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Iridescence in beef caused by multilayer interference from sarcomere discs
Meat Science 90 (2012) 398–401
Contents lists available at SciVerse ScienceDirect
Meat Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m e a t s c i
Iridescence in beef caused by multilayer interference from sarcomere discs H.J. Swatland ⁎ University of | Guelph, Guelph, Ontario, Canada N1G2W1
a r t i c l e
i n f o
Article history: Received 16 May 2011 Received in revised form 6 August 2011 Accepted 12 August 2011 Keywords: Iridescence Interference Reflectance Sarcomere
a b s t r a c t Microscope photometry of raw and cooked iliocostalis was used to test the hypothesis that interference colours in beef may originate from reflections from sarcomere discs. Evidence in support of the hypothesis was, firstly, that interference colours were not altered by rotating a polarizer in the illumination pathway, or by rotating a polarizer in the measuring pathway. But when both pathways contained polarizers, iridescence was completely extinguished when the polarizers were crossed. Secondly, the reflectance spectra of interference colours all showed multiple interference peaks, with a major peak possibly originating from the top sarcomere and minor peaks originating from lower sarcomeres. Thirdly, major peaks were strongly dependent on the angle of measurement. Iridescence in beef is quite rare, but reflections from sarcomere discs may be a ubiquitous source of light scattering in meat. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction The appearance of meat is important in consumer purchasing (Horowitz, 2006; Kropf, 1993; Mancini & Hunt, 2005) and meat colour is routinely measured using commercial colorimeters such as the Minolta (Brewer, Novakofski, & Freise, 2006; Nollett, 2007). Colorimeters designed for opaque surfaces also respond to subsurface factors in the translucent depth of the meat. For example, Elliott (1967) discovered that pork paleness is affected by muscle fibre orientation. Fibres cut transversely create darker surfaces than fibres cut longitudinally. Meat surfaces have been measured in countless routine studies, but we may be missing some important subsurface effects. Light may be reflected directly from meat surfaces as specular or mirror-like reflectance following Fresnel equations (Wolff, Shafer, & Healey, 1992). Specular reflectance is polarized to some extent, and may be partly extinguished by rotating a polarizer to an appropriate angle. Thus, a polarizer may be useful in quantification of marbling fat using video image analysis, otherwise it is difficult to separate marbling from bright flecks of specular reflectance from surface irregularities (Swatland, 1995). Light entering the meat is scattered. Some of it scatters back to the meat surface to appear as diffuse or Lambertian reflectance. Lambertian reflectance appears similar at all angles, whereas specular reflectance, like a mirror, has a strong angular effect (Wolff et al., 1992). Understanding subsurface effects could improve meat spectrophotometry as well as explaining iridescence on cut surfaces.
⁎ Tel.: + 1 519 821 7513. E-mail address: [email protected]. 0309-1740/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.meatsci.2011.08.006
Iridescence may appear on cut meat surfaces, especially on sections of raw beef semitendinosus (Kukowski, Wulf, Shanks, Page, & Maddock, 2004) and cured beef and pork products (Fulladosa, Serra, Gou, & Arnau, 2009; Lawrence, Hunt, & Kropf, 2002; Realini, Guàrdia, Garriga, Pérez-Juan, & Arnau, 2011). Most consumers ignore iridescence, but it may cause concerns, as indicated by numerous questions posted on the internet. The iridescent colours of meat are interference colours (Swatland, 1984), but what is generating the interference? Many sources have been proposed. Warriss (2000) considered that iridescence on bacon and ham originated from refraction due to the different refractive indices of water and fat at the surface. Dennis (1996) and Mancini (2007) attributed iridescence to diffraction. Iridescence also has been attributed to pigments in the meat (USDA, 2008). Refraction, diffraction and pigments may cause iridescence in feathers and insects, often with interactions between different sources (Berthier, 2007). But there is scope for another possibility. Long before electron microscopy and the discovery of sliding filaments, there were several schools of thought regarding the cause of transverse striations in skeletal muscle fibres (Swatland, 1985). The idea that muscle fibres contain striped longitudinal structures (myofibrils) proved correct, but here we will resurrect an incorrect idea that muscle fibres contain a stack of discs (Bowman, 1840). In modern terms, we may see how early microscopists treated their samples in such a way that weakening of the Z lines allowed fibres to fragment into Bowman's discs centered on the A band (Benda & Guenther, 1895; Schäfer & Thane, 1898). Thus, to explore subsurface optical effects, we will treat the muscle fibre as a stack of alternating discs composed of I bands (low refractive index) and A bands (high refractive index) and search for evidence of boundary reflections causing thin-film interference, as in the iridescent mineral labradorite (Raman & Jayaraman, 1950; Rayleigh, 1923).
