55
However, freeze drying also has disadvantages such as high capital and operating
costs and long processing time (Hammami et al. 1999). This makes the freeze-dried
product a more expensive product for the consumer than the products processed by
other methods.
In freeze drying, water in the products is removed by sublimation at low temperature and pressure below the triple point. First, the water in the products is frozen
in the freezing step and then removed by sublimation of ice in the first stage of drying. Freeze drying requires very low pressures or high vacuum to achieve a satisfactory drying rate. If the water is pure, freeze-drying can be carried out at or near 0 °C
under an absolute pressure of 4.58 mmHg. However, since water is usually present
in combination or in a solution, it must be cooled below 0 °C to keep the water in
the solid phase. The most critical stage of freeze drying is freezing. Freezing should
be made fast at this stage. Because of the slow freezing occurs large ice crystals and
this also deteriorates the quality of the final product. In the first stage of drying, the
pressure is reduced by applying a high vacuum and heat is applied to provide the
energy required to sublimate the ice. The water of 95% removes the first drying
stage. This process can take from a few hours to 2 days. Moorjani and Dani (1968)
dried the shrimps by freeze-drying method and compared to air drying method.
Drying process took 12 h and the moisture content of dried shrimp was 3.0%.
According to the sensory analysis results freeze dried shrimp were found better
quality than air dried shrimps.
Freeze drying was also used in combination with other drying methods for
shrimp drying. In a study combining freeze drying with electrohydrodynamic drying (EHD), a group of shrimps were dried at 18 °C by EHD, while one group was
dried by freeze-drying (FD) and the other group was dried by combination of both
methods (EFD). The samples were dried with EHD for 3 h and then the water content of the product was reduced to 12% by FD method. Less drying time was determined by the combined procedure compared to FD and EHD alone. The shrimp
dried by the combined drying treatment exhibited lower shrinkage, higher rehydration rate and better sensory properties (Hu et al. 2013). In another study, freeze
drying was combined with air drying. Similar results were reported with combined
drying method (Bai et al. 2013).
Microwave Assisted Drying
Microwave radiation is a part of the electromagnetic spectrum with wavelengths
ranging from 1 mm to 1 m. The frequency used for food applications is 2.450 MHz.
Microwaves are not ionized in nature and are known as a fourth-generation technology. Microwave-assisted dehydration is increasingly noteworthy. Microwaveassisted drying is applied in conjunction with other techniques to solve problems
encountered in conventional drying processes. It is reported that microwave assisted
drying is applied in various types of food and good results are obtained. In the studies conducted with microwave-assisted drying, decrease in drying time and energy
consumption, and increase in drying speed have been reported. Less space requirements and better overall process control are among the advantages of this method
2.1 Shrimps/Prawns
However, freeze drying also has disadvantages such as high capital and operating
costs and long processing time (Hammami et al. 1999). This makes the freeze-dried
product a more expensive product for the consumer than the products processed by
other methods.
In freeze drying, water in the products is removed by sublimation at low temperature and pressure below the triple point. First, the water in the products is frozen
in the freezing step and then removed by sublimation of ice in the first stage of drying. Freeze drying requires very low pressures or high vacuum to achieve a satisfactory drying rate. If the water is pure, freeze-drying can be carried out at or near 0 °C
under an absolute pressure of 4.58 mmHg. However, since water is usually present
in combination or in a solution, it must be cooled below 0 °C to keep the water in
the solid phase. The most critical stage of freeze drying is freezing. Freezing should
be made fast at this stage. Because of the slow freezing occurs large ice crystals and
this also deteriorates the quality of the final product. In the first stage of drying, the
pressure is reduced by applying a high vacuum and heat is applied to provide the
energy required to sublimate the ice. The water of 95% removes the first drying
stage. This process can take from a few hours to 2 days. Moorjani and Dani (1968)
dried the shrimps by freeze-drying method and compared to air drying method.
Drying process took 12 h and the moisture content of dried shrimp was 3.0%.
According to the sensory analysis results freeze dried shrimp were found better
quality than air dried shrimps.
Freeze drying was also used in combination with other drying methods for
shrimp drying. In a study combining freeze drying with electrohydrodynamic drying (EHD), a group of shrimps were dried at 18 °C by EHD, while one group was
dried by freeze-drying (FD) and the other group was dried by combination of both
methods (EFD). The samples were dried with EHD for 3 h and then the water content of the product was reduced to 12% by FD method. Less drying time was determined by the combined procedure compared to FD and EHD alone. The shrimp
dried by the combined drying treatment exhibited lower shrinkage, higher rehydration rate and better sensory properties (Hu et al. 2013). In another study, freeze
drying was combined with air drying. Similar results were reported with combined
drying method (Bai et al. 2013).
Microwave Assisted Drying
Microwave radiation is a part of the electromagnetic spectrum with wavelengths
ranging from 1 mm to 1 m. The frequency used for food applications is 2.450 MHz.
Microwaves are not ionized in nature and are known as a fourth-generation technology. Microwave-assisted dehydration is increasingly noteworthy. Microwaveassisted drying is applied in conjunction with other techniques to solve problems
encountered in conventional drying processes. It is reported that microwave assisted
drying is applied in various types of food and good results are obtained. In the studies conducted with microwave-assisted drying, decrease in drying time and energy
consumption, and increase in drying speed have been reported. Less space requirements and better overall process control are among the advantages of this method
2.1 Shrimps/Prawns