Drying behavior of microwave dried ginger and their energy consumption

Abstract

Fresh ginger slice was dehydrated in a microwave oven to study the drying behavior and energy consumption of dried products. The ginger was dried at different microwave power and slice thickness. Moisture content as well as the drying time both decreased with a decrease in slice thickness in most of the cases. In majority of the cases, the Henderson and Pabis model was found to fit the best to the experimental data. The linear model for moisture content was found significant at p≤0.01.The regression model for the energy consumption was significant at p≤0.01. The removal of moisture from food products is one of the oldest methods of preservation. By reducing the water content of food products to very low levels, the likelihood of microbial deterioration is minimized and the rates of other deteriorative reactions are reduced significantly. In addition to preservation, dehydration reduces mass and volume of product by a significant amount and improves the efficiency of their transportation and storage. Often, the dehydration of food results in a product that is more convenient in use. Ginger is herbaceous perennial plant known as Zingiber officinale (Rosc.), which belongs to the order scitamenase and the family, zingiberacease. It is a tropical herb extensively grown for its pungently aromatic underground stem or rhizome which is an important export crop valued for its powder, oil and oleoresin (NEPC, 1999). The ginger is used as spice in culinary, beverage, confectionery, pharmaceutical and perfumery industries. Dry ginger contains 1-3 % essential oil, 5-10 % oleoresin, 50-55 % starch and 7-12 % moisture, with protein, fiber; fat and ash as its other constituents (Ebwele and Jimoh, 1981). Ginger contains moderate amounts of magnesium, calcium, protein, calcium, iron, sodium, potassium and phosphorus. Together they act as are medicine for muscle spasms, depression, hypertension, muscle weakness, convulsions, confusion, personality changes, nausea and gastrointestinal disorders. The high level of potassium in ginger protects the body from bone fragility, paralysis, sterility, muscle weakness, kidney damage and damage to the heart. Ginger also has a high content of antioxidants which means it also has anti-inflammatory properties. Ginger is a seasonal product but it is used extensively in many food herbal medicine preparations. Hence, dried ginger or ginger powder is used in large quantity during off-season. Okafor and Okafor (2007) studied the effect of pricking and sun drying on colour retention of Nigerian yellow bark ginger. The interior colour of fresh ginger surface was 24% lighter than the skin surface colour. Drying decreased the external surface colour by 12.5%, pricking darkened the interior surface of ginger by 7.2% and 2.6% respectively for black and gray backgrounds respectively. Balladin et al. (1996) used a wire basket dryer (1.8 m x 0.9 m x 0.2 m) to dry sliced (0.15 cm) West Indian ginger rhizome to a moisture content of 10.2% (db) over a 3 day period. The optimum charge size was 14.97 kg, with a packing density of 462.04 kg m-3 and a specific drying rate of 0.446 h-1. The quantities of the main pungent principles extracted from fresh, non-steam-distilled solar-dried and steam-distilled solar-dried ginger rhizomes showed increases of 0.068, 0.46 and 0.67 g/per 100 g (db), respectively, with a decrease in the oleoresin quality (reflected in pungency profile) of the same order. In microwave drying, electromagnetic energy is directly converted into kinetic energy of the water molecules, thus generating heat within the product, and energy transport is not affected by conductivity barriers, especially in high viscosity or lumpy materials (Bouraoui et al., 1994; Kiranoudis et al., 1997; Kudra