Impact of Pretreatment and Drying Methods on Quality Attributes of Onion Shreds

Experiments were conducted on dry untreated onion shreds (2 mm thickness) or treated with salt (5% solution) and potassium metabisulphite (0.5% solution) in convective drier at 50 °C ((46±4) % relative humidity (RH)), 55 °C ((35±4) % RH), 60 °C ((28±4) % RH) and 65 °C ((20±4) % RH), heat pump-assisted convective drier at 35 °C ((32±2) % RH), 40 °C ((26±2) % RH), 45 °C ((19±2) % RH) and 50 °C ((15±2) % RH) and microwave-assisted convective drier at four microwave power levels, i.e. 120, 240, 360 and 480 W. The quality parameters of the dried onion shreds, namely rehydration ratio, colour difference, pyruvic and ascorbic acid contents and sensory scores were evaluated. The quality of dehydrated onion shreds was observed to be comparatively better when treated in heat pump drier at 50 °C, followed by that in microwave-assisted convective drier at 240 W and 50 °C, and last in convective drier at 60 °C. The onion shreds pretreated with potassium metabisulphite retained better colour of the dried product irrespective of drying methods. Therefore, heat pump drying may be recommended as one of the best drying methods for onion shreds, because it maintains the final product quality, which has practical importance for the food industry.