Valorisation of grape juice wastes to produce citric acid by Aspergillus niger

Citric acid, which is used as an additive in many industries, including food, beverage, and pharmaceutical, is commercially produced from microorganisms by fermentation. Although many types of bacteria, yeast, and fungi are used in the production of citric acid, the most preferred species in industrial production are Aspergillus niger species, also known as the black mold. Carbon sources with high sugar content are preferred in fermentation in the production of citric acid. Therefore, especially fruit wastes are suitable substrates for fermentation. Grape pomace, which is an important waste of wine and fruit juice factories, has a sugar content of 50-80%. For this reason, dried and ground grape pomace was chosen as the substrate in this study. In order to obtain citric acid in high yield, the fermentation parameters must be optimized. However, testing all parameters is a negative situation in terms of time and cost. For this reason, the optimum conditions of the parameters can be determined by some statistical methods. In this study, the optimum conditions of substrate concentration, initial pH, fermentation temperature, and time were investigated using the Response Surface Methodology Box Behnken Design. 30-120 g/L for substrate concentration, 2-8 for pH, 25-35⁰C for fermentation temperature, and 48-168 hours for fermentation time were determined for minimum and maximum values. Optimum conditions for substrate concentration, initial pH, fermentation temperature, and time were determined as 120 g/L, 5, 30⁰C, and 168 hours respectively, and the citric acid concentration was found to be 7.57 g/L. The statistical analysis showed that the created model was significant and the R2 was found to be 0.936. Keywords: Aspergillus niger, Box Behnken Design, citric acid optimization, grape pomace