Traditional slaughter procedures for sheep and goats require more man-hours per given weight compared with beef, pork and poultry—many mechanical adaptations ensure efficiency. Head only electrical stunning without cardiac arrest occurs by passing a current through the head only ensuring the animal is insensible and is acceptable for Halal slaughter. Additional current pathways can cause cardiac arrest. Bleeding/exsanguination occurs by a throat cut severing the carotid arteries, jugular veins and esophagus and usually occurs after a stun. Carcasses are then suspended by either front or rear legs and the pelt removed manually or mechanically. Low voltage electrical stimulation may be applied after stunning or high voltage stimulation applied after weighing and grading of the carcass.

We describe and evaluate published mathematical models that address the conversion of muscle to meat. These models include mechanistic approaches that attempt to describe the underlying processes of muscle energy metabolism and/or the action of proteases during muscle degradation post-rigor for the purpose of predicting meat quality outcomes. We also discuss empirical (data-driven) approaches that develop models largely based on observations with little or no consideration of underlying mechanisms. We make the case for the development of mechanistic models that consider the interactions of energy metabolites driving calcium dynamics and hence protease activity. We present a schema that depicts these key pathways and interactions. As an illustration of the potential, we discuss results from simulations from a proprietary holistic model that shows how electrical stimulation immediately after slaughter changes the fall in pH. This accelerates rigor mortis onset, enhances the calpain/calpastatin system degrading the cytoskeletal proteins, and improves tenderness. We discuss limitations in current models and where future modeling could improve and extend the literature, including consideration of the heterogeneity of muscle and muscle fibers.

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Traditional harvest procedures for sheep and goats often use mechanical adaptations to ensure efficiency. Head-only electrical stunning occurs by passing a current through the head only to ensure the animal is insensible at the time of bleeding and is halal. Additional current pathways causing cardiac arrest are not suitable for halal slaughter. Exsanguination occurs through severing carotid arteries, jugular veins, and esophagus. Carcasses are suspended by either front or rear legs for manual or mechanical pelt removal. Low-voltage electrical stimulation may be applied after stunning or high-voltage stimulation applied after weighing and grading.

Optimization of sensory aspects of meat quality does not lie with a single palatability trait but embodies mixtures of numerous attributes. Tenderness alone is not enough and other aspects of palatability such as juiciness, texture, and flavor development are also necessary. Factors affecting the optimum sensory components extend from the animal production side and include genetics, animal age, stress reduction, nutrition, and process control to avoid toughening as well as procedures such as electrical stimulation to enhance tenderization and ensure that meat ages to optimum tenderness. Factors to be considered also depend on the end use of the meat and the way meat is cooked.

High voltage electrical stimulation (1130 V peak, 14.28 bidirectional half sinusoidal pulses/s) or low voltage stimulation (45 V peak, 36 alternating square wave pulses/s) was used on cattle: (1) low voltage stimulation applied for 10 or 40 s with fast and slow chilling or high voltage stimulation for 60 s with normal chilling, applied to 100% Bos taurus cattle, (2) low voltage stimulation (40 s) and high voltage stimulation (60 s) with normal chilling applied to mixed Bos indicus and B.taurus cattle, (3) high voltage stimulation (54 s) with normal chilling applied to B. taurus and B. indicus cattle of 0-100% B. indicus composition, and (4) high voltage stimulation (60 s) applied to 100% B. taurus and 100% B. indicus cattle. All stimulation parameters enhanced the tenderness of steaks from M. longissimus thoracis et lumborum (LTL) aged at 1°C up to 28 days compared with non stimulated LTL. Short low voltage stimulation of 10s was marginally more effective than no stimulation and longer durations of 40s were very effective and high voltage stimulation was most effective. The shear force values for non stimulated B. indicus LTL are much greater than for B. taurus, but following high voltage stimulation LTL of B. indicus were similar to B. taurus and all had lower shear force values than from non stimulated carcasses. Thus adequate electrical stimulation removes any toughness of LTL related to B. indicus genetic composition.

A mathematical model of anaerobic muscle energy-metabolism was developed to predict pH and the concentrations of nine muscle metabolites over time. Phosphorous-31 Nuclear Magnetic Resonance was used to measure time-course data for some phosphate metabolites and pH in anoxic M. semitendinosus taken from three slaughtered sheep. Muscles were held at 35 degrees C during the experiment. Measurement commenced 25 min post mortem and concluded before rigor mortis. The model was fitted to these data within experimental error, by simultaneously varying model parameter values and initial substrate concentrations. The model was used to simulate the period from death until metabolic activity ceased, in order to predict the different stages of metabolic response to anoxia. The model suggested that alkalinisation would occur in all three muscles in the first few minutes after the onset of anoxia, followed by a steady decline in pH. For two of the muscles this decline continued until rigor, with final pH values of 5.60 and 6.07. For the other muscle, pH reached a low of 5.60 near rigor but then increased to a final value of 5.73. A rise in pH after rigor has been observed but not previously explained in the literature. The modelling results suggest it was caused by the alkalising effect of adenosine monophosphate deamination being greater at low pH than the acidifying effect of inosine monophosphate dephosphorylation.

