Microwave, Vacuum, and Air Drying Characteristics of Collard Leaves

Abstract

Collard leaves (Brassica oleracea L. var. acephala) with an initial moisture content of 6.65 on percentage dry basis (%db) were dried by three different drying methods: microwave, air, and vacuum. Samples of fresh leaves, 25 g each, were dried until their moisture was down to 0.1 on a dry basis. The following drying levels were used in each of the drying processes: 350, 500, 650, 750, 850, and 1000 W for microwave drying; 50, 75, 100, 125, 150, and 175 C for air drying; and 0.4, 50, and 100 mmHg at 50 and 75 C for vacuum drying, respectively. Drying times ranged between 2.5 to 7.5 min, 8 to 210 min, and 35 to 195 min for microwave, air, and vacuum drying, respectively. The data obtained compared well with a thin-layer drying model. Microwave drying at 750 W provided optimal results with respect to drying time, color, and ascorbic acid content (vitamin C).

Convective, vacuum and microwave drying kinetics of mallow leaves and comparison of color and ascorbic acid values of three drying methods

“…Determination of the investigated components was carried out in three replicates. Mean differences vacuum drying had an important effect on the reduction of drying time (Alibas, 2007(Alibas, , 2009). The extent of drying realised at 50 °C temperature and 7 kPa vacuum with the longest drying period was 3.42 times higher compared with the drying process realised at 75 °C and 3 kPa, with the shortest drying period.…”

“…The moisture content of the material was very high during the initial phase of the drying, which resulted in a higher absorption of microwave power and higher drying rates because of the higher moisture diffusion. As the drying progressed, the loss of moisture in the product caused a decrease in the absorption of microwave power and resulted in a fall of the drying rate (Alibas, 2009;Soysal, 2004;Sharma & Prasad, 2001). The average convective drying rates during the drying of mallow leaves at 50, 75, 100 and 125 °C were 0.009, 0.015, 0.029 and 0.044 kg water.…”

“…All weighing processes were completed in 10 s during the drying process. Drying process continued until the moisture content of mallow fell down to 0.1±0.005 on dry basis (Alibas, 2009). Energy consumption of microwave, air and vacuum oven (with vacuum pump) was determined using a digital electric counter (Kaan, Type 101, Turkey) with 0.01 kWh precision.…”

“…Ascorbic acid was determined by extracting the samples with oxalic acid (0.4%) and then reading and calculating the absorbency values at 520 nm with a spectrophotometer (Shimadzu UV-120-01, Shimadzu Co., Duisburg, Germany) (Alibas, 2009). …”

“…Moreover, microwave drying reduces the drying time and saves energy during drying in producing high quality dry products (Balbay et al, 2011;Li et al, 2010). Vacuum drying includes lower process temperatures, less energy usage and hence greater energy efficiency, improved drying rates, and in some cases, less shrinkage of the product (Alibas, 2007(Alibas, , 2009). …”