- Email: [email protected]
Py—a parameter for meat quality
Meat Science 65 (2003) 1429–1437 www.elsevier.com/locate/meatsci
Py—a parameter for meat quality U. Pliquetta,*, M. Altmannb, F. Pliquettc, L. Scho¨berleind a
University of Bielefeld, Faculty of Chemistry, D-33615 Bielefeld, Germany Martin-Luther-University Halle, Faculty of Agriculture, Halle, Germany c University of Leipzig, Institute for Medical Physics and Biophysics, Leipzig, Germany d Saxonian State Center of Agriculture Leipzig, Institute of Agricultural Investigations (LUFA), Leipzig, Germany b
Received 15 October 2002; received in revised form 20 February 2003; accepted 1 March 2003
Abstract The Py is a parameter which assesses the integrity of the cell membranes. It is a direct indicator for the volume fraction of cells surrounded by insulating cell membranes. The Py has been shown to correlate well with meat quality parameters like the drip loss or pH. It is a useful parameter for the discrimination between normal suited meat and PSE meat. The measurement is instantaneous and nondestructive. Due to aging of meat, Py depends on the time post mortem. It shows the highest significance between 4 and 24 h p.m. # 2003 Elsevier Ltd. All rights reserved. Keywords: Meat quality; Electrical impedance spectroscopy; Py
1. Introduction Meat is a natural product which shows a great variability in its properties. The assessment of meat quality is not only a question of pricing but also the basis for optimal processing. For instance, PSE (pale soft exudative) and DFD (dark firm dry) quality is especially problematic for ham production. PSE loses a considerable fraction of juice and DFD does not sufficiently develop taste. These problems can be avoided by sorting the meat prior to its use. While DFD quality is good for sausages, PSE would be recommended for fresh use. When meat ages, it undergoes several chemical processes like lactosis. This decreases the pH from originally 6.8 to 5–6. If this process is hindered, for instance due to a lack of ATP, the meat becomes dry and firm. On the other hand, several breeds (i.e. Pietrain) tend to an accelerated glycolysis and thereby loosing the ability of holding water, identified by a fast decrease in pH and a very high drip loss. Meat consists of cells surrounded by an insulating cell membrane. The conductivity of the cytoplasm electrolytes is approximate l=0.7 S/m and is mainly governed
by small ions (Cl, K+, Na+). Additionally, intracellular membrane structures (i.e. Golgi apparatus, mitochondria, endoplasmatic reticulum) and macromolecules like proteins or nucleic acids contribute to the electrical properties but in a frequency range not considered for meat quality measurement. In the extra-cellular space the glycocalyx and the collagen matrix (extra-cellular matrix) gives rise to the mechanical strength of the cell. The electrical properties outside the cell depend mainly on the ionic strength of small ions like Cl and Na+. Since the integrity of cell membranes correlates with quality parameters for meat, it is obvious to characterize it by its passive electrical properties. The effort ranges from conductivity measurements, G=I/U, at a single frequency up to capturing the entire impedance spectrum (complex resistance depending on frequency) in a wider range. Many of the procedures presented are not practicable or the results are not sufficiently reproducible. Here we show, that the Py-parameter is well defined, easy to measure and correlates highly with quality parameters like drip loss and pH 45 min post mortem. 1.1. Passive electrical properties of meat
* Corresponding author. Tel.: +49-521-1066261; fax: +49-5211062981. E-mail addresses: [email protected] (U. Pliquett). 0309-1740/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0309-1740(03)00066-4
If the meat is contacted with electrodes, a direct current (dc) can only flow around the cells because of their
1430
U. Pliquett et al. / Meat Science 65 (2003) 1429–1437
Nomenclature U I dc j Y G B Z R X f o z DFD PSE ATP
voltage, [U]=V (volt) current, [I]=A (ampere) direct current pffiffiffiffiffiffiffi imaginary unit, j ¼ 1 admittance, complex conductance, Y=G + jB conductance (real part of admittance), [G]=S (siemens) conductivity (specific conductance, independent of geometry), []=S/m susceptance (imaginary part of admittance), [B]=S impedance, Z=R + jX resistance (real part of impedance), [R]= (ohm) reactance (imaginary part of impedance) frequency, [f]=Hz (hertz) angular frequency, [o]=s1 geometry constant of an electrode configuration dark firm dry pale soft exudative adenosine-tri-phosphate
insulating plasma membranes. When cell membranes are permeabilized, for instance due to aging of the meat, the current can flow also through the interior of the cells and thus the conductivity increases. Therefore the dcconductivity could serve as an indicator for the integrity of the cell membranes. This however exhibits several problems: 1. The measurement of dc-conductivity is technically problematic because of the electrode polarization. Reversible electrodes are not preferable for use in a rough environment like a slaughter house. 2. Since not the conductivity but the conductance between the electrodes is measured, the measurement depends on the electrode geometry. If a needle electrode is calibrated to conductivity k, this calibration is valid only for a fixed insertion depth. Moreover the measurement is forged by the surface resistance of the electrode. 3. Biological tissue exhibits considerable non linearity in the voltage/current relationship, especially at direct current, which causes measurement artifacts. 4. The meat conductivity is a function of temperature. Technically, some of these problems can be avoided. For instance, using a temperature sensor gives the
opportunity to automatically compensate the temperature dependence of the conductivity. Since almost all the problems are associated with dc-current, the use of higher frequencies will avoid problems like electrode polarization or the influence of non linearity. Practically, frequencies between f=400 Hz and f=2 kHz are used (i.e. LF-Star, Mattha¨us, Germany). The use of one single frequency for conductivity measurement exhibits an uncertainty owing to the frequency dependence of meat conductivity (Pliquett & Pliquett, 1998). 1.1.1. Frequency dependence of meat conductivity The insulating membrane contacted by conductive electrolytes behaves like a capacitor. The cell is surrounded by electrolytes giving rise to a dc-conductivity of the meat. This can be modeled as an electrical equivalent circuit with three elements: a resistor Rex for the extra-cellular pathway in parallel with a RC combination (resistor, Rin, capacitor, Cm) modeling the pathway through the cell interior and thereby crossing the cell membrane (Fig. 1). The conductance, G, of a resistor is inverse proportional to its resistance, R:G=1/R. It relates to the conductivity by the electrode geometry G=z, where z is an geometrical electrode constant. For a plate capacitor with the area A and the plate distance of d we find z=A/d. Direct current (dc) does not cross the cell membranes, except when it is switched on or off, yielding a temporary current which charges up the cell membrane (displacement current). When the membrane is charged, a current reversal causes the membrane to recharge with the opposite polarity. If this is periodically repeated a continuous time dependent displacement current results. This current is called imaginary because no net transport of charge carriers (ions) occurs which is opposite to the ‘real’ ion transport in an aqueous electrolyte solution. The resistance of a capacitor, XC, depends on the frequency by XC=-j/oC. o Is the angular frequency, o=2pf,pwhere ffiffiffiffiffiffiffi f is the frequency, and j is the imaginary unit j ¼ 1. This means, the capacitor has a purely imaginary resistance. Note, that this resistance is negative. Since the resistance of meat has a real part R, and an imaginary part X, it can be described as complex number. This complex resistance is called impedance Z. Z ¼ R þ jX: There are several presentations of the frequency dependent impedance. The locus diagram is the imaginary part of the impedance versus the real part (Fig. 2). The locus diagram of the equivalent circuit is a semicircle with the low frequencies at the right side. The resistance at dc is labeled R0. With increasing frequency the real part decreases while the magnitude of the imaginary part increases. The imaginary part reaches a minimum at the characteristic frequency f0 and vanishes at high frequencies. At high frequencies all capacities
U. Pliquett et al. / Meat Science 65 (2003) 1429–1437
1431
Fig. 1. A simple equivalent electrical circuit for meat. Rex and Rin are resistors modeling the behavior of the electrolyte solutions and Cm is the capacitor formed by the cell membranes.
Fig. 2. Locus diagram of pork meat (M. longissimus dorsi, dots) and for the equivalent circuit (line). R0 and R1 are indicated for the equivalent circuit.
