Influence of carcass temperature , glycogenolysis and glycolysis 45 min postmortem on the development of PSE pork

This study investigated the effect of slaughter stress, scalding and process time from stunning to chilling on carcass temperature, muscle glycogen and lactate content, and the development of PSE meat (pH,<5.B). Blood creatine kinase (CK) activity was positively (P<0.001) related to carcass temperature at slaughter. During scalding, carcass temperatureraised by 1.2± I.4°C, 13 %of muscle glycogen was broken down and lactate level elevated by 5 %. Furthermore carcass temperature fell by 0.7±0.8°C, 5 % of muscle glycogen was consumed and lactate content increased by 35 % between scalding and chilling. The time elapsed from stunning to splitting and further to chilling had a minor effect on carcass temperature, muscle glycogen or lactate content. Whereas, lactate production was positively correlated with the increase in carcass temperature both during scalding (PcO.Ol) and between stunning and chilling (P<0.01), and with muscle glycogen breakdown (PcO.OOl). Consequently, the enhanced glycogenolysis during scalding, the accelerated glycolysis between scalding and chilling, and the elevated carcass temperature 45 min postmortem (p.m.) resulted in the development of PSE meat. Index words: slaughter, creatine kinase, postmortem glycogenolysis, glycolysis, PSE meat


Introduction
The efforts to imrove pork quality should include the proper handling and management of pigs all along the line from farm to chilling.
At the abattoir, the treatments just before slaughtering are easier to control than the p.m. biochemical reactions triggered by stress- ful handling.
Wismer-Pedersen and Briskey (1961a)   presented four types of p.m. pH falls: a slow gradual, a gradual, a relatively rapid and a sharp, significant decrease.PSE meat resulted when pH, (45 min p.m.) was decreased to about 5.4 while tissue temperature remained above 25°C.However, the accelerated chill- ing rate had no significant effect on final lac- tic acid content but it did reduce the rate of lactate formation (Wismer-Pedersen and Briskey, 1961 b).
The heat production and loss from muscles were brought to equilibrium 30 min p.m. (Bierning-Sorensen, 1976).Moreover, the rigor and temperature measurements 45 min p.m. gave information about the intensity of treatment before and during slaughtering (Sybesma and van Logtestun, 1966).Finally, the rapid pH fall combined with a high carcass temperature developed PSE meat (Woltersdorf and Troeger, 1987   T Earlier papers (Honkavaara, 1988, 1989a   and 1989 b) considered the effect of porcine stress on blood composition and early p.m. meat quality in pigs of different halothane genotypes; the influence of selection phase, fasting and transport on porcine stress and on the development of PSE; and the influence of lairage on blood composition of pig and on the occurrence of PSE.
The purpose of this study was to evaluate the effect of blood CK activity and slaughter line on carcass temperature, muscle lactate and glycogen content, and the development of PSE.

Test animals
38 Finnish Landrace (L) pigs, 8 Finnish Yorkshire (Y) pigs and 56 LxY crosses were slaughtered at three abattoirs.The Central As- sociation of Artificial Insemination Societies determined the halothane genotype of 52 animals, and the results were combined by the Finnish Animal Breeding Association to get the probability for the genotype, the rest were of unknown genotype.The collection and treatment of animals are described earlier (Honkavaara, 1989 a, 1989 b).
Measurements at the abattoir The temperature and relative humidity were measured with a portable hygrometer (Humicap HMI 31, Vaisala, Finland) in the lairage and on the slaughter line at splitting and be- fore carcass chilling.Moreover, it was mea- sured scalding temperature and time, the time elapsed from stunning to splitting, and from splitting to chilling.

Evaluation of meat quality
Methods used for the evaluation of meat quality are described earlier (Honkavaara, 1988).Furthermore, the increase in carcass temperature during scalding and that from stunning to chilling were calculated by sub- tracting the temperature of 0 min p.m. from that of 10 min p.m. and from that of 45 min p.m., respectively.In addition, the rate of muscle glycogen breakdown and lactate for- mation were calculated from the samples of M. longissimus dorsi (LD) by the formulas: micromoles (pmol) of glycogen (MG) broken down per 100 g of wet tissue in one minute = MG 0 min p.m. (pmol/100 g)-MG 45 min p.m. (pmol/lOO)  time elapsed from exsanguination to chilling (min).

Statistical analyses
Conventional statistical methods were used to calculate means, standard deviations and standard error of the means (SEM).The re- lations between the measured variables and meat quality traits were analysed by simple regression.The difference among the means were evaluated by analysis of variance.More- over, stepwise regression analyses were used to examine the relative predictive value of the measured variables for carcass temperature, muscle glycogen, lactate and pH value 45 min p.m. (statistical program PATO for micro- computers, Mikrovuo, Finland).

