Fermentation monitoring is the systematic acquisition and interpretation of measurements that describe the physical, chemical, and biological state of a culture during operation. Online instruments can follow temperature, pH, pressure, agitation, gas flow, dissolved oxygen, oxygen uptake, carbon dioxide evolution, and exhaust-gas composition without removing broth ^[1]. At-line and off-line analyses provide variables that are not yet measured continuously, including cell density, viable biomass, residual substrates, organic acids, product concentration, metabolites, and contaminants ^[2]. The measurements form time series that reveal growth phases, substrate exhaustion, oxygen limitation, metabolic shifts, and the approach of a harvest endpoint. Monitoring strategy is selected according to the organism, process risk, sensor response time, sampling burden, and the decisions that must be made from the data ^[3]. A single bulk measurement can conceal gradients caused by imperfect mixing, especially in large vessels or highly viscous cultures. For this reason, monitoring may combine direct sensors, soft sensors, spectroscopy, imaging, and mass-balance calculations ^[4].