Development and Evaluation of semi-continuous microwave convective dryer for food products

Abstract

The research work aimed to develop a semi-continuous microwave convective dryer for foods and evaluate the performance of the dryer for drying of shrimp and oyster mushroom. The study also aimed to assess the shelf-life of the dried products under different packaging technologies and packaging materials. The quality of conventionally dried shrimp and oyster mushrooms were compared and evaluated with the hot air assisted microwave (HAMW) dried products. The developed HAMW drying system comprised of a drying chamber, conveyor belt, magnetron to generate microwaves at frequency of 2450 MHz±50 MHz, hot air generation system with air heater, axial fan and other controls. The drying experiments for shrimp were performed according to a second-order Box- Behnken design (BBD) with three factors at three levels: microwave power (600, 800 and 1000 W), air temperature (50, 60 and 70 °C), and air velocity (0.5, 1.0 and 1.5m/s). Drying time, water activity, and rehydration ratio was selected as the response variables. A three-factor, three-level Box-Behnken design (BBD) experimental design was used in optimizing the drying conditions for shrimp in a hot air-assisted microwave (HAMW) drying system. The optimization was performed to optimize the conditions with minimum values of drying time and water activity, and maximum value of rehydration ratio. The design plan consisted of 17 runs with 5 centre points. The optimum conditions of process variables were derived using the desirability function. Design Expert Statistical Software package 9.0.0 (Stat Ease Inc., Minneapolis, MN, USA) was used to perform statistical analysis. The moisture content of shrimp decreased from an initial value of 80.55% to a final value of 16.5% within 2.9 h of drying. HAMW drying of shrimp was represented by reduction in moisture percent as a function of drying time. Moisture ratio obtained during drying were fitted into thin layer drying models by non-linear regression analysis. Page model was identified as the best fit model with higher R2 value of 0.9984, lower χ2 value of 0.000134 and RMSE value of 0.01552. Drying efficiency of the dryer under HAMW mode was observed to be 35.71%. This was a result of volumetric hearing effect of microwave radiation combined with convective effect of hot air. The SEC value for HAMW of shrimp was found to be 1.75 kWh/kg. The total value of colour change (ΔE) determined for dried shrimp was 16.95 ± 2.14. The ‘L’ value of the dried shrimp (41.31 ± 1.63) decreased during drying whereas the ‘a’ and ‘b’ values during the initial stages in drying that can be due to the higher moisture content of sample that created more friction and heat generation due to dipole rotation. Drying efficiency of the dryer under HAMW mode for oyster mushroom was observed to be 23.12%. The SEC value for HAMW of oyster mushroom was found to be 2.71 kWh/kg. Solar drying efficiency of the collector was calculated using the whole collector area and solar irradiation received instantaneously during drying. The instantaneous collector efficiency values varied in the range of 30.9 to 42.4%. The L*, a* and b* colour values of fresh mushrooms were determined to be 84.12±0.46, 3.3±0.24 and 15.57±0.62 respectively. The colour values of the microwave dried mushrooms under optimized conditions were recorded as 56.15±0.25 (L*), 4.95±0.24 (a*) and 20.85±0.51 (b*) respectively. The ‘L’ value of the dried oyster mushroom decreased whereas the ‘a’ and ‘b’ values increased during microwave convective drying. The drying rate of SD and HAMW dried oyster mushrooms were found to be 3.47 kg/kgh and 2.14 kg/kgh at the beginning of drying. Drying rate exhibited a maximum value of 3.47 during the initial stages of drying that can be due to the higher moisture content of a sample that created more friction and heat generation due to dipole rotation. Drying reduced the TPC value of oyster mushroom to 1.2 × 104 and 3.9 ×105 CFU/g respectively under MW and SD drying conditions. The optimized sample was analysed for microstructure to study the pore size distribution in the dried product. The pore size for microwave dried oyster mushroom samples ranged from 8.65 – 32.4 µm. Scanning electron microscopy analysis of microwave dried oyster mushroom showed the formation of pores of diameters ranging from 2.06 – 15.7 µm. Storage studies of hot air assisted microwave dried and solar dried oyster mushrooms were carried out under MAP and vacuum conditions in three types of packaging materials namely LDPE (150 µ), polyester polyethylene laminate (72 µ) and metallised polyester (84 µ). Shelf life of the dried oyster mushroom was quantified with respect to the microbial growth and other quality parameters and maximum value of 9 months was achieved for SD and HAMW dried oyster mushroom in polyethylene-polyester (72 µ) laminated packaging material.