Citric acid production from the integration of Spanish-style green olive processing wastewaters with white grape pomace by Aspergillus niger

Abstract

This study aimed to optimize an integrated simple process for citric acid production using Spanish-style green olive processing wastewaters enriched with sugars from white grape pomace and the robust Aspergillus niger B60. Mild mixing of equal quantities of the above streams governed satisfactory amount of appropriate carbon sources (equimolar mixture of glucose and fructose, 111.5 g/L) in the sugar-enriched wastewater and its neutralization. Various nutrients and fermentation conditions were investigated and maximum citric acid content (85 g/L) and yield (0.56 g/g) were obtained in liquid surface culture after minimum regulation by adding sucrose and NH₄NO₃ (100 g/L and 1.1 g/L, respectively). Scale-up experiments (5 L-scale) verified findings from small scale (250 mL). The chemical oxygen demand value and phenolic content of the treated wastewater were reduced by 78% and 64%, respectively. Findings support the potential for clustering the respective enterprises in a biorefinery plant for citric acid fermentation.

Keywords: Aspergillus niger; Citric acid; Grape pomace; Surface fermentation; Table olive wastewaters.