Raman spectroscopy for the analysis of food quality at the producer site of the food chain

The development of different lab-synthesized food related chemicals (e.g. artificial sweeteners or food dyes) that can replace naturally occurring additives, reducing the total price of food production or food processing, while keeping the taste of the food raised awareness for the requirement for food analysis and the assessment of the food composition and consumption safety. In addition to this it also soon became clear that every food product require to be analyzed at specific times during the production – processing – market - consumption food chain. That is, vegetables would be ideally analyzed on the plantation field or before entering supermarkets while cereals and soft drinks should be analyzed during the industrial processing procedure. Moreover, in the latter case in-line, non-invasive procedures should be established to efficiently and reliably access the as prepared food or beverage. By doing so, the food quality and safety can be ensured. This is, however, seldom is the case in the current analysis chain and development still required towards this goal. We propose the use of surface enhanced Raman spectroscopy (SERS) for the assessment of the presence of different chemicals in solid food and drinks. By performing SERS measurements the Raman signal of the molecules is enhanced by several orders of magnitude as a consequence of the increased electric field excited on a metallic nanoparticle, in the vicinity of the molecule. Moreover, SERS allows for fast analysis, is easy to perform and provides high specificity. These makes it a good candidate for food analytics [1]. Within this contribution, analytical procedures are introduced for the assessment of different analytes in food [2-4]. Depending on the complexity of the food product and on the expected concentration of the targeted analyte a choice was made between performing Raman or SERS measurements and full analytical procedures were developed. An example of such a procedure is depicted in Figure 1. In this particular situation the abundance of two carotenoids in tomatoes belonging to different ripening stages was analyzed by SERS [2]. For this, a food extraction protocol was first applied (i) and the resulting solutions (ii) were used for incubating the SERS active substrates. Upon this step the substrates were dried using an N2 and SERS measurements were performed. Considering the complexity of the food matrix interfering in the measurements, different statistical analyses of the data were performed. These include, but are not limited to principal component analysis (PCA) and partial least squares regressions (PLSR). The final result was then compared with the ones obtained by the gold standard analytical tool (within this example high performance liquid chromatography – HPLC measurements). Similar protocols were also developed for the analysis of B vitamins in fortified cereals [3] and dyed in soft drinks [4]. The obtained results suggest that Raman and SERS based applications can be further developed and adapted towards routine, on-field or in-line (industrial) analysis, potentially filling the existing gab in the food production-distribution chain. Figure 1. Schematic representation of the analysis chain. As depicted, the first step consists of the preparation of the analytes to be measured. To this end, the described extraction protocol was applied (i) and the resulting solutions (ii) were used for incubating the SERRS active substrate (iii). Upon incubation the substrates were dyed with N2 (iv) and measured by means of SERRS. Acknowledgement: Funding of the project “TG PRORAM” within the framework “Photonik Plus – Neue optische Basistechnologien“ by the Federal Ministry of Education and Research, Germany (BMBF) is gratefully acknowledged. [1] Jahn, M.; Patze, S.; Hidi, I.; Knipper, R.; Radu, A.I.; Mühlig, A.;. Yüksel, S; Peksa, V.; Weber, K.; Mayerhöfer, T.; Cialla-May, D.; Popp, J. Analyst 2016, 141, 756–793. [2] Radu, A.I.; Ryabchykov, O.; Bocklitz, T.W.; Huebner, U.; Weber, K.; Cialla-May, D.; Popp, J. Analyst 2016, 141, 4447–4455. [3] Radu, A.I.; Kuellmer, M.; Giese, B.; Huebner, U.; Weber, K.; Cialla-May, D.; Popp, J. Talanta 2016, 160, 289–297. [4] Peksa, V.; Jahn, M.; Štolcová, L.; Schulz, V.; Proška, J.; Procházka, M.; Weber, K.; Cialla-May, D.; Popp, Anal. Chem. 2015, 87 (5), 2840–2844.