Use of electrical conductivity to predict water-holding capacity in post-rigor pork

Transcription

1 Meat Science 55 (2000) 385±389 Use of electrical conductivity to predict water-holding capacity in post-rigor pork S. Lee a, J.M. Norman b, S. Gunasekaran b, R.L.J.M. van Laack c, B.C. Kim a, *, R.G. Kau man b a Department of Animal Science, Korea University, Seoul, , South Korea b Muscle Biology Laboratory, University of Wisconsin-Madison, Madison, WI 53706, USA c Department of Food Science, University of Tennessee, Knoxville, TN 37901, USA Received 29 June 1999; received in revised form 4 October 1999; accepted 16 November 1999 Abstract This study was designed to re-examine the interrelationships among ultimate ph (ph u ), electrical conductivity (EC) and waterholding capacity (WHC), and to test if EC, using one of two EC instruments, and ph u can be used to predict WHC. The % drip (PD) was used as measure of WHC. Forty-seven pork loins were classi ed either as PSE (pale, soft, exudative), RSE (reddish-pink, soft, exudative), RFN (reddish-pink, rm, non-exudative) or DFD (dark, rm, dry), based on L* values, PD and ph u. The measurement of EC, conducted using the instrument developed at the University of Wisconsin-Madison (UW-EC) was highly correlated to PD at 24 h post mortem (CD=66%), but the ph (NWK) measure of EC was not as highly related to WHC as UW-EC. The predictive value of ph u and EC for WHC were similar. When 47 samples were divided into three di erent groups, such as low (< 2% drip), medium (2±6% drip) and high (> 6% drip) drip loss, UW-EC alone grouped 80% of the samples correctly. Thus, EC seemed to be an accurate predictor of WHC in pork muscle when measured at 24 h post mortem. It is conceivable that EC can be used independently, or with even better success in combination with ph u, to classify WHC of pork carcasses. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Pork quality; Water-holding capacity (WHC); Electrical conductivity (EC); ph u ; PSE; RSE 1. Introduction In producing desirable quality pork, the aim is to provide a lean product with a consistent, acceptable quality, desirable economic value and maximum safety. The quality of pork is de ned by consumers as being attractive in appearance and possessing satisfying palatability. The three important parameters used to de ne pork quality are surface exudate, color, and ultimate ph (ph u ) of post-rigor muscles. These parameters are related to drip loss, attractiveness, shelf-life, shrinkage during storage, water holding capacity (WHC) during processing, and palatability (Joo, Kau man, Kim & Kim, 1995; Kau man et al., 1993; Kau man & Warner, 1993; Van Laack & Smulders, 1992; Warriss, Brown, Lopez-Bote, Bevis & Adams, 1989). * Corresponding author. Tel.: ; fax: address: meat@kuccnx.korea.ac.kr (B.C. Kim). Pork quality has been divided into four categories PSE (pale, soft, exudative), RSE (reddish-pink, soft and exudative), RFN (reddish-pink, rm and non-exudative) and DFD (dark, rm and dry) (Kau man, Cassens, Scherer & Mecker, 1992 ; Warner, 1994). One of the major limitations to predict the pork quality has been the presence of RSE pork. Since the color is similar to RFN pork, most chemical or physical procedures have failed to di erentiate RSE from RFN. Pork plants must be able to di erentiate among the variations of pork quality accurately, quickly and economically according to WHC for sorting the carcasses on quality as they do for weight, composition and wholesomeness prior to processing or merchandising (Kau man et al., 1993). It is likely that when the classes of PSE, RSE, RFN and DFD are accurately identi ed for all pork carcasses, quality variations can be either eliminated or minimized. Since 1980, the electrical properties of muscle have been investigated to determine or predict pork quality /00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S (99)

