Optimization of microwave-convective drying of Oyster mushrooms (Pleurotus ostreatus) using response-surface methodology

Ergothioneine, a potent antioxidant, has been found in highest concentration in oyster mushroom (Pleurotus ostreatus) among other mushroom species. The purpose of the present study was to optimize the drying air temperature and residual moisture content (wb) for the maximization of ergothioneine content, total phenolic content (TPC), 2,2-diphenyl-1-picryhydrazyl (DPPH) radical scavenging activity, total reductive power activity (potassium ferricyanide), and the reduction of colour difference (ΔE) using response surface methodology (RSM). Oyster mushrooms were dried in a microwave-convective dryer, the temperature range was varied from 60 to 80°C and residual moisture content (wb) was varied from 5 to 20%, each at 5 levels. A rotatable central composite design (RCCD) consisting of 13 experimental runs with five replicates at the central points were applied and second-order polynomial models were used to describe the experimental data regarding the responses. Ergothioneine content was found to be highest at 70°C and 12.5% residual moisture content (wb). Total phenolic content was found to decrease with increase in temperature, 11.8 to 4.6 mg GAE/gm dw going from temperature 60 to 80°C. The IC50 for DPPH and total reductive power activity showed a gradual increase, showing decline in the antioxidant activity with rise in temperature, 9.9 to 0.1 mg/ml and 0.0033 to 0.0012 mg/ml, respectively. A rise in drying temperature caused darkening. Thus, the optimized values of responses can be obtained at drying temperature of 69°C and 9% (wb) residual moisture content, yielding an ergothioneine content of 1.73 mg/gm dw, total polyphenols of 8.9 mg GAE/gm dw, IC50 for DPPH activity of 0.09 mg/ml, IC50* (absorbance at 0.5 AU) for total reductive power of 0.0027 mg/ml and colour difference (ΔE) of 18.