30 May 2024
Eben van Tonder
Introduction
This article explores the role of cortisol in stress management and its impact on meat quality. Cortisol mobilizes energy by promoting gluconeogenesis, protein catabolism, and lipolysis. It also has anti-inflammatory effects and helps maintain homeostasis by regulating blood pressure and cardiovascular function. The article discusses how chronic stress and elevated cortisol levels lead to muscle wasting, poor weight gain, and inferior meat quality. It also covers the mechanisms behind DFD and PSE meat and emphasizes the importance of rapid cooling and stress management to preserve meat quality.
Christa Berger, who by now has become a collaborator with me in this work recounts the following memories of growing up on a farm in Austria. She writes, “I can briefly report from experience about the slaughter of animals and the stress associated with it: We used to have chickens on our farm for our own use, one of which was slaughtered every now and then. Before this happened, you grabbed the chicken tightly and spun around in circles a few times very quickly so that they would lose their orientation. In winter they were briefly placed in a snow field, where they also lost their orientation. They were then briefly unconscious. Only then they were slaughtered. The pigs were also for our own use. They were used to roaming in the yard, which was later the place of slaughter. It was a square courtyard with a large entrance. A picture of the courtyard type is included. The pig was also stunned with a captive bolt gun before slaughter
Then we had cattle, cows for milk production and bulls as well as calves, a total of around 50 – 60 animals. The cows had long lives; I remember one cow that left the farm at 18 years old. Every cow had a name, and every one of us knew the cow and its quirks. She then described the stress of cows if they were led away by the butcher for slaughter. The discussion of the affect of stress on animals progressed from there.
She sent me an article by Lea Sprügel and Jasmin Peschke titled “On-farm slaughter: animals’ stress hormones reduced by a factor of twenty”, published on July 6, 2023, on the Section for Agriculture website. The study mentioned in the article reveals that animals slaughtered on the farm have twenty times less cortisol in their blood compared to those slaughtered in abattoirs. This significant reduction in stress hormones is linked to improved meat quality, as stress during slaughter can negatively impact tenderness and water-holding capacity. (For more insights and the full study details, you can visit the original article here: On-farm slaughter: animals’ stress hormones reduced by a factor of twenty.)
This was news to me! I knew that conditions like Pale, Soft, Exudative meat and Dry, Firm and Dark Meat refer to poor meat quality brought on by stress in animals. I thought that this was due to a build-up of lactic acid in their muscles which causes damage to the muscle structure. I even dedicated a chapter in Bacon & the Art of Living to the subject, describing conditions at the Cape Town City Abattoir in The Shambles – Meat Quality and the Human Mind.
After my correspondence with Christa on the subject I realised that I actually know nothing about it and what I discovered was that I was completely wrong in how I understood the matter.
Cortisol
Lea Sprügel and Jasmin Peschke gave me the education on the subject I needed by identifying the issue not as lactic acid in the muscles, which turns out to be indispensable for good meat quality, but cortisol.
Cortisol is a steroid hormone produced by the adrenal glands in response to stress and low blood glucose levels. It is the vehicle of the body to summon all available energy resources. Cortisol increases blood sugar through gluconeogenesis, mobilizes fat stores, and stimulates protein catabolism to provide immediate energy for the body. This involves breaking down proteins into amino acids, which are then converted into glucose. So, the issue in meat quality deterioration is high cortisol levels.
Let’s look a bit closer. Cortisol mobilizes energy for the body under stress. The reason it breaks down muscle proteins is to access amino acids, which are then transported to the liver and converted into glucose through gluconeogenesis. This glucose is released into the bloodstream, providing a readily available energy source. Once in the bloodstream, glucose enters cells throughout the body, where it undergoes glycolysis, breaking down into pyruvate and producing ATP. The pyruvate enters the mitochondria and is further processed through the Krebs cycle and oxidative phosphorylation, generating a significant amount of ATP to fuel various cellular activities and maintain vital physiological functions.
Lets diagram the various stages out as follows:
Cortisol-Induced Pathway
- Cortisol Release
- Stress triggers the release of cortisol.
- Muscle Protein Breakdown
- Cortisol breaks down muscle proteins into amino acids.
- Gluconeogenesis in the Liver
- Amino acids are converted into glucose in the liver.
- Glucose Release into Bloodstream
- Glucose is released back into the bloodstream.
- Glucose Uptake by Cells
- Glucose enters cells throughout the body.
- Glycolysis
- Inside the cells, glucose is broken down into pyruvate, producing ATP.
- Pyruvate Enters Mitochondria
- Pyruvate enters mitochondria.
- Krebs Cycle and Oxidative Phosphorylation
- Pyruvate is processed through the Krebs cycle and oxidative phosphorylation, generating large amounts of ATP to fuel cellular activities.