The effect of electrical stimulation of lamb carcasses (n=269) or its absence (n=257) on shear force of m. longissimus thoracis et lumborum (LT) was monitored during ageing in pasture-fed merino lambs (n=526). The lambs were slaughtered on four different days allowing durations of between one to 10 days of recovery from pre-slaughter handling (yarding, weighing and crutching) that affected ultimate pH (pH(u)). The right LT was removed 20-40min post-slaughter, tightly-wrapped in cling film (prevents the muscle cross-section increasing and thus minimising shortening) and rapidly cooled to 15°C to enter rigor mortis and age. At 0, 4, 24 and 72h post-slaughter, pH measurements and samples for shear force measurement were taken. Pre-slaughter handling had a significant negative effect on pH(u) and several days recovery were required for pH(u) to reach values associated with optimal meat quality as reflected by pH(u). Lambs with one and three days recovery (no significant difference between them) had a pH(u)>5.7 in 50% of the muscles and 19.4%>pH(u) 5.8. Whereas, in lambs with 8-10 days recovery (no significant difference between them), only 8% had a pH(u)>5.7 and 3.1%>pH(u) 5.8. Within each slaughter day electrically stimulated lambs were always more tender than non-stimulated lambs. For non-stimulated muscles at 72h, shear force values >40N occurred for 11.2% of the muscles: for electrically stimulated muscles at 72h, shear force values >40N occurred for 1.9% of the muscles. The rates of tenderisation were slower for intermediate pH(u) values resulting in higher shear force values at all ageing durations. With ageing at 72h for intermediate pH(u), non-stimulated muscles (n=38) 17.64% were >40N and for stimulated muscles (n=34), 7.9% were >40N.

Previous investigations indicated that a correlation exists between the preprocessed dual energy X-ray images of an airport security scanner and subjective tenderness in meat samples [PCT International Patent Application No. PCT/NZ01/00108, 2001. A method for the non-invasive measurement of properties of meat. Filed 11 June 2001]. We performed studies to confirm the patented claims and to pin-point the correlation factors based on unprocessed X-ray images. In two separate trials the peak shear force was determined on cooked samples as a measure of objective tenderness, and DEXA images scanned. The results of Trial 1 suggest that DEXA scans of whole steaks are suitable for tenderness estimation, as a coefficient of determination of R2 = 0.69 was calculated for a multiple non-linear regression. These results need to be considered in light of high within-steak coefficients of variation (mean = 16% and max = 27%). An additional, though weak correlation (R2 = 0.26), was found between a single DEXA parameter correlated to composition and mechanical tenderness. Apparently small sample size and scanning of cooked and frozen meat samples rather than raw steaks were at least partially responsible for the inconclusive results of the second trial, where no correlation could be found exceeding R2 = 0.12. More research is encouraged using state-of-the-art scanning techniques and optimized experimental design.

The tenderness (objective measure is shear force) of sheep meat ( longissimus thoracis et lumborum) during post-rigor ageing at 15°C can be predicted using near infrared (NIR) analysis ( r2 = 0.85). Measurements ( n = 260 from 65 lambs) were made at 0, 8, 24 and 72 h post-rigor using an interactance system operating over the spectral range from 400–1700 nm. There was no predictive power in the NIR models to segregate tenderness measurements made at any one particular post-rigor time. NIR measurements could segregate the post-rigor ages with good accuracy ( r2 = 0.91). The OH combination band at 840 nm is the dominant feature of the predictive models and suggests that the underlying chemistry is related to the post-rigor changes in the water status of the meat. Observations on another similar data set ( n = 48 from 12 lambs), involving NIR and water content measurements, confirmed there are significant water changes in meat during aging. We speculate that the NIR models may be sensitive to the accumulation of free water arising from the degradation of cytoskeletal proteins during ageing, releasing the water normally tightly bound up in the tertiary protein structures.

