Drying time calculations
The total drying time is the summation of time required for a constant rate period and falling rate period drying up to a certain moisture content.
equation(6)
Results and discussion
The plots of variation of moisture contents with dehydration time are shown in Fig. 1a,b and compared with untreated samples. The rate of change of moisture content was obtained from the graph and plotted against average moisture contents as per Eq. (2) (Fig. 2a,b). Fig. 2a,b shows that a large portion of the moisture from paprika is removed when the rate of drying is constant (constant rate period) and further drying rate was reduced with respect to moisture contents (falling rate period). The relevant values of critical moisture contents are reported in Table 1 for all treatments.The cell disintegration indices (Zp) used as an approximation of the extent of cell permeabilisation were compared for the various pre-treatments as shown in Table 1. Hot water blanching treatment resulted in the highest cell disintegration index of 0.88 due to thermal effect and 0.61 and 0.58 cell disintegration indexes were recorded for HELP and HHP, respectively. For the skin treatments (NaOH and acid pre-treatments), the cell permeabilisation was 0%, indicating that the treatments only affected the skin permeability but did not permeabilise the tissue.
The calculated values of mass and heat transfer coefficients during the constant rate period are reported in Table 1. It can be inferred from Table 1 that all the pre-treatments increased the drying rate during the constant rate period as compared to the control samples. The heat and mass transfer coefficients were also found to increase due to these treatments, except for 5% HCl at 25°C, which did not significantly increase the mass transfer coefficient (Table 1). The increase as a result of acid and 5% NaOH at 25°C pre-treatments might be due to partial removal of the waxy layer from the skin surface, thus reducing the resistance to mass transfer. The results were similar to the tomato treatment (Shi et al., 1997).
The 5% NaOH treatment at 35°C resulted in the separation of skin from the paprika and provided an increased surface area and less resistance to heat and mass transfer, which resulted in high heat and mass transfer coefficients (even though the cell disintegration index was zero).
Physical pre-treatments such as HHP and HELP treatments were equally effective regarding drying rates as compared to blanching, without the disadvantages of blanching and other processes used as a reference. This showed that non-thermal permeabilisation of paprika cells was beneficial as a pre-treatment to increase the drying rates as well as heat and mass transfer coefficients.
Comparison of drying times (as calculated as per , and ) during the constant rate and falling rate periods as well as total drying time (Fig. 3) for the different pre-treatments showed that the drying time during the constant rate period was not significantly different for the different pre-treatments, except for HELP and hot water blanching pre-treatments with slightly lower times, whereas the drying time during the falling rate period was significantly different. The drying time during the falling rate period was shortest for HELP pre-treated paprika samples. For HHP pre-treated paprika samples, drying times were comparable with blanching treatment. The NaOH treatment at 35°C could further reduce the drying time as compared to the blanching pre-treatment. However, the shortcoming of this treatment was the reduction in the colour intensity of the peeled sample, which eventually impaired the quality of dehydrated paprika products.