Relationships between Japanese pork color standards and optical properties of pork before and after frozen storage

Food Research International 25 (1992) 21-30

Relationships between Japanese pork color standards and optical properties of pork before and after frozen storage M. Irie Livestock Division, Osaka Agricultural Research Centre, Shakudo, Habikino-Shi, Osaka 583, Japan &

H. J. Swatland* Department of Food Science, University of Guelph, Guelph, Ontario, Canada, NIG 2 WI

Optical properties of pork before and after freezing were examined in relation to Japanese Pork Color Standards (JPCS) and various aspects of meat quality measured after frozen storage. Measurements made with a xenon arc on 2%mm pork chops equilibrated to 22°C 3 days post-mortem were correlated (p I 0.01) with a number of aspects of thawed meat quality made after 14 to 70 weeks of frozen storage; r = 4.78 for JPCS, r = -0.78 for fluid loss during Japanese-style thin slicing, r = Xl.85 for drip loss, r = 0.91 for pH, r = a.97 for water holding capacity (WHC), and r = 0.94 for cooking loss of meat slurry. Scatter coefficients measured with a red laser on 25mm pork chops equilibrated to 22°C 3 days post mortem ranged from a.03 1 to Xl.077 with a mean of Xl.058 and were correlated with JPCS (r = 4.63, p 5 0.05) with pH (r = -0.72, p 2 0.025) with WHC (r = 0.84, p < 0@05) and with cooking loss (r = 0.80, p 5 0.01). For Colormet measurements made on thawed pork after 68 to 70 weeks of frozen storage, the most reliable predictors of JPCS were 700 nm reflectance, L* (r = -0.95 and r = 4.95, respectively, p I 0.01). The refractive index of fluid that dripped from pork slices was inversely related to paleness (r = 4.94 for 700 nm reflectance, p 5 0.01). These results show that it may be possible to use optical measurements on fresh pork to predict its ultimate quality after prolonged frozen storage. Keywords: pork, meat quality, fiber-optics, lasers. color.

INTRODUCTION

(Bendall & Swatland, 1988) so that affected carcasses can be removed from premium shipments. The Japanese Pork Color Standards (JPCS) are the basis of one of the most widely used methods for the subjective assessment of PSE. The JPCS are six plastic disks with a meat-like appearance that were developed using objective calorimetry (Nakai et al., 1975). They are widely used as the references for the subjective sorting of pork to be shipped to Japan. However, objective sorting by optical methods is possible and could provide a superior method of quality control for premium exports, since it would facilitate standardization and automation. Most of the objective methods for detecting PSE involve hand-held devices, but optical probes

Pale, soft, exudative (PSE) pork generally has less appeal to consumers and meat processors than pink pork with a somewhat firm texture and little or no evidence of fluid loss (Bendall & Swatland, 1988). Not only are fluid losses unsightly and wasteful in fresh PSE pork, but they continue during secondary processing, causing serious fluid losses, as well as detracting from product color and structural integrity. Numerous methods have been developed for the detection of PSE pork *To whom correspondence

should be addressed

Food Research International 0963-9969/92/$05.00 0 1992 Canadian Institute of Food Science Technology 21

A4. Irie. H. J. Swatland

22

have also been automated. Since the systems we have at present, both subjective and objective, are far from perfect, there is considerable interest in finding new ways to measure PSE which might be used to enhance existing methods. There are numerous possible methods on which automated sensors for the prediction of pork quality might be based and the potential combinations of light sources, measuring systems and optical geometry have almost no limit. This survey identifies a few possibilities that may be worthy of future investigation in more detail.

equilibrated to an ambient temperature of approximately 22°C. A similar optical lay-out was used with a xenon arc instead of a laser as a light source, as described by Swatland (1991). With the xenon arc, a grating monochromator was used in front of the photometer to enable both spatial and spectral scanning. In other words, it was possible to measure the extent to which different wavelengths of light were transmitted or scattered through the depth of the meat sample. Samples were measured 3 days post mortem after they had equilibrated to an ambient temperature of approximately 22°C.

