The Role of pH in Meat Quality: Normal, DFD, and PSE Meat

When you buy a piece of meat at the market, its colour, juiciness, and tenderness have already been largely determined – not by the cook, but by a single biochemical factor: pH. The pH of meat governs how proteins behave after an animal is slaughtered, and this in turn shapes everything from shelf life to flavour. Understanding how pH works in meat – and what happens when it goes wrong – is essential for anyone studying meat science, working in the meat industry, or simply wanting to make better purchasing decisions.

What is pH and why does it matter in meat? 🔗

pH is a measure of acidity or alkalinity on a scale of 0 to 14, with 7 being neutral. In living muscle, the pH sits at approximately 7.0 to 7.2 – slightly alkaline. After slaughter, however, a series of biochemical changes begins that steadily lowers this value. This decline is one of the most significant events in the conversion of muscle to meat, and it directly influences colour, water holding capacity (WHC), tenderness, texture, and even microbial safety.

The reason pH matters so much is its effect on muscle proteins. Proteins like myosin and actin are responsible for binding water, reflecting light, and creating texture in meat. Their structure and function change dramatically depending on the acidity of their environment. A rapid drop, a slow drop, or an incomplete drop in pH after slaughter each produces very different outcomes in the final product.

Post-mortem pH decline: the normal process 🔗

Once an animal is slaughtered, blood circulation stops and oxygen is no longer delivered to the muscles. The muscle cells shift from aerobic (oxygen-dependent) metabolism to anaerobic glycolysis – a process where stored glycogen is broken down without oxygen. The end product of this anaerobic breakdown is lactic acid, which accumulates in the muscle tissue and causes the pH to fall.

In a well-rested, properly handled animal, this pH decline happens gradually. The pH drops from around 7.0 in living muscle to an ultimate pH (pHu) of about 5.4 to 5.8 over a period of 6 to 24 hours, depending on the species. In beef, this process typically takes 24 to 36 hours, while in pork and poultry it happens faster.

This gradual acidification continues until the glycolytic enzymes become inactive at low pH, or until the glycogen supply is depleted – whichever happens first. The point at which the pH stabilises is called the ultimate pH, and it is one of the most widely used indicators of meat quality in the industry.

What normal pH decline produces 🔗

When the rate and extent of pH decline are both within normal ranges, the result is meat that the industry classifies as RFN – Reddish-pink, Firm, and Non-exudative. This is the ideal outcome. RFN meat has a bright, appealing colour, firm texture, minimal drip loss, good water holding capacity, and desirable tenderness after aging. According to the Pork Information Gateway, the preferred range for ultimate pH in pork is roughly 5.6 to 6.0, which ensures optimal protein functionality and consumer satisfaction.

The isoelectric point: a critical pH threshold 🔗

To understand why pH variations cause such different meat qualities, you need to know about the isoelectric point (pI) of meat proteins. The isoelectric point of the major myofibrillar proteins (actin and myosin) is approximately pH 5.0 to 5.1. At this pH, the proteins carry no net electrical charge – they have equal numbers of positive and negative charges, which means they have the least ability to attract and bind water molecules.

As the pH moves away from this isoelectric point – either higher or lower – the proteins gain a net charge and their water-binding ability increases. This is why meat with a very low ultimate pH (close to 5.0) loses excessive moisture, and meat with a high ultimate pH (above 6.0) retains water very well. The relationship between pH and WHC is one of the most fundamental principles in meat science.

Dark, firm, and dry (DFD) meat 🔗

DFD meat is a quality defect that occurs when the pH of meat fails to decline sufficiently after slaughter. Instead of reaching a normal ultimate pH of 5.4 to 5.8, DFD meat remains at a pH above 6.0 – and in some cases can be as high as 6.8.

What causes DFD? 🔗

The primary cause of DFD is prolonged or chronic stress before slaughter. When animals experience extended periods of stress, physical exertion, fighting, poor handling during transport, long fasting periods, or adverse weather conditions, they use up their muscle glycogen reserves while still alive. By the time the animal is slaughtered, very little glycogen remains in the muscles. Without sufficient glycogen, the post-mortem glycolysis process cannot produce enough lactic acid to lower the pH to normal levels.

