Introduction to laser induced breakdown spectroscopy imaging in food: Salt diffusion in meat
Introduction
Salt curing is an ancient technique which has been used over centuries for preserving perishable food products such as meat. Salt curing can be of the following types: dry, wet and a combination of both. Sodium chloride (NaCl) is the main ingredient used in meat curing. It offers various functionalities such as preservation, improved technological yields, and influences meat tissue properties such as water-holding capacity and protein solubilisation, which in turn defines meat texture (Sharedeh et al., 2015). NaCl diffusion in meat during wet (immersion) curing, also known as brining, is defined by Fick's diffusion theory. The theory of diffusion in isotropic substances is based on the hypothesis that the rate of transfer of diffusing substance through unit area of a section is proportional to the concentration gradient measured normal to the section (Crank, 1979). NaCl diffusion into the muscles is usually rapid and an equilibrium is reached in about 48 h. However, salt diffusion is slower in meats with a close tissue micro-structure when immersed in a weak brine solution (Lawrie, 2006). It is important to study salt diffusion in meat in order to optimize brining time, brine concentration as well brine temperature, which can have a direct effect on the microbiological, physico-chemical and sensorial characteristics of meat.
Laser-induced breakdown spectroscopy (LIBS) is an emerging elemental technique for mineral analysis of food (Andersen et al., 2016, Bilge et al., 2016b, Cama-Moncunill et al., 2017, Casado-Gavalda et al., 2017, Dixit et al., 2017b). LIBS provides numerous advantages such as minimal or no sample preparation, chemical free, rapid detection, portability and spatial information (Abdel-Salam et al., 2017, Bilge et al., 2016a, Er et al., 2016, Moncayo et al., 2016, Singh et al., 2017, Wang et al., 2016).
Various studies have been conducted regarding NaCl diffusion in meat during immersion-brining with or without other additives (Graiver et al., 2006, Graiver et al., 2009, Hansen et al., 2008). However, the literature reveals no study has been reported using LIBS for analysing salt diffusion in meat. In the current study, an experiment was conducted for imaging salt diffusion in beef using a LIBS system combined with an automatic sample chamber. Chlorine emission peaks are not easily resolvable under normal atmospheric conditions due to interference with nitrogen (N) and oxygen (O) peaks, thus emission peaks related to sodium (Na) were utilized to create salt diffusion images (Weritz et al., 2007). The aim of this study was to illustrate the ability of LIBS as a novel chemical free technique for imaging NaCl diffusion in beef.
Section snippets
Sample preparation
Fresh round beef steak weighing approximately 1.5 Kg was purchased from a local butchers shop in Dublin city and transferred to the School of Food Science and Environmental Health in the Dublin Institute of Technology, Dublin, Ireland. On the same day, the steak was carefully cut in order to separate the lean from the fat beef trimmings. The lean beef was cut into three cubes of approximately 4 cm edge length. A 6% salt brine solution was prepared using laboratory grade NaCl (cas no: 7647-14-5,
Image analysis
Fig. 3 shows the Na distribution for the cross-section of the control, 2 h brined and 24 h brined samples with respect to the Na peak at 589.05 nm which directly represents the salt distribution. The colour scale represents the normalized intensity of Na at various locations of the meat cross section. Fig. 3 (a) illustrates the cross-section image of the control sample which clearly shows the emission intensities of salt (Na) equally distributed throughout the sample at the lowest level of the
Conclusions
This study illustrates the ability of LIBS as a novel spectral technique for imaging NaCl diffusion in beef by utilizing the Na emission peak at 589.05 nm in LIBS spectra. Spectral images of the cross-section of brined beef samples (2 h and 24 h) along with a control sample were generated with respect to the Na emission peak at 589.05 nm. As expected, the control sample showed the lowest salt (Na) distribution whereas 2 h brined sample showed Na distribution along the sample edges which
Acknowledgements
The authors would like to acknowledge funding from the Food Institutional Research Measure administered by the Department of Agriculture, Food and the Marine, Ireland (Grant agreement: 13/F/508).
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