Optimization of microwave assisted extraction (MAE) of pectin from black mulberry (Morus nigra L.) pomace

Microwave assisted extraction (MAE) tech-nique was employed for the extraction of pectin from black mulberry (Morus nigra L.) pomace in this research. The effects of process variables namely microwave power (300-900 watt), irradiation time (10-30 min) and liquid to solid ratio (15-30 mL/g) on the yield, degree of esterification (DE) and galacturonic acid content (GalA) of pectin were investigated and optimized by using response surface methodology (RSM) with a face canter central composite design. Depending on the extraction conditions, the yield, DE and GalA of pectins varied in the range of 8.87-14.47% (dry weight basis), 44.44-57.06%, and 29.17-43.13%, respectively. The results showed that both the microwave power and irradiation time had highly significant effects on the pectin yield, while the effect of liquid to solid ratio was found to be not significant, in this regard. The optimal extraction conditions for maximum yield, DE and GalA of pectin were power of 900 watt, irradiation time of 18.17 min and liquid to solid ratio of 15 mL/g. Under these conditions, 13.16% of pectin with DE of 55.07% and GalA of 36.94% was extracted. The intrinsic viscosity and viscosity average molecular weight of a representative pectin sample was also determined and it was found to be 1.22 dL/g and 32.78 kDa, respectively. The black mulberry pomace pectin showed a viscoelastic behavior in the frequency sweep test and the crossover between G' and G" occurred at low frequency region (~ 0.07 rad/s).