Microwave sensing for an objective evaluation of meat ageing

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Abstract

Monitoring changes in muscle structure during ageing of bovine meat is a major industrial challenge. During ageing, bovine muscle becomes tender through muscle fibre de-structuring, and full control of this process is essential. To improve competitiveness, and to meet consumer quality demand, muscle structure needs to be evaluated in-line. We present here a broadband microwave study (0.3–24 GHz) involving contact reflection coefficient measurements using coaxial and rectangular probes. This study is based on the measurement of dielectric properties of tissues with direction of polarisation parallel and perpendicular to muscle fibre directions, as muscles have anisotropic dielectric properties. Findings show the feasibility of a simple microwave sensor.

Introduction

Electromagnetic sensors, and particularly microwave sensors, are already used in industrial sectors such as wood processing (Goy et al., 1992), civil engineering materials (Adous et al., 2006) and chemicals (Hauschild et al., 1995 and Gradinarsky et al., 2006), but are not widely exploited in the food industry. The food industry works on often complex and delicate products and needs a high degree of control to optimise processing. In food matrices the relevant variables are structure, composition (particularly water content), water state and water distribution in the product and particularly water content profiles. There are already many tools available to measure these variables, but they are often invasive and difficult to use in-line. Electromagnetic sensors offer a non-invasive solution.

Several research laboratories internationally are working on the potential applications of microwave sensors in the food industry. Concerning water content, Sheen and Woodhead (1999) are working on the dielectric properties of milk products. The US Department of Agriculture has also published work on water content, water state and density of grain and seed (Trabelsi et al., 1997, Trabelsi et al., 1998, Trabelsi and Nelson, 2006, Nelson et al., 1998, Nelson and Trabelsi, 2006). Their techniques use transmission–reflection measurements between non-contacting horns to respond to industry needs. Besides water content, Kent and Anderson (1996) have focused on water state in food products and particularly meat products to detect added water, a classical fraud in the food industry. Water state has also been studied at INRA (Clerjon et al., 2003) with a spectroscopic technique. Dielectric relaxation frequency changes according to the nature of the water molecules bound to the matrix. Other compounds in food products also have to be controlled, such as salt (Shiinoki et al., 1998), and parasites (Nelson et al., 1997).

In seafood quality evaluation field, the European project SEQUID (Kent et al., 2006) has lead to the development of a microwave sensor based on the time domain reflectometry method and multivariate analysis for the classification of fish’s according to flesh quality. As a direct consequence of that project an instrument is now nearing production (Sequid Gmbh, 2008). Because this sensor uses a coaxial probe, it does not exploit dielectric fibres anisotropy properties.

Many efficient methods of non-destructive monitoring, based on the measurement of reflection and transmission coefficients of electromagnetic waves at microwave frequencies, have also been proposed for medical applications (Aimoto and Matsumoto, 1996) and process control in the pharmaceutical industry (Smith et al., 1995).

Applications listed above used coaxial probe methods. Because of the axial symmetry of the coaxial sensor, the material under test is exposed to the electromagnetic field in all direction and therefore there is no potential for directional measurement. The originality of our study is to use, in addition to a traditional coaxial probe, polarimetric rectangular probes emitting linearly polarized microwaves.

Some industrial applications have also been based on the use of polarized microwave with the objective of the characterization of anisotropic material. In the development of new materials for electronic applications, Clerjon et al. (1999) developed an asymmetric cavity sensor to measure non reciprocal microwave propagation in magnetic material. The wood and paper (Kaestner and Baath, 2005, Vepsalainen et al., 1998) and the leather (Osaki et al., 1993, Niitsuma et al., 2007) industries also used microwave polarimetry for fibres orientation measurement. Osaki et al. (2002) worked on the determination of the orientation of collagen fibres in human bone and Kim et al. (1997) on the molecular orientation angle of biaxially stretched poly(ethylene terephthalate) films. There are few industrial applications using microwave polarimetry and all of them are based either on cavity, antenna or probe measurements.

The microwave sensor for food structure evaluation under development at the Clermont-Ferrand INRA Research Centre (French National Institute for Agricultural Research) is presented here in an application to meat process control. The sensor for structure characterization studied at INRA is original, being based on linearly polarized microwaves, i.e. microwaves with a set electric field direction. This feature makes it possible to obtain direction-dependent information, as microwave–sample interaction is not the same according to the direction of the material fibres with respect to the electric field direction. We focus here on muscle ageing during muscle-to-meat transformation.

The most important striated muscle quality is meat tenderness, which is directly linked to meat structure. After slaughter, bovine carcasses are stored in chilling rooms for 7–12 days. Tendering processes occur during this ageing. Studies of the enzymatic (Koohmaraie et al., 1988, Dransfield et al., 1992) and non-enzymatic (Takahashi et al., 1995) ageing processes have shown that the mechanisms involved are very complex, making optimal ageing time difficult to predict. This ageing time depends on the animal. The tenderisation rate of bovine meat is particularly low, imposing storage and so adding cost. Meat is usually sold within one or two weeks and not all pieces are fully aged at this time, because the tenderisation rate ranges widely among animals. Consequently, meat can be tough (resultant tenderness adds to that associated with connective tissue) if sold too early, failing to meet consumer demand for constant tender meat. Conversely, a longer ageing time is more costly and so reduces competitiveness (Lepetit and Hamel, 1998). Reducing the variability in tenderness obtained by a controlled ageing process is an important step towards marketing meat of constant quality. The solution is a sensing method that can predict the best ageing duration for each piece of meat. Such sensing needs to be very fast (for in-line measurements), non-destructive, low-cost, hygienic and safe.