are shortened and thus only the parallel circuit of Rex and Rin (Fig. 1) remains, labeled as R1. The locus diagram of meat is a depressed semicircle. This results from a distribution in relaxation processes, each giving rise to one characteristic frequency. A mathematical model describing this behavior is the Cole–Cole distribution, but this is beyond the scope of this paper. Moreover it is clear, the curve does not achieve the x-axis along the semicircle, which hints to additional frequency dispersion at low as well as at high frequencies. The frequency dispersion depending on the cell membranes is called -dispersion (Schwan, 1963). Large molecules are polarized at higher frequencies, which is called g-dispersion. The origin of the dispersion at low frequencies (a-dispersion) is not clear, but it is probably produced by movement of charges along the cell surface, the microtuboli and intracellular conductive pathways (e.g. gap junctions). 1.1.2. Locus diagram and meat quality The drip loss depends directly on the integrity of the cell membranes. Thus, if the b-dispersion is a direct measure of the membrane behavior, it should predict the drip loss. If there are no insulating membranes, one would expect an extraordinary high drip loss. The locus
diagram will merge in R1 since no capacitors (insulating cell membranes) are present. The other extreme case would be, that the entire meat consists of cells with insulating membranes but leaving no extracellular space. In this case the R0 is infinite, which means, that also the circumference of the b-dispersion is infinite. Meat is somehow between these limiting cases. It should be clear that the more insulating cell membranes are present the greater is the b-dispersion. Just measuring at a single low frequency will be sensitive to the membrane integrity, but what is the right frequency? Since each sample of meat behaves differently, it is impossible to find a unique frequency for everything. Hence, the choice of the frequency for conductivity meters is a compromise between the sensitivity what is best at low frequencies and the rejection of electrode effects (electrode polarization) which decreases as the frequency increases. A better approach would be the difference between R0 and R1. Both can be fitted from the locus diagram and thereby rejecting departures from the semicircle due to high and low frequency dispersion effects. This difference is very sensitive to the membrane integrity and therefore to the ability of meat to hold water. Unfortunately this difference depends on the insertion depth of the electrodes as well as on the temperature. If we normalize this difference to the dc-conductivity, we get a parameter which is still sensitive to the membrane integrity but independent of the insertion depth of the electrodes. If the temperature coefficient of the intraand extra cellular electrolyte is equal, it is also independent of the temperature. This parameter is called Py and is calculated as Py ¼
R0 R1 100 R0
The Py ranges for fresh meat between 85 and 95, depending on the kind of meat and declines within several days to small values (Py < 10). Physically, the Py is a
1432
U. Pliquett et al. / Meat Science 65 (2003) 1429–1437
monotonically increasing function of the cell volume fraction surrounded by intact cell membranes (Pliquett, Pliquett, Scho¨berlein, & Freywald, 1995). The passive electrical behavior cannot only presented as impedance, but also as admittance, Y, which is the complex conductance and therefore connected to Z as Y=1/Z. The real part is G and the imaginary part is B (Y=G + jB). The locus diagram is a semicircle as well, but in the first quadrant (Fig. 3). The Py can be found in the Y-plane as well: Y1 Y0 Y1 It is worthwhile to note, that Py is closely connected to the quotient R1/R0 (Oliver et al., 2000) by
Py ¼
Py ¼ 1
R1 : R0
1.2. Measurement of Py A great variety of instruments for impedance measurements exist, usually sophisticated and expensive. They can measure the impedance or admittance as a function of frequency. Then the Cole–Cole function can be fitted in order to find R1 and R0. Another approach is the use of time domain-based measurements. In opposite to the variation of frequencies (frequency domain) only one signal with a broad bandwidth is applied and the response measured. If, for instance, at time t0 a voltage step with the voltage U0 is applied, the capacitors start to charge up. In this moment no voltage drops across the capacitor, i.e. the current, I0, flows through the parallel circuit of Rex and Rin. Than, with charging of the capacitor the current decreases after an exponential function. Finally, if the capacitor is charged, the current through Rin becomes zero and all the remaining current, I1, flows through Rex. The current response to a voltage step is shown in Fig. 4. If we assume only the influence of the ß-dispersion and a length of the voltage step enough to charge the capacitor completely, Py is simply
Fig. 3. Locus diagram of meat in the Y-plane. Note that Y0 is left while Y1 is far from the root, which is due to the fact, that the conductivity increases with frequency.
Fig. 4. Current through meat due to a voltage step of 100 mV at t=0 s.
Py ¼ 1
I1 ; I0
where I0 is the current at t=0 and I1 is the current after infinite time or for practical reason after about 10 times the time constant of the current decay. It was found for meat, that 250 ms is sufficient to meet this requirement. Since Rex is the dc-resistance R0 and the parallel circuit of Rex and Rin is R1, we get simply I1 ¼ U0 =Rex ¼ U0 =R0 and I0 ¼ U0 ðRex þ Rin Þ=ðRex Rin Þ ¼ U0 =R1 : Inserting into the equation for Py we get: Py ¼ 1
I1 U0 =R0 R1 R0 R1 ¼1 ¼1 ¼ I0 U0 =R1 R0 R0
2. Devices for measurement of Py The use in the slaughter house requires devices which are robust, cheap and easy to use. Moreover, an instantaneous measurement is needed. Therefore, the practical achievement is always a compromise between accuracy and robustness. This is acceptable, because the reproducibility of passive electrical behavior even within the same muscle at the same animal is not better than 90–95%. Hence, for further processing of the Py a scale with no more than 100 units is necessary. Based on this requirement we constructed a very simple device using the current response to a voltage square wave. Py was then presented either as a single number by I1 Py ¼ a 1 þb I0
U. Pliquett et al. / Meat Science 65 (2003) 1429–1437
where a and b are the scaling constants. The electrodes were needle shaped (Ø=2.5 mm) with a distance of 2 cm and a length of 4 cm. Several configurations of devices were manufactured by Sigma Electronic Erfurt.
3. Materials and methods Pork and beef of different breeds and cross-breeds were used. The Py measurements utilized a Meatcheck 162 (Sigma Electronic Erfurt, Germany). For the conductivity measurements we used a LF-Star while a pH-Star was employed for the pH measurements. 3.1. Experiment 1 The objective of this experiment was the relationship between pH45 min and Py depending on the time post mortem. The measurements were performed at a total of 52 pig carcasses in a commercial slaughter house. The Py measurement was conducted at varying times between 45 min and 12 h at the thirteenth rib of the M. longissimus dorsi, the same location as for the pH45 min measurement. 3.2. Experiments 2 and 3 Here we focused on the relationship between storageand cooking loss and the Py-parameter. In experiment 2 (133 pig carcasses) and experiment 3 (59 pig carcasses) the Py was measured 24 h post mortem on an excised part of M. longissimus dorsi between the twelfth and the fourteenth thoracic vertebra with and without bones and fat respectively. Moreover, in experiment 2 the Py was measured 5 and 12 h post mortem at the thriteenth rib of the half carcass. For determination of the drip loss a 3 cm thick slice was stored in a foil bag at 4 C for 3 and 6 days, respectively. The relative weight of the juice was than calculated. For the grill loss, a 2.5 cm thick slice was grilled in a contact grill until a core temperature of 70 C and 5 min cooled. The color L* (meat brightness) (Honikel, 1999) was quantified by a Chroma-Meter CR 300 (Minolta, Osaka, Japan) while the shear force was measured using the Warner-Bratzler-method (Honikel, 1999). The intramuscular fat (IMF) was determined by Soxhlet (Arneth, 1999).
1433
the parameters investigated in experiments 2 and 3 the cooking and frying loss was determined. For the cooking loss a cube of m=15 g was placed for 15 min in 150 ml boiling water and afterwards cooled for 15 min. For the frying loss the cube was fried in 170 C fat for 2 min and cooled for another 15 min. The drip loss was determined between the sixteenth and the twenty-third day post mortem on a 2.5 cm thick slice, which was stored at 4 C under vacuum seal.
4. Results and discussion 4.1. Influence of the time post mortem (experiment 1) After slaughtering Py decreases (Fig. 5). The kinetics of Py decrease varies with the kind of meat and its quality. There is no significant difference in Py immediately after slaughtering owing to almost 100% of insulating cell membranes. However, after 45 min the difference between PSE and normal quality appears with a tendency of lower Py for PSE. This early measurement is not recommended for safe distinction since only the carcasses with extreme PSE quality can be detected at this time. The following declension of Py until 140 min is most pronounced for PSE quality. In opposite, carcasses with normal quality show only slight changes within this time. Thus, the differentiation between various quality increases with time. For a clear interpretation of Py in terms of quality, the measurement should not be done before 3 h, better later. This is further supported by the time dependent relationship between Py and pH (Table 1). The correlation at 45 min is with r=0.51 significantly lower than at later times (r=0.73–0.77). This is confirmed by Geisler (1999), where the correlation between Py, drip loss and pH was significant better at 3 h compared with measurements at 45 min post mortem. Other authors, who used the conductivity instead of the Py, also favor the measurement
3.3. Experiment 4 Here we assessed the correlation between the Py and other quality parameters in beef. 168 samples of M. longissimus dorsi between the twelfth and fourteenth thoracic vertebra with and without bones and fat respectively were used for the Py measurement. Besides
Fig. 5. Py in porcine M. longissimus dorsi of different quality depending on time post mortem (for statistics see Table 5).
1434
U. Pliquett et al. / Meat Science 65 (2003) 1429–1437
Table 1 Correlations between Py-value and pH45
min
in pig carcasses
Time of Py-measurement
N
Correlation to pH45 min
45 min 2 h 20 min 5h 12 h
52 52 52 52
0.51 0.78 0.73 0.77
at later times (Brunken & Glodek, 1985; Fortin & Raymond, 1988; Greshake, Schmitten, & Schepers, 1988; Jaud, Weisse, Gehlen, & Fischer, 1992; Mussmann, Ju¨ngst, Tholen, & Schmitten, 1992; Tho¨lking & Brenner, 1990; Warris, Brown, & Adams, 1991; Whitman, Forrest, Morgan, & Okos, 1996). Forrest et al. (2000) who used a special equipment for meat quality based on a tetrapolar conductivity measurement at 1 kHz (Purdue Tetrapolar Porbe, Sheiss, 1998) found a correlation to the drip loss of r=0.5. Unfortunately, only measurements after 30 min were reported. The ratio of the real part of the impedance at dc, R0, and high frequencies, R1 in the M. semimembranosus after 24 and 36 h post mortem shows a correlation to pH45 min of 0.78 and 0.66 (Oliver et al., 2001; Riu, Elvira, Oliver, Gobantes, & Arnau, 2001).
