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1. Introduction
Onion (Allium cepa L.) is the most widely cultivated species of the genus Allium [1].
The plant portion commonly used is the bulb, which is utilized as a food ingredient to give flavour and aroma to a great variety of dishes.
Onions are an important source of several phytonutrients as flavonoids, fructooligosaccharides (FOS), and thiosulfinates and other sulfur compounds, recognized as important elements of the Mediterranean diet [2].
In fact, onions contain high levels of phenolic compounds, which have antioxidant properties besides beneficial effects against different degenerative pathologies (cardiovascular and neurological diseases, dysfunctions based on oxidative stress) [3].
Flavonoids are the major phenolics in onions, which can be classified to different subclasses (flavones, flavanones, flavonols, isoflavones, flavanonols, flavanols, chalcones, and anthocyanins) on the basis of the degree of unsaturation and the degree of oxidation of the central ring. Flavonoids subclasses can be further differentiated on the basis of the number and nature of substituent groups attached to the rings [4].
Flavonols are the most abundant in onions, present as their glycosides, that is, quercetin and kaempferol [5, 6], in higher concentration (280–400 mg/kg) than other vegetables (i.e., 100 mg/kg in broccoli, 50 mg/kg in apple) [7]. Anthocyanins, belonging to anthocyanidins, are mainly present in red onions (250 mg/kg), besides having a composition rich in flavonols as yellow onions [7].
FOS represent another source of phytochemicals in onions bulbs. They are mainly inulin, kestose, nystose, and fructofuranosylnystose. The health benefits of these carbohydrates have been widely reported in the past years due to their prebiotic effect [8].
In onions, sulfur compounds are responsible for typical odour and flavour and are also active antimicrobial agents [9]; hence, onions may be used as natural preservatives to control microbial growth [10]. Furthermore, they have also protective effects against cardiovascular diseases.
The precursors of sulfur-containing compounds are S-alk(en)yl-L-cysteine sulfoxides (ACSOs, i.e., methiin, propiin, and isoalliin) which are hydrolysed by means of alliinase enzyme into pyruvate, ammonia, and a mixture of both volatile and nonvolatile sulfur compounds [11], after the breakage of the tissue caused by cutting, mastication, and cooking.
The concentration...