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
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:
- 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 ac