Table 2 Mean value and standard deviation for the experiments 2–4
N Py5 h* Py12 h* Py24 h** Py24 h*** pH45 min pH24 h
Experiment 2 (pork)
Experiment 3 (pork)
Experiment 4 (beef)
Mean
S.D.
Mean
S.D.
Mean
19.9 16.7
59 47.7 44.5 33.1 30.3
15.1 17.0 15.8 21.1
133
31.7 15.6
S.D.
168
6.23 5.54
0.4 0.4
6.24 5.44
0.3 0.1
6.0 4.6
2.0 1.1
5.0
26.1
50.3 31.0
12.4 19.0
5.50
0.1
1.6
1.7 2.3 3.2 3.3
0.9 0.9 1.2 1.1
4.01
27.1 28.3 29.1
4.1 3.0 3.4
Cooking loss% 2nd day
45.0
1.8
4.2. Relation between Py-value and other quality parameters (experiments 2–4)
Frying loss% 2nd day
35.2
1.9
The mean values and standard deviation of the data are shown in Table 2. Concerning pH and the drip loss (second–third day) the carcasses show a great variety in their quality. The drip loss in pork ranges from 1.5 to 11.1%, the pH45 min from 5.4 to 6.8. Beef showed pH24 h values between 5.4 and 6.0 which did not give any hint to DFD-quality. The drip loss varies between 0.3 and 7.2% between 24 and 48 h which points to some relatively dry samples. Compared with pork, beef exhibits the higher Pyvalues. However, due to cutting out the sample from the half carcass and boning, the Py of the M. longissimus dorsi declines significantly, which results from mechanical stress on the cell structure. Before dissection the muscles are fixed at the bones, which supports the tissue mechanically. If this support is lost, the structure collapses partially, changing the geometry of cells and thus the relationship between intra- and extra-cellular pathways. This is supported by investigations by Altmann, Geissler, Scho¨berlein, Pliquett, and Pliquett (2000). Moreover the decline in Py was observed by separation of the whole cutlet row (Geisler, 1999) as well as by cutting out a part of the cutlet between twelfth and fourteenth thoracic vertebra (Schoeberlein, Grimm, & Janetschke, 1990). The difference was between 17 and 20 Py-units. This tendency was confirmed in experiment 3, where the Py12 h,half carcass Py24 h,separated muscle was around 11. In these experiments also the initial values of Py have been
Color L*
Drip loss% 2nd–3rd day 3rd–6th day 2nd–16th day 2nd–23rd day Grill loss% 2nd day 16th day 23rd day
Shear force/kp 2nd day 16th day 23rd day IMF%
50.3
3.0
48.5
3.1
33.1
2.1
4.6
0.9
4.4
1.1
5.9 4.0 3.4
1.1 0.9 0.7
1.2
0.5
1.2
0.4
2.3
1.7
* Measured at the hanging half carcass. ** Measured at the separated muscle with bones and fat. *** Measured at the separated muscle without bones and fat.
smaller than in other experiments. After removal of bones and fat from the M. longissimus dorsi Py further decreases by 10–12 units (Kraa, Geissler, Scharner, Scho¨berlein, & Pliquett, 1997; Schoeberlein, Scharner, Honikel, Altmann, & Pliquett, 1999). The variety in the experiments presented here was higher, so that the Pydecrease by dissection was found between 3 and 20 units. Schwaegele (1993) observed an increase in conductivity due to separation of cutlet and ham around 3.5 mS/cm. The Py measurement between 5 and 24 h post mortem allows a good estimation of the drip loss between the second and third day at the M. longissimus dorsi. Therefore, the measurement yields also reliable results at separated muscles with or without bones and fat. For pork we found correlations for the Py measurement on the carcass at r=0.58 and r=0.56 (Table 3). For the separated M. longissimus dorsi the correlations have
U. Pliquett et al. / Meat Science 65 (2003) 1429–1437 Table 3 Correlations of pH45 min and Py-value with other quality parameters in pork pH45 min Py5 h* Py12 h* Py24 h** Py 24 h*** Experiment 3 N=59 Drip loss 2nd–3rd day Grill loss 2nd day Color L* Shear force IMF
0.58
0.58 0.56
0.68
0.62
0.02 0.46 0.11 0.31
0.05 0.06 0.52 0.46 0.19 0.28 0.21 0.30
0.06 0.51 0.36 0.28
0.16 0.48 0.41 0.28
0.77 0.28
0.68 0.28
0.01 0.52 0.29 0.14
0.02 0.49 0.33 0.11
Experiment 2 N=133 Drip loss 2nd–3rd day 3rd–6th day Grill loss 2nd day Color L* Shear force IMF
* Measured at the hanging half carcass. ** Measured at the separated muscle with bones and fat. *** Measured at the separated muscle without bones and fat.
Table 4 Correlations of pH24 beef
h
and Py-value with other quality parameters in pH24 h
Py24 h**
Py24 h***
168
168 0.07
168 0.09
0.17 0.12 0.16 0.21
0.54 0.40 0.09 0.27
0.53 0.39 0.20 0.30
0.12 0.11 0.11
0.19 0.06 0.10
0.08 0.11 0.05
0.06
0.02
0.01
0.08
0.17
0.22
0.10
0.19
0.07
Shear force difference 2nd day 16th day 23rd day 2nd–23rd day
0.23 0.20 0.37 0.00
0.28 0.14 0.16 0.23
0.42 0.27 0.25 0.36
IMF
0.00
0.03
0.12
N pH24 h Drip loss 2nd–3rd day 3rd–6th day 2nd–16th day 2nd–23rd day Grill loss 2nd day 16th day 23rd day Cooking loss 2nd day Frying loss 2nd day Color L*
** Measured at the separated muscle with bones and fat. *** Measured at the separated muscle without bones and fat.
1435
been between 0.62 and 0.77 while beef gave values between 0.53 and 0.54 (Table 4). Schoeberlein et al. (1999) found for separated muscles higher correlations than for carcasses as well. The Py of dissected muscles assesses the fraction of cell membranes becoming permeable due to accelerated glycolysis and mechanical stress. That’s why these correlations are higher. Others (Byrne, Troy, & Buckley, 2000) reported at separated muscles from beef a correlation of 0.45 between the drip loss and Py. The drip loss after longer storage becomes less correlated to the Py measured at 24 h post mortem. For instance, the drip loss between the second and third day at pork correlates to the drip loss after longer storage (until 23 days) with r=0.36. This correlation is higher for cattle, between 0.47 and 0.65 (data not shown). Here we used samples with different packing in order to assess the influence of initial quality to the changes during storage. Obviously, besides the condition of the cell membranes after 24 h, there are additional factors influencing the weight loss during storage. A proposition of weight loss due to cooking using the Py at 24 h is impossible for pork as well as for beef, because of insignificant correlations. This was confirmed by Byrne et al. (2000) too. An explanation is the total destruction of the cell membranes due to heating, which levels the differences detected by Py or drip loss at uncooked meat off. Moreover the cooking loss shows much less variety than the drip loss. The correlation to the color parameter L* (meat brightness) was for pork 0.46 to 0.52. Lighter meat has a tendency to PSE-quality. As confirmed by Schoeberlein et al. (1999) and Byrne et al. (2000), the correlation to the shear force is positive but at a low level ( 0.20). The pH45 min is often used for PSE-detection in pork. Its correlation to drip loss and L* is similar or less compared with the Py. Because meat quality is also influenced by conditions of cooling a measurement at a later phase (after several hours) allows a better assessment of the meat quality than by measurements of pH 45 min post mortem. The pH24 h in beef correlates much less to the drip loss after 6 days (r=0.17 to 0.21) than the Py. A similar level was found for other traits. Because in our experiments no samples with an pH24 h > 6 were found, its use for prediction of the drip loss is limited. However, DFD-meat cannot be detected by means of Py alone (Schoeberlein, 1995; Warris, Brown, & Adams, 1996). Hence, we recommend the combination of Py and pH for assessment of meat quality. 4.3. Py limits for quality assessment in pork Based on the drip loss between the second and the third day, the material was divided into three qualities: PSE,
1436
U. Pliquett et al. / Meat Science 65 (2003) 1429–1437
Table 5 Py-value of the M. longissimus dorsi in pork with different drip loss Drip loss <4% (normal) N Py5 h* Py12 h* Py24 h** Py24 h***
18 18 44 44
Mean a
57.9 55.7a 51.4a 40.4a
S.D. 10.9 13.3 11.6 18.2
Drip loss 4–6% (indifferent) N 20 20 59 59
Mean a
50.6 48.3a 39.8b 24.1b
S.D. 11.1 13.4 14.2 16.6
Drip loss >6% (PSE) N 21 21 89 89
Mean b
36.2 31.2b 17.6c 7.4c
S.D. 14.2 14.1 10.9 9.5
Table 7 Correlations of Py-value and conductivity 24 h post mortem with pH45 min
Py Conductivity
N
M. longissimus dorsi
M. semimembranosus
133 133
0.59 0.61
0.44 0.32
indifferent and normal. The Py for each group is listed in Table 5. Between the groups highly significant differences appear. Using the frequency of occurrence we defined the Py-limits for the evaluation of the M. longissimus dorsi. The significance of the classification is given in Table 6. For instance the right evaluation of carcasses with normal quality was 61.1% by using the Py after 5 h while PSE classification was right at 71.4% of all cases. A more significant classification was possible at the separated muscle (24 h post mortem) with 79.5 and 78.6%, respectively. The limits depend on the time of measurement and the degree of dissection. They are valid for M. longissimus dorsi in the region of the twelfth–fourteenth thoracic vertebra. Caudally or cranially displaced measurements showed higher Py values (Geisler, 1999; Kraa et al., 1997). An extrapolation to other muscles is not recommended or possible. Schoeberlein et al. (1999) reported limits for the Py measurement on half carcasses at 24 h post mortem. The Py limit for PSE was < 30 and for normal quality > 50. Oliver et al. (2001) evaluated the quality of ham with the quotient of R0/R1 36 h post mortem. They achieved a fraction of right classification of 88.5% for normal quality and 81.8% for PSE.
