Description

Electrical stimulation is a process that involves passing an electric current through the body or carcass of freshly slaughtered animals. This current causes the muscles to contract, increasing the utilization of muscle energy reserves, and leads to acceleration of glycolysis and subsequent rigor development. Electrical stimulation as a means of attempting to increase tenderness has been used experimentally on pigs, deer, goats, sheep, cattle, and various poultry species and, until recently, used commercially on all of these except pigs. This has changed as it has been shown that if stimulation is applied at 20 min postslaugh-ter and the chilling rate is sufficiently fast (eg, deep leg to 10°C in 5-10 h), then the meat in those animals susceptible to pale, soft, and exudative (PSE) does not show it, and the remaining meat avoids cold shortening (15).

There are numerous physical methods by which electrical stimulation could be applied many different possible electrical specifications, and in reality many different perceptions of the response. Regardless of species, stimulation can be applied immediately after slaughter or at any point in time thereafter until the muscles become unresponsive. The time until muscles fail to respond is related to the natural rate of glycolysis and the voltage being applied, the duration of stimulation, and the type of response expected.

Most commercially used electrical stimulation systems employ the conveying rail as ground, and a live electrode contacts some other point of the body, carcass, or side. In the most basic systems, the live electrode contact is a clip manually applied to the head end of the animal body that is suspended by one or both hind legs, resulting in a current flow to the grounded rail support. More sophistication and protection is required as voltages increase and as application of the electrode becomes automated.

Procedures range from stimulating stunned but not bled animals, whole bodies, skinned bodies, carcasses, or sides. The electrical characteristics of the waveforms used are often poorly described, defying anyone to reproduce the results. Voltages used vary from 32 to 3,600 V and may be described in various ways. The value specified might be that of the peak or the rms (root mean square) voltage or in some cases the average over the total time. The rms voltage is the effective value or heating capacity of a waveform. For a sine wave, the rms value is the peak voltage divided by -J 2 (ie, 1130 V peak 50 Hz is 800 V rms), but for derived waveforms it may be quite different. For the MIRINZ waveform, which uses every seventh half-sine wave of a 50 Hz sine wave, the rms voltage is the peak voltage divided by V14.

Figure 1 sets out to illustrate the meaning of the different terms used to describe voltages and waveforms. Defining a waveform with a frequency (Hz) is likely to lead to confusion unless the waveform is defined in terms of shape, duration, and pulse spacing. Figure 2 shows some different waveforms. The applied waveforms may be unipolar or bipolar and applied as discrete pulses or as pulse trains. Again, their description has often defied interpretation. Some of the waveforms and pulse shapes used have been well researched and described, whereas others seem to have been determined merely by extending the use of available equipment, for example, electric stunning tongs.

Safety has been of utmost importance during experimentation and implementation of electrical stimulation in New Zealand, Australia, the UK, and France, to the point that in some instances safety concerns have effectively prevented commercial adoption of the process. Although not considered of overriding importance in some countries, safety of personnel can always be assured if the normal electrical safeguards are applied.

In New Zealand, practical on-line sheep operations stimulating up to 56 carcasses/min are a common operation. Less than 30 min after slaughter, dressed carcasses are suspended by metal skids and gambrels from a grounded rail and moved through a stimulation tunnel to make contact, at shoulder level, with an electrode supplied with high voltage pulses (16,17).

Similar systems can be applied for stimulation of beef bodies and sides of carcasses, and a novel system has been developed whereby sides contact a moving chain electrode curtain that maintains contact as the side curls (18) (Fig. 3). A range of other systems has also been developed for beef stimulation (19) covering both batch and continuous operations. The batch systems may involve manually inserted electrodes or electrode bars that move out to make contact with the body or carcass. In these systems the carcass or carcasses are enclosed within a shielded cabinet during stimulation. Continuous systems consist of stationary rubbing electrodes, or, where the stimulation is applied to carcasses prior to inspection, the electrode system consists of a moving series of electrodes that are sterilized

Time (msec)

Figure 1. Terminology used to describe pulses and waveforms illustrated by sinusoidal (a) and (c) and derived waveforms (b). It is important to give the peak height in terms of either current or voltage while a carcass is being stimulated. The pulse shape must be specified either by description of the shape (eg, sinusoidal or square) or by specification of rise and fall times. The pulse width (mark) and space between pulses are also needed. The mark-to-space ratio helps specify a single cycle, which is the period from start to finish of the repetitive unit. The polarity of pulses is also necessary. The number of cycles per second then completes the description. The waveforms (b) and (c) both have the same period (inverse of wavelength) and peak but have different shape characteristics. The mark-to-space ratio for (b) is 1:7, whereas for (c) is 1:0. The time scale is given as an example only. The same-shaped waveforms could be given with different time scales.

Time (msec)

Figure 1. Terminology used to describe pulses and waveforms illustrated by sinusoidal (a) and (c) and derived waveforms (b). It is important to give the peak height in terms of either current or voltage while a carcass is being stimulated. The pulse shape must be specified either by description of the shape (eg, sinusoidal or square) or by specification of rise and fall times. The pulse width (mark) and space between pulses are also needed. The mark-to-space ratio helps specify a single cycle, which is the period from start to finish of the repetitive unit. The polarity of pulses is also necessary. The number of cycles per second then completes the description. The waveforms (b) and (c) both have the same period (inverse of wavelength) and peak but have different shape characteristics. The mark-to-space ratio for (b) is 1:7, whereas for (c) is 1:0. The time scale is given as an example only. The same-shaped waveforms could be given with different time scales.

between carcasses. In most instances electrical stimulation is applied with the aim of ultimately improving tenderness, but in the United States considerable attention has been devoted to color changes that improve grading of carcasses (20).

High or low voltages can be used for stimulation. Low voltages (less than 80 V peak) are commonly used within 5 min of slaughter in small operations where electrodes are manually applied and removed. There is often an overlap in use of electrical currents to immobilize carcasses and the use of electrical currents to accelerate glycolysis—the name is different but the process is the same. Although, in general, plants stimulate with an electrode on the nose or stick wound and ground via the rail from which the carcass is suspended, the resistance of the hind leg can be very high, due to the high bone and tendon but low muscle content of the lower narrow portion of the leg. This high

50-Hz waveform

MIRINZ waveform

Food Fanatic

Food Fanatic

Get All The Support And Guidance You Need To Be A Success At A Food Business. This Book Is One Of The Most Valuable Resources In The World When It Comes To Turning Your Love For Cooking Into A Money Maker.

Get My Free Ebook


Post a comment