MATERIALS

Freezing and thawing

AND METHODS

Samples A shipment of eight pork loins was purchased from a commercial abattoir and various optical measurements were made between approximately 20 h and several days post mortem. The loins were selected by abattoir staff to give a range of coloration from pale to dark (from color standards 2 to 4 in the Agriculture Canada method of grading pork color).

The loins were subdivided into quarter lengths and placed in Cryovac bags (tightly wrapped but not vacuum packed) and frozen in a freezer at -10°C. After storage at -10°C (times given below), subsets of samples were thawed in a meat cooler at approximately 4°C. The conditions employed in the present study differ from those found in industry (as discussed later), primarily because of a longer storage time. Unfortunately, there were unexpected delays in getting the research partnership operational after the meat was frozen.

Measurements before freezing Measurements after thawing Internal reflectance was measured with a probe Colormet (Instrumar Engineering Ltd, St. John’s, Newfoundland) at ten points along each loin. Following the suggestion of Conway et al. (1984), internal reflectance measured via optical fibers is called interactance. The optical scatter coefficient of slices of fresh pork was measured with a red laser using the method of Birth et al. (1978) adapted for a fiberoptic scanning sensor (Swatland, 1991). The laser beam was directed at the top of the slice of pork and the spreading pattern of scattered light was measured from beneath the slice with an optical fiber moved by servo motor along a slit in the platform supporting the sample. The meat slices were 25 mm thick and cut in a plane perpendicular to the vertebral column so that muscle fibers crossed the optical axis at approximately 45” (as in the original protocol of Birth et al., 1978). The length of the light path through the sample (from the point of incidence on the upper surface to the center of the measured area on the lower surface) was calculated trigonometrically. Samples were measured 3 days post mortem after samples had

After 14 weeks of storage, samples were thawed and pH was measured with a Chemcadet 5984-50 pH meter (Cole-Parmer, Chicago) in triplicate after sample comminution. For the determination of water holding capacity (WHC) and cooking loss, samples were cornminuted for 1.5 min, between 0 and 5”C, in a bowl cutter (Schneidmaster SMK40, Germany) set for maximum speed. Sodium chloride was added to the meat slurries to extract some of the salt-soluble functional proteins responsible for fluid retention and binding during heat processing. Salt concentration in the aqueous phase was kept constant at an ionic strength (IS) of 0.42. Two tests were used to assess the commercial properties of the meat slurries. Firstly, centrifugation fluid loss was used to obtain a measure of WHC (adapted from Wardlow et al., 1973). Triplicate 10 g samples per treatment were incubated with 16ml NaCl solution (IS = 0.42) for 30 min at 0°C and centrifuged at 7 X 103 g for 15 min. Fluid loss was determined as the proportion of fluid released from the sample. For the second test, cooking loss was determined as the amount

