ABSTRACT
    
   High hydrostatic pressure (HHP) processing consists of applying pressure to food with an intensity that can vary between 50 and 1000 MPa. HHP technology is considered one of the most economically viable within the non-thermal technologies and allows the obtention of safe and wholesome food products with a high sensory and organoleptic quality. Most studies on the potential and limitations of HHP food processing have focused on microbial and enzyme inactivation. However, the effect of this technology on the microstructure and nutritional components, vitamins and bioactive food compounds has been less studied. The beneficial effect of nutrients and bioactive compounds present in plant foods depends on their bioavailability, which in turn depends on many factors such as the variety, maturity, location and structure of the vegetable matrix, interaction with other components, processing, etc. In this regard, HHP processing may influence the extractability and bioavailability of these compounds. Therefore, the main objective of this PhD thesis is to advance in the understanding of the relationship between the microstructure of plant tissue and improved functionality (extractability) of their bioactive compounds by applying HHP. In this work, a fruit, persimmon (Diospyros kaki L.f.) and a vegetable, onion (Allium cepa L.) have been selected as model plant products because of their content of functional compounds. Persimmon 'Rojo Brillante' has undergone a major expansion in the Valencian Community and is an important source of phenolic compounds (mainly tannins, responsible for the characteristic astringency of the fruit), dietary fiber and carotenoids. Onions, meanwhile, noted for their high content of polyphenols with significant antioxidant activity. To study the microstructure of the different plant samples submitted to HHP different microscopic techniques have been used, Optical Microscopy (LM), Confocal Laser Scanning Microscopy (CLSM), Scanning Electron Microscopy at Low Temperatures (cryo-SEM), Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM). Several physicochemical properties of the samples have also been examined (soluble tannin content, total soluble phenol content, soluble protein percentage, total soluble solids content, pH, color and textural properties).
   The results showed that HHP processing has an important effect on the microstructure of persimmon and onion, affecting the integrity of cell walls and membranes. This cell damage is greater as more degraded is the starting plant tissue and it seems to cause diffusion of soluble components from the interior of the cells into the apoplast and thus favor their extractability. The soluble fiber and carotenoid fractions also experience greater dispersion throughout the plant tissue caused by the HHP treatment. The application of HHP also affects some physicochemical properties of the plant as the soluble tannin content, the total soluble phenol content, the percentage of soluble protein, the total soluble solids content, pH, color and texture.
   The most suitable conditions for the application of HHP in persimmon require an early ripening stage and absence of previous deastringency treatment, being the treatments with 200 MPa at 37.5 °C for 3 or 6 minutes the ones providing the best results, as these treatments improve the extractability of some components and cause less disruption of the microstructure and the physicochemical properties. Regarding the onions, the most appropriate treatments are 100 MPa at 50 °C or 400 MPa 25 °C for 5 minutes. Storage causes structural degradation in the HHP-treated tissues and the evolution of the physicochemical properties studied varies depending on the treatment conditions applied and the duration of the storage.