Effect of various pretreatments on dehydration of onion

Onion was pretreated with citric acid (CA), potassium metabisulphite (KMS) and NaCl for different pretreatment levels and dried in mechanical tray dryer at 50 oC temperature. The quality parameters of dried onion viz. rehydration ratio, pyruvate content and colour (L, a and b) was analysed and the process variables were optimized by I-optimal technique of multiple regression used. In pretreated onion samples, drying period was minimum 680 min at 0.5 percent KMS pretreated level while it was minimum 1000 min at 20 per cent NaCl pretreatment level for drying upto 8 % (db) moisture content. Moisture diffusivity of pretreated onion samples ranged from 1.09 × 10-8 to 1.72 × 10-8 m2/s. The rehydration ratio of CA, KMS and NaCl pretreated onion was in the range from 3.76 to 4.06, 4.53 to 4.83 and 2.87 to 3.73 respectively. Pretreated onion with 0.75 per cent KMS had lighter colour (L = 85.9) than with other pretreatments while onion with 12.5 per cent NaCl showed was dark colour (L = 75.28). Similarly, yellowness was maximum (b = 28.32) for 5 per cent NaCl pretreatment. The pyruvate content ranged from 10.2 to 10.9, 11.9 to 12.8 and 9.8 to 8.6 µmol/g for CA, KMS and NaCl pretreated onions respectively.