H.J. Swatland / Meat Science 90 (2012) 398–401
399
The hypothesis to be tested is based on the following assumptions. 1. Sarcomere discs will reflect incident light at refractive index boundaries (A-band N I-band). 2. Light reflected from lower discs will have a longer light path than light reflected from upper disks. 3. Light waves following long light paths will often be out of phase with waves following short paths. 4. Waves out of phase will produce either destructive or constructive interference. 5. Interference will alter the spectrum of reflected light, thus producing interference colours (iridescence). 2. Materials and methods A Zeiss Universal microscope (Carl Zeiss, D-7082 Oberkochen, Germany) was fitted with accessories identified here by a Zeiss part number. Zeiss documentation is reported by bulletin number. The bulletin for the part numbers is A-41-824-e. Accessories were operated from a type MPC 64 control module (Zeiss 477469) on a bus (IEEE 488). Programming instructions are given in Zeiss bulletin G-41-912-e, and algorithms are given by Swatland (1998). The main controller was a personal computer running HP BASIC (HewlettPackard, Palo Alto, CA). Components were controlled via a HP VXI mainframe (HP E1421B). The photomultiplier was a side-window Hamamatsu (1126 Ichino-Cho, Hamamatsu City, Japan) HTV R 928 with S-20 characteristics operated as described in Zeiss bulletin G-419000.1/ll-e. A grating monochromator (Zeiss 474345) with stray-light filters (Zeiss 477215) was mounted under the photomultiplier and scanned from 400 to 700 nm in steps of 10 nm with a 10 nm bandpass. Illumination was from a 100 W halogen source with a stabilized power supply (HP 6642A) directed through a solenoid shutter and into a vertical illuminator (Zeiss II-C with H-PL-POL beam splitter). The photometer head was a type 03 (Zeiss 477304). The objective was a LD-Epiplan x 8 with numerical aperture 0.2 (no detectable strain birefringence) which provided sufficient depth above the specimen to allow tilting. Interference colours were named from a Michel-Lévy colour chart (Zeiss S41-500.0-e). The chart is named after French geologist Auguste Michel-Lévy (1844–1911) who developed it to aid in the identification of birefringent minerals. Standardization of reflectance microscopy for diffuse samples is a notorious problem (Piller, 1977). Here the system was standardized on Teflon tape (polytetrafluoroethylene; Weidner & Hsia, 1981) at 50% of the dynamic range of the photomultiplier, knowing that constructive interference might exceed the reflectance standard. The experimental material was taken from six prime rib roasts before and after domestic cooking in a gas oven (final internal temperature approximately 60° C). Iliocostalis muscles were sliced transversely to the long axes of muscle fibres at a thickness of approximately 1 mm using a long sharp blade. The roasts conformed to the specification of Canada Grade A beef and were aged in vacuum packs for a minimum of 2 weeks then allowed to dry for 4 days at 4 °C. Surface slices of iliocostalis had a relatively high pH (5.85 ± 0.13) because of their aerobic exposure. Mean sarcomere length was 1.94 ± 0.41 μm giving relatively narrow I bands. Samples were kept for several days in closed Petri dishes at 4° C as measurements were taken (at an ambient temperature of approximately 15° C with a heat filter in the optical light path). Ambient light was low and was cancelled by the method of standardization.
