Postharvest characteristics and pesticide residue levels of conventionally and organically grown fruits and vegetables in selected districts in Uganda
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The study was undertaken to assess the effect of production systems (organic vs conventional) on the physicochemical and nutritional properties of selected fruits and vegetables (tomato, scarlet eggplant, apple banana and passion fruit) as well as the pesticide residue levels in the fruits and vegetables available on the Ugandan market. A total of ninety six (96) individual crops (tomato, apple banana and passion fruits) were used for the storage experiments under ambient temperatures whereas 256 crops were used for scarlet eggplants from each of the production systems such that enough sample for testing would be obtained after extraction. Tomatoes were obtained from organic and conventional farms in Mpigi district, scarlet eggplant from Rubirizi district whereas apple bananas and passion fruits were obtained from Nakaseke district. The fruits and vegetables from the two production systems were harvested at the same stage of physiological maturity and were analyzed for physicochemical properties at timed intervals during storage at the Department of Food Technology, Makerere University laboratory. For mineral and pesticide residue analysis, 10 individual items constituted one sample and up to 15 and 10 samples were analyzed respectively for fruits from two production systems at Uganda National Bureau of Standards and Chemiphar Laboratories. The physicochemical parameters measured were, pH, total soluble solids (TSS), ascorbic acid (AA), titratable acidity (TA), firmness, and dry matter (DM) content. Minerals analyzed include sodium (Na), potassium (K), magnesium (Mg), calcium (Ca) and phosphorus (P). For pesticide residues chlordane, chlorphyrifos, heptachlor, dimethoate, malathion and dithiocarbamate were analyzed. Results showed that pH, TSS and DM content for organically grown tomato (4.51, 3.8 °Brix and 7.5%) and scarlet eggplants (5.39, 4.5 °Brix and 14.69%) were significantly different (P ≤ 0.05) from conventionally grown tomato (4.47, 3.5 °Brix and 7.32%) and scarlet egg plants (5.18, 4.3 °Brix and 13.39%). There were no significant difference (P ≥ 0.05) registered for the commodities between the production systems with regard to TA, AA and firmness. For apple bananas, only DM content showed significant difference (P ≤ 0.05) between the production systems. The DM for conventionally grown apple banana was 32.55% whereas organically grown apple banana was 32.68%. The other parameters were not significantly different (P ≥ 0.05) between the production systems. For passion fruits, pH, TSS and TA in organic produce were significantly higher than conventional produce whereas AA and DM were not significantly different (P ≥ 0.05) between the production systems. All the main mineral constituents (Na, K, Ca, P and Mg) were more concentrated in organic tomato, scarlet eggplants and passion fruits than their conventional counterparts. In apple banana, only Na, K and Ca content was significantly higher (P ≤ 0.05) in organic than conventional fruits with P and Mg remaining the same. The mineral concentration (Na, K and Ca) for organically grown apple bananas was 13.77, 424.68 and 1.65 mg/100g, respectively whereas for conventionally grown apple bananas, 12.78, 416.00 and 1.57 mg/100g respectively, was recorded. The pesticide residues were <0.01 mg/kg in all the fruits and vegetables except dithiocarbamate in conventional tomato that was exceedingly high at 23.69 mg/kg compared to <0.01 mg/kg in organic tomato. The results from this study contribute towards the already on-going efforts aimed at developing and fully exploiting the fruits and vegetables value chain in Uganda and promotion of good postharvest management activities to ensure quality and safe produce. This study suggests the need to establish the consumer safety aspects related to consumption of conventionally grown tomatoes considering the high residue levels of dithiocarbamate registered which is of great public health concern in Uganda.