Optical properties of pork before and after freezing

of fluid (fat, water and solutes) released from three 25 g samples during cooking in closed test tubes (26 mm diameter). Temperature was monitored from two thermocouples inserted into the centre of the sample. The rate of heating (0.66”C min-1) was controlled automatically from 20 to 70°C with a Haake PG20 controller (Haake, Germany) connected to a heating coil immersed in a water bath. For spectrophotometry, a double illuminator was used to provide strong light intensity from 380 to 1000 nm. Both a Zeiss 12 V 60 W tungsten filament lamp with a stabilized power supply (Zeiss 487334, Oberkochen, Germany) and a stabilized 75 W xenon short-arc lamp (Zeiss 487333) were focused on one branch of a bifurcated light guide (BLGS1875-36-M, PBL Electra-optics, New London, New Hampshire). The optical fibers of the two branches of the light guide were spliced so that light was emitted from a semicircular area of the common trunk and was collected for measurement by the other semicircle of optical fibers of the common trunk. The common trunk of the light guide had a flat surface (7 mm diameter) in contact with the sample. The light returned from the sample (interactance) passed through a grating monochromator (Zeiss 474321 with 474346) through a stray-light filter (Zeiss 477215) and onto the silicon detector of a radiometer (SEEO15/F and 700A, International Light, Newburyport, MA). Ambient illumination (which was very low) and the dark-field response of the photometer were measured at each wavelength using a solenoid-operated shutter in front of the illuminator. The sample was scanned from 380 to 1000 nm in increments of 10 nm with a 10 nm bandpass. Measurements were made at about 22°C. The optical system was standardized against a newly pressed plate of optical-quality barium sulphate. The height of the common trunk of the light guide over the standard plate was adjusted to give maximum sterance at 550 nm. This wavelength was near to the peak response of the silicon detector and was also used to maximize the dynamic range of the radiometer. After 68 weeks (replicate 1) and 70 weeks (replicate 2) of storage, samples were thawed and graded subjectively by the JPCS method. JPCS standards are numbered from 1 (pale) to 6 (dark). Samples that were intermediate between two standards were given intermediate values (steps of 0.5). For example, a sample between standards 3 and 4 was graded 3.5. For replicate 1, interactance was measured with

23

a probe Colormet and reflectance was measured with a flat-face Color-met. For replicate 2, only reflectance measurements were made. Measurements were made in triplicate and averaged. Measurements from 400 to 700 nm in increments of 10 nm were collected together with immediate results from the instrument display panel. For the first replicate, the flat-face Color-met was used on the major surface of slices of meat (approximately 15 mm thick) and the probe Color-met was used in the interior of chunks of the sample. For the second replicate, flat-face Colormet measurements were made on a stack of 3-mm meat slices. Slicing losses were obtained by weighing samples of longissimus dorsi several centimetres in length, slicing the muscle at a thickness of approximately 3 mm, and reweighing the slices. Fluid losses on slicing were obtained by subtraction of the second weight from the first and were expressed as a percentage of the first weight. Thus, slicing loss included drip loss, evaporation loss, and minute shreds of meat lost on slicing. Drip losses were measured by suspending five or six 3-mm thick slices of pork on a string inside a partly inflated plastic bag (total weight from 42 to 50 g). At intervals of 24 h for 3 days, the fluid that dripped from the meat was removed and measured volumetrically. Drip losses were measured in subsets of samples at 5°C and 22°C approximately, and expressed as percentages of initial sample weights. The refractive index of the fluid that dripped from the meat was measured with an Abbe Refractometer (model B, Zeiss, 7082) at an ambient temperature of approximately 22°C. Analysis of data Several methods were used to obtain an overall assessment of paleness by the integration of transmittance, reflectance and interactance spectra. Spectral data (at 10 nm increments) were integrated using the weighted ordinate method to obtain chromaticity coordinates x and y and %Y (Billmeyer & Saltzman, 198 1). Alternatively, L*, a* and b* values were obtained from the data display panel of Colormet instruments. Interactance spectra obtained via optical fibers differ fundamentally from true reflectance spectra collected above the sample surface according to CIE (Commission International de 1’Eclairage) procedures. There are also minor differences between the optical geometry of the flat-face Col-

24

M. Irie, H. J. Swatland

ormet instrument and CIE standards. It is important to note that weighted ordinate transformations were used here simply as a convenient method of integrating spectra and the results reported here are not the same as CIE values. As a reminder of this point, prefixes are used for transformations of spectra by weighted ordinate methods: ‘i’ for interactance, ‘r’ for reflectance and ‘t’ for transmittance. Spectra were also integrated as a ratio (400/700 nm) or by taking the mean value of all wavelengths. A Hewlett-Packard 9826 microcomputer with a Multiprogrammer was used as a controller for the collection and analysis of data. Statistical tests were adapted from Steel and Torrie (1980) with a one-tail acceptance level for the t-statistic. It was predicted that, in pale pork, short wavelengths would have higher scattering than long wavelengths and vice verm (Swatland, 1989~).

