Effect of osmotic drying as pretreatment on some physical and chemical quality properties of sliced frozen red pepper

The aim of this study was to investigate the influence of osmotic and hot air pre-drying treatments on some quality characteristics and energy efficiency of red pepper (Capsicum annuum L.) for frozen sliced red pepper production. For this purpose, frozen peppers directly, osmodehydrofrozen and convective dehydrofrozen peppers were stored at -18±2°C for 60 days. Some physical (water activity, color, texture) and chemical (moisture, ascorbic acid, antioxidant activity, carotenoid, peroxidase and lipoxygenase enzyme activity) analyses were performed at the storage days of 0., 15., 30. and 60. Osmotic dehydration treatments of red peppers were optimized by using response surface method according to Box-Behnken experimental design. Temperature (25°C, 40°C, 55°C), sorbitol/NaCl (1:1, 2:1, 3:1) and solution ratio/raw material (5:1, 10:1, 15:1) were chosen as independent variable and treatment was optimized by maximum water loss and minimum solid gain. Optimum processing conditions were determined as 25°C treatment temperature, 2,41 sorbitol/NaCl ratio and 15:1 solution ratio:raw material in this optimization study. Research results showed that moisture content value of frozen directly and osmodehydrofreezing with convective dehydrofreezing peppers changed between 87,05-88,12% and 66,27-73,41%, respectively during storage period. According to analyses results of water activity, osmodehydrofreezing peppers had lowest value (0,92) and frozen directly red peppers had highest value (0,99). All of applied treatments and storage period did not show significant effect on L, a, b color values of samples. Beginning of storage, skin puncture values of samples ranged from 16,15-17,83 N depending on treatments and these values decreased end of the storage. Ascorbic acid content of convective dehydrofrozen and osmodehydrofrozen samples were determined lower than the peppers frozen directly and ascorbic acid content of all samples decreased by storage period. Antioxidant activity determined by using two different method (DPPH ve ORAC) of frozen peppers were higher than convective dehydrofrozen and osmodehydrofrozen samples. Although peroxidase enzyme inactivation of samples was completely provided, lipoxygenase activity was partially provided by blanching. Carotenoid content losses were determined as 57,97, 47,48 ve 46,93% in frozen directly, osmodehydrofrozen and convective dehydrofrozen peppers, respectively at the end of the storage compared to beginning. Compared to the amount of spent energy in pre-drying treatment, because of no heating required for osmotic dehydration, the amount of spent energy for osmotic drying (947,2 kJ/kg) was significantly lower than convective drying (23.034,5 kJ / kg). According to obtained the freezing temperature of the graphics, when temperature of frozen directly red peppers reached to -25°C in about 270 minutes, temperature of osmodehydrofrozen and convective dehydrofrozen peppers reached to -25°C in about 105 minutes. Thus, predrying provided 1/3 advantage in terms of energy consumption during freezing.. When the results are evaluated, low energy consumption and improved product quality during storage period were provided by pre-drying processes for frozen red pepper production. Pre-drying processes under the specified conditions were found to be preferred approach for industrial scale frozen red pepper production. Additionally shortened process time were obtained by pre-drying applications compared to directly freezing of red pepper.