Impacts of microwave and freeze-drying methods on drying kinetics, physicochemical properties and antioxidant activity of pine nut (Pinus pinea L.)

Abstract

Freeze-drying and microwave drying (180, 360, 540, 720 and 900 W) were used to investigate drying behavior, and evaluate impacts of both drying methods on color, texture, rehydration, total phenolic content (TPC) and antioxidant activity of pine nuts. Microwave drying yielded higher effective moisture diffusivity coefficients (Deff) (5.60 x 10(-9) - 9.78 x 10(-9) m(2 )s-(1)) than the Deff value (1.35 x 10(-10) m(2) s(-1)) in freeze-drying. Microwave drying time was affected by microwave power level, leading to almost 50% reduction in the drying time when the microwave power level increased from 180 to 900 W. The total color difference (Delta E*) and browning index (BI) values increased with increasing microwave power levels while hardness, TPC and antioxidant activity decreased. The pine nut that was freeze-dried (FD) had the highest TPC and antioxidant activity whereas the pine nut that was microwave-dried (MWD) had a lower level of TPC and antioxidant activity. Antioxidant activity measurements correlated well (R-2 = 0.954) with the phenolic content. The antioxidant activity assay revealed that phenolic compounds in pine nut had a good antioxidant activity. Higher rehydration temperatures caused higher rehydration ratio (RR) values for FD and MWD pine nuts, and the FD pine nut yielded the highest rehydration ratios. The quality characteristics of FD pine nuts were very close to those of raw pine nuts and were superior to those of MWD pine nuts. It should be stated that freeze-drying is a promising method of drying pine nuts.