Whey fermentation: on-line analysis of lactose and lactic acid by FTIR spectroscopy.

The batch fermentation of whey permeate by Lactobacillus helveticus 8652, was monitored on-line by Fourier transform infrared (FTIR) spectroscopy. Substrate (lactose) and product (lactic acid) levels were measured over a period of 47 h. A method for the quantitative analysis of the two components was first established. Fifteen standard solutions containing both lactose and lactic acid were used. The method developed was tested using validation samples of known composition. Mean errors of 1.1% and 0.9% were attained in the measurement of lactose and lactic acid respectively. Sample analysis is fast (approx. 3 min), simple and almost completely automated. The results obtained with FTIR spectroscopy compared favourably with samples analysed off-line using enzyme kits.