Fig.1. Second-order interference colours from within muscle fibres of a fasciculus of cooked beef iliopsoas (small divisions of bar scale = 10 μm). Surface specular reflectance is white. Both iridescence and surface specular reflectance disappeared when the sample was viewed between crossed polarizers.
exhibited the same second-order interference colour, most often orange but sometimes greenish blue. However, numerous fasciculi were observed where adjacent fibres exhibited a variety of different second-order interference colours including deep red, purple, indigo, sky blue and greenish yellow (Fig. 1). Interference colours were not extinguished by rotating a polarizer in the illumination pathway, or by rotating a polarizer in the measuring pathway. But when both pathways contained polarizers, iridescence was completely extinguished (fibres became dark) when the polarizers were crossed. Surface specular reflectance (white) also disappeared when both polarizers were crossed. Thus, like surface specular reflectance, iridescence originated from Fresnel reflectance. This supported the working hypothesis that iridescence originated from reflectance at A bands in a stack of sarcomere discs. The most common interference colour was second-order orange seen in both raw and cooked samples (Fig. 2). Cooking caused an increase (P b 0.01, except at 440 nm) in reflectance intensity and a shift in the major reflectance peak from 630 to 640 nm (a shift from yellow-orange to orange). The minor peaks before (b630 nm) the major reflectance peak were not caused by monochromator error. When the system was tested with coloured-glass filters the spectra were smooth (b1% error in re-measuring the Teflon standard). Minor peaks were also observed in the spectra for second-order yellow (Fig. 3) and for other colours . Thus, a secondary working hypothesis was devised: in each spectrum, a major interference peak originates from the top sarcomere disc, while minor peaks originate from lower discs. In support of this hypothesis, fibres with second-order blue
3. Results and discussion Iridescence was observed in iliocostalis muscles of all six roasts. Iridescence was easier to find in cooked samples than in raw samples. Iridescence originated from within individual muscle fibres which were optically isolated, probably by internal reflections at the plasma membrane. Sometimes all the fibres of several adjacent fasciculi
Fig. 2. Reflectance spectra of second-order orange interference in muscle fibres from raw and cooked beef iliocostalis. Lines are means (n = 10) with a standard deviation subtracted (■). Except at 440 nm, P b 0.01.
400
H.J. Swatland / Meat Science 90 (2012) 398–401
Fig. 3. Reflectance spectrum of second-order yellow interference in muscle fibres from cooked beef iliocostalis. Line is a mean (n = 10) with a standard deviation subtracted (■).
interference had their minor peaks at higher wavelengths than the major peak (Fig. 4). The regularity of the sequence of major and minor peaks was extremely well developed in many single spectra and was blurred by averaging. Another test of the hypothesis that interference originates from Fresnel reflectance from sarcomere discs was to search for angular effects. In all data presented up to this point, the meat samples were tilted at 45° with vertical illumination and photometry. This configuration was relatively insensitive to the angle of tilting, so the illumination axis was changed and samples were illuminated by an optical fibre fixed to the tilting stage and giving a constant lateral illumination at 45°. As seen in Fig. 2, the minimum and maximum reflectance of orange iridescence was at 540 and 640 nm, respectively. Reflectance at 640 nm increased to a maximum at 25° then dropped sharply at 30° whereas reflectance at 540 nm showed a much weaker angular effect (Fig. 5). Angular effects were detected for all interference peaks.
4. Conclusion Myofibrillar refraction has a strong effect when incident light is perpendicular to the long axes of muscle fibres (Swatland, 2008). Results reported here show that when incident light follows the long axes of muscle fibres, then reflective effects may predominate. In most meat, multilayer interference is probably a source of light scattering and visible interference colours only survive when the optical geometry is relatively simple. In response to a reviewer who asked how this research might relate to meat colorimetry in general, here is a personal conclusion. Meat colorimetry with apparatus giving chromaticity coordinates is
Fig. 4. Reflectance spectra of second-order blue interference in muscle fibres from cooked beef iliocostalis. The single spectrum has a more conspicuous sequence of peaks than the mean (n = 10) with a standard deviation subtracted (■).
Fig. 5. Tilting a fasciculus of cooked iliocostalis with constant illumination from one side by an optical fibre at 45°.
likely to dominate routine research for many years to come. But new approaches are starting to appear. This is not the place for a detailed review but a typical format is as follows. Digital images of meat surfaces are processed to obtain derivative indices which are then fed into a multivariate statistics package and correlations with aspects of meat quality such as tenderness are discovered. Although admirable technologically, this is not science, if science is taken as the pursuit of understanding. Meat iridescence may be of trivial importance industrially, but shows quite clearly the existence of subsurface microstructural optics. For example, one would expect sarcomere length to affect the depth and separation of sarcomere discs and, hence, interference effects which, almost certainly, would be detectable with appropriate processing of a digital image, thus explaining a possible prediction of meat toughness. To elevate empirical technology to science we need ideas to test. This is what this study contributes.