pH measurement allows an early evaluation at the time of carcass classification. Changes due to the following cooling are not assessed. Often the pH measurement is poorly done. Sources of artifacts are a wrong or not performed adjustment, an insufficient measurement time and a too short insertion of the pH-probe. If the M. longissimus dorsi is not reached, but just the preliminary M. multifidus, than the results are by 0.1–0.2 pH units higher. pH-devices need frequent maintenance and are fragile due to the glass corpus. The use of ISFETdevices (ion sensitive field effect transistor) is promising, but their lack in stability limits practical application. The measurement of Py or other electrical parameters like conductivity makes sense only at later times after carcass classification. The measurement time is short compared with the pH measurement. The metal electrodes itself are robust and do not need much attention. A calibration like for the pH devices is not necessary. However, the electrical measurements are not suitable for the detection of DFD quality. This requires additional pH measurement after 24 h. The advantages of the Py-measurement (or the factor R0/R1 as well) over the conductivity are mentioned at the beginning. Py and conductivity correlate in some experiments with r=0.8. The relationship of both methods to pH45 min is shown in Table 7. While the correlation (Py/ pH, conductivity/pH) for the M. longissimus dorsi differs insignificantly, the assessment of the Py/pH is better performed at the M. semimembranosus. Byrne et al. (2000) reported higher significance of the Py value too.
4.4. Comparison between Py value and established fast methods for assessment of meat quality
5. Conclusions
Today the pH45 min and the conductivity are the best established methods for the detection of PSE meat. The
The Py is a suitable parameter for the assessment of PSE quality. It correlates well to the pH and the drip
Significant differences at mean values signed with different letters (a=0.001). * Measured at the hanging half carcass. ** Measured at the separated muscle with bones and fat. *** Measured at the separated muscle without bones and fat.
Table 6 Py-limits and percentage of right classification for PSE, indifferent, and normal quality in pork Time post mortem 5h 12 h 24 h 24 h
Degree of dissection
Half carcass Half carcass Separated muscle with bones and fat Separated muscle without bones and fat
Limits
Percentage of right classification
Normal
Indifferent
PSE
Normal
Indifferent
PSE
>57 >55 >45
40–57 35–55 25–45
<40 <35 <25
61.1 66.6 79.5
40.0 40.0 44.1
71.4 61.9 78.6
>35
15–35
<15
65.9
35.6
85.4
U. Pliquett et al. / Meat Science 65 (2003) 1429–1437
loss. Measurements should be done 3 h post mortem, or better even later. Because of its simple handling it is well suited for routine measurements. A defined time post mortem and the degree of dissection are necessary for comparative proposition. The measurement on the carcass yields the fraction of intact cell membranes which changes with proceeding glycolysis. For a proper detection of PSE quality at pork measurements should be done on the carcass because dissected muscles are additionally influenced by mechanical stress. Measurements on carcass cuts are suitable in the meat industry for controlling the quality of the raw material or for classification and sorting of meat prior to further processing. PSE quality in beef can be found rarely. However, Py is highly correlated to drip loss until 3 days post mortem also in the range of normal quality. Therefore, Py is useful to evaluate the drip loss in beef too.
References Altmann, M., Geissler, A., Schoeberlein, L., Pliquett, U., & Pliquett, F. (2000). Einfluß von Transport und Zerlegung auf die Fleischstruktur. Fleischwirtschaft, 80, 94–98. Arneth, W. (1999). Repra¨sentative probenentnahme fu¨r die chemische analyse. In Analytik bei Fleisch (pp. 13–24). Kulmbach, Germany: Bundesanstalt fu¨r Fleischforschung. Brunken, H., & Glodek, P. (1985). Untersuchungen zur Fleischbeschaffenheit im Hinblick auf die Leitfa¨higkeit und Halothanreaktion von Schweinen der Deutschen Landrasse (DL). Zu¨chtungskunde, 57, 357–361. Byrne, C. E., Troy, D. J., & Buckley, D. J. (2000). Postmortem changes in muscle electrical properties of bovine M. longissimus dorsi and their relationship to meat quality attributes and pH fall. Meat Science, 54, 23–34. Forrest, J. C., Morgan, M. T., Borggaard, C., Rasmussen, A. J., Jespersen, B. L., & Andersen, J. R. (2000). Development of technology for the early post mortem prediction of water holding capacity and drip loss in fresh pork. Meat Science, 55, 115–122. Fortin, A., & Raymond, D. P. (1988). The use of the electrical characteristics of muscle for the objective detection of PSE and DFD in pork carcasses under commercial conditions. Canadian Institute Food Science and Technology Journal, 21, 260–265. Geissler, A. (1999). Beurteilung der Fleischqualita¨t von Schweinen durch passiv elektrische Gewebseigenschaften. Thesis, Leipzig. Greshake, F., Schmitten, F., & Schepers, K. H. (1988). Untersuchungen zur Bestimmung des Tropfsaftverlustes von Schweinefleisch und dessen Zusammenha¨nge zu anderen Fleischqualita¨tskriterien. Fleischwirtschaft, 68, 765–769. Honikel, K. O. (1999). Standardisierung physikalischer Messverfah-
1437
ren. In Analytik bei Fleisch (pp. 167–192). Kulmbach, Germany: Bundesanstalt fu¨r Fleischforschung. Jaud, D., Weisse, K., Gehlen, K. H., & Fischer, A. (1992). pH-Wert und Leitfa¨higkeit—vergleichende Messungen an Schweineschlachtko¨rpern und ihre Beziehung zum Tropfsaftverlust. Fleischwirtschaft, 72, 252–259. Kraa, S., Geissler, A., Scharner, E., Schoeberlein, L. & Pliquett, F. (1997). Vera¨nderungen des passiv elektrischen Verhaltens von Schweinemuskeln post mortem. In 38. Arbeitstagung der Dt. Vet.Med. Ges. (pp. 557–562). Garmisch-Partenkirchen, Teil 1. Mussmann, T., Ju¨ngst, H., Tholen, E., & Schmitten, F. (1992). Untersuchungen zur Ermittlung der Fleischbeschaffenheit beim Schwein an verschiedenen Meßstellen und zu verschiedenen Meßzeitpunkten. Weihenstephan: Vortragstagung DGfZ/GfT. Oliver, M. A., Gobantes, I., Arnau, J., Elvira, J., Riu, P., Grebol, N., & Monfort, J. M. (2001). Evaluation of the electrical impedance spectroscopy (EIS) equipment for ham meat quality selection. Meat Science, 58, 305–312. Pliquett, U., & Pliquett, F. (1998). Kritische Bemerkungen zur Leitfa¨higkeit als Qualita¨tsmerkmal fu¨r Fleisch. Fleischwirtschaft, 78, 1010–1012. Pliquett, F., Pliquett, U., Schoberlein, L., & Freywald, K. H. (1995). Impedanzmessungen zur Charakterisierung der Fleischbeschaffenheit. Fleischwirtschaft, 75, 496–498. Riu, P., Elvira, J., Oliver, M. A., Gobantes, I. & Arnau, J. (2001). Insitu assessment of the technical quality of meat. In XI. ICEBI (pp. 619–622). Oslo. Schoeberlein, L. (1995). Unpublished material. Schoeberlein, L., Grimm, S., & Janetschke, P. (1990). Untersuchungen zur Bestimmung der Muskelfleischqualita¨t beim Schwein in der Produktion. In 21. Jahrestagung Prophylaxe und Beka¨mpfung von gesundheitlichen Sto¨rfaktoren in der Tierproduktion (pp. 553–560), Leipzig. Schoeberlein, L., Scharner, E., Honikel, K. O., Altmann, M., & Pliquett, F. (1999). Der Py-Wert als Fleischqualita¨tsmerkmal. Fleischwirtschaft, 79, 116–120. Schwaegele, F. (1993). Qualita¨tsmerkmale - Erfassung nach dem Schlachten. Fleischwirtschaft(73), 228–238. Schwan, H. P. (1963). Physical techniques in biological research, 6. In W. L. Nastuk (Ed.), Determination of biological impedances (pp. 323–407). New York: Academic Press. Sheiss, E. B. (1998). Evaluation of tretapolar impedance measurement for the detection of quality abnormalities in pork. MS thesis, Purdue University, West Lafayette, IN 47907, USA. Thoelking, L., & Brenner, K. V. (1990). Vergleich der Aussagekraft von Methoden zur Messung der Fleischbeschaffenheit bei Schweinen am Schlachtband. Schweinezucht u. Schweinemast, 38, 11–14. Warris, P. D., Brown, S. N., & Adams, S. J. M. (1991). Use of the Tecpro pork quality meter for assessing meat quality on the slaughterline. Meat Science, 30, 147–156. Whitman, T. A., Forrest, J. C., Morgan, M. T., & Okos, M. R. (1996). Electrical measurement for detecting early postmortem changes in porcine muscle. J. Anim. Sci., 74, 80–90.
