Submillimetre and THz spectroscopy: an effective tool for monitoring the freshness of foods of various kinds?

During the first period of the Terafood project (https://terafood.iemn.fr/) several relevant freshness indicator gasses have been identified via lab tests on Atlantic salmon (Salmo salar) (more info available in first Terafood newsflash [1]. Now the gasses indicating early spoilage are known a new approach is used to analyse surrounding gasses in order to determine the microbial activity (and linked to that the freshness) of a packaged target food product. For this purpose submillimeter and THz absorption spectroscopy is potentially a very attractive solution, as it allows to obtain extremely clear signatures (at least at low pressure) of the different gasses in complex mixtures. In principle, the THz and submillimetre wavelength offers excellent selectivity in gas detection. To detect and identify the gasses tracers with a strong dipole moment to be suitable for trace detection are needed. To illustrate this, an existing spectrometer developed at the ‘Physico-Chimie Laboratory of the Atmosphere’ in Dunkerque has been adapted for the temporal monitoring of a target molecule within a gas mixture. The principle of the experiment is shown in Figure 1. A microwave source multiplied in frequency is used to cover the frequency range of 100 to 900 GHz (3 to 0.333 mm wavelength). An absorption cell ensures the interaction between the radiation and the gas under study. A Schottky diode at room temperature is used for signal detection. The sweeping frequency of the radiation source and the detection of the associated signal allow to reveal absorptions at characteristic frequencies of the target molecules present in the gas cell (see Figure 2).

Figuur 1

Figure 1 : Experimental setup to measure molecular absorption in the submillimeter THz range

Provided that the spectroscopic parameters of the target molecule are known, it is then possible to deduce its concentration. In this example, formaldehyde was clearly identified and quantified at the ppm level. In order to improve the signal-to-noise ratio of this technology a frequency modulation technique with a second harmonic demodulation can be used. The next step in the project consists of carrying out a feasibility experiment on a sample of Atlantic salmon measuring the identified spoilage gas molecules in the head space of a packaged sample in order to improve the technology and to propose a first prototype of the system.

Figuur 2

Figure 2: Absorption signal of formaldehyde in a complex mixture

With support of the European Regional Development Fund

Terafood


[1] Available on http://food2know.org/en/news/terafood-eng