RESULTS AND DISCUSSION Time-temperature interactions When the study was commenced (November 1989), there was little reason to suspect that complete development of paleness would not be complete in pork loins within 20 h post mortem. The loins were ranked subjectively by experienced staff at the abattoir from which they were purchased and the rank order appeared correct when the loins were first unpacked. However, after probe Colormet measurements were made on the boxed loins at approximately 4°C the subjective rank order for paleness began to change. This was confirmed by several observers after samples were cut into chops that had equilibrated to room temperature for laboratory optical measurements. It was too late by this time to remeasure the samples with the probe Colormet since samples had either been frozen or sliced. Pommier et al. (1990) have since reported that complete development of paleness may take several days in veal. Late developing PSE has also been demonstrated in pork from the Hampshire breed (Monin & Sellier, 1985). Although the rank order for probe Colormet measurements was perfectly correlated with the subjective rank order for paleness when the loins were first unpacked, these initial probe Colormet measurements were only weakly correlated with aspects of meat quality measured after frozen storage. In view of this unsuspected problem that

clearly needs further investigation, Colormet data were discarded.

initial probe

Optical measurements after thawing Table 1 shows the correlations of Colormet reflectance and interactance with JPCS when both measurements were made after thawing. For measurements on thick slices and chunks of meat (replicate l), reflectance measured with the flatface Colormet was generally of more use than interactance measured with the probe Colormet for the prediction of JPCS. Flat-face Colormet reflectance measurements made on piles of Japanesestyle pork thin slices were almost as reliable as measurements made on thick slices of meat. The most reliable transformation for both interactance and reflectance was L*. Results for interactance and reflectance at 400 and 700 nm were rather variable. Hence, sometimes the 400/700 nm ratio was very strongly correlated with JPCS quality while at other times it was not. This point, which also has been noted in earlier research (Swatland, 1988), explains why 400/700 nm ratios are scrutinized in the results that follow. Table 1 shows correlations of Colormet reflectance measurements with the refractive index of the fluid that dripped from the pork when sliced. Correlations were mostly negative so that features associated with paleness, such as rL* and reflectance, were negatively related to refractive index. Thus, pale pork tended to produce fluid with a lower refractive index. A similar pattern of correlations existed for fluid that dripped from the meat slices. This supports earlier research using low-angle X-ray diffraction (Swatland et al., 1989) in which Table 1. Simple correlations of transformations of interactance spectra (probe Colormet) and reflectance spectra (flat-face Colormet) with Japanese Pork Color Standards (JPCS) and with refractive index (RI) of drip loss fluid (all measurements made after thawing) JPCS

Colormet

iL* ia* ib* i or i or i or

or rL* or ru* or ib* r 400 nm r 700 nm r 400/700 nm

“p I 0.05;

‘p 5 0.01.

RI

Interactance Reflectance Reflectance Replicate 1 Replicate 1 Replicate 2

Reflectance Replicate 2

-0.89~ 0.23 -0.54 -0.91c -0.85~ 0.13

4.92~ a.53 -0.63 a.790 +94c -0.42

4.96~ 0.47 -0.17 4.97c +94c G97C

-0.930 -0.68” -0.71a -0,77y -0.95c 4.38

25

Optical properties of pork before and after freezing Table 2. Drip losses from sliced pork

Temperature

Day 1

3

2

Percent drip loss 5°C 22°C Refractive

-.88 500

400

Wavelength

600

1. Spectral

.4

400

500 600 700 Wavelength

800 nm

index ((RI

0.78 SD 0.42 0.63 SD 0.27

0.67 SD 0.45 0

- 1.35) X 10000)

700

distribution of correlations of reflectance (flat-face Colormet, A) and interactance (probe Colormet, B) with refractive index of meat fluid. Fig.

