It's meat, but not as we know it: Mechanically recovered chicken detected by marker proteins

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  • Published: May 15, 2011
  • Author: Steve Down
  • Channels: Proteomics & Genomics
thumbnail image: It's meat, but not as we know it: Mechanically recovered chicken detected by marker proteins

Mechanically recovered paste is not meat

Mechanically recovered meat (MRM) ranks low down the meat scale as far as consumers are concerned, being regarded as a cheap and inferior foodstuff. The dislike originates from two sources, namely the method of production and the historical use of mechanically covered beef.

In the 1990s, beef MRM was still made from vertebral columns, so could be contaminated with traces of spinal cord, raising the possibility of BSE infection. This practice was banned in the UK in 1995.

MRM is recovered from animal carcasses and bones once they have been separated from the bulk of the meat by hand. The bones are forced through machines such as sieves under high pressure to detach the residual meat, which looks like a smooth paste.

In 2001, the EU declared that MRM can no longer be defined as meat, so products containing it should be labelled accordingly. The most common method involves histological staining but it lacks reliability and quantitative capabilities.

Last year, Peter Bramley and colleagues from Royal Holloway, University of London, published a metabolomics procedure for detecting MRM in food products. Now, they have reported a different approach that relies on proteomics to highlight differences in the protein composition of MRM and hand-deboned meat (HDM).

Separating differentiating proteins from chicken meat types

The research team, comprising Bramley, Kaisa Koistinen, Paul Fraser and senior reporter Izabella Surowiec, applied their new method specifically to chicken meat. Samples of MRM and HDM chicken were prepared according to EC regulations.

Initially, the proteins were extracted from each meat sample and subjected to SDS-PAGE but the resolution was insufficient for the reliable identification of protein differences between the meat types. Two-dimensional separation of proteins would be required to gain the necessary resolution and help to identify low-abundance proteins.

However, the researchers were not particularly enamoured of 2D gel electrophoresis due to its laborious nature. As an alternative, they employed off-gel electrophoresis as a first step. This process employs immobilised pH gradient isoelectric focussing to resolve proteins based on their pI values and has the great advantage that, unlike conventional IEF, it delivers the fractions into the liquid phase.

The proteins were fractionated over pH 3-10 and the 24 fractions obtained were each subjected to a second fractionation step involving SDS-PAGE, to improve the resolution further.

Now, when the gels for MRM and HDM were compared, clear differences in the protein patterns could be observed. In particular, one intense protein band at a molecular mass of 15 kDa was present in abundance in MRM but was very weak in HDM. A second band on the gels at about 130 kDa was also more abundant in MRM.

These bands were excised from the gel for protein identification using HPLC coupled with tandem mass spectrometry.

Protein markers of mechanical recovery

The heaviest differential protein at 130 kDa was slow-type myosin binding protein C, which is a thick filament-associated protein localized to the crossbridge-containing C zones of striated muscles. The team postulated that it is more abundant in MRM due to the dispersion of enzymes during the mechanical production process, which help to release it from the muscle tissue.

The second MRM-abundant protein at 15 kDa was related to haemoglobin subunits, which probably originated from the bone marrow. However, haemoglobin can also derive from residual blood in HDM products, so it cannot be used on its own as a marker of MRM.

Haemoglobin has been proposed previously as a marker for chicken MRM but it would be better used in conjunction with other markers such as slow-type myosin binding protein C.

At this stage, the success of these two potential markers can only be judged after more rigorous testing against chicken products from different producers, then developing an accurate method, preferably with protein quantification.

The researchers recommended that abundance ratios of the marker proteins or peptides versus other proteins found in chicken muscle should be used in a MRM/HDM discriminatory test, to counter variations resulting from the meat source or production process.

This could be achieved by HPLC-tandem mass spectrometry following tryptic digestion of the off-gel fractions, the speed and reproducibility of off-gel electrophoresis being of great benefit in method development.

Alternatively, the marker proteins could be used to produce specific antibodies for an ELISA, a more simple procedure with which many labs are familiar.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

Mechanically recovered chicken meat can be distinguished from hand-deboned chicken using particular proteins as marker compounds, say scientists in the UK, who used off-gel electrophoresis for preliminary fractionation in a proteomics comparison of the two types of meat product
Peter Bramley

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