For the first time, a method capable of monitoring and quantifying the proteolytic activity of wine yeast in real-time, directly in fermenting grape must, is reported. When azocasein was added to the bioreactors, optical density in- creased over time in the presence of M. pulcherrima and Metschnikowia spp. No variation was observed in non-inoculated must (data not shown), suggesting that the optical density variation resulted from the yeast metabolism. Monitoring proteolytic activity by measuring the degradation of azocasein made it possible to determine the maximum activity period for each strain, as well as variations over time. This method revealed that proteolytic activity started at the beginning of growth, except in the case of strain Y6259. Further experiments with this strain are re- quired to explain this difference. This method highlighted inter-strain differences that were much more difficult to detect using previously- described techniques. n farmina nrotaing ce in the nresence of