A Non-Contact Electronic Nose System Based on Off-Gas Response for Real-Time NH₄⁺ Monitoring in Fermentation

Real-time online monitoring of key parameters such as the ammonium nitrogen (NH₄⁺) concentration during biological fermentation is important for the optimization of a biological fermentation process. Traditional offline detection technologies like spectrophotometry need contact sampling, with drawbacks of monitoring lag, risk of contamination, etc. In this work, taking the gentamicin fermentation process as an example, we developed an intelligent electronic nose non-contact monitoring system on the basis of fermentation off-gas signals. We captured the typical signals of off-gas and established a quantitative relationship between the signals and the NH₄⁺ concentration in fermentation broth; the system then realized non-contact real-time monitoring. The whole system consists of a gas-switching module, a sensor array module, a signal-processing module, and an intelligent prediction module. The system adopts a five-phase gas switching strategy to suppress sensor drift in metal-oxide-semiconductor (MOS) sensors and a light neural network for prediction, which improve both prediction speed and accuracy. Experiments were conducted using a 5 L fermenter, and the prediction result was consistent with the offline measured value (coefficient of determination, R² = 0.9871, root mean square error, RMSE = 0.0317 g/L). This technique provides a new method for the non-contact measurement of key fermentation parameters, and it can be expanded to other fermentations.