Estimation of shrinkage and rehydration in apple slices dried by infrared radiation using intermittent heating method

Introduction: Dehydration of fruits and vegetables is a widely used unit operation (Funebo et al., 2000). Intermittent infrared radiation, in which the product surface maintained at constant temperature, is a novel drying method to produce high quality dehydrated fruits and vegetables (Zhu & Pan, 2009; Zhu et al., 2010). One of the most important physical changes in the quality of food during drying is the decrease in external volume. This change in volume is called shrinkage and is an unavoidable phenomenon during the dehydration process. Shrinkage is usually expressed by the ratio between the volume of the sample after and before drying (Yan et al., 2008). Shrinkage also affects the porosity of dried materials and it is the parameter determining about the mass transfer properties, mechanical properties, and the texture of food product (Witrowa-Rajchert & Rząca, 2009). The three qualitative parameters such as the degree of shrinkage, density and firmness of the product are related to each other (Funebo et al., 2000). Maximal shrinkage and structural collapse were shown to decrease until kept quality characteristics (Barzaghi et al., 2008). The factors affecting shrinkage include volume of water extracted from the product, mechanical mobility of the solid matrix, drying rate, and process conditions such as temperature, air flow rate, and relative air humidity (Moreira et al., 2000). The important consequences of shrinkage occurring during drying result in a loss of rehydration ability. Dehydrated products are usually rehydrated prior to their use (Krokida, M, & Marinos-Kouris, 2003). The rehydration behavior is studied as an indicator of texture damage (Giraldo et al., 2006). Rehydration is a complex process that is intended to restore the properties of the fresh product by contacting dehydrated products with a liquid phase. This process to be composed of three simultaneous steps: (1) absorption of water into the dry material, (2) swelling of the rehydrated product, and (3) loss or diffusion of soluble components. Rehydration ability or capacity measures the ability of a dehydrated product to rehydrate (Maldonado et al., 2010). Mathematical models of rehydration will be useful in designing and optimizing this operation that have been applied as exponential models or capillary absorption theory or Fick’s diffusion laws (Lee et al., 2006). In this research, the effect of this heating method on the physical-qualitative characteristics of dried apple slices, including volume, density, shrinkage and behavior of rehydration were studied.

Materials and Methods: Apples (Golden Delicious variety) were purchased from a local market and according to Acevedo et al., 2008 kept in 0°C±1°C and relative humidity ranging from 90% to 95%. The samples were skinned manually and then cut into slices with different thicknesses of 5mm, 9mm and 13mm and all with 20mm in diameter. The average moisture content of apple was measured using oven (Binder FD53) at 103 °C for 24 hours and it was equal to 84.11% based on wet weight (AOAC, 2000). Operation of infrared radiation with intermittent heating method was performed at three constant temperatures of 70, 75 and 80 °C using the infrared dryer similar to Liu et al., 2014. Shrinkage coefficient of product was estimated with both theoretical (βtheo) and practical (β) methods according to Talla et al., 2004. Also, the mathematical models as Peleg (Reyes et al., 2011) and Exponential (Krokida et al., 2003) equation were compared to describe the rehydration process in dried product using adjusted R-squared (Adj.R2) and root mean squared error (RMSE). Statistical analysis of the effect of thickness and temperature on the shrinkage coefficients (βtheo, β) was performed in SPSS software in version of 19. To achieve this aim, a completely randomized design (CRD) with factorial arrangement with two factors as thickness (at three levels) and temperature (at three levels) was used. Also, two methods of computing the shrinkage coefficients (theoretical and practical) were compared together with one-way F test. Mean comparison was performed by Duncan test with 95% confidence level (P<0.05). All experiments were performed in three replications.

Results and Discussion: The results showed that increasing of the temperature caused greater volume and density changes during rapid dehydration of the product. The density changes were more intense for the thin product (reducing the final density to about 0.6 g/cm3). Also for thicker products, the density changes were slower during the process (reducing the final density to about 0.7 g/cm3). During process, the variation of product shrinkage indicated an exponential rising trend which was influenced by the amount of evaporated water. Shrinkage coefficient is significantly increased at higher processing temperature and thickness of product (β equal to 0.164), representing that the product is heavily affected by the structural collapse phenomenon. The observations were similar to those reported by many researchers (Mayor & Sereno, 2004; Nowak & Lewicki, 2004; Katekawa & Silva, 2006). The temperature of 80 °C showed significantly higher theoretical and practical shrinkage coefficient than the other two tested temperatures (70, 75 °C). The difference between the two methods of calculation in shrinkage coefficient (theoretical and practical) was also significant and the difference increased with the increase of temperature (more than 6%). Jannot et al., 2002 reported that the error in the practical method can be due to the noisy data (deviation from the actual value). Limitation of rehydration capacity (RC) in slices with high shrinkage stress confirms the damage of the texture of the product (reduction in predicted equilibrium moisture content: Xe from 6.523 to 4.148 kg/kg, db). Rehydration process in different product thicknesses is described better using Peleg model (higher Adj.R2 and lower RMSE) compared with the exponential one. The observations were similar to those reported by many researchers (Krokida, M, & Marinos-Kouris, 2003; Nathakaranakule et al., 2010).

Conclusion: The physical properties including shrinkage and density in the dried product are affected by process and product conditions. The low occurred shrinkage that reflects the preservation of the internal structure of the product and will be effective in rehydration behavior. In fact, reabsorption cannot be simply defined as the returned process of dehydration. Because of moisture reabsorption is completely affected by the texture, structure and properties of the dried product. Therefore, mild thermal conditions prevent the structural breakdown of apple slices and, at low thicknesses, due to the reduced volume of water evaporated from the slices, the shrinkage coefficient is reduced, thus preserving the internal physical structure of the infrared dried product.