5.69 SD 2.20 9.49 SD 2.73

900

1000

Fig. 2. Interactance spectra of thawed pork samples obtained via optical fibers on the meat surface. The range from spectrum A to B shows the range from pale to dark pork.

it was concluded that fluid from between thick and thin myofilaments makes a major contribution to the exudate from pale pork. Thus, as pH declines post mortem, the birefringence of myofibrils increases (Swatland, 1990a) and their transmittance decreases (Swatland, 1990b) so that increased back-scattering (reflectance or interactance) increases concurrently with a reduction in the negative electrostatic repulsion between myofilaments that releases fluid from the meat. One would expect the refractive index of the fluid from between myofilaments to have a lower refractive index than that of sarcoplasm with a high solute content. The only exception to this concordance, were the results of change in refractive index with time (Table 2). From the working hypothesis outlined so far, one would expect refractive index to decrease progressively as more fluid dripped from the meat slices (as high-solute sarcoplasm is flushed out by low-solute interfilament fluid). The changes in refractive index shown in Table 2 are inconclusive so that further investiga-

5°C 22°C

83.4 SD 40.0 86.4 SD 41.5

89.9 SD 42.3 99.2 SD 40.9

56.5 SD 24.0

~

tion on this topic is needed. A counter-hypothesis to the above is that refractive index may start to increase as a result of proteolysis that might be faster in samples with a low pH than in samples with a high pH. As shown in Fig. 1, negative correlations with refractive index were stronger for interactance measurements made with the probe Color-met than for reflectance measurements made with the flat-face Color-met. This finding supports earlier research on the effect of direct ver.sus indirect optical interfaces (Swatland, 19896). Thus, for the probe measurements made within the meat, the medium linking the meat to the optical system was the meat fluid from which the refractive index measurements were made. For the flat-face Colormet, on the other hand, there was a glass window and an air-filled space between the meat surface and the optical system. Hence, correlations of instrument output with refractive index were stronger for the probe Colormet than for the flat-face Colormet. The patterns of irregularities seen in Fig. 1, particularly in line A for reflectance measurements, match the absorbance spectra of myoglobin (Swatland, 1989a). It is not unreasonable for there to be an interaction between myoglobin content and refractive index of meat exudate. However, this is another topic that requires examination in more detail. The percent drip loss of sliced pork is shown in Table 2. Drip losses were greater in total amount and earlier at 22°C than at 5°C. With analysis of variance, the effects of time and temperature were significant (p 5 0.01 and p I 0.05, respectively) and there was a significant interaction of time and temperature (p I 0.01). However, few of the simple correlations of Colormet reflectance parameters or JPCS with fluid loss were significant (Table 3). In other words, although there was a strong correla-

A4 Irie. H. J. Swatland

26

Table 3. Simple correlations of JPCS and flat-face Colormet measurements with fluid loss (all measurements made after thawing) .5 -

Color 0-A - B

-.5

-

300

400

500 600 700 Wavelenath

900 nm

9001000

Fig. 3. Spectral distribution of simple correlation coefficients of interactance (from Fig. 2) with drip loss at 22°C (line A) and with JPCS (line B).

tion of some Color-met reflectance measurements with JPCS (Table l), neither of these methods was reliable for predicting fluid losses. Figure 2 shows interactance spectra obtained with a light guide on the sample surface. These interactance spectra were correlated negatively with JPCS (Fig. 3, line B). A similar but positive pattern of correlations of interactance with drip loss at 22°C was also detected (Fig. 3, line A). However, none of the correlations of interactance with drip loss at 5°C or with slicing loss was significant at the 5% level. These spectra, that extend below and above the range of Colormet instruments, show that there is unlikely to be any advantage in extending the spectral range of the Colormet. Correlations at wavelengths < 400 nm were of no value, while correlations at wavelengths > 700 nm were very similar to those at 700 nm. Optical measurements made before freezing Unfortunately, as outlined earlier, it is not possible to report on Color-met measurements as longterm predictors of meat quality after frozen storage. The other methods examined are more experimental in nature than the commercially available Colormet instruments. Figure 4 shows the pattern of red laser light transmitted and scattered through the pork samples before freezing. Each line represents one sample. The optical axis (0”) is where the laser beam strikes the upper surface of the sample to give a path length of 25 mm through the 25-mm thick sample. In Fig. 4, each line starts 30 mm to one side of the optical axis with the recording optical fiber on the lower surface of the sample. At this off-axis position, the path length through the meat is 39 mm, as indicated by the distances shown on the x-axis of Fig.

