Evaluation of physicochemical, antioxidant, antibacterial activity, and sensory properties of watermelon rind candy

Watermelon (Citrullus lanatus) is consumed all over the world that contains a large number of seeds and rind, which is discarded. These by-products contain phytochemical compounds with great nutritional potential. This study aims to evaluate physicochemical properties and sensory values of watermelon rind candy. In this study in order to make the waste of watermelon a more sustainable and value-added food product, the watermelon rind was dried using an osmotic dehydration technique which comprises gradual impregnation of syrup (50 and 70% w/w – 1 to 5 h) before drying at 40 and 60 °C in 8 and 10 h. Various variables such as moisture content, chemical composition, water loss, solid gain, rehydration water, acidity, pH, antioxidant activity, antibacterial activity, residual toxins, phenolic and flavonoid contents during osmotic dehydration of watermelon were investigated. Results showed by rising temperatures, dehydration becomes more severe. Increasing the temperature in both osmotic samples in a concentrated solution (70%) and in osmotic samples with a dilute solution (50%) can enhance the mass transfer, water loss, solid absorption, as well as dehydration intensity. However, antioxidant activity, phenolic and flavonoid content significantly decreased after osmotic dehydration. TPC decreased from 35.83 mg/100 g to 27.45 mg/100 g and TFC of the watermelon rind (8.71 ± 0.01 mg/100 g) decreased to 2.63 ± 0.02 mg/100 g and also antioxidant activity after the osmotic process decreased from 61% to 40%. Also, osmotic dehydration had no significant impact on acidity and pH. The watermelon rind dehydrated sample (osmosis temperature: 40 °C, osmotic solution concentration: 70%, immersion duration: 5 h) was the best choice of panelists due to the highest score in the sensory evaluation including taste, texture, and overall acceptability. By determining the hardness of the watermelon rind candy and comparing it with the results of texture analysis of other dried products, it can be concluded that this product can be used as a healthy snack with longer shelf life properties.