Phytochemical characterization and evaluation of the in vitro bioaccessibility and in vivo bioavailability of black onion (Allium cepa L.) and black garlic (Allium sativum L.)

Black onion is a product analogous to black garlic and is obtained by a similar production process involving the ageing of the product. The fresh onion (Allium cepa L.) is subjected to controlled temperature and humidity conditions for a certain period of time. This process causes a series of physico-chemical changes in the final product, resulting in a foodstuff with sensory qualities substantially different from those of the initial bulb and with important changes in its phytochemical composition. To date, black onion has not been the subject of research despite the success of the commercialisation of black garlic and the beneficial health properties with which it has been associated. This makes the phytochemical characterisation of this new product necessary, for which the methods of extraction and analysis must be validated and optimised, as well as evaluating in vitro and in vivo, the possible changes produced in the profile of phytochemical compounds during gastrointestinal digestion, colonic fermentation and their metabolisation and excretion once they are consumed. In this Doctoral Thesis, two selective, sensitive and accurate chromatographic methods for the identification and quantification of flavonoids, organosulfur compounds and amino acids in fresh onion and black onion were optimised and validated and applied to study the influence of the processing of this product on the profile of phenolic compounds, organosulfur compounds, amino acids, sugars and organic acids from three different varieties of onion. This allowed to conclude that the processing of black onion has a significant influence on the profile of nutritional and functional compounds, and that these changes are influenced by the sensitivity of each of the compounds to heat treatment, pH, the presence of oxygen and the composition of the matrix. On the other hand, the impact of in vitro gastrointestinal digestion on the phytochemical profile of fresh garlic, black garlic, fresh onion and black onion was evaluated and the bioaccessibility index of these compounds was determined. This showed that the production process by which black garlic and black onion are obtained could have a positive effect on the bioaccessibility of the organosulfur compound profile, although this is not the case for the phenolic compound profile. The evolution of the phenolic and organosulfur compound profile during in vitro colonic fermentation of fresh garlic, black garlic, fresh onion and black onion was also studied, which allowed establishing that the processing of black garlic and black onion could also be influencing the metabolism of these compounds at the colonic level, with different trends observed between the original and the final products. Finally, the bioavailability of phenolic and organosulfur compounds present in black garlic and black onion was addressed by means of an acute intake intervention study, for which ii urine samples were collected, making it possible to locate the majority absorption of these compounds in the small and large intestine.