## 1 Introduction  
_Allium_ is the larger genus of the Alliaceae family, with approx. 450 species [1]. Among these species, onion (_Allium cepa_ L.) is widely used in various forms all over the world as part of the international cuisine, in food processing but also in traditional medicine [2]. It has been cultivated for more than 4000 years, with USA, China, Turkey, Russia and Egypt as the main producers [3]. Some of the reported properties of onion are antidiabetic, antibiotic, antioxidant, antimicrobial, antifungal,antiashmatic, anticancer, anti-inflammatory, hypolipidemic and anticholesterolemic [2; 4; 5; 6; 7; 8] but also the usefulness of consuming dried onions in preventing viral infections, coronary hearth diseases, cataract and disturbances of the gastrointestinal tract have been described [1; 9]. Fresh onion is very perishable and the postharvest action of enzymes as pectin methylesterase and polygalatturonase lead to onion softening [10], thus, in order to extend onion shelf life, drying fresh onion for obtaining chip or powder is one of the most used processes [11; 12]; dried onion has gained increasingly attention as an alternative to the fresh onion as an ingredient for sauces, snacks and frozen foods, for both industrial and domestic uses, also thanks to several characteristics meeting the consumers' expectations [13; 14]. In the powdered form, dried onion could be a very easy-to-use functional food ingredient [12]. In 2017, according to Food and Agriculture Organization (FAO), the global production of dried onion accounted for about 97 million tons, with China and India as the main producers, while the Italian production accounted for approx. 0.4 million tons (\(\approx\)0.4% of the global production). The characteristic flavor and biological properties at the basis of these uses of onion have been mainly attributed to the sulfur compounds present in the volatile fraction of onion. The volatile compounds emitted by onion also demonstrated anti-browning activity, with trisulfides as better inhibitors than disulfides [4; 15]. It has been reported that the antimicrobial activity of sulfides is affected by the number of \(C\)-atoms of the alk(en)yl groups and \(S\)-atoms and by the concentration of the different sulfur compounds in the volatile profile of onion, pointing out the importance of characterizing the volatile profile of onion in the different used forms, in that the composition of the sulfur compounds varies among the different flavored forms of onions [4; 12; 14]. Another class of volatile compounds detected in fresh or treated onion is that of aldehydes [12; 14]. The type of sulfur compounds found in onions and other _Allium_ species is strongly affected by the nature of the \(S\)-alk(en)yl-L-cysteine-\(S\)-oxide precursors (Figure 1). For example, in garlic, propini is almost absent and alli is the major compound, while in onion, alli is present at a very low concentration [16] and iso-alli is the major metabolite. Iso-alli and its derivatives containing the 1-propenyl moiety are usually present as a mixture of the \(E\) and \(Z\) isomers. In onion, some derivatives of iso-alliin are also transformed in \(Z\)-propanethi \(S\)-oxide (the lachrymatory factor) and its isomer \(E\)-propanethi \(S\)-oxide, _cis_ and _trans_-zwiebelane and cepaenes [1], which largely predominate in fresh onion. Analysis by SPME-GC-MS of fresh, frozen, freeze-dried and sterilized onions pointed out that, in the transformed samples, most of the sulfur compounds present in fresh onion (thiosulfinates and zwiebelanes) degraded to form other sulfur volatiles--mainly disulfides and trisulfides--with the consequent changes in sensorial and nutraceuticals properties [17]. For example, frozen onions were reported to be rich in 2 methyl-2-pentenal (a degradation product of the LF) and three disulfides, while in dried onions a strong diminution of disulfides, a certain increase of some trisulfides and the presence of tetrasulfides were reported [12; 17].