Results
Prediction of the development of PSE meat Table 1 shows the combined effects of studied variables on muscle pH, value and on the changes in carcass temperature, mus- cle glycogen and lactate content 45 min p.m. Thus, the coefficient of determination (R 2 100) of the prediction equations were for the rate of lactate formation and glycogen break- down, increase in carcass temperature from Table 1.The best stepwise regression models11 for predicting biochemical changes in the M. tongissimus dorsi 45 min postmortem.
Carcass temperature during scalding The average increase in carcass temperature during scalding was 1.2+ I.4°C.Carcass tem- perature 0 min p.m. was highly significantly positively correlated with log CK at exsanguination (r = 0.47, PcO.OOl).Moreover, this CK activity contributed 12.9 % of the variation in carcass temperature 0 min p.m. (Honkavaara, 1989 a). Figure 1 shows the differ- ence in carcass temperature before and after scalding between the pigs with a low and elevated CK activity.Thus carcass tempera- tures of 38.5 and 39.6°C at slaughter, X, led to the respective temperature increases of 1.3 and O.4°C during scalding, Y (Y = 35.985- 0.902 X, R : 100=45 %, PcO.OOl).On the other hand, the higher the blood CK activity at exsanguination, X the smaller was the increase in carcass temperature during scalding, Y (Y = 3.757 -0.72810gX, R 2 100 =8 %, Pen 05).
The most contributing components of the increase in carcass temperature during scald- ing were carcass temperature 0 min p.m., se- rum glucose and glycerol, and muscle glycogen 0 min p.m. (partial R 2 100 = 45.3, 5.9, 4.9 and 4.3 °7o, respectively, Table 1).In the present study, neither stunning order nor car- cass hot weight had no significant influence on the increase in carcass temperature during scalding.
The lactate content of the LD muscle of the carcasses with the highest CK values were 45.6 pmol/g before scalding and 62.2 pmol/g af- ter scalding.The former value was in the mean range of 45.5 + 12.2 pmol/g, whereas the lat- ter was significantly (Pc0.05)higher than the respective average value of 47.8 pmol/g in this study.So it was suggested that the great oc- currence of reactors (20 °7o) led to a high heat generation due to accelerated glycolysis dur- ing scalding in carcasses with the highest CK values (Fig. 1).Furthermore, the effect of halothane genotype on carcass temperature is discussed more accurately earlier (Honkavaa- ra, 1988).
Carcass temperature from stunning to chilling The average carcass temperature 0 and 45 min p.m. was 38.6+1.0 and 39.0±1.1°C,respectively.Moreover, the mean increase in carcass temperature during scalding and that from stunning to chilling was respectively 1.2± 1.4 and 0.5 ± I.3°C.Consequently, car- cass temperature fell by 0.7 ±O.B°C between scalding and chilling.
In order to present the influence of carcass temperature on meat quality, the collected data were classified into three groups of nearly the same number of pigs according to the car- cass temperature rise between stunning and chilling.Consequently, Table 2 shows the variation in those variables that differed significantly between the groups.
The present results suggested that the degree of metabolic stimulation of muscles at exsanguination determined the temperature of carcass 10 and 45 min p.m. Actually the car- casses with the most prominent temperature rise between stunning and chilling had the highest (P< 0.001) increase in temperature during scalding.In contrast, the time elapsed from stunning to splitting and from splitting to chilling had a minor effect on carcass tem- perature.On the other hand, of prime grading class.Finally, the group that included all reactors had the most prominent increase in temperature 45 min p.m. and the highest PSE frequency (Table 2).

Rate of muscle glycogen breakdown
The average muscle glycogen content 0, 10 and 45 min p.m. was 28.4± 12.3, 24.7 + 12.3 and 23.5 + 13.6 pmol/g, respectively.This showed the most prominent decrease in glyco- gen during scalding and a minor decline af- terwards.Thus 17.4 % of the glycogen was broken down 45 min p.m. which corre- sponded to a breakdown rate of 9.9±23.5 qmol/(100 g X min).
The present results suggested that it was dif- ficult to predict meat quality on the basis of muscle glycogen content which, however, will have an important influence on the development of PSE pork through the glycolytic pathway.

Discussion
In general, blood CK activity and carcass hot weight correlated negatively with carcass temperature rise during scalding.Moreover without reactors, carcass temperature fell between stunning and chilling.Whereas in reac- tors, carcass temperature raised due to accelerated glycolysis during scalding and 45 min p.m. which resulted in the development of PSE.This agreed with Schneider et al.  (1980) who found that reactors had the highest CK activity and the lowest pH, value.In ad- dition, Sybesma and van Logtestijn (1966) found that a high carcass temperature was related to a rapid pH fall and the onset of rigor.
The rate of glycogenolysis was fastest dur- ing scalding after that it reduced up to the chilling.Carcasses with the most prominent rate of glycogen breakdown had the highest muscle glycogen content 0 min p.m. and the greatest PSE frequency.This was consistent with Wismer-Pedersen and Briskey (1961a) that a muscle with a sharp pH fall of 5.1 at 1.5 h contained more glycogen at slaughter than those with a slower pH fall.
Lactate production was low during scald- ing but it enhanced from scalding to chilling.
Thus it was suggested that, during scalding, a major part of glycogen was broken down to glucose, whereas a minor part of glucose was degraded to yield lactate.After scalding, a major part of glucose was consumed in glycolysis to produce lactic acid.This production was accelerated by elevated carcass tem- perature.On the other hand, the carcasses with the fastest rate of lactate formation included a great amount of reactors, had the smallest proportion of prime grading class and the highest occurrence of PSE.A similar re- sult was obtained by Lawrie (1960) that a fast rate of glycolysis was associated with a great degree of exudation.
In summary, the results suggested that to avoid the development of PSE meat the rate of glycogenolysis and glycolysis should be reduced so that the carcass temperature will raise as low as possible during scalding and there will be a total fall in carcass tempera- ture 45 min p.m. SELOSTUS Ruholämpötilan, glykogenolyysin ja glykolyysin vaikutus PSE-lihan muodostumiseen 45 min postmortem

Fig
Fig. I. Effect of CK activity (logarithmic transformation) at exsanguination on carcass temperature.

Table 2 .
Effect of carcass temperature on studied variables.