Citric acid production using immobilized conidia of Aspergillus niger TMB 2022

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Abstract
Conidia of Aspergillus niger TMB 2022 were immobilized in calcium alginate for the production of citric acid. A 1‐mL conidia suspension containing ca. 2.32 × 108 conidia were entrapped into sodium alginate solution in order to prepare 3% Ca–alginate (w/v) gel bead. Immobilized conidia were inoculated into productive medium containing 14% sucrose, 0.25% (NH4)2CO3, 0.25% KH2PO4, and 0.025% MgSO4·7H2O with addition of 0.06 mg/L CuSO4·5H2O, 0.25 mg/L ZnCl2, 1.3 mg/L FeCl3·6H2O, pH 3.8, and incubated at 35°C for 13 days by surface culture to produce 61.53 g/L anhydrous citric acid. Under the same conditions with a batchwise culture, it was found that immobilized conidia could maintain a longer period for citric acid production (31 days): over 70 g/L anhydrous citric acid from runs No. 2–4, with the maximum yield for anhydrous citric acid reaching 77.02 g/L for run No. 2. In contrast, free conidia maintained a shorter acid‐producing phase, ca. 17 days; the maximum yield for anhydrous citric acid was 71.07 g/L for run No. 2 but dropped quickly as the run number increased.

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