Molecular dynamics study of incipient protein fouling in fallingfilm evaporators: implications for alternative protein processing

Licentiatavhandling, 2026

Falling-film evaporators (FFEs) are central to concentrating protein-rich streams in dairy, food, and emerging alternative-protein processes, yet fouling of stainless-steel heat-transfer tubes reduces energy efficiency, increases cleaning downtime, and elevates chemical and water consumption. This licentiate thesis sheds molecularlevel light on protein fouling in FFEs by integrating two complementary studies: (i) incipient adsorption of hen egg-white lysozyme under process-relevant conditions and (ii) the reversal of this attachment under industrial alkaline Cleaning-in-Place (CIP). All-atom molecular dynamics (MD) simulations were performed using CHARMM36/TIP3P with 100 ns production trajectories across multiple initial orientations, using Cr₂O₃ as a representative passive oxide layer on stainless steel, and the computational descriptors were qualitatively compared with QCM-D measurements. 

Under near-neutral conditions (293–333 K), lysozyme adsorption is strongly orientation-dependent and only weakly temperature-sensitive (ANOVA p = 0.61 across temperatures). Stable attachment requires that a surface-facing protein patch enriched in basic residues (Lys, Arg, His) forms persistent multi-residue electrostatic and hydrogen-bond contacts with negatively charged oxygen sites on Cr₂O₃, whereas acidic residues (Asp, Glu) contribute net repulsion. Bound and solvated states are discriminated by contact persistence, minimum separation, and a pronounced Coulombic dominance in the interaction energy (often >70% of the total), while the global fold remains largely intact (backbone RMSD within the fluctuation range of the solvated protein). These molecular descriptors align qualitatively with observed QCM-D mass-uptake trends on stainless-steel sensors, supporting their relevance as mechanistic indicators of incipient fouling. 

Under alkaline CIP conditions (pH 13.0–13.8 at 333 K; ~1 wt% NaOH, pH ≈ 13.4 as industrial reference), progressive deprotonation reduces the net positive character of lysozyme (0 → -9), weakens electrostatic anchoring, increases the mean minimum separation (3.7 → 4.8 Å), and decreases protein–surface contact counts. Detachment is accompanied by hydration recovery, reflected by increased solventaccessible surface area and more favorable solvation energetics. The simulations further indicate that hydroxide ions enrich at the Cr₂O₃–water boundary, competitively occupying interfacial coordination sites and producing an ionscreened interface that suppresses sustained protein binding and re-adsorption. Importantly, alkalinity disrupts the same basic-residue anchoring network that drives fouling onset, without requiring bulk unfolding. 

Overall, the thesis establishes a unified mechanistic picture connecting patchcontrolled electrostatic adsorption during operation with pH- and ion-mediated interfacial disruption during CIP. This framework provides residue-level and interfacial insight for anticipating fouling propensity of novel protein formulations and for guiding the design of lower-chemical, energy-efficient cleaning strategies in sustainable FFE-based downstream processing.