This condition is commonly seen in cattle (where it is also called “dark cutting” beef) and in pigs. It is less frequent in lambs. Research published in the journal Food Science & Nutrition has confirmed that factors such as transport duration, lairage holding time, mixing with unfamiliar animals, and changes in environment all contribute to glycogen depletion and subsequent DFD occurrence.

Characteristics of DFD meat 🔗

DFD meat has several distinctive features that set it apart from normal meat:

Dark colour: The high pH allows muscle proteins to bind more tightly together, creating a closed, dense structure. This structure absorbs more light and reflects less, giving the meat an unusually dark, almost purplish-black appearance instead of the expected bright red.

Firm texture: Because the proteins retain more water at high pH and are not denatured, the muscle structure stays dense and firm. The texture feels noticeably drier to the touch despite the high internal moisture content.

High water holding capacity: Since the pH is well above the isoelectric point, the muscle proteins carry a strong net charge and hold onto water effectively. This means DFD meat actually has good WHC and low drip loss – which makes it suitable for use in certain processed products like cooked sausages.

Poor shelf life: This is the most serious commercial problem with DFD meat. Most spoilage bacteria thrive in environments with pH values above 6.0. The elevated pH of DFD meat creates ideal conditions for rapid microbial growth, meaning the meat spoils much faster than normal meat. This drastically reduces its commercial viability for fresh retail.

Reduced flavour: DFD meat tends to have less characteristic meat flavour compared to normal meat, which limits its appeal in fresh-meat applications.

Pale, soft, and exudative (PSE) meat 🔗

PSE meat represents the opposite extreme of the pH problem. While DFD results from too little acidification, PSE occurs when pH drops too rapidly in the early post-mortem period – often falling below 5.8 within just 45 minutes of slaughter, while the carcass temperature is still high (above 35°C).

What causes PSE? 🔗

PSE meat is primarily caused by acute, short-term stress immediately before slaughter. This includes rough handling, use of electric goads, overcrowding in lairage, fighting among animals just before slaughter, and heat stress. These sudden stressors trigger an accelerated metabolic response that speeds up glycogen breakdown enormously.

Genetics also plays a major role. In pigs, a mutation in the ryanodine receptor gene (RYR1) – associated with a condition known as porcine stress syndrome (PSS) – causes abnormal calcium release within muscle cells. This leads to exaggerated glycolysis, excessive lactic acid production, and a rapid rise in muscle temperature. Pigs carrying this gene are significantly more susceptible to developing PSE meat.

While PSE was historically associated mainly with pork, it is now recognised in poultry and, to a lesser extent, in beef as well. The incidence in poultry has reportedly increased in recent decades, likely due to intensive breeding for rapid growth and larger breast muscles.

The biochemistry behind PSE 🔗

The key problem in PSE is the combination of low pH and high temperature occurring simultaneously. Under normal conditions, by the time pH reaches its lowest point, the carcass has cooled considerably and the proteins are protected. In PSE conditions, the pH plummets while the muscle is still warm. This combination causes protein denaturation – particularly of myosin and sarcoplasmic proteins like myoglobin.

Denatured proteins lose their normal structure and function. Myosin that has been denatured cannot bind water effectively, and myoglobin that has been denatured loses its ability to impart the normal red colour. The myofibrils shrink and contract excessively, squeezing water out of the muscle cells into the extracellular space, where it is eventually lost as drip.

Characteristics of PSE meat 🔗

Pale colour: The denatured proteins scatter and reflect more light from the meat surface, giving it an abnormally light, washed-out appearance. Research has established that lightness values (L*) above 53 on the CIE colour scale are indicative of PSE conditions.

Soft texture: The denatured protein structure loses its firmness, making the raw meat feel soft and mushy. When sliced, PSE meat tends to fall apart easily.

Exudative (excessive drip loss): Because the denatured proteins cannot hold water, PSE meat loses moisture rapidly. This is visible as excessive fluid on the surface of packaged meat and as significantly higher cooking losses. Despite the term “soft” sounding positive, cooked PSE meat is actually tough and dry because so much moisture is lost during cooking.

Reduced flavour: The combination of moisture loss and protein denaturation results in meat with notably less flavour compared to normal meat.

Poor processing functionality: PSE meat performs poorly in processed products. Its denatured proteins have reduced ability to bind water, emulsify fat, and form stable gels, making it a challenge for sausage and other processed meat manufacturers.

