The application of near infrared (NIR) spectroscopy to measure simultaneously the concentration both of substrates and products of acetic fermentation, i.e. ethanol, acetic acid and biomass concentration, is reported. All these three parameters are closely involved in the kinetics of the process. Monitoring of these variables is a valuable tool to model, optimise and control industrial processes. Furthermore, the NIR technique is also applied to measure the concentrations of ethyl acetate and acetoin, two compounds of decisive relevance for the sensory characterisation of commercial vinegar. The monitoring of these compounds is fundamental to the global optimisation of the process both in economic and sensorial terms. The analysed samples were taken directly from the industrial fermentors of a vinegar producer in La Rioja and their NIR spectra were measured in order to serve as the basis for the subsequent development of separate PLS regression models for each studied response. Several pre- processing methods, such as mean centring, autoscaling, derivation, SNV, MSC and OSC were applied to the data set, testing and comparing the quality of the respective calibration models constructed. The simplest model that provided the best results in terms of predictive ability was selected for each parameter. Thus, the NIR instrument was integrated into a pilot fermentation system to monitor the concentration of ethanol, acetic acid, ethyl acetate, acetoin and biomass (critical parameters to model and optimise the industrial process) by using the proposed regression models.

Keywords: NIR spectroscopy, on-line monitoring, bioprocess, acetic fermentation, PLS, OSC