Taking the guesswork out of electrical stimulation

New technology jointly developed by Realcold Milmech in conjunction with Applied Sorting Technologies and Meat and Livestock Australia (MLA) has taken the guesswork out of applying electrical stimulation to cattle and sheep carcasses. The system is already operating in a number of sheep and beef plants throughout Australia.

Electrical stimulation accelerates pH decline, the onset of rigor mortis and the natural ageing process. This allows meat to reach an acceptable eating quality in a significantly shorter period of time and can alleviate problems caused by the faster chilling of carcasses. The challenge for processors is to match the level of electrical stimulation (ES) to the rate of chilling and the time the meat is scheduled to reach the consumer.

Muscles derive their energy from glycogen, which during exercise is broken down as a fuel producing lactic acid. In living muscles the pH remains just above 7.0 but can vary between 7 and 6.4 during exercise.

After death, the breakdown in glycogen to lactic acid continues, but with blood flow having ceased the lactic acid cannot be removed. Thus the lactic acid gradually accumulates and the muscles become more acidic with a lower pH.

Without any intervention either:

• An ultimate pH of 5.5 is reached - at which point the cell becomes too acidic for the cellular enzymes to continue functioning and the residual glycogen remains in the muscles; or
• All available glycogen in the cell is used up before the pH has fallen to 5.5. Without the fuel the cell ceases to function.

At these situations, the muscles are still and the animal is in rigor.

The MSA tenderness criteria determine that the glycogen decline/lactic acid formation needs to lower the pH to 6.0 with the muscle temperature between 35°C and 12°C, followed by standard ageing. This means that the time and temperature needs be controlled to ensure that a pH of 6.0 is achieved within this 'window of opportunity'.

For any given chilling rate, too much ES results in too rapid a pH decline. If pH 6 is achieved at a temperature above 35°C 'heat shortening' occurs and the natural ageing enzymes are destroyed. With inadequate ES where the carcass does not reach pH 6 before the temperature falls below 12°C, 'cold shortening' occurs and ageing of the meat is delayed.

The standard intervention system has been the application of electrical stimulation, post slaughter, as an electrical current applied to the carcass. This application of electric current mimics the natural 'contract/relax' signals in living muscles and this accelerates the breakdown of glycogen and serves to hasten the onset of rigor.

This intervention is important for:

• The chilled meat trade to provide a consistent and correct level of tenderness for local trade. For export trade, the natural ageing rate is very slow at 0°. Hence again during transport it cannot be relied on to produce consistent and correct tenderness, particularly if the initial processing time/temperature regime was not correct.
• For the frozen meat trade, the natural ageing process is halted when the meat is frozen. Further, if the meat is rapidly chilled and cold shortened this can interfere with the ageing enzymes. Thus the meat will be tough on thawing unless it has been correctly stimulated prior to freezing.

The effective level of ES is a function of the carcass type and the total electrical load applied to the carcass during the slaughter process. Electrical inputs can come from immobilisers, bleeders, back stiffeners and stimulators with the effect of each depending on the voltage, its duration and the waveform.

The new system accounts for the total electric load (including immobilisation and back stiffeners etc) and can be adjusted on the basis of the ideal electrical inputs for particular carcass types. Its main feature is its use of test pulses to determine the resistance of a carcass and the use of this information to apply the same, precise electrical dose control to all carcasses. The optimal pH decline for any given chilling rate can then be obtained to maximise the benefits of rapid ageing.