The hormonal status of animals, including deer, during the biochemical events that transform muscles into edible meat has been clearly shown to affect the eating quality of meat. While several hormones are secreted during this period, there are two that have been most frequently linked to the major biochemical changes that can occur in muscles. These two hormones are adrenaline (aka epinephrine) and cortisol, both of which are known as “stress hormones”. Both hormones are secreted in live animals as a result of exposure to stress that occurs prior to death, and which continue to affect muscle biochemistry after death and during the conversion of muscle to meat. Both play a role in the adaptation of animals to stress but the biochemical changes that occur in response to stress are different and may, or may not, affect meat quality depending on the intensity of the stress and the length of time of exposure to stress.

Adrenaline is a catecholamine secreted by the inner (medulla) portion of the adrenal gland and is widely known as the “flight or fight” hormone because it is very quickly released in response to perceived danger, excitement or extreme stress. Adrenaline release results in increased blood flow to muscles, increased heart rate, increased respiration rate, heightened alertness, dilated eye pupils and decreased pain sensitivity, all of which are preparation for the “flight or fight” response. These physiological changes are well-known for their potential dramatic effects on meat quality, depending on the intensity of the response and the length of time over which it occurs.

There are two extreme meat quality effects that can occur in response to adrenaline secretion. For example, if a buck or doe is running hard from a deer drive, adrenaline will be stimulating muscle activity and rapid utilization of glucose for energy production. Increased blood flow will be actively removing the end products of the energy production, including lactic acid, from the muscles. If the deer is killed at close to the point of exhaustion, there will be little energy production capacity remaining in the muscle and after blood flow is stopped, very little lactic acid will remain. The result is an abnormally high muscle pH. The meat quality results will include greater toughness, less flavor, very dark color (in beef, known as dark-cutting beef) and meat that will spoil more quickly due to the higher pH.

On the other hand, if the deer is killed earlier in the drive but still after an extended run, when the blood flow stops but while there are still significant energy stores (glucose) remaining, the muscles will continue to actively, and rapidly, metabolize the glucose and produce lactic acid. This means that the lactic acid will be produced at a fast rate and will remain in the muscles to result in a rapid pH decline to a low final muscle pH while the muscle temperature is still high. The rate of pH decline is important and occurs because the muscle has been stimulated to produce energy quickly but the combination of a low pH and high muscle temperature results in protein denaturation and loss of meat quality. Meat quality effects in this case will include a soft texture (different from tenderness) that will not retain moisture well and a lighter color. While the color may be somewhat lighter in venison or beef, it is especially problematic in pork where the color can be easily seen as extremely pale. The commercial meat industry goes to great lengths to minimize the stress on domestic livestock that occurs during transportation, handling and slaughter of meat animals. Because some degree of stress is unavoidable, the commercial meat industry also incorporates rapid chilling of carcasses to minimize the negative effects of reduced muscle pH combined with high muscle temperature.

Finally, if the deer escapes the drive and has some time to rest and recuperate, then suddenly killed by a still-hunter, a lack of adrenaline release will not accelerate lactic acid production to the same extent. Muscle pH will still decline, but at a slower rate with little effect on meat quality, providing that appropriate dressing procedures and carcass chilling rates are implemented.

Cortisol, a glucocorticoid, is also secreted by the adrenal gland but by the outer (cortex) portion of the gland. Cortisol is responsible for many basic, everyday biological functions including regulation of blood pressure, metabolism of lipids and proteins for energy production and blood glucose concentrations and circadian rhythms (sleep-wake cycles). In human beings, blood cortisol concentration affects the circadian rhythm by increasing the cortisol concentration prior to waking up in the morning and by lowering the concentration at the end of the day to encourage sleep. Cortisone, a metabolic byproduct of cortisol, is also a glucocorticoid and is closely related to cortisol but is not active as a hormone. Cortisone has anti-inflammatory properties and is often used for treatment of arthritis, allergies and skin conditions.

When cortisol is released in response to stress, the release (peaking at about 15 minutes after exposure to stress) is slower than for adrenaline, and is designed to maintain the energy needed for stress response for a longer period of time. The physiological changes include mobilization of glucose from the liver, converting proteins to amino acids and mobilization of energy from fat stores. These changes provide for a longer term, more sustained response to stress than that of adrenaline. Blood cortisol concentrations typically return to a baseline level about 60-90 minutes following a single stressful event but will be maintained at an elevated level if stress continues. Thus, cortisol is important for biological adjustment to chronic stress whereas adrenaline is focused on short-term acute stress response.

