with-slaughter/
2. https://pmc.ncbi.nlm.nih.gov/articles/PMC8277181/
3. https://www.sciencedirect.com/science/article/pii/S2213453018300600
4. https://www.sciencedirect.com/science/article/abs/pii/S0308814606003037
5. https://fppn.biomedcentral.com/articles/10.1186/s43014-021-00062-0
6. https://pmc.ncbi.nlm.nih.gov/articles/PMC8460324/
7. https://pmc.ncbi.nlm.nih.gov/articles/PMC4872334/
8. https://www.kingson-foodtech.com/en/a3-2068/Meat-aging.html