Drying and Storage Engineering

Lesson.20 Miscellaneous Drying

20.2 REFRACTANCE WINDOW® DRYERS

Refractance Window drying is a relatively new indirect drying technology that is used to evaporate moisture from foods (Nindo and Tang, 2007; Nindo et al., 2007; Vega-Mercado et al., 2001). This approach uses a plastic fi lm to facilitate thermal energy transfer between a heating medium (i.e., water below the film) and a suspension (on the surface of the film). The thin plastic film, fabricated from Mylar, allows the transmission of infrared radiation in a wavelength that matches the absorption spectrum of water in the product. Refractance Window was developed to provide an alternative to long drying times and/or the use of high temperatures. Although the water is maintained at temperatures just below boiling at 95 to 98°C, product temperatures do not exceed 70°C. Shorter drying times are demonstrated by a reduction from hours (tray and freeze-drying) to 5 min (Refractance Window) for strawberry puree (Nindo and Tang, 2007). This system is applicable to the drying or preconcentration of liquid foods. It is also being pursued for its potential to process fruits, vegetables, and herbs into value-added powders, flakes, and concentrates.

20.3 Hybrid Drying

The use of hybrid drying technologies is another approach to combine the advantages of different drying methods which are in practice.

The combination of the osmotic dehydration and hot air drying is one of the important hybrid drying techniques.

The osmotic dehydration of foods prior to hot air drying partially removes water and thus reduces water removal load at the dryer. Also, solute gain creates elevated temperature during hot air drying resulting in faster drying rates as well as solid uptake by the root pieces modify the composition (sugar to acid ratio), prevent the enzymatic and oxidative browning, and improves the sensory attributes (colour, flavour, texture, taste and overall acceptability) of the final product.

Foods like onions can be dried using combination of osmotic dehydration and fluidized bed drying to get the better quality dehydrated onions with less energy consumption ^[19]. Recently, osmotic dehydration is combined with microwave drying.

Microwave vacuum drying of osmotically pre-treated foods combines the benefits of both the operations and high quality product can be obtained. The combined osmotic and microwave drying results in more homogeneous heating of the product by modification of its dielectric properties due to the solute uptake, slightly reduced drying time, reduced shrinkage, high porosity and improved rehydration characteristics ^[49,50] .

Literature shows some studies on the combined osmotic microwave vacuum dehydration of carrots and potatoes ^[19,51].

 The value addition in the orange coloured carrots can be done by increasing its sweetness using osmotic pre-concentration and further it can be dried by microwave vacuum drying.

The probable benefit of the osmotic pre-treatment using sucrose solution is simultaneous sugar gain and osmotic dehydration which reduces the water removal load during finish drying by microwave vacuum drying.

The hybrid drying techniques of foods involve low energy unit operations and result into high quality product ^[52].