JPCS ra* rb* rL* 400 nm 700 nm 4OO/700 nm

Slicing loss -0.36 0.65a 0.47 0.02 -0.19 0.14 -0.46

5°C drip loss -0.01 0.16 0.19 0.07 -0.06 0.15 -0.24

22T drip loss -0.47 0.21 0.36 0.63a 0.52 0.68‘~ -0.20

ap 5 0.05.

4. As the servomotor moved the recording optical fiber towards the optical axis, the path length through the meat decreased and the recording optical fiber became more closely aligned with the optical axis to detect light that had penetrated straight through the meat rather than being scattered sideways. At a path length of 25 mm, the same as the depth of the sample, the recording optical fiber was in the optical axis at 0” (directly in line with the laser beam coming through the sample from the upper surface) and maximum transmittance was measured. Then, as the recording optical fiber moved across to the other side of the sample, the path lengths through the meat increased and less light was captured by the optical fiber. For pork that was relatively dark (A in Fig. 4) more light was transmitted through the meat than for pork that was relatively pale (B in Fig. 4). In other words, transmittance gave the opposite result to interactance (internal reflectance): pale pork had a high reflectance and a low transmittance and vice versa for dark pork. The data in Fig. 4 show considerable anisotropy (hysteresis-like effect) relative to slices of turkey breast muscle with muscle fibers perpendicular to the optical axis (Swatland, 1991). In other words, more light was scattered to one side of the optical axis relative to the other side. In the original protocol of Birth et al. (1978) this phenomenon was missed because only unilateral measurements were made. In the present study, this optical anisotropy (which originated from the uncontrolled orientation of angles of muscle fibers in the optical axis) was cancelled by using a linear regression of log T (T=Transmittance) on path length to calculate scatter coefficients. Scatter coefficients ranged from -0.031 to -0.077 with a mean of -0.058. The laser scatter coefficient was correlated significantly with JPCS (r = -0.63, p I 0.05), with pH (r =

Optical properties of pork before and after freezing Ql

27

,

E

c

m

-2t"""""""""'1 25 20

!I

30 Distance

35

40

mm

Fig. 4. Pattern

of red laser light transmitted and scattered through pork samples ranging from moderately dark (A) to moderately pale (B).

-0.72,

with WHC of meat slurry (r = and with cooking loss of meat slurry (r = 0.80, p I 0.01) but not with slicing loss (r = -0.09), drip 1oss at 5°C (r = -0.02) or drip loss at 22°C (r = 0.62). When white light from a xenon arc rather than a monochromatic laser was directed through the samples, longer wavelengths (up to 700 nm) were transmitted more than shorter wavelengths (down to 400 nm), as shown in the example in Fig. 5. When measurements were made in the optical axis, so that the recording optical fiber was directly under the point of illumination on the upper surface of the sample, transmittance at 700 nm was relatively high. When measurements were made 12 mm to one side of the optical axis with a path length of 28 mm through the meat, transmittance at 700 nm was low. Transmittance of light at 400 nm was similar at both positions. In other words, red light has a high forward transmittance through the meat whereas violet light was scat-

0.84,

0

0

I

0

Mean

Transmittance

I

1

I

5 110

Fig. 6. Transmittance

spectrum of white light through 25-mm thick pork sample (all samples pooled) transformed to show the relationship between mean transmittance (all wavelengths combined) and the ratio transmittances at 400 and 700 nm.

p I 0.023,

p I

0.005)

.