We investigated the relationship between stress responsiveness and meat quality in cattle. The cattle were 16-19-month-old Mixed (n = 37, 303 kg) or Non-mixed (n = 23, 279 kg) Friesian bulls and Friesian cull cows (1.5-7 years) that were classified Unfinished (n = 133, 195 kg) or Finished for 3 months (n = 34, 252 kg). A portion of in. longissimus lumborum (LL) muscle was obtained from consecutive animals for a single days slaughter of 227 cattle processed through a hot boning plant. Catecholamines were obtained from bladder urine. Ultimate pH, lactate, glycogen. glycolytic potential and representative sarcomere lengths were determined from 20-h muscle samples. Shear force measurements were from meat aged at 15 degreesC for 20 and 90 h.Urinary noradrenaline was similar between Mixed (14.86 ng/mumol creatinine) and Non-mixed bulls (14.07 ng/mumol creatinine) and Finished cows (15.24 ng/mumol creatinine) and elevated in the Unfinished cows (22.28 ng/mumol creatinine). Urinary adrenaline was higher in Mixed bulls (9.5 ng/mumol creatinine) than Non-mixed bulls (5.7 ng/mumol creatinine) and higher in both Unfinished cows (16.67 ng/mumol creatinine) and Finished cows (14.19 ng/mumol creatinine).For bulls that are growing well on-farm, with a only short period of Fasting prior to slaughter, pH(u) is a good predictor of meat tenderness. In this situation, stressors that lower muscle glycogen pre-slaughter can have a significant effect on meat shear force, and individual animals with elevated urinary adrenaline were the most susceptible. However, in the case of cows, adrenaline responses are not necessarily associated with glycogen depletion and pH(u) is less affected than for bulls. Furthermore, the ability of meat to tenderise fully, related to reduced proteolytic turnover, is reduced in nutritionally compromised animals such as the Unfinished cows. (C) 2003 Elsevier Ltd. All rights reserved.

Application of electrical stimulation in the sheep and beef processing industry has been erratic around the world and this may reflect an incomplete knowledge of how to optimise the technology. Although it is well established that stimulation increases the rate of post-mortem glycolysis, other biochemical and biophysical effects have been implicated with the use of this technology. This review seeks to examine the current theories about the effect of stimulation on post-mortem muscle. The classical view that stimulation prevents muscle from shortening excessively during rigor development has been expanded to include the possibility that it also results in physical disruption of muscle structure. The interaction of these effects with the acceleration of the rate of proteolysis through activation of the calpain protease system has not been comprehensively reviewed in the past. Thus there are two mechanisms which could explain the effect of stimulation on tenderisation, reduced 'cold-induced' shortening and alteration of protein structure. A secondary effect is the enhancement of the rate of proteolysis stimulated by release of Ca (2+) at a higher temperature. As a result of this review we highlight several areas that may prove fruitful for further research. The challenge for further development of electrical stimulation systems is optimisation of the activation of the enzyme systems in parallel with manipulation of chilling regimes so as to ensure rigor mortis is achieved at temperatures which minimise shortening. These optimal temperatures largely established at a fixed incubation temperature for detached muscle may be different when measured in intact carcasses. The potential of regional stimulation of sections of the carcass to achieve this outcome is worthy of study given the different fibre composition of muscles and temperature gradients. In addition, to ensure that appropriate amounts of energy are applied to individual carcasses, development of self-response stimulation units, which are able to determine carcass resistance and apply appropriate durations or strengths of stimulation is worthy of future research. This would lead to more effective electrical stimulation practices.

The effect on shear force of skeletal restraint and removing muscles from lamb m. longissimus thoracis et lumborum (LT) immediately after slaughter and electrical stimulation was undertaken at a rigor temperature of 18°C (n=15). The temperature of 18°C was achieved through chilling of electrically stimulated sheep carcasses in air at 12°C, air flow 1–1.5 ms−2. In other groups, the muscle was removed at 2.5 h post-mortem and either wrapped or left non-wrapped before being placed back on the carcass to follow carcass cooling regimes. Following rigor mortis, the meat was aged for 0, 16, 40 and 65 h at 15°C and frozen. For the non-stimulated samples, the meat was aged for 0, 12, 36 and 60 h before being frozen. The frozen meat was cooked to 75°C in an 85°C water bath and shear force values obtained from a 1 × 1 cm cross-section. Commencement of ageing was considered to take place at rigor mortis and this was taken as zero aged meat. There were no significant differences in the rate of tenderisation and initial shear force for all treatments. The 23% cook loss was similar for all wrapped and non-wrapped situations and the values decreased slightly with longer ageing durations. Wrapping was shown to mimic meat left intact on the carcass, as it prevented significant prerigor shortening. Such techniques allows muscles to be removed and placed in a controlled temperature environment to enable precise studies of ageing processes.