Role Of Electrical Stimulation In Hot Boning

Hot boning, widely practiced in the past and common in many underdeveloped countries, is now experiencing revival in industrialized countries because of its economic advantages (savings in energy, space, labor, and time). The major constraints to the use of hot boning have been the contraction of prerigor excised muscles, the slippage of one muscle relative to another, and the extra shortening of excised muscles subjected to rapid chilling and in the interest of microbiological control. It has been suggested (84) that hot-boned beef should be chilled below 8°C within 4 h if boned at 37°C. However, it has been noted (85) that a reduction to below 21°C within 9 h would be acceptable. When muscle shortens by approximately 20%, it can still age and be tender (86), so hot boning is feasible if temperatures are controlled (see the article Meat science).

The contraction and consequent toughening of most prerigor excised muscles can be minimized by adequate holding at above 10°C (conditioning). In an early study (87), conditioned muscles were boned at 2 h postmortem, and then the vacuum-packaged primals were aged at 15°C. This gave a product that was as tender as, or more tender, than control samples from normally chilled and boned sides. However, boning at 2 h postmortem does not readily fit with a continuous operation; therefore, commercial processors bone earlier. These processors must use controlled chilling and tight packaging to minimize contraction and toughening; thus, there is interest in being able to reduce processing time and restraints without influencing product quality.

One study considered the physics of rapid chilling and cold shortening and concluded that cold shortening in rapidly chilled beef cannot be prevented without electrical stimulation (88). The situation was even more critical with hot boning because of the more rapid chilling of isolated product.

Electrical stimulation has been used to minimize or eliminate potential tenderness problems in hot-boned beef. The tenderness of product boned from stimulated carcasses at 1 h postmortem has been compared with product boned after 24 h of chilling (89). It was found that the stimulated hot-boned meat, even though chilled rapidly to 7°C, was as tender as that from sides conventionally chilled and boned at 24 h. If the electrically stimulated sides were held for 5 h so that they had entered rigor before boning, the product was also tender (90). Vacuum-packaged primals were prepared for studies of hot boning with and without electrical stimulation (91). It was found that cold-shortening toughness was avoided by either a delayed and slow chilling (21 h to 7°C) or by electrical stimulation of the carcass before boning followed by rapid chilling (17 h to 7°C).

Some attempts have been made to use high-temperature conditioning (eg, holding for 3 h at 37°C) as a means of ensuring that hot-boned beef does not toughen. It was shown that this high-temperature conditioning of hot-boned cuts gave the same results as did prior electrical stimulation of the carcass (92). The effects differed between the three muscles tested, with improvement noted in the m. longissimus, no change in the m. semimembranosus, and a decrease in tenderness of the m. semitendi-nosus compared with nonstimulated controls. It was theorized that the high-temperature conditioning caused increased proteolysis in the m. longissimus but connective tissue shrinkage in the m. semitendinosus. This theory does not seem to be completely substantiated by the results.

In general, hot boning has resulted in an increase in the toughness of beef m. psoas major muscles, which are normally stretched when carcasses are supported by the Achilles tendon; but if the product was aged for a day at 20°C, the differences between hot and cold boning were removed (93). The sarcomere length of cold-boned product was 3.3 //m (stretched), whereas that of the hot-boned product was 1.95 //m, close to that of rest length of most muscles in the body. Other cuts showed a lesser difference. High-temperature conditioning is not necessary to ensure tenderness if an effective electrical stimulation system was employed (92,93). It should be noted that the effect of a small degree of passive shortening can be overcome by additional aging. The observation that rigor at 15°C produces the most tender meat whether hot boned or restrained (34), suggests that further temperature controls will result in significant tenderness improvements.

Hot boning of pork for whole-hog sausage gives lower cooking losses and increased juiciness scores than does postrigor processed pork (94). For primal cuts, hot boning is reputed to give poorer initial color and appearance, but the differences from cold-boned product disappear over the following 120 h (95). Some studies have considered the use of electrical stimulation to aid hot processing of pork, but the results are conflicting due to the PSE condition, which is increased in susceptible pigs (46,96). Electrically stimulated pork hot boned at 1 h postmortem and then rapidly chilled produces cuts that give less purge and are more juicy than slowly chilled product (15,97). If the chilling is slower in the case of hot-boned pork, stimulation resulted in no tenderness advantage and increased drip and was not be desirable (98).

The role of electrical stimulation in hot-boning applications is clearly to hasten the onset of rigor mortis, so that cold shortening is minimized in cuts that will cool more rapidly than the corresponding cuts on the carcass. Rapid cooling, with its greater control of microbial proliferation can then be used without irrevocably toughening the product. The sooner and more rapid the chilling, the more efficient the electrical stimulation must be to be effective. Very early boning and rapid chilling, even after electrical stimulation, will result in cold shortening and toughness. If a little stimulation is good, a lot is not necessarily better. Future work will refine the temperature conditions to pro duce the most tender meat enabled by precisely controlled stimulation for each carcass. Future work should be aimed at refining the whole process to increase the uniformity of tenderness across muscles and animals.

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