The effects of adrenaline on meat quality have been researched extensively, particularly in domestic livestock where the potential changes in meat quality are well understood. However, the case is not so clear for cortisol. Measurement of blood cortisol concentration as part of animal research is often used as an indicator of stress response but the effects on meat quality have not been studied as thoroughly. Further, the research that is available has not been conclusive. The few studies available involving cortisol, mostly in swine, have included reports of positive effects on meat quality, negative effects on meat quality and no effects on meat quality. There has been ample research on the effects of long-term elevated cortisol levels on human health where high blood glucose, high blood pressure, and weight gain as well as muscle and bone weakness (Cushing’s Syndrome) are some of the negative results. Because cortisol is known to increase blood glucose and metabolize proteins and lipids to produce energy, one can speculate that meat quality is likely to be affected depending on the muscle glucose concentration at the point of death of an animal. If the animal is killed while muscle glucose is high such as in the case of chronic stress with elevated cortisol, loss of meat quality would be likely, due to rapid muscle pH decline, similar to the effects of adrenaline following acute stress.

While the research in the U. S. on effects of chronic stress/cortisol on deer has been very limited, there have been some studies in Europe and other countries where the commercial production of venison is more prevalent. In a study in New Zealand (Pollard, et al., 2002), the effect of pre-slaughter handling and cortisol levels on red deer were evaluated for a group that were head-shot while pasturing (representing little or no stress) relative to a group that were commercially slaughtered after 18 hours of handling and holding (representing significant stress). Blood cortisol was measured and meat quality assessed by pH, color, shear tenderness and eating quality by a sensory panel. Results showed that blood cortisol concentration was about 3-fold greater in the stressed group while muscle lactate (lactic acid) was about twice as great, resulting in a somewhat lower muscle pH. Tenderness, both by mechanical shear and sensory panel score was slightly better for venison from the unstressed group but overall eating quality was only slightly improved. The authors concluded that reduced stress as shown by significantly less cortisol secretion produced “marginal” improvement in eating quality of venison. However, the potential effects of chronic stress and long-term elevation of cortisol concentration were not part of this study.

A comparison of cortisol levels produced in wild game in Sweden as a result of different forms of stress included shooting injuries, collisions with vehicles, being caught in fences and hunting methods (still-hunting, deer drives or hunting with dogs) (Gintsch et al., 2018) included red deer and fallow deer. Blood cortisol levels were increased by about 3-fold by stress or trauma, with vehicle collisions resulting in more cortisol elevation than shooting injuries or entanglement in fences. Cortisol levels also increased with length of time that the stress was experienced, as might be expected. Cortisol secretion was also increased by the degree of “post-stress disturbance”, described as animals pursued and disturbed by hunters or dogs following an injury. Comparison of hunting methods showed that still-hunting resulted in the least amount of elevated cortisol while deer drives and hunting with dogs both resulted in about a 3-fold increase. The anatomical site of wounds or injuries was also a factor in cortisol levels resulting from hunting, with injuries to the central nervous system, skeleton or muscles resulting in lower cortisol secretion than injuries to the chest or abdominal organs. The authors did not offer an explanation for this except to suggest that pain might be a factor.

The bottom line relative to cortisol effects on venison quality is not very clear. While cortisol is well-recognized as a stress-response hormone that is elevated several-fold in response to stress, it is also slower to respond than adrenaline. Further, cortisol’s primary role is to maintain homeostasis in response to chronic stress over an extended period of time, which has not received very much research attention for effects on venison quality.

References

Gintsch, R., et al., 2018. Cortisol response of wild ungulates to trauma situations: hunting is not necessarily the worst stressor. European Journal of Wildlife Research 64:11. doi.org/10.1007/s10344-018-1171-4.

Pollard, J. C., et al., 2002. A comparison of biochemical and meat quality variables in red deer (Cervus elaphus) following either slaughter at pasture or killing in a deer slaughter plant. Meat Science 60 (1): 85-94. doi.org/10.1016/s0309-1740(01)00110-3.