Subcritical Water Extraction of Phenolic Compounds from Onion Skin Wastes (Allium cepa cv. Horcal): Effect of Temperature and Solvent Properties

 

1. Introduction

Subcritical water extraction (SWE) has emerged as a powerful green-technology approach for recovering high-value phytochemicals from agro-industrial residues. Onion skins, often discarded as waste, are exceptionally rich in phenolic compounds such as quercetin and its derivatives, which exhibit strong antioxidant, anti-inflammatory, and antimicrobial properties. This study explores how temperature and solvent characteristics affect the extraction yield and chemical profile during SWE of Allium cepa cv. Horcal onion skins. By investigating these relationships, researchers aim to optimize sustainable extraction strategies capable of transforming agricultural waste into valuable raw materials for the food, cosmetic, and nutraceutical industries.

2. Influence of Temperature on Phenolic Yield

Temperature is a critical factor in subcritical water extraction due to its strong influence on water’s dielectric constant, diffusivity, and solvation behavior. As temperature rises, water behaves more like an organic solvent, enhancing its ability to dissolve and mobilize phenolic compounds. This topic discusses how various temperature levels impact extraction efficiency, degradation of thermolabile compounds, and the balance between yield and structural stability of polyphenols from onion skins.

3. Role of Solvent Properties in Extraction Efficiency

The solvent characteristics of subcritical water—including polarity, viscosity, and ion product—change significantly under elevated temperature and pressure. These shifts directly affect the solubility of phenolic compounds and the selectivity of extraction. This topic explores how modifying solvent properties can target specific phenolic families, improve antioxidant activity, and support more eco-friendly extraction processes without the need for organic solvents.

4. Antioxidant Activity of Extracted Phenolics

Phenolic compounds recovered from onion skins display strong antioxidant potential, making them valuable for applications in functional foods, skincare formulations, and pharmaceutical products. This section highlights analytical techniques such as DPPH, ABTS, and FRAP assays used to evaluate the antioxidant power of SWE extracts. It also examines how extraction conditions affect the biochemical potency and stability of phenolic components.

5. Valorization of Onion Skin Waste in Circular Bioeconomy

Onion skins represent a major agricultural by-product that often ends up in landfills despite being highly concentrated in health-promoting phytochemicals. This topic emphasizes how SWE can transform this low-value waste into high-value bioactive extracts. Such valorization supports circular bioeconomy models, reduces environmental burden, and provides industries with sustainable sources of natural antioxidants and pigments.

6. Industrial and Functional Applications of SWE Phenolic Extracts

Phenolic-rich extracts obtained through SWE have diverse industrial applications due to their antioxidant, anti-aging, antimicrobial, and protective attributes. This paragraph explores their use in clean-label food preservation, natural cosmetic formulations, and nutraceutical development. It also addresses the scalability of SWE technology, regulatory considerations, and the need for further research to optimize extraction systems for commercial production.

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