Py—a parameter for meat quality U. Pliquetta,*, M. Altmannb, F. Pliquettc, L. Scho¨berleind a
University of Bielefeld, Faculty of Chemistry, D-33615 Bielefeld, Germany Martin-Luther-University Halle, Faculty of Agriculture, Halle, Germany c University of Leipzig, Institute for Medical Physics and Biophysics, Leipzig, Germany d Saxonian State Center of Agriculture Leipzig, Institute of Agricultural Investigations (LUFA), Leipzig, Germany b
Received 15 October 2002; received in revised form 20 February 2003; accepted 1 March 2003
Abstract The Py is a parameter which assesses the integrity of the cell membranes. It is a direct indicator for the volume fraction of cells surrounded by insulating cell membranes. The Py has been shown to correlate well with meat quality parameters like the drip loss or pH. It is a useful parameter for the discrimination between normal suited meat and PSE meat. The measurement is instantaneous and nondestructive. Due to aging of meat, Py depends on the time post mortem. It shows the highest significance between 4 and 24 h p.m. # 2003 Elsevier Ltd. All rights reserved. Keywords: Meat quality; Electrical impedance spectroscopy; Py
1. Introduction Meat is a natural product which shows a great variability in its properties. The assessment of meat quality is not only a question of pricing but also the basis for optimal processing. For instance, PSE (pale soft exudative) and DFD (dark firm dry) quality is especially problematic for ham production. PSE loses a considerable fraction of juice and DFD does not sufficiently develop taste. These problems can be avoided by sorting the meat prior to its use. While DFD quality is good for sausages, PSE would be recommended for fresh use. When meat ages, it undergoes several chemical processes like lactosis. This decreases the pH from originally 6.8 to 5–6. If this process is hindered, for instance due to a lack of ATP, the meat becomes dry and firm. On the other hand, several breeds (i.e. Pietrain) tend to an accelerated glycolysis and thereby loosing the ability of holding water, identified by a fast decrease in pH and a very high drip loss. Meat consists of cells surrounded by an insulating cell membrane. The conductivity of the cytoplasm electrolytes is approximate l=0.7 S/m and is mainly governed
by small ions (Cl, K+, Na+). Additionally, intracellular membrane structures (i.e. Golgi apparatus, mitochondria, endoplasmatic reticulum) and macromolecules like proteins or nucleic acids contribute to the electrical properties but in a frequency range not considered for meat quality measurement. In the extra-cellular space the glycocalyx and the collagen matrix (extra-cellular matrix) gives rise to the mechanical strength of the cell. The electrical properties outside the cell depend mainly on the ionic strength of small ions like Cl and Na+. Since the integrity of cell membranes correlates with quality parameters for meat, it is obvious to characterize it by its passive electrical properties. The effort ranges from conductivity measurements, G=I/U, at a single frequency up to capturing the entire impedance spectrum (complex resistance depending on frequency) in a wider range. Many of the procedures presented are not practicable or the results are not sufficiently reproducible. Here we show, that the Py-parameter is well defined, easy to measure and correlates highly with quality parameters like drip loss and pH 45 min post mortem. 1.1. Passive electrical properties of meat
* Corresponding author. Tel.: +49-521-1066261; fax: +49-5211062981. E-mail addresses: [email protected] (U. Pliquett). 0309-1740/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0309-1740(03)00066-4
If the meat is contacted with electrodes, a direct current (dc) can only flow around the cells because of their
1430
U. Pliquett et al. / Meat Science 65 (2003) 1429–1437
Nomenclature U I dc j Y G B Z R X f o z DFD PSE ATP
voltage, [U]=V (volt) current, [I]=A (ampere) direct current pffiffiffiffiffiffiffi imaginary unit, j ¼ 1 admittance, complex conductance, Y=G + jB conductance (real part of admittance), [G]=S (siemens) conductivity (specific conductance, independent of geometry), []=S/m susceptance (imaginary part of admittance), [B]=S impedance, Z=R + jX resistance (real part of impedance), [R]= (ohm) reactance (imaginary part of impedance) frequency, [f]=Hz (hertz) angular frequency, [o]=s1 geometry constant of an electrode configuration dark firm dry pale soft exudative adenosine-tri-phosphate
insulating plasma membranes. When cell membranes are permeabilized, for instance due to aging of the meat, the current can flow also through the interior of the cells and thus the conductivity increases. Therefore the dcconductivity could serve as an indicator for the integrity of the cell membranes. This however exhibits several problems: 1. The measurement of dc-conductivity is technically problematic because of the electrode polarization. Reversible electrodes are not preferable for use in a rough environment like a slaughter house. 2. Since not the conductivity but the conductance between the electrodes is measured, the measurement depends on the electrode geometry. If a needle electrode is calibrated to conductivity k, this calibration is valid only for a fixed insertion depth. Moreover the measurement is forged by the surface resistance of the electrode. 3. Biological tissue exhibits considerable non linearity in the voltage/current relationship, especially at direct current, which causes measurement artifacts. 4. The meat conductivity is a function of temperature. Technically, some of these problems can be avoided. For instance, using a temperature sensor gives the
opportunity to automatically compensate the temperature dependence of the conductivity. Since almost all the problems are associated with dc-current, the use of higher frequencies will avoid problems like electrode polarization or the influence of non linearity. Practically, frequencies between f=400 Hz and f=2 kHz are used (i.e. LF-Star, Mattha¨us, Germany). The use of one single frequency for conductivity measurement exhibits an uncertainty owing to the frequency dependence of meat conductivity (Pliquett & Pliquett, 1998). 1.1.1. Frequency dependence of meat conductivity The insulating membrane contacted by conductive electrolytes behaves like a capacitor. The cell is surrounded by electrolytes giving rise to a dc-conductivity of the meat. This can be modeled as an electrical equivalent circuit with three elements: a resistor Rex for the extra-cellular pathway in parallel with a RC combination (resistor, Rin, capacitor, Cm) modeling the pathway through the cell interior and thereby crossing the cell membrane (Fig. 1). The conductance, G, of a resistor is inverse proportional to its resistance, R:G=1/R. It relates to the conductivity by the electrode geometry G=z, where z is an geometrical electrode constant. For a plate capacitor with the area A and the plate distance of d we find z=A/d. Direct current (dc) does not cross the cell membranes, except when it is switched on or off, yielding a temporary current which charges up the cell membrane (displacement current). When the membrane is charged, a current reversal causes the membrane to recharge with the opposite polarity. If this is periodically repeated a continuous time dependent displacement current results. This current is called imaginary because no net transport of charge carriers (ions) occurs which is opposite to the ‘real’ ion transport in an aqueous electrolyte solution. The resistance of a capacitor, XC, depends on the frequency by XC=-j/oC. o Is the angular frequency, o=2pf,pwhere ffiffiffiffiffiffiffi f is the frequency, and j is the imaginary unit j ¼ 1. This means, the capacitor has a purely imaginary resistance. Note, that this resistance is negative. Since the resistance of meat has a real part R, and an imaginary part X, it can be described as complex number. This complex resistance is called impedance Z. Z ¼ R þ jX: There are several presentations of the frequency dependent impedance. The locus diagram is the imaginary part of the impedance versus the real part (Fig. 2). The locus diagram of the equivalent circuit is a semicircle with the low frequencies at the right side. The resistance at dc is labeled R0. With increasing frequency the real part decreases while the magnitude of the imaginary part increases. The imaginary part reaches a minimum at the characteristic frequency f0 and vanishes at high frequencies. At high frequencies all capacities
U. Pliquett et al. / Meat Science 65 (2003) 1429–1437
1431
Fig. 1. A simple equivalent electrical circuit for meat. Rex and Rin are resistors modeling the behavior of the electrolyte solutions and Cm is the capacitor formed by the cell membranes.
Fig. 2. Locus diagram of pork meat (M. longissimus dorsi, dots) and for the equivalent circuit (line). R0 and R1 are indicated for the equivalent circuit.