These are the important points so far related to muscle structure because we learned that cortisol breaks down muscle proteins into amino acids which already tells us that long-term stress harms meat quality. Remember that there are other functions of cortisol. In addition to its role in energy mobilization, cortisol has significant anti-inflammatory effects. It suppresses the immune system’s inflammatory response, reducing potential tissue damage during stress. This helps maintain homeostasis by regulating blood pressure, cardiovascular function, and the body’s overall response to stress. By managing these processes, cortisol ensures that the body can cope effectively with stress while maintaining vital physiological functions.
A. Effect of Cronic Stress and Accompanying Constant High Cortisol Levels
We ended by noting that there will be an effect on the body’s muscles if the animal is under constant stress. Let’s delve into this first. Proper regulation of cortisol levels is essential for health, as chronic high levels also adversely affect meat through muscle wasting and general metabolic imbalances. Not only is the muscle structure impacted, but metabolic imbalances due to stress can negatively impact weight gain and feed conversion in animals.
Stress diverts energy resources towards coping mechanisms (e.g., gluconeogenesis) rather than growth, which reduces the feed conversion efficiency. Chronic stress elevates cortisol levels, which can alter metabolism by increasing protein catabolism which is the process we looked at in the previous section but it also affects muscle synthesis. It is therefore not just stress immediately pre-slaughter that affects meat quality, but cortisol affects the meat quality of animals who are constantly under stress such as in a feedlot.
The breakdown of muscle proteins by constantly high levels of cortisol affects the structural integrity of the meat, resulting in poorer texture, lower water-holding capacity, and overall inferior meat quality. The reason remains the same namely that chronic stress and high cortisol levels cause protein catabolism, breaking down muscle proteins into amino acids. This can lead to reduced muscle mass and quality.
B. Impact on High Cortisol Immediately Pre-Slaughter
As I mentioned, I thought the issue with poor meat quality was too much lactic acid. This is not the case at all. It is too little lactic acid and too high a temperature in an environment where homeostasis can no longer be maintained because the body is dead.
Glycogen Depletion and pH
High cortisol levels lead to more rapid glycogen depletion in muscles. Glycogen, being the primary energy reserve in muscle tissues, is crucial for post-mortem biochemical processes. Upon slaughter, this glycogen is converted into lactic acid, which plays a significant role in lowering the pH of the meat.
Glycogen Conversion to Lactic Acid Post-Slaughter
Upon slaughter, the stored glycogen in muscle tissues is converted into lactic acid. This biochemical process is crucial for lowering the pH of the meat, which significantly affects meat quality. When an animal is slaughtered, the oxygen supply to the muscles ceases, forcing the muscles to switch from aerobic to anaerobic metabolism. In the absence of oxygen, glycogen is broken down through anaerobic glycolysis, resulting in the production of lactic acid.
Post-slaughter, the conversion of glycogen to lactic acid is a natural and essential process. The enzyme lactate dehydrogenase facilitates this conversion, leading to the accumulation of lactic acid within the muscle tissues. This increase in lactic acid causes a drop in pH, creating an acidic environment in the meat.
The decrease in pH due to lactic acid is beneficial for several reasons. Firstly, it inhibits the growth of spoilage microorganisms and pathogens, enhancing the meat’s safety and shelf life. Secondly, the acidic environment promotes the denaturation of muscle proteins in a controlled manner, which improves meat tenderness and texture. The lower pH also stabilizes the colour of the meat, resulting in a more appealing bright red hue in beef.
Lactic acid plays a crucial role in improving the water-holding capacity of meat. As the pH drops, the muscle proteins denature and form a gel-like structure that can trap water within the muscle fibres. This reduces drip loss and ensures the meat remains juicy and tender.
The production of lactic acid from glycogen post-slaughter is therefore a vital process for achieving high-quality meat. (For further reading on the biochemical processes involved in meat quality post-slaughter, you can refer to the detailed explanations provided in Lehninger Principles of Biochemistry and research articles available on PubMed.)
When cortisol levels are high due to prolonged stress, the glycogen stores in muscle tissues are depleted more rapidly. Lower glycogen levels result in reduced lactic acid production, leading to a slower and less pronounced drop in pH. This slower acidification process is bad for meat quality and affects the meat’s texture and water-holding capacity.
The reduction in lactic acid leads to a higher ultimate pH in the meat, resulting in a condition known as Dark, Firm, Dry (DFD) in beef. DFD meat is characterized by being darker, drier, and tougher due to insufficient acidification that normally tenderizes the meat and enhances its quality. The higher pH negatively impacts the meat’s texture and water-holding capacity, making it less appealing to consumers. (For more detailed information, you can refer to Lehninger Principles of Biochemistry and research articles available on PubMed.)