2 386 S. Lee et al. / Meat Science 55 (2000) 385±389 (Feldhusen, Neuman & Wenzel, 1987; PfuÈ tzner, Fialik, Krause, Kleibel & Hopperwieser, 1981; Schmitten, Schepers, Jungst, Reul & Festerling, 1984; Swatland, 1980). Schmitten et al. (1984) reported that conductivity measurement was better than ph measurement to determine pork quality, regardless whether the measurements were made on the slaughter line or on the chilled carcass at 24 h post mortem. Electrical conductivity (EC) measurement may be an accurate method for di erentiating the variation of pork quality; compared to RFN and DFD, PSE (and RSE) pork contains signi cantly more `free' uid which leaks out during postmortem storage. The uid contains charged compounds that should enhance conductivity of an electrical current (Schmitten, Schepers & Festerling, 1987). Thus, it seems likely that, compared to RFN, PSE and RSE pork will be less resistant to a current, and DFD muscle will be more resistant. According to Schmitten et al., (1987), measurement of EC at 40 min post mortem was e ective in di erentiation of meat quality on the slaughter line. In addition, measurement of EC at 24 h post mortem was a suitable tool for quality control in cold carcasses and in meat processing (Schmitten et al., 1987). However, assessment of pork quality with EC may yield variations depending on the type of EC instruments (Feldhusen et al., 1987). The objectives of the current study were to re-examine the relationship between EC and WHC, and to test the additive e ects of ph u in prediction of WHC as well as in classi cation of pork into four categories (PSE, RSE, RFN and DFD). The possibility to select RSE with EC was also investigated, and two di erent types of EC instruments were compared. 2. Materials and methods 2.1. Samples At 24 h post mortem, 47 pork loins were selected from a commercial pork plant. Loins were selected using a visual assessment of color and exudate, followed by measurement of ultimate ph (ph u ) using a NWK ph meter (NWK-THIEN GmbH, Model ph* K21, Landsberg, Germany). Loins were deboned and sampled for quality analysis. Based on L* values, % drip (PD) and ph u, samples were classi ed either as PSE (N=13), RSE (N=14), RFN (N=13), or DFD (N=7) (Kau man et al., 1992; Warner, 1994) Physical measurements of meat quality Muscle ph u was measured in triplicate. L* values were measured in triplicate on a freshly cut surface after 10 min bloom time using a Minolta Chromameter 200b (0 viewing angle, di use illumination and 8 mm optical port, standardization using four color tiles and set to `auto-select'). The WHC, based on PD, was measured on a size-standardized sample (3 cm thick and 4 cm in diameter) and expressed as the weight loss over 48 h at 4 C, during suspension of a sample (50 g) in an in ated plastic bag (Honikel, Kim, Hamm & Roncales, 1986) Electrical conductivity The two instruments used to measure EC of the muscle at 24 h post mortem were the NWK LT K21 (NWK- THIEN GmbH, Landsberg, Germany) with a frequency of 4 khz, and a probe-type device developed at the University of Wisconsin-Madison (UW). The probetype device was attached to a digital impedance meter (Model 252, Tegam Inc., Geneva, OH44041, USA) that permits rapid measurement of inductance (L), capacitance (C), resistance (R), conductance (G) and dissipation factor (D), at a test frequency of 1 khz ( 1%). The probe combined two needles, insulated by sleeves to ensure that the surface area of the electrodes in contact with muscle tissue was constant for every measurement. Measurement of EC was conducted using the two instruments inserted to a depth of 2.5 cm Statistical analysis Appropriate statistical procedures including correlations, coe cients of determination (CD=r 2 100), regressions, logistic discriminate analysis (LDA) and ANOVA of SAS (1985) were used to determine the e ectiveness of the measurements to predict WHC and quality classes. Di erences were de ned as signi cant at P Results The characteristics of the four classes of pork loins are presented in Table 1. Signi cant di erences were found in PD, L* values and ph u (P<0.05) for the four quality classes. This was expected, as their class identity was de ned by these traits. Also, samples in each quality class were di erent in UW-EC (P<0.05), whereas NWK-EC of PSE and RSE meat was not signi cantly di erent from that of RFN (P<0.05). In Table 2, the relationship between PD and the various independent predictors of WHC, when PSE and DFD samples were either included or excluded is shown. The NWK measure of EC was not as highly related to WHC as UW EC, whereas ph u was highly related to WHC (CD=76%). However, when DFD and PSE samples were excluded, CD with UW-EC (58%) was higher than with ph u (47%).

