ABSTRACT Onion drying is an essential post‐harvest process that extends shelf life and preserves the nutritional, functional, and sensory qualities of onions. This review evaluates the influence of various pretreatments (brining, thermal, non‐thermal, and potassium metabisulfite) and drying methods (open sun, solar, convective, microwave, and freeze drying) on drying kinetics, quality characteristics, energy efficiency, and cost economics. Among pretreatments, blanching and ultrasound notably improved drying efficiency by enhancing moisture diffusion, reducing drying time, and maintaining product quality. Conventional drying methods, such as sun and hot‐air drying, are simple and low‐cost but often lead to nutrient degradation, color changes, and reduced sensory appeal. In contrast, advanced and hybrid drying technologies (vacuum, freeze, and low‐pressure superheated steam drying) ensure better retention of bioactive compounds, thiosulphinates, and aroma while improving rehydration capacity. However, these techniques are often limited by high energy requirements and scalability issues. Hybrid drying systems provide the most effective compromise between product quality and energy efficiency, achieving higher moisture removal with reduced specific energy consumption. Mathematical models, particularly thin‐layer (Page, Logarithmic) are used for predicting drying behavior and optimizing parameters. High‐temperature drying can cause color darkening and volatile losses, whereas low‐temperature or vacuum‐based drying better preserves phenolics, vitamin C, and aroma. Overall, integrating optimized pre‐treatments with hybrid or energy‐efficient drying technologies can produce superior‐quality dried onion products while minimizing energy use and processing costs, supporting sustainable and scalable industrial drying operations.
Jeevarathinam et al. (2026) studied this question.