Sampling of the submerged fermentation entail the asceptic extration of 5 mi of the fermentation broth ................Citric acid quatification ( Xu et al., 1989 0 .Citric acid levels were determined by spectrophotometry at 420 nm after adding pyridine and acetic anhydride. For each 3 ml sample, 1. 7 ml of pyridine and 5.6 mi of acetic anhydride were added to develop color ( https;//www.shodhgganga.infibnert.ac.in Also consult https://www.scielo.magazyme.com - booklet.

I have used the Marier and Boulet Method with pyridine and acetic anhydride.

Here is the reference.

I have tried the pentabromoacetone method (used in clinical trials) but it didn't work in fermented foods.