**4. Conclusions**

These intriguing results may be explained by the different bioactive compounds identified in each fertilized red onion. SBOR onion, which showed the best positive effects on the cell culture treatment, contained the highest amount of total phenols, single phenolic acids, kaempferol, anthocyanidins, S-methyl cysteine sulfoxide, and volatile compounds correlated to a better in vitro antioxidant capacity as determined by the DPPH, ABTS and ORAC assays. The correlation data evidenced a diversity of action, at the metabolic level, of the different classes of secondary metabolites and mostly of the single compounds belonging to the different classes, evidencing that the chemical structure of a biocompound can determines its reactivity versus free radicals and other ROS, influencing the antioxidant activity.

Considering the data of the relative cellular vitality, the positive effects could be due to the total phenols and, in particular, to the great presence of specific phenolic acids such as chlorogenic and *p*-coumaric and, also, to the flavanol kaempferol, which were more correlated with ORAC and more present in red onions treated with SBOR(LP) than in the other fertilized red onions. In addition to their great ability as scavengers, free phenolic acids, unlike flavonoids, have a high bioavailability and good water solubility [55] and can be absorbed in the stomach, contrary to flavonoids that cannot be absorbed, and only their small quantity can be transported passively through the intestinal wall into the blood [56].

Our study describes, for the first time, the antioxidant effect of the bioactive phenolic fraction from red onion bulbs fertilized with sulphur-bentonite enriched with orange residue or olive pomace on rat cardiomyocytes and primary human fibroblasts. This work is a pilot study that highlighted significant and useful data of how sustainable fertilization can lead to the improvement of the quality of red onions that can be used to develop functional foods or nutraceuticals for the prevention and management of numerous diseases.

From this manuscript emerges how the use of sulphur-bentonite-based fertilizers can represent a tool to increase, also in other species, phytochemicals with beneficial effects on human health and how the antioxidant activity/capacity of functional foods is important in preventing and treating numerous diseases. The phytochemicals enhance the medical and economic values of crops with important consequences on the bio and green economy, creating new opportunities for business.

Further investigations are in progress to test the effects on the H9c2 rat cardiomyoblast cell line and primary human dermal fibroblasts, in terms of viability and oxygen radical homeostasis mammalian cells, of each single compound identified in the extracts, with the aim to verify if the positive effects are due to a single specific compound or to a synergic or additive effect of more compounds.

**Author Contributions:** Investigation, formal analysis, data curation, M.L.M.; conceptualization and methodology, A.V.; Investigation, formal analysis and data analysis, F.M.; Investigation, formal analysis and data analysis, C.M.; data curation, writing—review & editing, T.C.; project administration, funding acquisition, writing original draft, writing—review & editing, A.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Steel Belt Systems s.r.l., Venegono inferior, Varese, Italy (grant number 08092014).

**Institutional Review Board Statement:** This study involving human fibroblasts was approved by the local ethics committee at the University of Bari Medical School (Deliberation n. 847, 30 June 2011).

**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study as reported in Materials and Methods (3.8).

**Data Availability Statement:** Not applicable.

**Acknowledgments:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

**Sample Availability:** Samples of the compounds are available from all the authors.

#### **References**

