BRAHMAN NEWS JUNE 1999, Volume 22, No 4
Electrical stimulation (ES) of beef carcases has been shown to increase meat tenderness. But studies funded by the former Meat Research Corporation (MRC) found that the technique was not a “fix” in itself. It was a management tool that must be used properly and in conjunction with other slaughter floor and chilling procedures to deliver consistent tenderness. The MRC trials showed that ES could play a positive role in improving tenderness, provided it was fully integrated with slaughter and chilling procedures. Carcases must be chilled quickly following slaughter to minimise microbiological growth and to reduce weight loss. However, if the carcase is cooled too much before rigor mortis is achieved, muscle groups near the surface can become tough through exposure to the cold. This process is called cold shortening. The lower the temperature at which cold shortening is achieved, the more severe is the effect and consequent toughness. ES is the application of a specifically-designed electric current to a carcase, after slaughter, but before chilling. ES accelerate the decline in pH and the onset of rigor mortis. It causes muscle contractions which convert muscle glycogen (or sugars) to lactic acid. The acid build-up reduces the pH and the muscles enter rigor mortis. In this state the muscles have no further energy to contract and will not “cold shorten” when chilled to 12 degrees C. Over-stimulation can have a negative effect. If the pH decline is too rapid, muscles enter rigor mortis (pH 6) at above 30 degrees C and “heat-toughening” can occur. ES must be configured for individual slaughter floors. Processing plants use a variety of post-stunning immobilisers and hid-pulling equipment which used electric current to stiffen the carcase.
All this equipment has the effect of stimulating the carcase and impacting on pH decline. The new Agriculture and Resource Management Council of Australia and New Zealand chilling regulations, local and United States Mega Regs require quicker rates of chilling which will exacerbate the problem of cold shortening. Effective ES requires slaughter floor managers to ensure pH decline and chilling occur at appropriate rates. The correct integration of all these processes is essential to produce consistent results.
There are two ES systems: high voltage; and extra low voltage. Both result in effective stimulation and both have advantages and disadvantages. The appropriate system for a meatworks depends on the throughput and existing slaughter floor constraints. Extra low voltage ES should be applied as soon as possible after slaughter. High voltage systems are more time tolerant. Extra low voltage stimulation has lower initial costs, is safer in the work place and can result in extra blood yield. On the negative side it is more labour intensive because electrodes must be positioned manually. It must be applied soon after (less than 10 minutes) slaughter. The current is applied to the whole carcase usually before hide removal to comply to with the time constraint. With extra-low voltage ES, an electrode is inserted in the nostril with the second electrode being inserted in the rectum or a rubbing bar which makes firm contact with the leg. A minimum of 200 mA current, at 45 volts is applied for more than 40 seconds is required. High voltage stimulation requires greater initial outlay, primarily due to safety reasons but can be incorporated in to a high speed chain. It can be applied successfully up to 60 minutes after slaughter giving greater flexibility with siting of the ES module. Carcases can be stimulated after hide removal and splitting. Higher voltage systems use the hanging hook and roller as one electrode and a bar that makes contact with a point as near to the neck as possible. Carcases are stimulated at 800 volts for not less than 60 seconds with pulse of specified shape and frequency. Current is in the order of several amps depending if sides or if whole carcases are being stimulated. High voltage stimulation must be applied in a special safety booth incorporated on the chain to ensure worker safety.
After its introduction in the early 1970s, the use of electrical stimulation (ES) has declined in Australian meat works. The former Meat Research Corporation investigations showed that most poor or inconsistent meat quality outcomes resulted from poor ES techniques or failure to integrate the ES with hide removal, chilling and aging. Over time, misconceptions have grown about the effect of ES on carcases and meat quality. Following are the most common ones.
You cannot use ES if you have a downward hide puller. ES has been identified as the causes an increases in the number of “broken backs” when used in conjunction with downward hide pullers. Investigations show that there is no increase in the incidence of “broken backs” with ES. A number of works successfully use both processes and report no higher incidences of “broken backs”. Poorly adjusted hide pullers will cause high incidence of “broken backs,” and processors should ensure electrical and mechanical systems of the hide puller are in good working order.
High voltage stimulation can be applied up to one hour after slaughter. The rail provides one electrode while a rubbing bar provides the second contact. Carcases can be electrically stimulated in groups either whole or after splitting (as shown). High voltage stimulation uses 800 volts and several amps of current per carcase. The current is applied for about one minute. shorter shelf life and discolours more rapidly than unstimulated beef. There is no evidence to show ES beef has higher microbial counts than unstimulated beef and therefore a shorter shelf life. Trials in the mid 1980s, using light measuring instruments, showed ES beef experienced some colour change after three days on retail display. Further studies showed consumers could not detect this colour change. ES beef has colour performance and stability equal to un-stimulated beef. Display temperature is important for colour stability. Display temperatures today are 1-2 degrees C lower than in the mid 1980s, contributing to better shelf life and colour stability.
ES is not needed for “heavy” Japanese carcases. The rate of cooling is an important determinant in meat tenderness. While carcase weight is an important factor in determining cooling rate, fat cover also plays an important part because of its insulating effect. Heavy carcases that have had external fat trimmed, particularly over the loin, can experience cold shortening over this region due to the loss of the insulating effect.
ES should not be used on heavy carcases suitable for the Japanese trade. It is thought by some that ES increases weep in vacuum packs and colour variation and reduces shelf life. In practice, a number of companies electrically stimulate heavy carcases for this trade. Problems with weep and colour stability can invariably be traced to chilling problems, particularly to “hot spots” in chillers. The warmest part of a chiller must chill carcases to 20 degrees C (deep but) within 20 hours. ES of carcases will have no undesirable effect on these characteristics which re determined by chilling procedures.
Individual carcases which have been trimmed of fat, particularly over the loin, are at risk particularly when placed in chillers set for the rapid cooling of heavy carcases with greater fat covers. In this situation, beef is likely to benefit from ES which will reduce the risk of cold shortening of lighter carcases, and carcases which have been trimmed of fat.
ES increases discolouration problems like “heat ring” and two toning. Trials from Texas A & M show that discolouration is principally due to chilling rates and is alleviated or significantly reduced by ES. Es will significantly reduce variation in meat colour in the loin region of large carcases. This is particularly evident in those carcases where fat trimming has occurred.
Beef that is going to be aged does not need to be electrically stimulated. ES and aging are not the same process. ES and aging tend to have an additive effect and there is some evidence that ES will speed-up the aging process. To optimise tenderness both processes are desirable.
ES has no effect on tenderness of high pH (dark cutting) beef. Dark cutting meat usually has a high pH. High pH beef is normally tender without ES. Thus ES may not increase tenderness. The variable response of muscles, pH >5.8, is of little consequence to the over all effectiveness of ES. While ES has been shown to produce lighter coloured, brighter meat it will not reduce the incidence of dark cutting beef and does not reduce the darkness of dark cutting.