Our method makes use of the fact that muscles are very highly anisotropic dielectric materials owing to their structural organisation and their composition: from muscle to sarcomere filaments, the components are elongated and roughly parallel, forming bundles of conjunctive tissue and myofibres with widely different intrinsic dielectric properties (Gabriel et al., 1996). This confers anisotropic dielectric properties. After slaughter, during rigor mortis and ageing, structural damage appears (Koohmaraie, 1994 and Marsh and Carse, 1974) that reduces the dielectric anisotropy (Clerjon and Damez, 2000). Dielectric measurements done in parallel and perpendicular configuration change according to meat ageing, because insulating membrane disappearance, tending towards equal values. A complete description of the polarimetric method and a numerical model of these anisotropic properties are given, respectively, in Clerjon and Damez (2007) and Felbacq et al. (2002).

The objective of this study is to show that linearly polarized microwave sensing is a better tool for monitoring structural change in meat than non-polarized microwaves sensing. We also want to point out that a simple sensor using a rectangular probe between 4 and 6 GHz is able to classify muscle samples according to their toughness.

This paper first presents the muscle samples which have been studied and the numerous variables measured in this study. Next, we explain how data were proceeded to obtain linear combinations of dielectric variables which well predict mechanical resistance of muscle. We conclude with a classification of our samples by toughness using a very simple rectangular probe.

Section snippets

Materials and methods

In this part, we present materials (two raw bovine muscles) and variables which were measured on these muscles during the study. These variables are: mechanical resistance, temperature, and dielectric parameters. This term of “dielectric parameters” covers 62 dielectric variables listed in Table 2 (from variable 3 to variable 65) and described with more details in Section 2.3. As an overview of these dielectric variables, we have the real and imaginary parts of the permittivity at 10

Linear combination

We looked for linear combinations of variables to maximise correlation coefficients between data of interest. First we checked the correlation between samples temperature and permittivity.

Next, we compared coaxial and rectangular probes for discriminating ageing states. Lastly, we showed that a single probe, the 4–6 GHz rectangular probe, could be used alone for this task.

Conclusion

Dielectric measurements are not often used in the food industry, particularly in meat processing, even though they offer a rapid, non-destructive solution to monitoring product changes. We have shown the potential of a 4–6 GHz rectangular probe to classify meat into three ageing classes. The choice of a rectangular probe was prompted by the additional information given by this type of probe through microwave polarisation. For highly anisotropic products such as meat, microwave polarisation

Acknowledgements

The authors thank David Cormier, QuaPA Unit, INRA, for his technical assistance with dielectric measurements during this study, and Raphael Favier and Bernard Dominguez, QuaPA Unit, INRA, for mechanical resistance measurements.

References (43)

  • S. Ryynanen

    The electromagnetic properties of food materials: a review of the basic principles

    Journal of Food Engineering

    (1995)
  • N.I. Sheen et al.

    An open ended coaxial probe for broad-band permittivity measurement of agricultural products

    Journal of Agricultural Engineering Research

    (1999)
  • Y. Shiinoki et al.

    On-line monitoring of moisture and salt contents by the microwave transmission method in a continuous salted butter-making process

    Journal of Food Engineering

    (1998)
  • G. Smith et al.

    Dielectric relaxation spectroscopy and some applications in the pharmaceutical sciences

    Journal of Pharmaceutical Sciences

    (1995)
  • K. Takahashi et al.

    Relationship between the translocation of paratropomyosin and the restoration of rigor-shortened sarcomeres during post-mortem ageing of meat – a molecular mechanism of meat tenderization

    Meat Science

    (1995)
  • L. Zhang et al.

    Dielectric and thermophysical properties of meat batters over a temperature range of 5–85 °C

    Meat Science

    (2004)
  • M. Adous et al.

    Coaxial/cylindrical transition line for broadband permittivity measurement of civil engineering materials

    Measurement Science and Technology

    (2006)
  • S. Bakhtiari et al.

    Open-ended rectangular waveguide for non-destructive thickness measurement and variation detection of lossy dielectric slabs backed by a conducting plate

    IEEE Transactions on Instrumentation and Measurement

    (1993)
  • D.V. Blackham et al.

    An improved technique for permittivity measurements using a coaxial probe

    IEEE Transactions on Instrumentation and Measurement

    (1997)
  • Clerjon, S., Damez, J.L., 2000. Anisotropy and postmortem changes in the dielectric properties of semitendinosus...
  • S. Clerjon et al.

    Microwave sensing for meat and fish structure evaluation

    Measurement Science and Technology

    (2007)
  • Cited by (0)

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