## 1 Introduction  
Pickling--using food additives, i.e., a highly concentrated salt solution (brine)--is a well-known method for preserving foods including vegetables that has been used for thousands of years and remains in use today. Undoubtedly, the most unambiguous role of pickling for human nutrition is to make the nutrients naturally present in the original food materials more palatable while also preserving their quality. Nevertheless, pickling processes can have several effects on food nutritional qualities [1]. The effect of the pickling preservation method and storage time on vitamins and amino acid composition in garlic was reported by [PERSON] et al., 2004 [2]. In addition, the influence of pickling with and without fermentation was revealed, with fermented products showing higher \(\alpha\)-tocopherol, riboflavin, and total amino acid levels except for thiamine and arginine. Another study on Chinese cabbage reported that the nitrogenous content, viz. nitrate and nitrite, in vegetables was dramatically affected by fermentation [3]. Other studies [4; 5] demonstrated a decrease in the level of nitrates during the natural fermentation of cabbage and radish. The risk of the reduction of nitrates to nitrite lies in the possible induction of methemoglobinia [6]. The metabolic changes associated with pickling may thus develop some distinctive properties related to its organoleptic characters, shelf life, and/or safety [7; 8]. In terms of human health benefits, pickled foods produced by fermentation have been recognized as protective for the gut [9] via the microbial and enzymatic processing of food ingredients. Fermentation can improve protein and fiber digestibility by improving micronutrient bioavailability and decaying anti-nutritional factors [10]. Fermentation processes can also reduce toxic compounds such as aflat concurrent with the production of antimicrobial factors such lactic acid, bacteriocins, carbon dioxide, and ethanol, which prevent the growth of food-borne pathogens [11; 12]. _Citrus_ fruits are enriched in polyphenols which mainly include flavonones and, to a lesser extent, flavones, flavonols, coumarins, and phenolic acids, viz. ferulic, \(p\)-coumaric, sinapic, and caffe acids [13]. Phenolic compounds are well known for their myriad health benefits, as they exhibit antioxidant, anticancer, and anti-inflammatory properties, mostly attributed to their remarkable free radical scavenging activities [14]. A study on four Tunisian _Citrus_ varieties revealed the abundance of phenolic acids (73.13-86.40%) [15] among their secondary metabolite profiles, whereas camphene was found to be a major component in _C. limon. Citrus_ fruit is also abundant in coumarins, limonoids, sterols, and flavonoids, which all exhibit various bioactivities [16]. Onion bulbs (_Allium cepa_) encompass a myriad of bioactive natural product classes which are linked to its health benefits, i.e., anticancer or chemo-preventive agents [17]; antimicrobial [18], antiviral [19], and antioxidant activities [20]; protective activities against cardiovascular diseases [21], diabetes [22], neurodegenerative disorders, and cataract formation [23; 24]. Among the secondary metabolites found in onion, flavonoids, fructans, and organosulphur compounds were mainly found to mediate its health benefits [25; 26; 27]. We have previously reported on the flavor makeup of _A. cepa_ red cv. grown in the Siwa Oasis, Egypt, revealing its enrichment in sulphur compounds [28]. Solid-phase microextraction (SPME) is regarded as a powerful analytical tool for volatiles extraction without solvent consumption and with no heat application, which aids in maintaining the original food aroma without the formation of artifacts as compared to steam distillation [29]. Further, confined direct MS analysis with real-time ion source and its applications in the analysis of volatile organic compounds of _Citrus limon_ (lemon) and _Allium cepa_ (onion) was reported [30]. In terms the secondary non-volatile metabolite composition of _A.