Effect of microwave pretreatment on drying kinetics color shrinkage and rehydration of dried orange slices

Introduction: Moisture content promotes the growth of microorganisms and is a major cause of food spoilage. Therefore, decreasing the moisture content economically reduces foods spoilage (Subramanyam et al., 2017). Drying is a concurrent heat and mass transfer process that is widely used in fresh fruits and vegetables processing to prolong the shelf-life of agricultural products by decreasing moisture content and lowering water activity (Salehi, 2023). Microwave radiation is a type of non-ionizing radiation energy utilized in foods production, which can efficiently alter the structural and functional properties of foods ingredients. The operating frequency is 915MHz or 2450MHz and is widely used in the food industries (Yılmaz and Tugrul 2023). Microwave pretreatments can facilitate mass transfer in the dehydration procedure and reduce drying time for fruit and vegetable products (Wray and Ramaswamy 2015). Surface color and size of dried fruits are the main important quality parameters that determine the acceptance of these products. Procedure variables include pretreatment, dryer type, and drying conditions that are expected to influence the color and finish of the dried product (Kowalska et al., 2021). We found no report on the effects of microwave pretreatment on the hot-air drying kinetics of orange slices in the literature. Hence, the purpose of this study was to estimate the impacts of microwave pretreatment on the drying time, mass transfer kinetic, effective moisture diffusivity (Deff), shrinkage, color indexes, and rehydration of orange slices. In addition, the moisture ratio changes of orange slices during drying were modeled.
Material and methods: The average diameter of fresh orange slices was 7.3 cm. In this study, the water content of fresh and dried orange slices was calculated using an oven at 105°C for 4 h (Shimaz, Iran). In this research, the effect of microwave time on the drying time, effective moisture diffusivity coefficient, and rehydration of orange slices were investigated, and drying kinetics were modeled. To apply the microwave treatments on the orange slices, a microwave oven (Gplus, Model; GMW-M425S.MIS00, Goldiran Industries Co., Iran) was employed with a power of 330 W. To apply microwave pre-treatment, the orange slices were placed inside the microwave device for 0, 1, 2, and 3 minutes, and after leaving the device, the samples in thin layers were placed in the hot-air dryer (with a temperature of 70°C). The dehydration kinetics of orange slices have been explained using 4 simplified drying equations. Fick's second law of diffusion was used to calculate the moisture diffusivity of orange slices. The color of orange slices was analyzed using an Image J software (V.1.42e, USA). The L* (lightness), a* (greenness/redness) and b* (blueness/yellowness) values for each samples were recorded in triplicates. The rehydration tests were conducted with a water bath (R.J42, Pars Azma Co., Iran). Dried orange slices were weighed and immersed for 30 min in distilled water in a 250 ml glass beaker at 50°C. The experiments were conducted with a fully randomized factorial design using SPSS software version 21. All tests were run in triplicate and Duncan's test was utilized to compare the averages of observed responses at the 95% probability level.
Results and discussion: The results showed that microwave treatment, causes an increase in moisture removal rate from the orange slices, an increase in the effective moisture diffusivity coefficient, and as a result, reduces the drying time. By increasing the microwave time from zero to 3 min, the average drying time of orange slices in the hot-air dryer decreased from 216.67 min to 186.67 min. Sharma and Prasad (2006) used a laboratory microwave for drying of garlic. They confirmed that the microwave drying of garlic cloves decreased drying duration compared to hot air dehydration, and the quality attributes of dehydrated product by microwave methods, was found higher. The average effective moisture diffusivity coefficient calculated for the samples placed in the hot-air dryer was equal to 5.70×10-10 m2/s. Increasing the microwave time from 0 to 3 minutes increased the average effective moisture diffusivity coefficient by 31.76%. Microwave pre-treatment did not have a significant effect on the surface shrinkage change of the product, but in terms of the total color difference index, the treated and dried samples had less changes in appearance color than the original sample. The average yellowness, redness, and lightness indexes of dried orange slices was 40.71, 2.02, and 52.65, respectively. By increasing the microwave treatment time from 0 to 3 min, the total color difference index of the dried orange slices significantly decreased from 12.70 to 6.38 (p<0.05). The time of microwave treatment had significant effects on the rehydration of dried orange slices.
Conclusion: Kinetic modeling of orange slices weight changes during drying was carried out by models in the sources, followed the Page model was selected as the best model to predict moisture ratio changes under the selected experimental conditions. Mean values of the sum of squares due to error, root mean square error, and r for all samples ranged from 0.0148-0.0329, 0.0249-0.0370, and 0.9930-0.9967, respectively. Generally, 2 min pretreatment by microwave is the best condition for drying orange slices.