A diode laser and Raman sensor system for in situ quality control
Bernd Sumpf, Heinar Schmidt, Martin Maiwald, and Heinz-Detlef Kronfeldt
At a time when food prices are soaring and tales of contamination make headlines, consumers expect not only value for their food dollars, but assurances that what they buy is fit to eat.
These concerns raise questions of quality control up and down the food-production chain. Unfortunately, determining food quality has often been based on experts’ empirical knowledge or on time-consuming and expensive laboratory analytical methods. Currently parameters monitored and documented on-line are limited. Temperature and time, for instance, are a few that can be recorded with (radio frequency identification) RFID tags.
However, in situ methods of characterizing food’s status in terms of “edible,” “ripened” or “spoiled” are scarce to non-existing. What’s needed are techniques capable of fast, non-invasive and through-packaging monitoring. Optical means, especially laser spectroscopic methods, are one solution to the problem.
“FreshScan," a research project funded by the German Bundesministerium für Bildung und Forschung consisting of five institutes¹ have joined to demonstrate the feasibility of using miniaturized optical sensor systems to monitor food quality using meat as an example. Because correlations between the spectroscopic features and the status of the meat (ripeness and spoilage) need to be established, time dependant spectroscopic measurements were employed using pork as a test sample, which was stored under well defined conditions.
At the same time, laboratory analytical methods were applied for reference analyses. Raman spectroscopy was selected because its fingerprinting capability and fluorescence provided selectivity and sensitivity. Another task was to develop a handheld scanner for the optical in situ measurement. Here, the work concentrated on the design and development of suitable excitation light sources, specific optical transducers (so called optodes), and detection schemes.
For the project we designed a Raman optode that contains a compact wavelength-stabilized micro-system diode laser. Spectroscopic measurements on meat proved the capability of the concept and the ability to probe the protein matrix and monitor changes in meat during storage time even through the packaging.
Designed Especially for Meat
The Raman optode was designed especially for measurements of raw meat integrating the micro bench ECL into the probe head. Using lenses and mirrors, the laser is focused on the sample. The backscattered light is collected and the excitation wavelength is blocked with filters. Only the Raman signal is focused into a fiber, which guides the light into a compact spectrometer with a resolution of about 10 cm-1.
Fig. 1: Raman spectra of meat measured through the package (upper trace), of the package foil (middle trace) and difference spectrum (lower trace)
The meat measurements included time dependant spectroscopic measurements with pork stored under well-defined conditions and measured through the package. The latter is shown in Figure 1. The top trace shows the measured spectrum of meat in the package, in the middle the spectrum of the package itself. The spectrum of the pure meat can be determined by calculating the difference of the spectrum. Meat can be identified, e.g. by broad peaks at 1650 cm-1 and 1300 cm-1.
The aging of meat (ripening and the spoilage) was also studied. The meat was stored at 5 °C. Raman spectra were recorded at different ages. The measured Raman spectra are shown in Figure 2. The biochemical and physical changes are leading to complex spectral changes, which cannot be seen directly and had to be evaluated with chemometrical methods. Principal components analysis (PCA)2,3 of the Raman data can identify changes of structure and composition of the protein matrix. This allowed us to distinguish ripened meat and incipient spoilage between day 8 and 10 for the storage at 5 °C. This result corresponded with reference analyses using conventional laboratory analytical methods.
Fig. 2: Raman spectra of meat at different ages: after 2 days (lower trace), after 9 days (middle) and after 15 days (upper trace), spectra are off set for clarity
The work demonstrated that Raman spectroscopy is a useful tool for food quality control. The method allows measurements through the package and can distinguish between ripened products and spoilage. The spectroscopic results obtained with the microsystem diode laser integrated in the Raman optode are promising. A further miniaturization will deliver a compact handheld laser scanner for the in situ food control. Although the method was demonstrated for meat, the concept can be transferred to other food by adapting the excitation source and the optical elements
Meet the Authors...
Heinz-Detlef Kronfeldt and Heiner Schmit are with the Institute for Optics and Atomics Physics at the Technical University of Berlin. Bernd Sumpf and Martin Maiwald are with the Ferdinand-Braun-Institute for Hoechstfrequenztechnik in Berlin.
1 Fraunhofer Institut für Zuverlässigkeit und Mikrosystemtechnik – IZM – Berlin
Forschungsschwerpunkt Technologien der Mikroperipherik der Technischen Universität Berlin
Institut für Optik und Atomare Physik (IOAP) der Technischen Universität Berlin
Max-Rubner-Institut – Bundesforschungsanstalt für Ernährung und Lebensmittel – Institut für Sicherheit und Qualität bei Fleisch - Kulmbach Agrartechnik Bornim
Ferdinand-Braun-Institut für Höchstfrequenztechnik Berlin
2 K. R. Beebe, R. J. Pell, M. B. Seasholtz, Chemometrics: A practical guide, John Wiley and Sons, New York, 1st edition, 1998
3 J. N. Miller, J. C. Miller, Statistics and Chemometrics for Analytical Chemistry, Person Education Limited, Harlow, 5th edition, 2005