References Benda, C., & Guenther, P. (1895). Histologischer Hand-atlas. Leipzig und Wien: Franz Deuticke. Berthier, S. (2007). Iridescences. The Physical Colors of Insects. New York: Springer. Bowman, W. (1840). On the minute structure and movements of voluntary muscle. Philosophical Transactions of the Royal Society of London, 130, 457–501. Brewer, M. S., Novakofski, J., & Freise, K. (2006). Instrumental evaluation of pH effects on ability of pork chops to bloom. Meat Science, 72, 596–602. Dennis, R.G. (1996). Measurement of pulse propagation in single permeabilized muscle fibers by optical diffraction. Ph.D. Thesis, University of Michigan. Elliott, R. J. (1967). Effect of optical systems and sample preparation on the visible reflection spectra of pork muscle. Journal of the Science of Food and Agriculture, 18, 332–338. Fulladosa, E., Serra, X., Gou, P., & Arnau, J. (2009). Effects of potassium lactate and high pressure on transglutaminase restructured dry-cured hams with reduced salt content. Meat Science, 82, 213–218. Horowitz, R. (2006). Putting Meat on the American Table. Baltimore: Johns Hopkins University Press. Kropf, D. H. (1993). Color stability: factors affecting the color of fresh meat. Meat Focus International, 2, 269–275. Kukowski, A. C., Wulf, D. M., Shanks, B. C., Page, J. K., & Maddock, R. J. (2004). Factors associated with surface iridescence in fresh beef. Meat Science, 66, 889–893. Lawrence, T. E., Hunt, M. C., & Kropf, D. H. (2002). Surface roughening of precooked, cured beef round muscles reduced iridescence. Journal of Muscle Foods, 13, 68–73. Mancini, R. A. (2007). Iridescence: a rainbow of colors, causes and concerns. http:// www.beefresearch.org/CMDocs/BeefResearch/Beef%20Iridescence.pdf Accessed May 7, 2011. Mancini, R. A., & Hunt, M. C. (2005). Current research in meat color. Meat Science, 71, 100–121. Nollett, M. L. (2007). Handbook of Meat, Poultry and Seafood Quality (pp. 347). Ames, Iowa: Blackwell Publishing. Piller, H. (1977). Microscope Photometry. Berlin: Springer Verlag. Raman, C. V. Sir, & Jayaraman, A. (1950). The structure of labradorite and the origin of its iridescence. Proceedings of the Indian Academy of Science, A32, 1–16. Rayleigh, Lord (1923). Studies of iridescent colour and the structure producing it. III. The colours of Labrador felspar. Proceedings of the Royal Society of London. Series A, 103, 34–45. Realini, C. E., Guàrdia, M. D., Garriga, M., Pérez-Juan, M., & Arnau, J. (2011). High pressure and freezing temperature effect on quality and microbial inactivation of cured pork carpaccio. Meat Science, 88, 542–547. Schäfer, E. A., & Thane, G. D. (1898). Quain's Elements of Anatomy (pp. 285–302). London: Longmans, Green & Co.
H.J. Swatland / Meat Science 90 (2012) 398–401 Swatland, H. J. (1984). Optical characteristics of natural iridescence in meat. Journal of Food Science, 49, 685–686. Swatland, H. J. (1985). Early research on the fibrous microstructure of meat. Food Microstructure, 4, 73–82. Swatland, H. J. (1995). On-line Evaluation of Meat. Lancaster, PA: Technomic Press. Swatland, H. J. (1998). Computer Operation for Microscope Photometry. Boca Raton, FL: CRC Press. Swatland, H. J. (2008). How pH causes paleness in chicken breast meat. Meat Science, 80, 396–400.
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USDA (2008). http://www.fsis.usda.gov/factsheets/ Color_of_Meat_&_Poultry/index. asp Accessed May 7, 2011. Warriss, P. D. (2000). Meat Science. An Introductory Text (pp. 239–240). Wallingford, Oxford: CABI. Weidner, V. R., & Hsia, J. J. (1981). Reflectance properties of pressed polytetrafluoroethylene powder. Journal of the Optical Society of America, 71, 856–861. Wolff, L. B., Shafer, S. A., & Healey, G. (1992). Radiometry (pp. 139). London: Jones & Bartlett.
