University of Guelph
Evaluation "on-line" implies both computer interfacing and meat-plant line applications. The pork may be in any form: intact carcasses on an overhead rail, cuts on a conveyer, comminuted product for manufacturing in a box or tray, or a paste or batter in a pipe or chopper. Evaluation must be sufficiently rapid to keep pace with processing line speeds in major plants, and must be objective (instrument based), without a human judgement. Also, it must be non-contaminating and relatively non-destructive, and should relate to current industry goals such as (1) feed-back information to producers, (2) payment for lean meat produced, (3) objective quality control and grading, (4) detection of tough meat, (5) detection of paleness, (6) prediction of fluid losses, (7) measurement of marbling, (8) sorting for niche marketing, and (9) feed-forward to improve meat processing.
PSE DETECTION AT 24 HOURS
At 24 hours after slaughter, there is a choice of optical or electrical methods for PSE detection, or a combination of both. Both methods may be reliable for the cut measured, but less reliable for predicting one cut from another because of intermuscular variation. Multiple wavelength optical methods are adequate for quality control at 24 hours, giving reliable prediction of fluid losses, even after freeze/thaw cycles. Robotics now are being developed for automated measurement to deal with the problem of intermuscular variation by making rapid repeat measurements in different muscles and parts of the loin. With a firm understanding of the principle biophysical causes of PSE, using x-ray diffraction, the problems of 24 hour measurement are economic or political rather than scientific. If anything, the optical technology in common use (CIE paleness) is retrogressive, because it transforms data using a weighted ordinate to match the green sensitivity of the human eye. In other words, critical information on wavelength-related scattering at 400 and 700 nm is discarded in favor of a myoglobin-dominated region of the visible spectrum. Thus, compared with the information provided by the now defunct Colormet probe, which was based on a xenon-flash and PDA spectrograph with its own internal microprocessor, the technology for 24-hour PSE detection is on the decline. In many supposedly scientific publications, there is a naive assumption that subjective paleness is a firm parameter against which on-line probes should be evaluated.
PSE DETECTION AT 45 MINUTES
At 45 minutes after slaughter, neither optical or electrical methods are yet sufficiently reliable for commercial use. Attempts are being made to improve electrical detection of PSE using a single-cycle low frequency pulse, and to improve optical detection by coupling angular or goniospectrophotometric measurements to a PDA spectrograph. There are plans to coordinate research on early detection of pork quality at the University of Guelph with similar research being undertaken at Purdue University by Dr. John Forrest and at the Danish Meat Research Institute by Dr. Jan Andersen. At present, early (0 to 3 hours postmortem) data from all three groups show the same types of complex events involving transient cyclic changes in optical and electrical properties. At present, it appears that glycogenolysis transiently increases the osmotic pressure within small groups of muscle fibers, which then take up intercellular fluid, producing transient electrical and optical changes which are opposite in sign to those that eventually prevail by 24 hours. It is hoped that improved understanding of the causes of these chaotic events at the biochemical and biophysical level may suggest ways to improve the reliability of prediction. Improved robotic sampling also may improve reliability by encompassing intermuscular variation, and joint research with the Robotics Laboratory of the University of Toronto (Dr. Andrew Goldenberg) is presently underway to investigate this possibility. Neural networks also are being investigated.
Although not a traditional pork quality problem, toughness could become important as pork gets leaner. Recent research shows that pork muscles may cold-shorten to produce tough, short sarcomeres. An optical probe to detect this has been developed based on the use of polarized near-infrared light. A UV fiber-optic probe developed for the detection of collagen toughness in beef also works in pork. It detects the abundance of collagen by its fluorescence, and also detects heat-stable pyridinoline cross-linking and elastin.
At 24 hours, marbling in pork can be detected with a fiber-optic probe. A critical factor is the angular geometry of the fiber-optic window relative to the meat. The best results are obtained with a small optical window, using wavelengths that provide the maximum separation between PSE lean and adipose tissue. In hot carcasses at 45 minutes, tissue distortion by the probe is a major problem. However, the dominant problem is that the human perception of marbling is a complex subjective response affected by distribution patterns, probably in a logarithmic manner. Thus, it is hardly a firm parameter against which on-line probes may be evaluated. Probably, it would be easier to ascertain what commercial trait subjective marbling is being used to predict, and then attempt to predict that commercial trait directly.
Commercially available technology for 24-hour PSE detection is now inferior that available a decade ago, although our scientific understanding of the biophysical causes of PSE and other attributes of pork quality continues to advance. Attempts are being made to improve 45-minute PSE prediction.
Swatland, H.J. 1995. On-line Evaluation of Meat. Technomic Publishing, Lancaster, PA. Library of Congress Catalog Card No. 95-61607. ISBN 1-56676-3339.
Swatland, H.J. 1995. Near-infrared birefringence and transmittance of pork in relation to pH, sarcomere length, cold-shortening and causes of paleness. Food Research International, 28: 153-159.
Swatland, H.J. 1995. Reversible pH effect on pork paleness in a model system. Journal of Food Science, 60: 988-995.
Swatland, H.J. 1996. Effect of stretching pre-rigor muscle on the back-scattering of polarized near-infrared. Food Research International, IN PRESS.
Swatland, H.J. 1996. Observations on rheological, electrical, and optical changes during rigor development in pork and beef. Journal of Animal Science, IN PRESS.