1

2 c, +

E-OiO; 500

Wavelength

600

nm

700

Fig. 5. Example of transmittance spectra of white light through a 25-mm thick pork sample. Line A shows transmittance in the optical axis (0”) with a path length of 25 mm through the sample. Line B shows transmittance through a light path of 28 mm at an angle of 26” to the optical axis. Unlabelled lines are at intermediate positions at 2 mm increments along the underneath of the sample.

tered fairly uniformly through the sample. This was confirmed from the relationship between the 400/700 nm transmittance ratio versus the mean transmittance of all wavelengths combined (Fig. 6). Thus, when mean transmittance was low (either the sample was PSE or the measurement was taken with a long path length to one side of the optical axis), the transmittance of light at 700 nm was low. Conversely, when mean transmittance was high (either the sample was DFD or the measurement was taken with a short path length in the optical axis) the transmittance of both short and long wavelengths was similar and the 400/700 nm ratio was low. Spectra such as those shown in Fig. 5 were integrated in a number of ways (Table 4) to search for relationships with meat quality. With a pooled data set (spectra at all angles pooled for each animal), a number of relationships were detected. As an overall indicator of transmittance, the t%Y transformation was correlated with drip loss at both 5 and 22°C. A similar result was found for transmittance at 700 nm. No significant correlations were detected for transmittance at 400 nm and consequently the 400/700 nm ratio gave no improvement over simple transmittance at 700 nm. Transformations for chromaticity coordinates tx and ty, being affected by transmittance at different wavelengths, bore some relationship to drip loss at 22°C. Notably stronger correlations for some of these relationships were detected when the data set was reduced to contain selected angles. For example, correlations of t”/)Y and transmittance at 700 nm with drip loss at 22°C were strengthened by using only the direct 25 mm light path. Of particular interest, was the fact that this revealed correlations of slicing loss with transmit-

-0.58 0.640 0.10 0.27 -0.61 0.48 0.60

Oblique 28 mm path, 26” to optical axis JPCS 0.60 Slicing loss (%) -0.05 5°C drip loss (%) 0.62 22°C drip loss (%) 0.02 0.32 PH WHC a.57 Cooking loss -0.54

ap I 0.05, bp I 0.025, cp I 041, dp 5 om1.

loss

-0.50 0.670 0.26 0.22 -0.54 0.48 0.34

PH WHC Cooking

Direct 25 mm path, 0” to optical axis 0.47 JPCS Slicing loss (X) 0.03 5°C drip loss (%) a.58 22°C drip loss (%) Xb85d 0.83’ PH WHC -0.776 Cooking loss -0.726

400 nm

0.13 0.03 0.14 0.22 0.240 0.22a 0.19

t% Y

@28b 0.01 -0.32c -0.49d o.sod -0.46d -043d

All path lengths combined JPCS Slicing loss (%) 5°C drip loss (%) 22°C drip loss (%)

Meat quality

0.64a 4.16 -0.03 XI.54 wld -0.97d -0.9Od

0.61 0.00 -0.51 -0.84d 0.87d -0.82c -0.82c

-0.31c 0.01 -0.24a -0.43d 0.45d -o&V -043d

700 nm

-0.78c 0.40 0.05 0.51 -0.88d 0.89d 0.89d

0.14 0.17 0.59 -0.83c 0.81~ wad

-0i’lb

4.35d 0.04 0.05 0.31c -043d 044d 0.46d

400/700

0.69“ -0.15 -0.40 +8Oc 0.9od -0.89d -0.84d

0.64a 0.03 -0.17 -0.63a 0.87d

0.34c O-02 -0.16 -@Iid 0.51d -@5od -@51d

tx

0.33 -0.78c 0.33 0.39 -0.25 0.21 0.23

444 XI.12 -0.22 0.19 -044 0.49 0.65a

-0.276 -0.11 0.10 0.33d 4b39d 0.39d 0.43d

tY

Table 4. Correlations of transmittance (T) with meat quality when measured at different path lengths and angles