Contents lists available at SciVerse ScienceDirect
Meat Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m e a t s c i
Iridescence in beef caused by multilayer interference from sarcomere discs H.J. Swatland ⁎ University of | Guelph, Guelph, Ontario, Canada N1G2W1
a r t i c l e
i n f o
Article history: Received 16 May 2011 Received in revised form 6 August 2011 Accepted 12 August 2011 Keywords: Iridescence Interference Reflectance Sarcomere
a b s t r a c t Microscope photometry of raw and cooked iliocostalis was used to test the hypothesis that interference colours in beef may originate from reflections from sarcomere discs. Evidence in support of the hypothesis was, firstly, that interference colours were not altered by rotating a polarizer in the illumination pathway, or by rotating a polarizer in the measuring pathway. But when both pathways contained polarizers, iridescence was completely extinguished when the polarizers were crossed. Secondly, the reflectance spectra of interference colours all showed multiple interference peaks, with a major peak possibly originating from the top sarcomere and minor peaks originating from lower sarcomeres. Thirdly, major peaks were strongly dependent on the angle of measurement. Iridescence in beef is quite rare, but reflections from sarcomere discs may be a ubiquitous source of light scattering in meat. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction The appearance of meat is important in consumer purchasing (Horowitz, 2006; Kropf, 1993; Mancini & Hunt, 2005) and meat colour is routinely measured using commercial colorimeters such as the Minolta (Brewer, Novakofski, & Freise, 2006; Nollett, 2007). Colorimeters designed for opaque surfaces also respond to subsurface factors in the translucent depth of the meat. For example, Elliott (1967) discovered that pork paleness is affected by muscle fibre orientation. Fibres cut transversely create darker surfaces than fibres cut longitudinally. Meat surfaces have been measured in countless routine studies, but we may be missing some important subsurface effects. Light may be reflected directly from meat surfaces as specular or mirror-like reflectance following Fresnel equations (Wolff, Shafer, & Healey, 1992). Specular reflectance is polarized to some extent, and may be partly extinguished by rotating a polarizer to an appropriate angle. Thus, a polarizer may be useful in quantification of marbling fat using video image analysis, otherwise it is difficult to separate marbling from bright flecks of specular reflectance from surface irregularities (Swatland, 1995). Light entering the meat is scattered. Some of it scatters back to the meat surface to appear as diffuse or Lambertian reflectance. Lambertian reflectance appears similar at all angles, whereas specular reflectance, like a mirror, has a strong angular effect (Wolff et al., 1992). Understanding subsurface effects could improve meat spectrophotometry as well as explaining iridescence on cut surfaces.
⁎ Tel.: + 1 519 821 7513. E-mail address: [email protected]. 0309-1740/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.meatsci.2011.08.006
Iridescence may appear on cut meat surfaces, especially on sections of raw beef semitendinosus (Kukowski, Wulf, Shanks, Page, & Maddock, 2004) and cured beef and pork products (Fulladosa, Serra, Gou, & Arnau, 2009; Lawrence, Hunt, & Kropf, 2002; Realini, Guàrdia, Garriga, Pérez-Juan, & Arnau, 2011). Most consumers ignore iridescence, but it may cause concerns, as indicated by numerous questions posted on the internet. The iridescent colours of meat are interference colours (Swatland, 1984), but what is generating the interference? Many sources have been proposed. Warriss (2000) considered that iridescence on bacon and ham originated from refraction due to the different refractive indices of water and fat at the surface. Dennis (1996) and Mancini (2007) attributed iridescence to diffraction. Iridescence also has been attributed to pigments in the meat (USDA, 2008). Refraction, diffraction and pigments may cause iridescence in feathers and insects, often with interactions between different sources (Berthier, 2007). But there is scope for another possibility. Long before electron microscopy and the discovery of sliding filaments, there were several schools of thought regarding the cause of transverse striations in skeletal muscle fibres (Swatland, 1985). The idea that muscle fibres contain striped longitudinal structures (myofibrils) proved correct, but here we will resurrect an incorrect idea that muscle fibres contain a stack of discs (Bowman, 1840). In modern terms, we may see how early microscopists treated their samples in such a way that weakening of the Z lines allowed fibres to fragment into Bowman's discs centered on the A band (Benda & Guenther, 1895; Schäfer & Thane, 1898). Thus, to explore subsurface optical effects, we will treat the muscle fibre as a stack of alternating discs composed of I bands (low refractive index) and A bands (high refractive index) and search for evidence of boundary reflections causing thin-film interference, as in the iridescent mineral labradorite (Raman & Jayaraman, 1950; Rayleigh, 1923).
