DRYING KINETICS AND ANTIOXIDANT ACTIVITY OF OREGANO

Abstract

A b s t r a c t . Oregano was dehydrated by three methods: freeze drying, convective drying and vacuum-microwave drying (VM). Freeze drying was conducted at temperature of -60 °C and under pressure of 65 Pa. The temperature of heating plate was 30 °C. Convective drying was performed at three temperatures: 50, 60 and 70 °C. Hot air velocity was 1 m s -1 . In VM dehydration the pressure in drying chamber was changing from 4 to 6 kPa and three microwave power levels were applied: 240, 360 and 480W. The decrease in moisture content of oregano dehydrated in convection was described by an exponential function, while in VM method by a linear function in th e first drying period and by an exponential functio n in the second period. Moisture content at the critical point which divides drying curves into two periods was about 1.4 kg kg -1 dm regardless of the drying temperature used. The s tudy revealed that the increase in drying temperature (convective dehydration) and in microwave power (VM) resulted in decreasing of drying time. Freeze drying was the least destructiv e method of drying because of the antioxidant activ ity. The most harmful method was convective drying at temperatures of 60 and 70 °C. It was observed that higher phenolic content in dried oregano may be reached by reduction of temperature in convective method or by increasing the microwave power in V-M method. K e y w o r d s : oregano, drying kinetics, antioxidant act ivity, polyphenols