3 S. Lee et al. / Meat Science 55 (2000) 385± Table 1 Characteristics of four pork quality classes a Electrical properties Quality class b N % Drip L* ph u UW-EC, ms/cm c NWK-EC, ms/cm PSE a 1.9 d 52.8a a a a 1.2 RSE b b b a a 1.6 RFN c c b b ab 1.4 DFD 7 1.3d d c c b 0.7 a Means within a column having di erent letters were signi cantly di erent (P<0.05). b PSE, pale, soft, exudative; RSE, reddish-pink, soft, exudative; RFN=reddish-pink, rm, non-exudative; DFD, dark, rm, dry. c ms/cm, millisiemens per centimeter. d Mean standard deviation. Table 2 Coe cients of determinations (r 2 100) for % drip loss (PD) with other measurements Electrical conductivity UW-EC NWK-EC ph u L* Total (N=47) 66 a PSE,RSE and RFN (N=40) b RSE and RFN (N=27) b a All coe cient of determination values not identi ed with superscripts are P<0.05. b Statistically non-signi cant. The UW-EC was highly correlated to PD (CD=66%), and UW-EC still showed high correlation (CD=63%) when DFD samples (N=7) were excluded from the analysis. The CD remained high (CD=58%) when PSE and DFD samples (N=20) were excluded from the analysis to prove the e ect of EC on di erentiating RSE from RFN. When EC was measured at 4 and 7 days post mortem, the correlation decreased signi cantly (CD=45%, 15% for 4 and 7 days, respectively, unpublished data). Figs. 1±4 are graphic representations of the closeness of t for the linear relationship between WHC and UW-EC, and the curvilinear correlation between WHC and ph u. The simple regressions are also shown (Fig. 1±4). The linear relationship between WHC and UW-EC was signi cant (P<0.05). When ph u and UW-EC were combined to predict WHC [% drip (PD)=229.6+(0.4mS/ cm) (73.1pH u )+(5.8pH u 2 )], the CD increased from 66 to 84%, suggesting an additive e ect. When UW-EC in combination with ph u were subjected to LDA (logistic discriminate analysis) to predict the four classes of pork using the same data, 70% of the samples were correctly classi ed. When the 47 samples were categorized into three di erent groups such as low (< 2% drip), medium (2±6% drip) and high (>6% drip) drip loss, UW-EC alone grouped 80% of the samples correctly. Fig. 1. Relation of water-holding capacity to electrical conductivity of porcine musculature: PSE (N=13), RSE (N=14), RFN (N=13), and DFD (N=7). Fig. 2. Relation of water-holding capacity to electrical conductivity of porcine musculature: RSE (N=14) and RFN (N=13).

