Fluorimetric detection of protoporphyrins as an indicator for quality monitoring of fresh intact pork meat
Introduction
During recent years global meat production has been continuously increasing to approximately 278 million tons in 2007. The greatest section is pork, with an output of nearly 106 million tons. To guarantee the required high and constant quality in industrial meat production, strict rules are necessary. In the European Union the regulation for meat hygiene constitutes a visual inspection of every carcass, but a microbial control is only randomly required (Regulation (EC) No. 854/2004).
Optical methods are particularly suitable to improve high throughput operations. They are fast and non-destructive and can be integrated directly into the production line. Absorbance and reflectance measurements in the visible and near-infrared region are already established for determining the main components as well as for providing detailed information on drip loss (Geesink et al., 2003), Warner-Bratzler shear force (Byrne, Downey, Troy, & Buckley, 1998), intra-muscular fat content (Hoving-Bolink et al., 2005) or quality factors such as PSE (pale, soft, exudative) meat (Xing, Ngadi, Gunenc, Prasher, & Gariepy, 2007). However, due to relatively high instrument costs, only a few devices are used commercially.
The limitation of reflectance spectroscopy is its selectivity and sensitivity when compared to fluorescence spectroscopy as even small amounts of substances can be fluorimetrically detected using relatively low energy light sources. Substance-specific differences can be evaluated by choosing diverse excitation and emission wavelengths for analytical identification. Ingredients of nutritional importance like polyphenols in fruits (Wulf, 2006); maillard products in heat treated milk (Kulmyrzaev & Dufour, 2002); riboflavin in yoghurt (Becker, Christensen, Frederiksen, & Haugaard, 2003); lipid oxidation in refrigerated chicken breast (Gatellier et al., 2007) or processing parameters like collagen content (Egelandsdal et al., 2002, Swatland, 1987, Wold et al., 1999), can be traced. The hygienic conditions along the slaughter line can be determined by detection of the fluorescent degradation products of chlorophyll located in the faeces (Ashby et al., 2003).
The fluorescent porphyrin protoporphyrin IX (PP), which has a structure similar to chlorophyll, is formed in the biosynthetic pathway of haemoglobin. After chelation of iron, the PP turns to heme and after combining with globin chains it becomes haemoglobin. In trace amounts during the synthesis, the enzymatic binding of zinc to PP with ferrochelatase leads to zinc protoporphyrin IX (Grinstein & Watson, 1943). The formation of zinc protoporphyrin IX (ZnPP) is of interest for meat processing (Veberg et al., 2006, Wakamatsu et al., 2004). Even though the influence of PP and ZnPP on the meat quality is not clear, the appearance of ZnPP during ageing of meat can potentially be used as an indicator for quality monitoring. The aim of this study was the fluorimetric detection of protoporphyrins in fresh and intact pork meat. To investigate the time and temperature dependencies of protoporphyrin release, different storage regimes were examined. Additionally, the fluorescence signals were tested for correlations with changes in pH, drip loss and colour.
Section snippets
Meat samples and porphyrins
Twelve porcine musculus longissimus dorsi (MLD) from the left side of the carcass were purchased directly from a local slaughterhouse (Vion Lausitz GmbH, Kasel-Golzig, Germany). MLD was chosen because of its relative homogeneity, low fat content and commercial importance. Female pigs from various breeds were stunned with CO2, slaughtered and cooled as half carcasses for approx. 1 h at −10 °C and then at 7 °C maximum until cutting. At 24 h post-mortem, the MLD was removed. The muscle was cut
Meat storage and fluorescence measurements
When exciting aged porcine MLD with light at 420 nm, three peaks located at 592 ± 3, 638 ± 2 and 705 ± 2 nm appeared, which are barely visible in fresh meat (Fig. 1). During storage at 5 °C, only a small peak was visible on day 1–6, while the intensity on day 12 had considerably increased (Fig. 1a). Storage at 5 °C led to a significant increase in all three peaks, starting from approx. day 11. However, in the normal marketing chain, meat is already sold by this time. Storage at 12 °C was chosen to
Conclusion and outlook
During the ageing process of fresh pork meat, the intensity of the fluorescence signal due to ZnPP increased. Using PCA, a differentiation between the 10th and 11th day at 5 °C and between the 5th and 6th day at 12 °C storage was seen. pH, drip loss and colour were not suitable to determine the course of ageing. No correlation was found between the fluorescence spectra and the parameters analysed. However, the ZnPP fluorescence emission correlated with both storage time and temperature and can
Acknowledgements
This study was performed within the research project “FreshScan” and is supported by the Federal Ministry of Education and Research and the VDI/VDE Innovation + Technik GmbH. The project is a cooperation between the Leibniz Institute for Agricultural Engineering Potsdam-Bornim (ATB), Ferdinand-Braun-Institut für Hoechstfrequenztechnik (FBH), the Max Rubner-Institute, Federal Research Centre for Nutrition and Food, Institute for Safety and Quality of Meat, location Kulmbach (MRI), the Institute of
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