H.J. Swatland / Meat Science 90 (2012) 398–401
399
The hypothesis to be tested is based on the following assumptions. 1. Sarcomere discs will reflect incident light at refractive index boundaries (A-band N I-band). 2. Light reflected from lower discs will have a longer light path than light reflected from upper disks. 3. Light waves following long light paths will often be out of phase with waves following short paths. 4. Waves out of phase will produce either destructive or constructive interference. 5. Interference will alter the spectrum of reflected light, thus producing interference colours (iridescence). 2. Materials and methods A Zeiss Universal microscope (Carl Zeiss, D-7082 Oberkochen, Germany) was fitted with accessories identified here by a Zeiss part number. Zeiss documentation is reported by bulletin number. The bulletin for the part numbers is A-41-824-e. Accessories were operated from a type MPC 64 control module (Zeiss 477469) on a bus (IEEE 488). Programming instructions are given in Zeiss bulletin G-41-912-e, and algorithms are given by Swatland (1998). The main controller was a personal computer running HP BASIC (HewlettPackard, Palo Alto, CA). Components were controlled via a HP VXI mainframe (HP E1421B). The photomultiplier was a side-window Hamamatsu (1126 Ichino-Cho, Hamamatsu City, Japan) HTV R 928 with S-20 characteristics operated as described in Zeiss bulletin G-419000.1/ll-e. A grating monochromator (Zeiss 474345) with stray-light filters (Zeiss 477215) was mounted under the photomultiplier and scanned from 400 to 700 nm in steps of 10 nm with a 10 nm bandpass. Illumination was from a 100 W halogen source with a stabilized power supply (HP 6642A) directed through a solenoid shutter and into a vertical illuminator (Zeiss II-C with H-PL-POL beam splitter). The photometer head was a type 03 (Zeiss 477304). The objective was a LD-Epiplan x 8 with numerical aperture 0.2 (no detectable strain birefringence) which provided sufficient depth above the specimen to allow tilting. Interference colours were named from a Michel-Lévy colour chart (Zeiss S41-500.0-e). The chart is named after French geologist Auguste Michel-Lévy (1844–1911) who developed it to aid in the identification of birefringent minerals. Standardization of reflectance microscopy for diffuse samples is a notorious problem (Piller, 1977). Here the system was standardized on Teflon tape (polytetrafluoroethylene; Weidner & Hsia, 1981) at 50% of the dynamic range of the photomultiplier, knowing that constructive interference might exceed the reflectance standard. The experimental material was taken from six prime rib roasts before and after domestic cooking in a gas oven (final internal temperature approximately 60° C). Iliocostalis muscles were sliced transversely to the long axes of muscle fibres at a thickness of approximately 1 mm using a long sharp blade. The roasts conformed to the specification of Canada Grade A beef and were aged in vacuum packs for a minimum of 2 weeks then allowed to dry for 4 days at 4 °C. Surface slices of iliocostalis had a relatively high pH (5.85 ± 0.13) because of their aerobic exposure. Mean sarcomere length was 1.94 ± 0.41 μm giving relatively narrow I bands. Samples were kept for several days in closed Petri dishes at 4° C as measurements were taken (at an ambient temperature of approximately 15° C with a heat filter in the optical light path). Ambient light was low and was cancelled by the method of standardization.
