Rheology in bioprocessing: Which dissolved or dispersed compound, when added at comparable mass fraction to an aqueous solution, will cause the greatest increase in apparent viscosity under gentle mixing conditions?

Introduction:
Viscosity changes in bioprocess media arise from the size, shape, and interactions of solutes. This question tests your understanding of how macromolecular architecture (elongated versus compact) controls hydrodynamic volume, entanglement, and thus the apparent viscosity of solutions relevant to fermentation, downstream processing, and formulation.

Given Data / Assumptions:

• All candidates are added at comparable mass fractions to water.
• Temperature and ionic strength are typical laboratory values.
• No gelation or chemical crosslinking occurs; we consider purely physical thickening.
• Shear rates are in the gentle mixing range typical of bench reactors.

Concept / Approach:
Apparent viscosity rises strongly with solute molecular weight and with anisotropic, chain-like shapes that create large hydrodynamic volumes and inter-chain interactions. Long-chain proteins (e.g., serum albumin when partially unfolded or behaving as an extended coil) produce higher solution viscosity than compact globular proteins of similar mass because they occupy more volume and entangle more readily. Small molecules like glucose contribute negligibly, and adding more water obviously cannot raise viscosity.

Step-by-Step Solution:
Identify which option provides the largest hydrodynamic volume per molecule: elongated chains > globular proteins > small sugars > water.Relate chain conformation to viscosity: extended coils increase entanglement and resistance to flow.Compare contributions at equal mass fraction: macromolecular coils dominate over small solutes.Conclude that a long-chain, flexible protein such as albumin causes the greatest viscosity increase.

Verification / Alternative check:
Empirical correlations (e.g., intrinsic viscosity [η]) scale strongly with molecular weight and coil dimensions. Solutions of flexible biopolymers (proteins in unfolded/extended states, polysaccharides) are demonstrably more viscous than those of compact globular proteins at the same mass fraction, while monosaccharides barely change viscosity within practical ranges.

Why Other Options Are Wrong:
Glucose: very small; contributes little to viscosity at typical concentrations.

Water: dilutes the solution; does not increase viscosity.

A compact, globular protein: increases viscosity somewhat but less than a long, flexible chain at comparable mass fraction.

Common Pitfalls:

• Assuming molecular weight alone determines viscosity; shape and flexibility are equally critical.
• Ignoring shear-rate dependence (shear thinning) which further accentuates differences for chain-like solutes.

Final Answer:
A long-chain, flexible protein such as serum albumin