Infrared Drying of Onion

Infrared radiation drying is the new word in the world of drying. Infrared radiation drying is fundamentally different from convection drying where material is dried directly by the absorption of IR energy rather than the transfer of heat from the air. When infrared radiation (IR) is used to heat or to dry moist materials, the radiation impinges on the exposed materials surface, penetrates it and the energy of radiation converts into heat. Infrared radiation drying is especially suitable to dry thin layers of material with large surface exposed to radiation.

The electromagnetic spectra within food wavelengths can be divided into three parts: long waves, medium waves and short waves. Spectral dependence of infrared heating needs to be considered because energy coming out of an emitter comprises of different wavelengths and the fraction of the radiation in each band is dependent on a number of factors such as the temperature of the emitter, emissivity of lamp, etc. Infrared radiation has some advantages over convective heating. Heat transfer coefficients are high, the process time is short and the cost of energy is low. In spite of these advantages, application of infrared energy in food processing is rather scarce. The radiation energy is absorbed by the surface layers and converted to heat. In wet bodies, the highest temperature occurs under the irradiated surface layer and depends on the extinction coefficient. The smaller the extinction coefficient, the larger the distance from the surface at which maximum temperature occurs. Hence, heat generated in a layer under the surface is conducted towards the center of the body as well as to its surface. Heat from the surface to the surrounding air is transferred by convection. The application of combined electromagnetic radiation and hot air heating is considered to be more efficient than radiation or hot air heating along as it gives a synergistic effect.

A laboratory scale infrared-convective dryer was developed for the present study wherein infrared power, air temperature and air velocity could be varied within the range of 0-500 W, 30-60oC and 0-1.5 m/s. Red onion, procured from the local market, was used in the present study. The onions were hand peeled, cut into slices of approximately 6 ± 0.1 mm thickness with sharp stainless steel knife. The initial moisture content of the onion slices was measured 7.641 gm water/gm dry matter. The mass of the onion was measured by a digital electronic balance at interval of 15 min. Drying time was defined as the time required to reduce the moisture content of the product to 6% (dry basis), the average moisture content of a commercial dry product.

The drying rates are typical to ones for food stuffs, i.e., moisture content of onion slices decreased exponentially with elapsed drying time. As the air temperature increased, other drying conditions being same, indicating higher moisture removal rates thus resulted into substantial decrease in drying time (t). At air velocity of 1.0 m/s and infrared power of 300 W, drying time for the onion slices at air temperature of 35, 40 and 45oC were about 8.5, 8.0 and 7.8 h, respectively. Similar drying trends were observed at air velocities of 1.25 and 1.5 m/s at other infrared power levels. The drying time reduced dramatically with increase in infrared power. The drying time to reduce the moisture content of onion slice to about 0.06 gm water/gm dry matter at infrared power of 300, 400 and 500 W was about 9,7 and 4 h, respectively. The increase in infrared power might have caused a rapid increase in the temperature at surface of product, resulting into an increase in the water vapor pressure inside the product and thus in higher drying rates. A similar drying trend was observed at other air velocities of 1.25, 1.5 m/s; and air temperatures of 40 and 45oC. At a given air temperature and infrared power, an increase in air velocity increased the drying time i.e. decreased the moisture removal rate. The moisture content at any time was found to be little higher with higher velocity. The increase in air velocity accelerated the cooling effect, reducing the temperature at the surface of product thus the water vapor pressure or the moisture driving force.

The average effective moisture diffusivity D for all drying conditions ranged from 0.2514 x 10-10 to 0.3233 x 10-10 m2 s-1. The values of D increased progressively for the same values of drying air temperature and air velocity as applied radiation intensity was increased. This reduced drying time dramatically. However, at an air velocity of 1.5 m s-1, the values of D decreased at all air temperature and applied radiation intensity compared to similar drying conditions at an air velocity of 1.0 and 1.25 m s-1, a trend contrary to convective drying. This is due to cooling of onion slices at higher velocity because product temperature remained higher than surrounding air resulting in negative temperature gradient.

The rehydration ratio was considered as one of the important quality attributes for the dried slices in the present study. It varied between 4.5 and 5.3, under different drying conditions.

Source: Invention Intelligence, September - October 2006