On-line monitoring of D-lactic acid during a fermentation process using immobilized D-lactate dehydrogenase in a sequential injection analysis system

A sequential injection analysis system (SIA) involving immobilized enzymes was set up to quantify D-lactic acid during fermentation processes. The analytical system is fully computerized and it operates by analysing aliquots removed from the bulk fermentation through a filtration setup. An assay cycle of 182 s includes 90 s of incubation time when the flow is stopped. The enzymes used in the analysis are D-lactate dehydrogenase co-immobilized together with L-alanine aminotransferase on porous glass. Cell free permeate was sampled from the permeate side of the filtration unit. The concentration of D-lactate was monitored during the entire fermentation processes. The data read were intermittently compared to off-line analysis. A good fit was obtained when comparing the results of the two ways of analysis used.