are shortened and thus only the parallel circuit of Rex and Rin (Fig. 1) remains, labeled as R1. The locus diagram of meat is a depressed semicircle. This results from a distribution in relaxation processes, each giving rise to one characteristic frequency. A mathematical model describing this behavior is the Cole–Cole distribution, but this is beyond the scope of this paper. Moreover it is clear, the curve does not achieve the x-axis along the semicircle, which hints to additional frequency dispersion at low as well as at high frequencies. The frequency dispersion depending on the cell membranes is called -dispersion (Schwan, 1963). Large molecules are polarized at higher frequencies, which is called g-dispersion. The origin of the dispersion at low frequencies (a-dispersion) is not clear, but it is probably produced by movement of charges along the cell surface, the microtuboli and intracellular conductive pathways (e.g. gap junctions). 1.1.2. Locus diagram and meat quality The drip loss depends directly on the integrity of the cell membranes. Thus, if the b-dispersion is a direct measure of the membrane behavior, it should predict the drip loss. If there are no insulating membranes, one would expect an extraordinary high drip loss. The locus
diagram will merge in R1 since no capacitors (insulating cell membranes) are present. The other extreme case would be, that the entire meat consists of cells with insulating membranes but leaving no extracellular space. In this case the R0 is infinite, which means, that also the circumference of the b-dispersion is infinite. Meat is somehow between these limiting cases. It should be clear that the more insulating cell membranes are present the greater is the b-dispersion. Just measuring at a single low frequency will be sensitive to the membrane integrity, but what is the right frequency? Since each sample of meat behaves differently, it is impossible to find a unique frequency for everything. Hence, the choice of the frequency for conductivity meters is a compromise between the sensitivity what is best at low frequencies and the rejection of electrode effects (electrode polarization) which decreases as the frequency increases. A better approach would be the difference between R0 and R1. Both can be fitted from the locus diagram and thereby rejecting departures from the semicircle due to high and low frequency dispersion effects. This difference is very sensitive to the membrane integrity and therefore to the ability of meat to hold water. Unfortunately this difference depends on the insertion depth of the electrodes as well as on the temperature. If we normalize this difference to the dc-conductivity, we get a parameter which is still sensitive to the membrane integrity but independent of the insertion depth of the electrodes. If the temperature coefficient of the intraand extra cellular electrolyte is equal, it is also independent of the temperature. This parameter is called Py and is calculated as Py ¼
R0 R1 100 R0
The Py ranges for fresh meat between 85 and 95, depending on the kind of meat and declines within several days to small values (Py < 10). Physically, the Py is a
1432
U. Pliquett et al. / Meat Science 65 (2003) 1429–1437
monotonically increasing function of the cell volume fraction surrounded by intact cell membranes (Pliquett, Pliquett, Scho¨berlein, & Freywald, 1995). The passive electrical behavior cannot only presented as impedance, but also as admittance, Y, which is the complex conductance and therefore connected to Z as Y=1/Z. The real part is G and the imaginary part is B (Y=G + jB). The locus diagram is a semicircle as well, but in the first quadrant (Fig. 3). The Py can be found in the Y-plane as well: Y1 Y0 Y1 It is worthwhile to note, that Py is closely connected to the quotient R1/R0 (Oliver et al., 2000) by
Py ¼
Py ¼ 1
R1 : R0
1.2. Measurement of Py A great variety of instruments for impedance measurements exist, usually sophisticated and expensive. They can measure the impedance or admittance as a function of frequency. Then the Cole–Cole function can be fitted in order to find R1 and R0. Another approach is the use of time domain-based measurements. In opposite to the variation of frequencies (frequency domain) only one signal with a broad bandwidth is applied and the response measured. If, for instance, at time t0 a voltage step with the voltage U0 is applied, the capacitors start to charge up. In this moment no voltage drops across the capacitor, i.e. the current, I0, flows through the parallel circuit of Rex and Rin. Than, with charging of the capacitor the current decreases after an exponential function. Finally, if the capacitor is charged, the current through Rin becomes zero and all the remaining current, I1, flows through Rex. The current response to a voltage step is shown in Fig. 4. If we assume only the influence of the ß-dispersion and a length of the voltage step enough to charge the capacitor completely, Py is simply
Fig. 3. Locus diagram of meat in the Y-plane. Note that Y0 is left while Y1 is far from the root, which is due to the fact, that the conductivity increases with frequency.
Fig. 4. Current through meat due to a voltage step of 100 mV at t=0 s.
Py ¼ 1
I1 ; I0
where I0 is the current at t=0 and I1 is the current after infinite time or for practical reason after about 10 times the time constant of the current decay. It was found for meat, that 250 ms is sufficient to meet this requirement. Since Rex is the dc-resistance R0 and the parallel circuit of Rex and Rin is R1, we get simply I1 ¼ U0 =Rex ¼ U0 =R0 and I0 ¼ U0 ðRex þ Rin Þ=ðRex Rin Þ ¼ U0 =R1 : Inserting into the equation for Py we get: Py ¼ 1
I1 U0 =R0 R1 R0 R1 ¼1 ¼1 ¼ I0 U0 =R1 R0 R0
2. Devices for measurement of Py The use in the slaughter house requires devices which are robust, cheap and easy to use. Moreover, an instantaneous measurement is needed. Therefore, the practical achievement is always a compromise between accuracy and robustness. This is acceptable, because the reproducibility of passive electrical behavior even within the same muscle at the same animal is not better than 90–95%. Hence, for further processing of the Py a scale with no more than 100 units is necessary. Based on this requirement we constructed a very simple device using the current response to a voltage square wave. Py was then presented either as a single number by I1 Py ¼ a 1 þb I0
U. Pliquett et al. / Meat Science 65 (2003) 1429–1437
where a and b are the scaling constants. The electrodes were needle shaped (Ø=2.5 mm) with a distance of 2 cm and a length of 4 cm. Several configurations of devices were manufactured by Sigma Electronic Erfurt.
3. Materials and methods Pork and beef of different breeds and cross-breeds were used. The Py measurements utilized a Meatcheck 162 (Sigma Electronic Erfurt, Germany). For the conductivity measurements we used a LF-Star while a pH-Star was employed for the pH measurements. 3.1. Experiment 1 The objective of this experiment was the relationship between pH45 min and Py depending on the time post mortem. The measurements were performed at a total of 52 pig carcasses in a commercial slaughter house. The Py measurement was conducted at varying times between 45 min and 12 h at the thirteenth rib of the M. longissimus dorsi, the same location as for the pH45 min measurement. 3.2. Experiments 2 and 3 Here we focused on the relationship between storageand cooking loss and the Py-parameter. In experiment 2 (133 pig carcasses) and experiment 3 (59 pig carcasses) the Py was measured 24 h post mortem on an excised part of M. longissimus dorsi between the twelfth and the fourteenth thoracic vertebra with and without bones and fat respectively. Moreover, in experiment 2 the Py was measured 5 and 12 h post mortem at the thriteenth rib of the half carcass. For determination of the drip loss a 3 cm thick slice was stored in a foil bag at 4 C for 3 and 6 days, respectively. The relative weight of the juice was than calculated. For the grill loss, a 2.5 cm thick slice was grilled in a contact grill until a core temperature of 70 C and 5 min cooled. The color L* (meat brightness) (Honikel, 1999) was quantified by a Chroma-Meter CR 300 (Minolta, Osaka, Japan) while the shear force was measured using the Warner-Bratzler-method (Honikel, 1999). The intramuscular fat (IMF) was determined by Soxhlet (Arneth, 1999).
1433
the parameters investigated in experiments 2 and 3 the cooking and frying loss was determined. For the cooking loss a cube of m=15 g was placed for 15 min in 150 ml boiling water and afterwards cooled for 15 min. For the frying loss the cube was fried in 170 C fat for 2 min and cooled for another 15 min. The drip loss was determined between the sixteenth and the twenty-third day post mortem on a 2.5 cm thick slice, which was stored at 4 C under vacuum seal.
4. Results and discussion 4.1. Influence of the time post mortem (experiment 1) After slaughtering Py decreases (Fig. 5). The kinetics of Py decrease varies with the kind of meat and its quality. There is no significant difference in Py immediately after slaughtering owing to almost 100% of insulating cell membranes. However, after 45 min the difference between PSE and normal quality appears with a tendency of lower Py for PSE. This early measurement is not recommended for safe distinction since only the carcasses with extreme PSE quality can be detected at this time. The following declension of Py until 140 min is most pronounced for PSE quality. In opposite, carcasses with normal quality show only slight changes within this time. Thus, the differentiation between various quality increases with time. For a clear interpretation of Py in terms of quality, the measurement should not be done before 3 h, better later. This is further supported by the time dependent relationship between Py and pH (Table 1). The correlation at 45 min is with r=0.51 significantly lower than at later times (r=0.73–0.77). This is confirmed by Geisler (1999), where the correlation between Py, drip loss and pH was significant better at 3 h compared with measurements at 45 min post mortem. Other authors, who used the conductivity instead of the Py, also favor the measurement
3.3. Experiment 4 Here we assessed the correlation between the Py and other quality parameters in beef. 168 samples of M. longissimus dorsi between the twelfth and fourteenth thoracic vertebra with and without bones and fat respectively were used for the Py measurement. Besides
Fig. 5. Py in porcine M. longissimus dorsi of different quality depending on time post mortem (for statistics see Table 5).
1434
U. Pliquett et al. / Meat Science 65 (2003) 1429–1437
Table 1 Correlations between Py-value and pH45
min
in pig carcasses
Time of Py-measurement
N
Correlation to pH45 min
45 min 2 h 20 min 5h 12 h
52 52 52 52
0.51 0.78 0.73 0.77
at later times (Brunken & Glodek, 1985; Fortin & Raymond, 1988; Greshake, Schmitten, & Schepers, 1988; Jaud, Weisse, Gehlen, & Fischer, 1992; Mussmann, Ju¨ngst, Tholen, & Schmitten, 1992; Tho¨lking & Brenner, 1990; Warris, Brown, & Adams, 1991; Whitman, Forrest, Morgan, & Okos, 1996). Forrest et al. (2000) who used a special equipment for meat quality based on a tetrapolar conductivity measurement at 1 kHz (Purdue Tetrapolar Porbe, Sheiss, 1998) found a correlation to the drip loss of r=0.5. Unfortunately, only measurements after 30 min were reported. The ratio of the real part of the impedance at dc, R0, and high frequencies, R1 in the M. semimembranosus after 24 and 36 h post mortem shows a correlation to pH45 min of 0.78 and 0.66 (Oliver et al., 2001; Riu, Elvira, Oliver, Gobantes, & Arnau, 2001).
Table 2 Mean value and standard deviation for the experiments 2–4
N Py5 h* Py12 h* Py24 h** Py24 h*** pH45 min pH24 h
Experiment 2 (pork)
Experiment 3 (pork)
Experiment 4 (beef)
Mean
S.D.