4 388 S. Lee et al. / Meat Science 55 (2000) 385±389 Fig. 3. Relation of ph u to water-holding capacity of porcine musculature: PSE (N=13), RSE (N=14), RFN (N=13), and DFD (N=7). Fig. 4. Relation of ph u to water-holding capacity of porcine musculature: RSE (N=14) and RFN (N=13). 4. Discussion Although the existence of pork which is acceptable in color but has high drip loss has been previously reported (Briskey, 1964; Warris & Brown, 1987), the RSE is a quality class which has been described only recently (Kau man et al., 1992). The closeness of the relationship between color and WHC (% drip) of pork has been questioned (Warris & Brown, 1987), and color is not always a reliable indicator of WHC in pork (Van Laack, Kau man, Sybesma, Smulders, Eikelenboom & Pinheiro, 1994). Because PSE and RSE pork may be less resistant to a current, whereas DFD musculature may be more resistant than RFN, the measurement of electrical conductivity (EC) may be useful for e ective differentiation of RSE from RFN. However, no data have been reported on EC and interrelationship between EC and WHC for di erentiation of RSE from RFN. In the present study, although it strengthens the case for EC in predicting WHC when PSE and DFD are included, we have attempted to exclude PSE and DFD samples from the analysis to prove the possibility of di erentiating RSE from RFN. The results showed that the correlations between UW EC and WHC still remained high (CD=58%). Thus, it seems that EC may be a good predictor of WHC. Schmitten et al. (1987) concluded that measurement of EC at 40 min post mortem was an e ective tool for di erentiation of pork quality on the slaughter line, and suggested that EC at 24 h post mortem was suitable for quality control in cold carcasses and in meat processing. SchwaÈ gele (1991) also reported that EC measurement at 24 h post mortem was considered to be a good indicator of WHC in pork. It is possible that di erent factors at which the EC measurements are taken can in uence objective values, and the variation in the muscle temperature may in uence the EC value. In this experiment, the EC measured at 24 h post mortem served as an accurate predictor for WHC. Nevertheless the EC measurement at 24 h post mortem can be in uenced by several factors such as chilling and transport conditions (Schmitten et al., 1987). The muscle temperature at 24 h post mortem remained fairly constant compared to early postmortem stage, indicating that EC values might be more precise and reliable for determination of pork quality. Instruments for EC measurement have been developed since 1980s. The Meat Structure Tester (MS-tester) did not give a precise di erentiation of pork quality (Schmitten et al., 1984). Schmitten et al. (1987) used the Konduktometer LF DIGI 550 and the new model LF 191, operating at a frequency of 4 and 1 khz, respectively. They observed that there were constant di erences in the EC measurements between the two instruments due to di erences in frequency. By modifying this instrument, LF 191 with low frequency (1 khz), UW-EC was developed and operated in the present experiment. It was more precise to predict the pork quality than the NWK instrument (4 khz). Furthermore, UW-EC was able to di erentiate RSE from RFN, whereas NWK was not. These results suggested that frequency of the EC instruments may be an important determinant for accuracy (predictive value) of conductivity. Fischer, Horeth & Dobrowolski, (1997) suggested that PSE could be sorted by EC measurement, but it might be better to classify pork quality, if EC combined with color measurement. Color measurement may be useful in sorting PSE and RFN but not in sorting RSE and RFN. The results of our study indicate that EC measurement in combination with ph u may be successful to classify RSE and RFN. With current packing plant technology and organization, it would be

5 S. Lee et al. / Meat Science 55 (2000) 385± advantageous to determine traits related to quality immediately after slaughter for the purposes of sorting for further processing and to determine value for payment to farmers before identity is lost. However, it is suggested that EC measured at 24 h post mortem appears to be one of the promising traits to quickly, accurately and economically measure post-rigor quality. Weissmann and Honikel (1999) and Fisher et al. (1997) also concluded that EC seemed to work best at 24 h post mortem. They reported that measuring EC on primals/cuts is less reliable than measuring in carcasses, whereas EC measured on wholesale cuts was a good predictor of WHC in our experiment. When we attempted to measure EC on the exact same muscle in the same anatomical location but in a di erent form (carcass, wholesale cuts and boneless muscle), the results were di erent. The accuracy in predicting groups increased to 94%, when three borderline samples were excluded from the analysis. When UW EC values were < 6 ms, the mean PD was 2.1%, and when UW EC values ranged from 6 to 9 ms, the PD was 8.3%. However, these observations must be guarded until the methods can be tested on an independent group of loins representing a commercial population. It can be concluded that EC may be a reliable predictor of WHC in pork muscle when measured at 24 h post mortem. Because EC measurement can be done quickly, economically, and precisely on a commercial line, it is conceivable that EC can be used independently, or with even better success in combination with ph u to classify quality of pork carcasses, especially RSE and RFN. For the routine application of this technique under practical conditions in the packing plant, further research and technical equipment for data processing combined with carcass grading system are desirable. References Briskey, R. G. (1964). 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