Effect of thermal processing on bioactive compounds contents and antioxidant capacities of unripe and ripe Solanum anguivi Lam. fruit accessions

The bioactive compounds contents (BCC), total antioxidant capacity (TAC), and thus the potential health-promoting capacity of fruits and vegetables may depend on their processing history. Solanum anguivi Lam. fruits (SALF) are consumed as vegetables and have been traditionally used to manage diabetes, possibly due to the possession of bioactive compounds, including phenolics and saponins. Data on the effect of thermal treatments versus ripeness stage and accession on the BCC and TAC of SALF is currently limited. The present study investigated the effect of drying temperature (35, 55, and 85 °C), and traditional cooking method (boiling and steaming) at 100 °C, and duration (15, 30, 45, 60, and 120 min) on the BCC and TAC of SALF. Spectrophotometry was used to analyse the TAC and BCC (total phenolic, flavonoid saponins, and vitamin C contents), and gravimetry for total alkaloids. The effect of drying on the BCC and TAC of SALF significantly varied with drying temperature, accession, ripeness stage, cooking method, and cooking duration. Drying at 35 °C increased the total phenolics and TAC of unripe SALF more than 2-fold, and retained the highest vitamin C and total flavonoid contents. The effect of drying on saponins and alkaloids was dependent on the accession and ripeness stage. The effect of cooking on the BCC and TAC was significantly influenced by the cooking method and duration. Boiling and steaming increased the total phenolics and TAC 4-fold and 3-fold, respectively. Cooking reduced the saponins and did not affect flavonoid contents. Exceptionally, 15 min of boiling did not affect the total saponins and increased the total flavonoids. Total alkaloids were decreased by steaming and increased by boiling. However, very long boiling and steaming time (120 min) greatly increased the alkaloids 2-fold and 1.5-fold, respectively. All the thermal treatments negatively affected vitamin C. Overall, the thermal treatment that resulted in the highest total phenolics and TAC of SALF was boiling (for 15 min), which may result in the highest health benefits of SALF upon consumption.