In the industrial production of yoghurt, measurement of pH is normally the only in-line technique applied as a real-time monitoring signalfor following the dynamics during the fermentation process. However, every dairy company would benefit from an in-line technique giving information about the chemical composition, physical/textural properties and/or microbial contamination. In this study lactic fermentation batches with the starter bacteria Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus are explored by in-line near infrared (NIR) spectroscopy. The dynamics obtained from NIR spectroscopy is elucidated in order to explain what causes the variation of the indirect measurements. The results show that viscosity change has a large impact on the scatter, which affects the NIR data. It seems that the variation found by NIR spectroscopy is caused by both scatter and absorption, where the scatter gives us information about the textural change happening, and the absorption gives us information about the biomass formation plus the conversion of sugar into lactic acid.