The emergence of Quality by Design (QbD) and Process Analytical Technology (PAT) paradigm supported by the FDA imposes a strong motivation for digital transformation in biopharmaceutical industry. The inherent complexity of bioprocess dynamics, batch-to-batch variability resulting from raw materials and process operations, as well as the need for accelerating product manufacturing, makes dynamic soft sensors such as Kalman Filters highly desirable for process development, monitoring, and control. In this work, we develop an Ensemble Kalman Filter framework in the context of monoclonal antibody bioprocessing, where the noise on physical sensors is mitigated for extracellular metabolite states by integrating the process’ dynamic mechanistic model and sensor measurements. More importantly, the framework accurately estimates the nucleotide sugar concentrations, an intracellular state of the cell that is not routinely measured in industry due to experimental complexity. The proposed EnKF soft sensor retrieves this knowledge through state inference, providing valuable insights for monitoring and control of key quality attributes such as glycan distribution.