Abstract
Currently, the food industry is facing many pressures and challenges, among which it is important to highlight the innovation as the only way to face the huge competition that characterizes the market. 
The innovation in process is oriented to the research and to the application of novel technologies. To improve and to innovate in the processes will lead to safety and better quality products, which are related to current consumer demands. In this context, the modern sensors based on the application of electromagnetic energy are great improvements in the quality control of Food Industry. 
This doctoral thesis studies the application of electromagnetic radiation to different products and processes in the food industry, in order to solve some of the major problems in the meat sector and in post-harvest industry, as well as to control the osmotic dehydration operation of meat and fruits. 
First, the use of electromagnetic energy to control the quality of fresh meat was analyzed; in this sense, the possibility of characterizing the meat quality class by using electromagnetic spectra was studied. Moreover, the viability of determining the meat ageing process by analyzing the evolution of the electromagnetic spectrum was determined. Two frequency ranges were studied: from 100Hz to 0.4MHz, and from 0.5GHz to 20GHz. For this purpose two equipment were used: the parallel plates Agilent 16451B connected to an Impedance Analyser Agilent 4194A for low-frequency measurements, and the coaxial probe Agilent 85070E connected to a vector network analyzer Agilent E8362B for higher frequencies. Different biochemical and physical-chemical parameters were also determined. All these measurements were made at different postmortem times ranged from 2 hours to 7 days after slaughter.
The main conclusion of this research was that the dielectric spectra of meat samples showed differences between the three pork quality classes studied: PSE, DFD and RFN. These differences were significant at different frequencies at different postmortem times. The differences were attributed to the different metabolisms of the different kinds of meat. The results obtained in this study confirmed the feasibility of developing an industrial sensor to assess the quality of pork loins during the first hours postmortem. Moreover, two Ageing Indexes were defined based on different frequencies of meat spectrum and its effectiveness was demonstrated. Its relationship with traditional indicators of meat ageing such as the K value, the free amino acids content and some parameters of meat texture was also demonstrated.
Secondly, dielectric spectroscopy studies were performed on raw and salted longissimus dorsi pork samples. Dielectric spectra were measured by an Agilent 85070E open-ended coaxial probe connected to an Agilent E8362B Vector Network Analyzer immediately after salting process and also after 16 hours of repose. Salt content, moisture, water activity and volume were also measured. It was demonstrated that the rigor mortis state affects meat behaviour during the salting operation. Moreover, a thermodynamic model in non-equilibrium has been developed for describing the different behaviors involved in salting process. Three steps were identified during the salting operation with sodium chloride saturated solution: an initial relaxation of tissue, compressed during rigor mortis process; a muscle contraction promoted by the fast relaxation of elastic fibers; and finally, the salted samples reached mechanical equilibrium. The last conclusion of this work was that dielectric properties are a good tool to analyze the share of the added sodium chloride molecules in the muscle tissue during the salting treatment (bounded and solved ions).
Thirdly, a preliminary standard solutions study, simulating the liquid phase of apple fruit, was made; the main conclusion was that this system can correctly describe the relationship between the apple components and electromagnetic radiation, and allows identifying the phenomena attributed to key chemical species in apple maturation process along the electromagnetic spectrum. Subsequently, a study was carried out in fresh apple tissue for analyzing its maturation process and for analyzing the feasibility of controlling this process through the application of electromagnetic radiation. A maturity index based on the dielectric properties was developed and good correlations with the Thiault Index were obtained in the standard solutions and in structured tissue of Granny Smith apple.
Fourthly, osmotic dehydration experiments of Granny Smith apple were carried out. Samples were immersed into 65% (w/w) sucrose aqueous solution at 30ºC during different times. Some physical-chemical parameters and also dielectric spectra were measured in fresh, treated and reposed (24h at 30ºC) samples. Osmotic dehydration operation was modelled by using a nonlinear thermodynamic approach. The results allow identifying three differentiated steps in the osmotic operation process. At the beginning of the treatment, the cells were turgid and the wall-membrane system stored high level of mechanical energy. In this step, the highest velocity of shrinkage occurred. In the second step, the main driving force was diffusional, with important concentration profiles in the tissue; in this step, the deformation of the tissue reached the maximum shrinkage level of approximately 60%. It produces an important structural change in the tissue removing the concentration profiles and changing the mechanism driving forces.  Last step shows internal relaxation of the tissue, increasing the mechanical transport behaviours. This work demonstrated that dielectric spectra allow controlling the dehydration process by using the loss factor at relaxation frequency.
Finally, the osmotic dehydration of kiwifruit (Actinidia deliciosa cv Hayward) was studied. The samples were introduced in sucrose aqueous solution of 65 °Brix at 30°C for different times. Some physical-chemical parameters and also dielectric spectra were measured in fresh, treated and reposed (24h at 30 º C) samples. The dehydration operation was modelled by using a nonlinear thermodynamic approach. The results allowed describing the diffusional and mechanical terms of the system during the treatment; the dehydration operation produces contraction/relaxation phenomena which result in important changes of volume and structure, and also provokes compositional changes. This work demonstrated that dielectric spectra allow controlling the dehydration process by using the loss factor at relaxation frequency.
The results of this document have led to the presentation of two patents; the first patent: “Method and device for the discrimination of food”, which consists in verifying if the foods accomplish predetermined quality criteria. The second patent: "Method of discrimination of fruit pieces according to their degree of maturity and calibration of the method", consists in a method that allows discriminating pieces of fruit according to their degree of maturity, for example to carry out a discrimination method for classifying fruits in postharvest line by using a non-destructive spectroscopic technique.