Optimization of air jet impingement drying of okara using response surface methodology

•Fresh okara was dried by an air jet impingement drier.•Drying okara at low temperature to retain the quality.•Drying variables were optimized by response surface methodology.•Synthetic evaluation index was used to identify optimal drying conditions.•The actual S value was close to the predicted S value.

Okara is a byproduct from the process of soybean foods, and is rich in proteins, fatty acids and dietary fibers. Drying of wet okara is essential for long-term storage and its value-added applications. This study applied response surface methodology and synthetic evaluation method to optimize the drying process of okara in an air jet impingement drier (AJID). Air temperature (50-70 °C), air velocity (1.3-2.3 m/s), and sample loading density (3-4 kg/m2) were considered as treatment factors in the optimization, while drying rate, color, trypsin inhibitor activity, soy isoflavone content and antioxidant activity were evaluated as responding quality parameters. All treatment variables showed significant effects on the drying rate and soy isoflavone content (P < 0.05). Higher temperature, higher air velocity and lower loading density contributed to higher drying rate. Temperature and air velocity showed quadratic and interactive effect on the antioxidant activity of okara. The optimum conditions for AJID of okara were identified as 70 °C, 2.3 m/s air velocity, and 3 kg/m2 loading density. This process control study provided baseline data for developing effective drying of okara using AJID.