3 Chemical Changes Associated with Slaughter

Prior to harvesting (slaughter), animals are vulnerable to stresses that can and do alter their pH (potential hydrogen). pH is measured on a scale of 0 to 14 (Figure 6).

  • pH above 7 = alkaline
  • pH of 7 = neutral
  • pH below 7 = acid
Figure 6 By Hans Kirkendoll (Own work) [CC0], via Wikimedia Commons http://commons.wikimedia.org/wiki/File:Power_of_Hydrogen_%28pH%29_chart.svg
Figure 6. Potential Hydrogen, pH Chart by Hans Kirkendoll (Own work) [CC0], via Wikimedia Commons


These changes are most likely to occur with cattle (beef cattle in particular) and pigs, and can cause discolouration that is visible in the finished product. Therefore, it is important to understand how these changes occur and how they may affect product presentation, colour, and flavour.

The amount of stress animals suffer depends on how they are handled before harvesting. For example, when animals are selected for harvesting, they may be separated from the herd, rested overnight, then loaded on a truck to be driven to the harvesting plant. Sometimes the animals have to be transported vast distances, especially in Canada. Once unloaded at the plant, they are rested, hopefully with the same group of animals they have been transported with. All these sudden changes are stressful to the animals, and each step of the process must be carefully handled. Excessive heat, dehydration, cramped conditions, and strange surroundings have a negative effect on most animals, with some finding the process more arduous than others.

At the time of slaughter, animals are moved from their holding pens into a specially designed S-shaped approach chute that helps to keep the animals calm. This then leads the animal into a tight holding box where it is stunned, bled, then winched up for skinning, eviscerating, splitting, and washing followed by rapid cooling in a special holding cooler.

Example: http://www.grandinlivestockhandlingsystems.com/about.html

The key to minimizing stress is to handle the animals as quickly and gently as possible to ensure that their pH remains stable prior to death—around 6.5 (neutral) and dropping to about 5.6 to 5.2 post mortem (after death) during the first 24 hours of cooling, when the carcass temperature is forced down to 4°C (40°F).

All the factors outlined above have some effect on the animal’s pH. As the animal ceases to breathe, and as blood leaves the animal with the heart still pumping, about 50% of the blood is removed. It takes about four to six minutes before the heart ceases to beat. As the pH begins to drop below 6.5, lactic acid is produced, increasing the acidity. Lactic acid serves as a preservative, lessening deterioration of the carcass until the temperature of the muscles reaches 4°C (40°F).

At this point, rigor mortis (the stiffening of the muscles in death) begins to set in. This usually takes between 12 and 24 hours depending on the size of the carcass and amount of exterior fat covering.

There are three stages to rigor mortis:

  1. Pre-rigor: The muscle fibres begin to shorten due to the depletion of adenosine triphosphate (ATP), causing the muscles to become less extendable while hanging under load. With less oxygen available, the myosin and actin proteins form actomyosin after death occurs. The actomyosin produces a cross bridge between the actin and myosin filaments. In the living animal, these cross bridges are broken during the relaxation phase of a normal contraction cycle (e.g., movement such as walking). However, after death (post mortem), cross bridges are formed permanently as the muscles shorten.
  2. Rigor maximum: The muscle fibres reach maximum shortening, resulting in stiff muscles. The cross bridges are now firmly in place.
  3. Rigor resolution: The now stiff muscle fibres begin to extend again and stretch out to almost their original length. As this extension occurs, the cross bridges create a tearing effect. This phase results in tenderization during dry aging (hanging) or wet aging (storing in vacuum packaging) of carcass meat and is most noticeable in prime meat cuts from the short loin, sirloin, and 7-bone rib (prime rib) of beef. Another chemical process develops during this phase in which the still-living cells begin to produce lactic acid. Lactic acid is normally removed by the circulatory system of living animals; however, in rigor resolution it remains in the muscles, causing the pH to drop until the core temperature of the carcass reaches 4°C (40°F).

Rigor mortis takes different times to activate depending on the size of the animal and, in some cases, the species (Table 1).

Species Time for Rigor Mortis to Activate
Beef 6 to 12 hours
Lamb 6 to 12 hours
Pork 15 minutes to 3 hours
Turkey Less than 1 hour
Chicken Less than half an hour
Fish Less than 1 hour

Table 1- Length of time required for rigor mortis to activate

To further understand the three stages of rigor mortis in relation to meat tenderness, consider the following example: A beef animal has endured a stressful separation from its home farm and a prolonged road trip to the harvesting plant. During the trip, the animal became very dehydrated, thus arriving at the plant in a weakened and agitated state, and could not be settled down prior to slaughter.

In this example, the animal’s pH could be above 7 (neutral) into the alkaline part of the pH scale (8-14) before harvest. This could cause the carcass (post mortem) to never reach rigor resolution, remaining in the rigor maximum stage, where the muscle fibres are at maximum stiffness. Therefore, the carcass would remain tough even after the normal dry or wet aging process.


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Meat Cutting and Processing for Food Service Copyright © 2015 by go2HR is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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