Fig.1. Second-order interference colours from within muscle fibres of a fasciculus of cooked beef iliopsoas (small divisions of bar scale = 10 μm). Surface specular reflectance is white. Both iridescence and surface specular reflectance disappeared when the sample was viewed between crossed polarizers.
exhibited the same second-order interference colour, most often orange but sometimes greenish blue. However, numerous fasciculi were observed where adjacent fibres exhibited a variety of different second-order interference colours including deep red, purple, indigo, sky blue and greenish yellow (Fig. 1). Interference colours were not extinguished by rotating a polarizer in the illumination pathway, or by rotating a polarizer in the measuring pathway. But when both pathways contained polarizers, iridescence was completely extinguished (fibres became dark) when the polarizers were crossed. Surface specular reflectance (white) also disappeared when both polarizers were crossed. Thus, like surface specular reflectance, iridescence originated from Fresnel reflectance. This supported the working hypothesis that iridescence originated from reflectance at A bands in a stack of sarcomere discs. The most common interference colour was second-order orange seen in both raw and cooked samples (Fig. 2). Cooking caused an increase (P b 0.01, except at 440 nm) in reflectance intensity and a shift in the major reflectance peak from 630 to 640 nm (a shift from yellow-orange to orange). The minor peaks before (b630 nm) the major reflectance peak were not caused by monochromator error. When the system was tested with coloured-glass filters the spectra were smooth (b1% error in re-measuring the Teflon standard). Minor peaks were also observed in the spectra for second-order yellow (Fig. 3) and for other colours . Thus, a secondary working hypothesis was devised: in each spectrum, a major interference peak originates from the top sarcomere disc, while minor peaks originate from lower discs. In support of this hypothesis, fibres with second-order blue
3. Results and discussion Iridescence was observed in iliocostalis muscles of all six roasts. Iridescence was easier to find in cooked samples than in raw samples. Iridescence originated from within individual muscle fibres which were optically isolated, probably by internal reflections at the plasma membrane. Sometimes all the fibres of several adjacent fasciculi
Fig. 2. Reflectance spectra of second-order orange interference in muscle fibres from raw and cooked beef iliocostalis. Lines are means (n = 10) with a standard deviation subtracted (■). Except at 440 nm, P b 0.01.
400
H.J. Swatland / Meat Science 90 (2012) 398–401
Fig. 3. Reflectance spectrum of second-order yellow interference in muscle fibres from cooked beef iliocostalis. Line is a mean (n = 10) with a standard deviation subtracted (■).
interference had their minor peaks at higher wavelengths than the major peak (Fig. 4). The regularity of the sequence of major and minor peaks was extremely well developed in many single spectra and was blurred by averaging. Another test of the hypothesis that interference originates from Fresnel reflectance from sarcomere discs was to search for angular effects. In all data presented up to this point, the meat samples were tilted at 45° with vertical illumination and photometry. This configuration was relatively insensitive to the angle of tilting, so the illumination axis was changed and samples were illuminated by an optical fibre fixed to the tilting stage and giving a constant lateral illumination at 45°. As seen in Fig. 2, the minimum and maximum reflectance of orange iridescence was at 540 and 640 nm, respectively. Reflectance at 640 nm increased to a maximum at 25° then dropped sharply at 30° whereas reflectance at 540 nm showed a much weaker angular effect (Fig. 5). Angular effects were detected for all interference peaks.
4. Conclusion Myofibrillar refraction has a strong effect when incident light is perpendicular to the long axes of muscle fibres (Swatland, 2008). Results reported here show that when incident light follows the long axes of muscle fibres, then reflective effects may predominate. In most meat, multilayer interference is probably a source of light scattering and visible interference colours only survive when the optical geometry is relatively simple. In response to a reviewer who asked how this research might relate to meat colorimetry in general, here is a personal conclusion. Meat colorimetry with apparatus giving chromaticity coordinates is
Fig. 4. Reflectance spectra of second-order blue interference in muscle fibres from cooked beef iliocostalis. The single spectrum has a more conspicuous sequence of peaks than the mean (n = 10) with a standard deviation subtracted (■).
Fig. 5. Tilting a fasciculus of cooked iliocostalis with constant illumination from one side by an optical fibre at 45°.
likely to dominate routine research for many years to come. But new approaches are starting to appear. This is not the place for a detailed review but a typical format is as follows. Digital images of meat surfaces are processed to obtain derivative indices which are then fed into a multivariate statistics package and correlations with aspects of meat quality such as tenderness are discovered. Although admirable technologically, this is not science, if science is taken as the pursuit of understanding. Meat iridescence may be of trivial importance industrially, but shows quite clearly the existence of subsurface microstructural optics. For example, one would expect sarcomere length to affect the depth and separation of sarcomere discs and, hence, interference effects which, almost certainly, would be detectable with appropriate processing of a digital image, thus explaining a possible prediction of meat toughness. To elevate empirical technology to science we need ideas to test. This is what this study contributes.
