Characteristic Effects of Drying Processes on Bioactive Compounds in African Eggplant (Solanum aethiopicum L.)

Abstract

African eggplant (Solanum aethiopicum L.) is a rich source of bioactive compounds and functional constituents that are beneficial to human health. However, the short (3-5 days) shelf life can be a major cause of postharvest losses especially during peak harvesting season. Drying technology is a convenient way of producing shelf stable food products, but can lead to thermal degradation of available nutrients and bioactive compounds depending on the drying method and temperature conditions. Monitoring the changes in bioactive compounds is important for optimizing and choosing drying methods and conditions.The objective of this study was to investigate the effect of harvesting maturity on the drying characteristics; to determine the effect of four drying methods (solar, hot air oven, vacuum oven and freeze) on the retention of total phenolics, beta carotene, antioxidant capacity, vitamin C and lycopene; to determine the degradation kinetics of total phenolics, beta carotene and antioxidant activity during hot air oven and vacuum oven drying; and to establish consumer acceptability of the dried and reconstituted products through sensory evaluation. Five African eggplant (Solanum aethiopicum) accessions (sangawili, manyire green, S00047A, AB2 and aubergine blanche) were used in the study. Harvesting was carried out at two maturity stages defined by the peel shininess, colour and number of days between planting and harvest. Stage 1 maturity had a shiny peel while stage 2 maturity had a non-shiny peel. Samples were dried up to ~10% moisture content in a single layer. Random sampling was done at regular intervals as the drying process progressed. Subsequently, fresh and dried samples were analyzed for moisture content, total phenolics, beta carotene content, vitamin C, lycopene content and antioxidant capacity. Affective testing using 7 point hedonic scale was used for sensory evaluation through a consumer panel to establish the likability scores for the dried and reconstituted samples. The results showed that harvesting maturity significantly (p<0.05) effected the moisture content and the drying rate of the five accessions. Moisture content decreased from stage 1 maturity to stage 2 by up to 2.01% with the exception of AB2. The drying rates were higher for stage 1 maturity in comparison to stage 2. The drying time for stage 1 and stage 2 maturities was 360-840 and 360-960 minutes, respectively. In the fresh state, beta carotene, total phenolics and antioxidant activity ranged between 14.75-29.50 mg/100g db, 751.21-1363.95mg/100g gallic acid equivalent (GAE) db and 99.58-325.61mg/ml db percentage inhibition at 50% (IC50) value, respectively. The results showed a significant (p=0.001) positive correlation (r=0.822) between the total phenolics and the antioxidant equivalent ascorbic acid capacity. However, drying processes significantly (p<0.05) reduced the total phenolics, beta carotene and antioxidant capacity with freeze drying presenting the highest retention rate. Overall, 36.26 – 95.05% (total phenolics) and 31.44 – 99.27% (beta carotene) was retained during freeze drying. Lycopene was only detected in the dried samples of the accession manyire green. Also, vitamin C was undetected in both fresh and dried samples. The kinetics of degradation of total phenolics, beta carotene content and antioxidant activity followed a first-order reaction for both hot air oven and vacuum oven drying. The range of reaction rate constant was 0.018-0.067, 0.016-0.226,