Publication: Characteristic effects of drying processes on bioactive compounds in african eggplant
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2012
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JKUAT
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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,
xx
and 0.0237-0.453 h-1
for total phenolics, beta carotene and antioxidant activity,
respectively. Activation energy range was 8.246-23.548, 15.994-60.845, 11.111-25.764
kJ/mol for total phenolics, beta carotene and antioxidant activity, respectively.
Regarding the sensorial evaluation, it was not possible to clearly distinguish the sensory
profiles of the five accessions statistically. The drying characteristics and degradation
kinetics information from this study may be applied by farmers and industrialists in the
development of optimum drying controls plan.
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Keywords
Solanum aethiopicum L, degradation, antioxidants