# Antioxidant and Potentially Anti-Inflammatory Properties in Pasta Fortified with Onion Skin
## 3 Results and Discussion  
### Phenolic Compound Content  
Onion is known for being one of the natural sources of phenolic compounds in the human diet, containing at least 25 different flavonoids. The most abundant are quercetin 3,4-diglucoside, isorhamentin 3,4-diglucoside, quercetin 3-glucose, quercetin 4-glucose, quercetin aglycone, and phenolic acids, for example, ferulic, gallic, and protocatechuric acids [40]. In _Allium cepa_, about 90% of the quercetin is located in the first and second layers of a bulb, which are treated as waste and generated during industrial and domestic peeling. Over half of the total content of quercetin from dry onion skin is present in potentially bioavailable aglycones [12; 13]. Therefore, it is worth using this source of valuable plant secondary metabolites, e.g., for pasta enrichment. As shown in Figure 1, all uncooked pastas fortified with onion skin powder (P2.5-P7.5) were characterized by relatively similar contents of total phenolics acids (PAC), statistically not significantly higher than the control sample (P0). Neither quercetin (Q) nor any other quercetin glucosides were detectable in that last sample (P0). However, in the case of pasta after cooking with the increase of OS, concentration of PAC and Q gradually and statistically significantly increased. From among the quercetin glucosides, only quercetin-4-glucose was identified. According to the literature, this compound is predominant among all the glycosides occurring in dry onion skins [12]. The amount of those phenolic compounds was the highest in pastas fortified with 5 and 7.5% (m/m) of onion skin (P5 and P7.5). In these fortified pastas, the main source of quercetin, quercetin 4\({}^{\prime}\)-glucose and phenolic acids, was dry onion skin, because semolina (flour), which is the basic ingredient of pasta, contained a significantly lower quantity of PAC and no quercetin levels (Table 1). Semolina is perfect for making pasta due to its unique characteristics--relatively high yellow pigment content, low lipoxygenase activity, and high gluten content, which makes it possible to obtain a strong and elastic dough [41]. Studies confirm that semolina is a poor source of phenolic compounds. Its concentration in semolina is approximately one third of that in bran [42]. A group of phenolic acids is represented by \(p\)-hydroxybenzoic and ferulic acids [43]. The process of cooking resulted in a statistically significant increase in the content of phenolic acids content (PAC), quercetin (Q), and quercetin 4\({}^{\prime}\)-gluciside (Q 4\({}^{\prime}\) G) in the samples of all fortified pastas to 4.52 \(\mu\)g/g DW, 0.50 mg/g DW, and 0.06 mg/g DW, respectively (Figure 1). No other quercetin glucosides (quercetin 3,4\({}^{\prime}\)-diglucoside and quercetin 3-glucose) were found in the extracts analyzed. A similar effect concerning the increase in the content of phenolic compounds was observed in the studies carried out on pastas fortified with phenolic-abundant ingredients, such as onion skin or fleshy husk powder [16; 17; 44], buckwheat [45], chia [46], linseed [47], artichoke [27], and olive oil industrial by-product [30]. It has already been proven that plant material processing can change the structure of the food matrix and may also affect either positively or negatively the plant secondary metabolites. Previous studies have demonstrated that onion flavonoids are relatively heat-stable, and onion outer scale remains the most abundant in flavonoids, even after cooking [15; 48]. Probably for this reason, all fortified pastas after cooking did not have lower contents of phenolic compounds than those that were uncooked. Additionally, thermal treatment may damage the cell walls, making compounds more accessible to absorption. Results obtained in the present manuscript correspond perfectly with the research conducted by [PERSON] et al.