References Benda, C., & Guenther, P. (1895). Histologischer Hand-atlas. Leipzig und Wien: Franz Deuticke. Berthier, S. (2007). Iridescences. The Physical Colors of Insects. New York: Springer. Bowman, W. (1840). On the minute structure and movements of voluntary muscle. Philosophical Transactions of the Royal Society of London, 130, 457–501. Brewer, M. S., Novakofski, J., & Freise, K. (2006). Instrumental evaluation of pH effects on ability of pork chops to bloom. Meat Science, 72, 596–602. Dennis, R.G. (1996). Measurement of pulse propagation in single permeabilized muscle fibers by optical diffraction. Ph.D. Thesis, University of Michigan. Elliott, R. J. (1967). Effect of optical systems and sample preparation on the visible reflection spectra of pork muscle. Journal of the Science of Food and Agriculture, 18, 332–338. Fulladosa, E., Serra, X., Gou, P., & Arnau, J. (2009). Effects of potassium lactate and high pressure on transglutaminase restructured dry-cured hams with reduced salt content. Meat Science, 82, 213–218. Horowitz, R. (2006). Putting Meat on the American Table. Baltimore: Johns Hopkins University Press. Kropf, D. H. (1993). Color stability: factors affecting the color of fresh meat. Meat Focus International, 2, 269–275. Kukowski, A. C., Wulf, D. M., Shanks, B. C., Page, J. K., & Maddock, R. J. (2004). Factors associated with surface iridescence in fresh beef. Meat Science, 66, 889–893. Lawrence, T. E., Hunt, M. C., & Kropf, D. H. (2002). Surface roughening of precooked, cured beef round muscles reduced iridescence. Journal of Muscle Foods, 13, 68–73. Mancini, R. A. (2007). Iridescence: a rainbow of colors, causes and concerns. http:// www.beefresearch.org/CMDocs/BeefResearch/Beef%20Iridescence.pdf Accessed May 7, 2011. Mancini, R. A., & Hunt, M. C. (2005). Current research in meat color. Meat Science, 71, 100–121. Nollett, M. L. (2007). Handbook of Meat, Poultry and Seafood Quality (pp. 347). Ames, Iowa: Blackwell Publishing. Piller, H. (1977). Microscope Photometry. Berlin: Springer Verlag. Raman, C. V. Sir, & Jayaraman, A. (1950). The structure of labradorite and the origin of its iridescence. Proceedings of the Indian Academy of Science, A32, 1–16. Rayleigh, Lord (1923). Studies of iridescent colour and the structure producing it. III. The colours of Labrador felspar. Proceedings of the Royal Society of London. Series A, 103, 34–45. Realini, C. E., Guàrdia, M. D., Garriga, M., Pérez-Juan, M., & Arnau, J. (2011). High pressure and freezing temperature effect on quality and microbial inactivation of cured pork carpaccio. Meat Science, 88, 542–547. Schäfer, E. A., & Thane, G. D. (1898). Quain's Elements of Anatomy (pp. 285–302). London: Longmans, Green & Co.
H.J. Swatland / Meat Science 90 (2012) 398–401 Swatland, H. J. (1984). Optical characteristics of natural iridescence in meat. Journal of Food Science, 49, 685–686. Swatland, H. J. (1985). Early research on the fibrous microstructure of meat. Food Microstructure, 4, 73–82. Swatland, H. J. (1995). On-line Evaluation of Meat. Lancaster, PA: Technomic Press. Swatland, H. J. (1998). Computer Operation for Microscope Photometry. Boca Raton, FL: CRC Press. Swatland, H. J. (2008). How pH causes paleness in chicken breast meat. Meat Science, 80, 396–400.
401
USDA (2008). http://www.fsis.usda.gov/factsheets/ Color_of_Meat_&_Poultry/index. asp Accessed May 7, 2011. Warriss, P. D. (2000). Meat Science. An Introductory Text (pp. 239–240). Wallingford, Oxford: CABI. Weidner, V. R., & Hsia, J. J. (1981). Reflectance properties of pressed polytetrafluoroethylene powder. Journal of the Optical Society of America, 71, 856–861. Wolff, L. B., Shafer, S. A., & Healey, G. (1992). Radiometry (pp. 139). London: Jones & Bartlett.