Link Between Glycogen and Low pH in Post-Slaughter Meat
Let’s see one more time what is expected to happen to have a good impact on meat quality. Glycogen, stored in muscle tissues, is a critical energy reserve. Upon slaughter, the muscle cells continue to metabolize, but due to the lack of oxygen, they switch to anaerobic respiration. During anaerobic respiration, glycogen is broken down into glucose, which is then converted into lactic acid. This process generates energy for the cells. The accumulation of lactic acid lowers the pH of the meat, creating an acidic environment. If this happens it is good!
How Cortisol Leads to Rapid Glycogen Depletion
We look one last time at what we do not want to see. High levels of cortisol increase metabolic activity, which further accelerates the breakdown of glycogen. The heightened energy demands caused by stress require more glucose, leading to a faster depletion of glycogen stores. This increased metabolic rate ensures that sufficient energy is available to cope with stress, but it can also compromise the animal’s ability to maintain energy reserves over the long term. The combination of rapid glycogen depletion and increased metabolic activity can negatively impact overall health and muscle condition, contributing to poorer meat quality in stressed animals.
Impact of Low pH and High Temperature on Post-Mortem Muscle
The combination of low pH and high temperature in post-mortem muscles causes rapid protein denaturation and increased enzyme activity, leading to accelerated autolysis. Low pH helps stabilize muscle proteins by inhibiting some proteolytic enzymes, but high temperatures disrupt hydrogen bonds and non-covalent interactions in proteins, causing them to denature quickly. This results in rapid protein breakdown and enhanced autolysis, as enzymes like calpains and cathepsins remain highly active at physiological temperatures immediately post-mortem.
Despite the low pH, these enzymes are not fully inhibited due to the high temperatures, leading to poor meat quality characterized by pale, soft, and exudative (PSE) texture. Denatured proteins lose their water-holding capacity, increasing drip loss and further degrading meat quality. Proper handling and rapid cooling post-slaughter are essential to manage these effects and ensure high meat quality and consumer satisfaction.
C. Mechanics of DFD (Dark, Firm, Dry) and PSE (Pale, Soft, Exudative) Meat
DFD meat is caused by long-term stress before slaughter. This prolonged stress results in elevated cortisol levels, which lead to the rapid depletion of glycogen stores in the muscles. As a result, the animal’s overall health and feed-to-protein conversion efficiency decline, negatively impacting meat quality. Without sufficient glycogen, post-mortem lactic acid production is limited, leading to a higher pH. This elevated pH prevents muscle proteins from breaking down properly, resulting in dark, firm, and dry meat with poor water-holding capacity. The high pH and inadequate protein breakdown contribute to the undesirable texture and quality of DFD meat.
PSE meat is associated with a rapid decline in pH along with warm carcass temperatures. The low pH is from lactic acid production, similar to the lactic acid generated in muscles during exercise. However, the key issue in PSE meat is not merely the presence of lactic acid but the combination of rapid pH decline and high muscle temperatures post-slaughter.
When an animal undergoes acute stress immediately before slaughter, its muscles produce lactic acid as part of the anaerobic glycolysis process, just like in human muscles during intense exercise, which is also due to the influence of cortisol. In the pre-slaughter stressed animal, this lactic acid production leads to a rapid drop in pH, which is generally a normal and necessary process for meat quality. The problem in PSE meat arises when the carcass remains warm while the pH drops.
Point of Clarrification
In a living animal, homeostasis maintains a stable internal environment through various mechanisms. For muscle pH, this involves buffering systems and blood circulation, which help to remove excess lactic acid and maintain pH levels within a narrow range. When muscles work hard and produce lactic acid, increased blood flow and the presence of buffering agents (such as bicarbonate) help neutralize the acid and prevent significant drops in pH.
Upon death, the circulation stops, and the muscle cells continue to produce lactic acid anaerobically until their glycogen stores are depleted. This causes the muscle pH to drop from around 7.0 to between 5.5 and 5.8. This acidification is part of the rigour mortis process and contributes to meat tenderization over time by activating enzymes that break down muscle proteins.
When high temperatures accompany this rapid pH decline, as can occur in situations like poor handling or stress before slaughter, proteins can denature more easily in a way that would not happen if the only factor was low acidity. By itself, it would not cause PSE.
During exercise, the body can manage the increased production of lactic acid through several mechanisms which we are familiar with when we exercise.
- Increased Blood Flow: Blood flow increases to active muscles, helping to remove lactic acid and supply oxygen and nutrients.
- Buffer Systems: The body has buffer systems (like bicarbonate) that help neutralize excess acid.
- Temperature Regulation: The body can regulate temperature through sweating and increased respiration, preventing the internal environment from reaching levels that would denature proteins.