Mean
S.D.
Mean
19.9 16.7
59 47.7 44.5 33.1 30.3
15.1 17.0 15.8 21.1
133
31.7 15.6
S.D.
168
6.23 5.54
0.4 0.4
6.24 5.44
0.3 0.1
6.0 4.6
2.0 1.1
5.0
26.1
50.3 31.0
12.4 19.0
5.50
0.1
1.6
1.7 2.3 3.2 3.3
0.9 0.9 1.2 1.1
4.01
27.1 28.3 29.1
4.1 3.0 3.4
Cooking loss% 2nd day
45.0
1.8
4.2. Relation between Py-value and other quality parameters (experiments 2–4)
Frying loss% 2nd day
35.2
1.9
The mean values and standard deviation of the data are shown in Table 2. Concerning pH and the drip loss (second–third day) the carcasses show a great variety in their quality. The drip loss in pork ranges from 1.5 to 11.1%, the pH45 min from 5.4 to 6.8. Beef showed pH24 h values between 5.4 and 6.0 which did not give any hint to DFD-quality. The drip loss varies between 0.3 and 7.2% between 24 and 48 h which points to some relatively dry samples. Compared with pork, beef exhibits the higher Pyvalues. However, due to cutting out the sample from the half carcass and boning, the Py of the M. longissimus dorsi declines significantly, which results from mechanical stress on the cell structure. Before dissection the muscles are fixed at the bones, which supports the tissue mechanically. If this support is lost, the structure collapses partially, changing the geometry of cells and thus the relationship between intra- and extra-cellular pathways. This is supported by investigations by Altmann, Geissler, Scho¨berlein, Pliquett, and Pliquett (2000). Moreover the decline in Py was observed by separation of the whole cutlet row (Geisler, 1999) as well as by cutting out a part of the cutlet between twelfth and fourteenth thoracic vertebra (Schoeberlein, Grimm, & Janetschke, 1990). The difference was between 17 and 20 Py-units. This tendency was confirmed in experiment 3, where the Py12 h,half carcass Py24 h,separated muscle was around 11. In these experiments also the initial values of Py have been
Color L*
Drip loss% 2nd–3rd day 3rd–6th day 2nd–16th day 2nd–23rd day Grill loss% 2nd day 16th day 23rd day
Shear force/kp 2nd day 16th day 23rd day IMF%
50.3
3.0
48.5
3.1
33.1
2.1
4.6
0.9
4.4
1.1
5.9 4.0 3.4
1.1 0.9 0.7
1.2
0.5
1.2
0.4
2.3
1.7
* Measured at the hanging half carcass. ** Measured at the separated muscle with bones and fat. *** Measured at the separated muscle without bones and fat.
smaller than in other experiments. After removal of bones and fat from the M. longissimus dorsi Py further decreases by 10–12 units (Kraa, Geissler, Scharner, Scho¨berlein, & Pliquett, 1997; Schoeberlein, Scharner, Honikel, Altmann, & Pliquett, 1999). The variety in the experiments presented here was higher, so that the Pydecrease by dissection was found between 3 and 20 units. Schwaegele (1993) observed an increase in conductivity due to separation of cutlet and ham around 3.5 mS/cm. The Py measurement between 5 and 24 h post mortem allows a good estimation of the drip loss between the second and third day at the M. longissimus dorsi. Therefore, the measurement yields also reliable results at separated muscles with or without bones and fat. For pork we found correlations for the Py measurement on the carcass at r=0.58 and r=0.56 (Table 3). For the separated M. longissimus dorsi the correlations have
U. Pliquett et al. / Meat Science 65 (2003) 1429–1437 Table 3 Correlations of pH45 min and Py-value with other quality parameters in pork pH45 min Py5 h* Py12 h* Py24 h** Py 24 h*** Experiment 3 N=59 Drip loss 2nd–3rd day Grill loss 2nd day Color L* Shear force IMF
0.58
0.58 0.56
0.68
0.62
0.02 0.46 0.11 0.31
0.05 0.06 0.52 0.46 0.19 0.28 0.21 0.30
0.06 0.51 0.36 0.28
0.16 0.48 0.41 0.28
0.77 0.28
0.68 0.28
0.01 0.52 0.29 0.14
0.02 0.49 0.33 0.11
Experiment 2 N=133 Drip loss 2nd–3rd day 3rd–6th day Grill loss 2nd day Color L* Shear force IMF
* Measured at the hanging half carcass. ** Measured at the separated muscle with bones and fat. *** Measured at the separated muscle without bones and fat.
Table 4 Correlations of pH24 beef
h
and Py-value with other quality parameters in pH24 h
Py24 h**
Py24 h***
168
168 0.07
168 0.09
0.17 0.12 0.16 0.21
0.54 0.40 0.09 0.27
0.53 0.39 0.20 0.30
0.12 0.11 0.11
0.19 0.06 0.10
0.08 0.11 0.05
0.06
0.02
0.01
0.08
0.17
0.22
0.10
0.19
0.07
Shear force difference 2nd day 16th day 23rd day 2nd–23rd day
0.23 0.20 0.37 0.00
0.28 0.14 0.16 0.23
0.42 0.27 0.25 0.36
IMF
0.00
0.03
0.12
N pH24 h Drip loss 2nd–3rd day 3rd–6th day 2nd–16th day 2nd–23rd day Grill loss 2nd day 16th day 23rd day Cooking loss 2nd day Frying loss 2nd day Color L*
** Measured at the separated muscle with bones and fat. *** Measured at the separated muscle without bones and fat.
1435
been between 0.62 and 0.77 while beef gave values between 0.53 and 0.54 (Table 4). Schoeberlein et al. (1999) found for separated muscles higher correlations than for carcasses as well. The Py of dissected muscles assesses the fraction of cell membranes becoming permeable due to accelerated glycolysis and mechanical stress. That’s why these correlations are higher. Others (Byrne, Troy, & Buckley, 2000) reported at separated muscles from beef a correlation of 0.45 between the drip loss and Py. The drip loss after longer storage becomes less correlated to the Py measured at 24 h post mortem. For instance, the drip loss between the second and third day at pork correlates to the drip loss after longer storage (until 23 days) with r=0.36. This correlation is higher for cattle, between 0.47 and 0.65 (data not shown). Here we used samples with different packing in order to assess the influence of initial quality to the changes during storage. Obviously, besides the condition of the cell membranes after 24 h, there are additional factors influencing the weight loss during storage. A proposition of weight loss due to cooking using the Py at 24 h is impossible for pork as well as for beef, because of insignificant correlations. This was confirmed by Byrne et al. (2000) too. An explanation is the total destruction of the cell membranes due to heating, which levels the differences detected by Py or drip loss at uncooked meat off. Moreover the cooking loss shows much less variety than the drip loss. The correlation to the color parameter L* (meat brightness) was for pork 0.46 to 0.52. Lighter meat has a tendency to PSE-quality. As confirmed by Schoeberlein et al. (1999) and Byrne et al. (2000), the correlation to the shear force is positive but at a low level ( 0.20). The pH45 min is often used for PSE-detection in pork. Its correlation to drip loss and L* is similar or less compared with the Py. Because meat quality is also influenced by conditions of cooling a measurement at a later phase (after several hours) allows a better assessment of the meat quality than by measurements of pH 45 min post mortem. The pH24 h in beef correlates much less to the drip loss after 6 days (r=0.17 to 0.21) than the Py. A similar level was found for other traits. Because in our experiments no samples with an pH24 h > 6 were found, its use for prediction of the drip loss is limited. However, DFD-meat cannot be detected by means of Py alone (Schoeberlein, 1995; Warris, Brown, & Adams, 1996). Hence, we recommend the combination of Py and pH for assessment of meat quality. 4.3. Py limits for quality assessment in pork Based on the drip loss between the second and the third day, the material was divided into three qualities: PSE,
1436
U. Pliquett et al. / Meat Science 65 (2003) 1429–1437
Table 5 Py-value of the M. longissimus dorsi in pork with different drip loss Drip loss <4% (normal) N Py5 h* Py12 h* Py24 h** Py24 h***
18 18 44 44
Mean a
57.9 55.7a 51.4a 40.4a
S.D. 10.9 13.3 11.6 18.2
Drip loss 4–6% (indifferent) N 20 20 59 59
Mean a
50.6 48.3a 39.8b 24.1b
S.D. 11.1 13.4 14.2 16.6
Drip loss >6% (PSE) N 21 21 89 89
Mean b
36.2 31.2b 17.6c 7.4c
S.D. 14.2 14.1 10.9 9.5
Table 7 Correlations of Py-value and conductivity 24 h post mortem with pH45 min
Py Conductivity
N
M. longissimus dorsi
M. semimembranosus
133 133
0.59 0.61
0.44 0.32
indifferent and normal. The Py for each group is listed in Table 5. Between the groups highly significant differences appear. Using the frequency of occurrence we defined the Py-limits for the evaluation of the M. longissimus dorsi. The significance of the classification is given in Table 6. For instance the right evaluation of carcasses with normal quality was 61.1% by using the Py after 5 h while PSE classification was right at 71.4% of all cases. A more significant classification was possible at the separated muscle (24 h post mortem) with 79.5 and 78.6%, respectively. The limits depend on the time of measurement and the degree of dissection. They are valid for M. longissimus dorsi in the region of the twelfth–fourteenth thoracic vertebra. Caudally or cranially displaced measurements showed higher Py values (Geisler, 1999; Kraa et al., 1997). An extrapolation to other muscles is not recommended or possible. Schoeberlein et al. (1999) reported limits for the Py measurement on half carcasses at 24 h post mortem. The Py limit for PSE was < 30 and for normal quality > 50. Oliver et al. (2001) evaluated the quality of ham with the quotient of R0/R1 36 h post mortem. They achieved a fraction of right classification of 88.5% for normal quality and 81.8% for PSE.
