Selective quantification of tryptophan in milk using a sensor based on l-tryptophan dehydrogenase

The selective determination of tryptophan in pasteurized milk is an indicator of quality, as its presence indicates that the process has been carried out at the correct temperatures. Tryptophan analysis is costly for industries, but the use of a sensor would reduce both costs and analysis times. A sensor using l-tryptophan dehydrogenase (TrpDH) was optimized, with the system immobilized using an electrode to quantify the tryptophan content in industrial milk. The stability of the enzyme was studied at a controlled temperature of 4 °C, and it was observed that it maintained 90% of its activity during the first 65.5 days. The pH range was studied between 6 and 10.5, with 7 being the value with the maximum activity observed. The linear range was between 0.01 and 0.1 mM with an R² = 0.9918. The voltage used for quantification was - 50 mV. The optimum temperature of the enzyme was found to be 37 °C. The system showed high specificity K_M = 0.033 mM and good sensitivity of 0.01 mM. It was very stable, and the membranes could be reused up to 11 times without loss of activity. The methodology is considered to have low environmental impact and shows good correlation with high-performance liquid chromatography (HPLC).