In contrast, after death, these regulatory systems are no longer functional. The inability to remove lactic acid and regulate temperature post-mortem results in an uncontrolled drop in pH, and if the carcass is not cooled properly, elevated temperatures can accelerate protein denaturation.
As I noted above, the reduced pH alone (to around 5.5 to 5.8) does not necessarily cause protein denaturation to the extent seen in PSE meat. Under normal circumstances, proteins can remain stable at this pH level. However, when this low pH is combined with high temperatures, the risk of protein denaturation increases significantly. This combination disrupts the protein structures and leads to the formation of PSE meat.
Proteins do have some intrinsic buffering capacity, but this alone is not sufficient to counteract the effects of post-mortem pH decline and elevated temperatures. The buffering systems in a living animal (e.g., bicarbonate) are much more effective in managing pH levels during life. After death, without the support of these systems and temperature regulation, the proteins are more susceptible to denaturation when exposed to the combined effects of low pH and high temperature.
The body’s ability to maintain homeostasis during life prevents the denaturation of proteins despite temporary increases in temperature and lactic acid during activities like exercise. However, after death, the loss of these regulatory mechanisms means that high temperatures and rapid pH decline can lead to undesirable changes in meat quality, such as PSE. Proper post-mortem handling, including rapid cooling of the carcass, is essential to prevent these issues. A drop in pH alone does not lead to PSE.
D. Simple Solution: Rapid Cooling and Stress Management
Rapid cooling post-slaughter reduces muscle temperature, which slows down enzymatic activity and autolysis, preserving meat quality by preventing excessive protein denaturation and water loss. Additionally, managing both long-term and short-term stress in animals reduces cortisol levels, which helps maintain glycogen reserves and prevents the rapid pH changes that lead to PSE and DFD meat. By combining rapid cooling with effective stress management practices, overall meat quality is significantly improved, resulting in better texture, water-holding capacity, and overall consumer satisfaction.
Conclusion
This article examines the effects of cortisol on stress management and meat quality. Cortisol mobilizes energy by promoting gluconeogenesis, protein catabolism, and lipolysis, while also having anti-inflammatory effects and maintaining homeostasis. Chronic stress and high cortisol levels result in muscle wasting, reduced weight gain, and poor meat quality. The mechanisms behind DFD (Dark, Firm, Dry) and PSE (Pale, Soft, Exudative) meat are explored, highlighting the importance of rapid cooling and effective stress management in preserving meat quality.
“As part of the Lea Sprügel and Jasmin Peschke study, eleven cattle from the farm were slaughtered in the nearby slaughterhouse (ten minutes drive). In comparison, ten cattle were killed on the farm. All three parameters – cortisol, glucose, and lactate – were significantly higher in the animals slaughtered in the slaughterhouse than in those killed on the farm: the cortisol level was on average twenty times higher. This unexpectedly large difference has not been measured in any study to date. The glucose and lactate levels were a quarter and twice as high respectively.” I am stunned by how sensitive the animals are to stress. It changes the way I view these matters considerably!
Willi Wurm, a Master of Meat Science and Processing at SORGO-Anlagenbau, Austria sent me the following anecdote which captures everything we discussed in this article, including Christa’s recollections of life on their farm. He wrote, “My grandfather was a master butcher and collected the animals directly from the farmers in the mountain areas, who were sometimes 20 and more kilometres away from my grandfather’s slaughtery. He already at that time knew to rest the animals for 1 or 2 days in our old stable for feeding and water them well and to give enough time to raise the glycogen in the muscles. At that time he walked the 20 or more kilometres beside the animals. ”
A final but important conclusion is that PSE (Pale, Soft, Exudative) meat and DFD (Dark, Firm, Dry) meat impact their nutritional value. It does so in different ways. In PSE meat, protein denaturation can reduce the digestibility and bioavailability of proteins, meaning the body may not absorb and utilize these proteins efficiently. Additionally, the loss of water in PSE meat can carry away water-soluble nutrients such as B vitamins and minerals, potentially lowering the meat’s nutritional value. Increased fat oxidation in PSE meat can also compromise fat quality, making it less beneficial and potentially harmful.
On the other hand, DFD meat retains its vitamins and minerals more effectively than PSE meat. However, the higher pH environment in DFD meat is more conducive to microbial growth, which can pose health risks if not handled and cooked properly. While DFD meat might maintain better protein and nutrient stability compared to PSE meat, the increased risk of spoilage and microbial contamination necessitates careful processing and handling to ensure safety.
Part of the Capillary Series by Eben van Tonder, EarthwormExpress.
Relationship Between Cortisol, Stress, and Meat Quality in Animals Pre-Slaughter
Capillaries as a Quality Marker
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