Osmo hydrocolloid assisted dehydration of onion slices: Effect on bioactive components, rehydration, microstructure

The present study aimed to develop pretreatments and dehydration strategies for onions to retain the structure so that improved rehydration could be achieved with better quality attributes and shelf life. Effect of osmotic treatment at NaCl concentrations of 5, 10, 15 and 20 % for the duration of 60, 120, and 180 min and hydrocolloid coating (5, 10, 15 and 20%) on onion slices was studied. Osmotic pretreatments with 10% NaCl for 120 min followed by dipping in 5% gum Arabic solution were found optimum in increasing the total soluble solids content in onion slices for further effective dehydration. The pretreated onions were dried at temperatures of 50, 60, and 70°C using a tray dryer (air velocity of ∼1.5 m/s), drying kinetics and dried onion quality were evaluated. The results revealed that osmotic pretreatment and higher drying temperatures significantly increased the drying rate and decreased drying time for onion slices. However, the drying temperature of 60°C was optimum for reducing the drying time with better retention of ascorbic acid (62%), and pyruvic acid (77%) with a nearly 2-fold increase in total phenolics and total flavonoids compared to fresh onions. A positive change in color attributes and rehydration ratio was observed for osmo-hydrocolloid-treated and dried onion slices. Scanning electron microscopic images of fresh onion slices showed elongated well-arranged cells whereas in dried onions, cell walls were disrupted due to the sudden expulsion of moisture from the onion cells. The microstructure of osmo-hydrocolloid pretreated and dried onion cells showed less structural damage due to the protective effect of hydrocolloid coating and an increase in the solid content during osmotic treatment. Prospects of the current research could focus on exploring different hydrocolloids with innovative drying techniques to further enhance the nutritional and structural retention in dehydrated onions and the potential application of these strategies to other vegetables.