0.63a 0.10 -0.21 -0.746 0.86d a+v’ -0.85d

0.56 0.01 -0.53 Xb84d 0.87d Xb81c -0.8Oc

0.31c 0.01 -0.286 -047d 0.48d Ab46d +45d

Mean

T

Optical properties of pork before and after freezing

tance at 400 nm and chromaticity coordinate y. Mean transmittance (all wavelengths combined) was similar to t%Y for all path lengths combined and for the direct 25 mm path, but was superior for the oblique 28 mm path. Implications Background investigations in Japan revealed that Canadian pork is normally stored for 3-6 months. This estimate includes a 4-5 week journey from eastern Canada or a 2-3 week journey from western Canada. A typical procedure for thawing pork in meat processing operations in Japan is to place the frozen loins (that have equilibrated to -10°C during transport to the packer) directly into a water bath at approximately 16°C. The water is stirred and changed once or twice a day. The length of time for thawing to a temperature of 2-5°C ranges from 12 to 24 h, with an average of about 18 h. The temperature of the pork is approximately 5°C for retailing or further processing. For pork that is cut for retail sale as fresh meat, thawing is usually done by placing primal cuts in a meat cooler. By law, pork must be handled at a temperature of less than 10°C. About 20-30% of Canadian pork is used for further processing, while the remainder is consumed as fresh meat, much of which is sliced at a thickness of 1-15 mm just before it becomes soft during thaw: ing. Pork that is presented for retail display in Japan is very uniform in color and the range from PSE to dark, firm, dry (DFD) pork that is often seen in Canadian retail displays would be considered unacceptable in Japan. The overall objective of this research is to contribute to the development of optical sensors with which measurements made in Canada might be used to predict the quality of pork exported to Japan. The survey reported here casts a wide net to search for relationships which might be exploited to develop or improve appropriate sensors. Thus, the results that are presented here are not intended as definitive proofs but rather as signposts pointing the direction for further investigation. The slicing properties of pork are extremely important for Japanese cuisine, yet this is a subject about which very little is known in Canada. None of the established methods of color assessment such as JPCS or Colormet reflectance was of any value in predicting fluid losses that occurred during slicing, even though the slicing measurements were made almost immediately after the color

29

measurements. However, as shown in column 2 of Table 4, there were weak correlations of transmittance at 400 nm with slicing losses measured over a year later. These results could be spurious or the first glimpse of a biophysical relationship of considerable commercial significance. The data in Fig. 6 may be of value in explaining why 400/700 nm ratios are sometimes strong indicators of pork paleness while, at other times, they are useless. Remember that Fig. 6 is for transmittance, which tends to give the opposite result to reflectance and interactance. For PSE pork with a low transmittance, the 400/700 nm ratio is a strong indicator of paleness because there is a large change in the 400/700 nm ratio for a small difference in mean transmittance or paleness. For DFD pork with a high transmittance, on the other hand, the ratio is a weak indicator because it changes very little for a big difference in paleness. A similar result (not shown) was obtained by comparing the 400/700 nm ratio with t%Y. In conclusion, there is tentative evidence that optical properties may be used to predict commercially important properties of pork after a prolonged period of storage. Both spatial and spectral scanning methods may be of value. Further investigations are needed to study time and temperature interactions when predictive measurements are made, refractive index changes in meat fluids and effects caused by the orientation of muscle fibers. ACKNOWLEDGEMENTS The authors thank the Governor of Osaka for allowing M. Irie to travel to Canada and Dr J. P. Mahone for providing funding for transportation and health insurance. Some of the meat quality data reported here were kindly provided by Dr Shai Barbut. REFERENCES Billmeyer, F. W. & Technology. John Birth, G. S., Davis, scatter coefficient

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