Comparing normal, DFD, and PSE meat 🔗

The three conditions can be summarised by looking at their pH behaviour and resulting quality traits:

Normal (RFN) meat has a gradual pH decline from ~7.0 to an ultimate pH of 5.4-5.8, producing bright reddish-pink colour, firm texture, good WHC, and desirable tenderness. DFD meat shows minimal pH decline with an ultimate pH above 6.0, resulting in dark colour, firm-dry texture, high WHC, short shelf life, and reduced flavour. PSE meat undergoes a rapid pH drop to below 5.8 within 45 minutes, leading to pale colour, soft texture, very low WHC with excessive drip loss, and poor flavour.

The economic impact of pH-related meat defects 🔗

These quality defects carry substantial economic consequences for the meat industry. PSE-related quality problems alone have been estimated to cost the US pork industry approximately $200 million annually. This figure accounts for increased drip loss (meaning less saleable weight), consumer rejection due to unappealing appearance, and reduced yields in processing. DFD meat, while having better water retention, suffers from poor shelf life and dark appearance that consumers find unappealing, reducing its commercial value for the fresh meat market.

Preventing DFD and PSE: the role of animal welfare 🔗

Since both DFD and PSE are fundamentally linked to pre-slaughter stress, prevention strategies focus heavily on improving animal handling and welfare throughout the supply chain.

To prevent DFD: Minimise long-distance transport where possible. Provide adequate rest periods (lairage) after transport so animals can replenish glycogen reserves. Avoid mixing unfamiliar animals. Ensure access to water and comfortable holding conditions. Reduce noise and unfamiliar stimuli in the slaughterhouse.

To prevent PSE: Avoid rough handling, electric goads, and overcrowding immediately before slaughter. Use low-stress stunning methods – research suggests two-phase gas stunning may be effective. In hot weather, provide ventilation and water showers during transport. Rapid post-mortem chilling of carcasses can also help by bringing the muscle temperature down before excessive glycolysis occurs.

Genetic selection is another important tool, particularly for PSE prevention. Breeding programmes that identify and eliminate the Halothane gene (HAL) and other stress-susceptibility markers have significantly reduced PSE incidence in commercial pig populations. DNA marker technology is now widely used to improve meat quality traits including pH, colour, and tenderness through more accurate genetic selection.

Practical implications for meat processing 🔗

Despite being quality defects, both DFD and PSE meat are perfectly safe to eat. The challenge lies in how to use them effectively. DFD meat, with its high water holding capacity, can be suitable for scalded or boiled sausages and other cooked products – but it should be used quickly due to its reduced shelf life. PSE meat can be improved through marinades and brining, which help restore some moisture retention, though results are limited. In commercial practice, blending DFD or PSE meat with normal meat is a common strategy to produce products of acceptable quality.

pH monitoring is now a standard practice in modern abattoirs. Measuring pH at 45 minutes post-mortem (pH₄₅) and at 24 hours (pHᵤ) allows processors to identify and sort carcasses into quality categories early, enabling them to route meat to the most appropriate end use.

What do you think? Given that both DFD and PSE conditions are largely preventable through better animal handling, how can the meat industry balance the pressure for efficiency and speed in slaughter operations with the need for low-stress conditions that produce better quality meat?

References

1. https://pmc.ncbi.nlm.nih.gov/articles/PMC10217372/
2. https://www.sciencedirect.com/science/article/abs/pii/S0309174018301207
3. https://porkgateway.org/resource/pork-quality-ph-decline-and-pork-quality/
4. https://porkgateway.org/resource/water-holding-capacity-of-fresh-meat/
5. https://www.fao.org/4/t0562e/t0562e02.htm
6. https://pmc.ncbi.nlm.nih.gov/articles/PMC10084969/
7. https://en.wikipedia.org/wiki/PSE_meat
8. https://www.davidpublisher.com/Public/uploads/Contribute/5dc27e08c78e6.pdf
9. https://www.researchgate.net/publication/279582972_Pale_soft_exudative_PSE_and_dark_firm_dry_DFD_meats_Causes_and_measures_to_reduce_these_incidences-A_mini_review
10. https://www.thepigsite.com/articles/influence-of-ultimate-ph-on-meat-quality-and-consumer-purchasing-decisions