pH measurement allows an early evaluation at the time of carcass classification. Changes due to the following cooling are not assessed. Often the pH measurement is poorly done. Sources of artifacts are a wrong or not performed adjustment, an insufficient measurement time and a too short insertion of the pH-probe. If the M. longissimus dorsi is not reached, but just the preliminary M. multifidus, than the results are by 0.1–0.2 pH units higher. pH-devices need frequent maintenance and are fragile due to the glass corpus. The use of ISFETdevices (ion sensitive field effect transistor) is promising, but their lack in stability limits practical application. The measurement of Py or other electrical parameters like conductivity makes sense only at later times after carcass classification. The measurement time is short compared with the pH measurement. The metal electrodes itself are robust and do not need much attention. A calibration like for the pH devices is not necessary. However, the electrical measurements are not suitable for the detection of DFD quality. This requires additional pH measurement after 24 h. The advantages of the Py-measurement (or the factor R0/R1 as well) over the conductivity are mentioned at the beginning. Py and conductivity correlate in some experiments with r=0.8. The relationship of both methods to pH45 min is shown in Table 7. While the correlation (Py/ pH, conductivity/pH) for the M. longissimus dorsi differs insignificantly, the assessment of the Py/pH is better performed at the M. semimembranosus. Byrne et al. (2000) reported higher significance of the Py value too.
4.4. Comparison between Py value and established fast methods for assessment of meat quality
5. Conclusions
Today the pH45 min and the conductivity are the best established methods for the detection of PSE meat. The
The Py is a suitable parameter for the assessment of PSE quality. It correlates well to the pH and the drip
Significant differences at mean values signed with different letters (a=0.001). * Measured at the hanging half carcass. ** Measured at the separated muscle with bones and fat. *** Measured at the separated muscle without bones and fat.
Table 6 Py-limits and percentage of right classification for PSE, indifferent, and normal quality in pork Time post mortem 5h 12 h 24 h 24 h
Degree of dissection
Half carcass Half carcass Separated muscle with bones and fat Separated muscle without bones and fat
Limits
Percentage of right classification
Normal
Indifferent
PSE
Normal
Indifferent
PSE
>57 >55 >45
40–57 35–55 25–45
<40 <35 <25
61.1 66.6 79.5
40.0 40.0 44.1
71.4 61.9 78.6
>35
15–35
<15
65.9
35.6
85.4
U. Pliquett et al. / Meat Science 65 (2003) 1429–1437
loss. Measurements should be done 3 h post mortem, or better even later. Because of its simple handling it is well suited for routine measurements. A defined time post mortem and the degree of dissection are necessary for comparative proposition. The measurement on the carcass yields the fraction of intact cell membranes which changes with proceeding glycolysis. For a proper detection of PSE quality at pork measurements should be done on the carcass because dissected muscles are additionally influenced by mechanical stress. Measurements on carcass cuts are suitable in the meat industry for controlling the quality of the raw material or for classification and sorting of meat prior to further processing. PSE quality in beef can be found rarely. However, Py is highly correlated to drip loss until 3 days post mortem also in the range of normal quality. Therefore, Py is useful to evaluate the drip loss in beef too.
References Altmann, M., Geissler, A., Schoeberlein, L., Pliquett, U., & Pliquett, F. (2000). Einfluß von Transport und Zerlegung auf die Fleischstruktur. Fleischwirtschaft, 80, 94–98. Arneth, W. (1999). Repra¨sentative probenentnahme fu¨r die chemische analyse. In Analytik bei Fleisch (pp. 13–24). Kulmbach, Germany: Bundesanstalt fu¨r Fleischforschung. Brunken, H., & Glodek, P. (1985). Untersuchungen zur Fleischbeschaffenheit im Hinblick auf die Leitfa¨higkeit und Halothanreaktion von Schweinen der Deutschen Landrasse (DL). Zu¨chtungskunde, 57, 357–361. Byrne, C. E., Troy, D. J., & Buckley, D. J. (2000). Postmortem changes in muscle electrical properties of bovine M. longissimus dorsi and their relationship to meat quality attributes and pH fall. Meat Science, 54, 23–34. Forrest, J. C., Morgan, M. T., Borggaard, C., Rasmussen, A. J., Jespersen, B. L., & Andersen, J. R. (2000). Development of technology for the early post mortem prediction of water holding capacity and drip loss in fresh pork. Meat Science, 55, 115–122. Fortin, A., & Raymond, D. P. (1988). The use of the electrical characteristics of muscle for the objective detection of PSE and DFD in pork carcasses under commercial conditions. Canadian Institute Food Science and Technology Journal, 21, 260–265. Geissler, A. (1999). Beurteilung der Fleischqualita¨t von Schweinen durch passiv elektrische Gewebseigenschaften. Thesis, Leipzig. Greshake, F., Schmitten, F., & Schepers, K. H. (1988). Untersuchungen zur Bestimmung des Tropfsaftverlustes von Schweinefleisch und dessen Zusammenha¨nge zu anderen Fleischqualita¨tskriterien. Fleischwirtschaft, 68, 765–769. Honikel, K. O. (1999). Standardisierung physikalischer Messverfah-
1437
ren. In Analytik bei Fleisch (pp. 167–192). Kulmbach, Germany: Bundesanstalt fu¨r Fleischforschung. Jaud, D., Weisse, K., Gehlen, K. H., & Fischer, A. (1992). pH-Wert und Leitfa¨higkeit—vergleichende Messungen an Schweineschlachtko¨rpern und ihre Beziehung zum Tropfsaftverlust. Fleischwirtschaft, 72, 252–259. Kraa, S., Geissler, A., Scharner, E., Schoeberlein, L. & Pliquett, F. (1997). Vera¨nderungen des passiv elektrischen Verhaltens von Schweinemuskeln post mortem. In 38. Arbeitstagung der Dt. Vet.Med. Ges. (pp. 557–562). Garmisch-Partenkirchen, Teil 1. Mussmann, T., Ju¨ngst, H., Tholen, E., & Schmitten, F. (1992). Untersuchungen zur Ermittlung der Fleischbeschaffenheit beim Schwein an verschiedenen Meßstellen und zu verschiedenen Meßzeitpunkten. Weihenstephan: Vortragstagung DGfZ/GfT. Oliver, M. A., Gobantes, I., Arnau, J., Elvira, J., Riu, P., Grebol, N., & Monfort, J. M. (2001). Evaluation of the electrical impedance spectroscopy (EIS) equipment for ham meat quality selection. Meat Science, 58, 305–312. Pliquett, U., & Pliquett, F. (1998). Kritische Bemerkungen zur Leitfa¨higkeit als Qualita¨tsmerkmal fu¨r Fleisch. Fleischwirtschaft, 78, 1010–1012. Pliquett, F., Pliquett, U., Schoberlein, L., & Freywald, K. H. (1995). Impedanzmessungen zur Charakterisierung der Fleischbeschaffenheit. Fleischwirtschaft, 75, 496–498. Riu, P., Elvira, J., Oliver, M. A., Gobantes, I. & Arnau, J. (2001). Insitu assessment of the technical quality of meat. In XI. ICEBI (pp. 619–622). Oslo. Schoeberlein, L. (1995). Unpublished material. Schoeberlein, L., Grimm, S., & Janetschke, P. (1990). Untersuchungen zur Bestimmung der Muskelfleischqualita¨t beim Schwein in der Produktion. In 21. Jahrestagung Prophylaxe und Beka¨mpfung von gesundheitlichen Sto¨rfaktoren in der Tierproduktion (pp. 553–560), Leipzig. Schoeberlein, L., Scharner, E., Honikel, K. O., Altmann, M., & Pliquett, F. (1999). Der Py-Wert als Fleischqualita¨tsmerkmal. Fleischwirtschaft, 79, 116–120. Schwaegele, F. (1993). Qualita¨tsmerkmale - Erfassung nach dem Schlachten. Fleischwirtschaft(73), 228–238. Schwan, H. P. (1963). Physical techniques in biological research, 6. In W. L. Nastuk (Ed.), Determination of biological impedances (pp. 323–407). New York: Academic Press. Sheiss, E. B. (1998). Evaluation of tretapolar impedance measurement for the detection of quality abnormalities in pork. MS thesis, Purdue University, West Lafayette, IN 47907, USA. Thoelking, L., & Brenner, K. V. (1990). Vergleich der Aussagekraft von Methoden zur Messung der Fleischbeschaffenheit bei Schweinen am Schlachtband. Schweinezucht u. Schweinemast, 38, 11–14. Warris, P. D., Brown, S. N., & Adams, S. J. M. (1991). Use of the Tecpro pork quality meter for assessing meat quality on the slaughterline. Meat Science, 30, 147–156. Whitman, T. A., Forrest, J. C., Morgan, M. T., & Okos, M. R. (1996). Electrical measurement for detecting early postmortem changes in porcine muscle. J. Anim. Sci., 74, 80–90.