Understanding Viscosity: A Critical Factor in Hygienic Pump Selection

Why Viscosity Is One of the Most Important, and Misunderstood Process Variables
When selecting hygienic pumping equipment, flow rate and pressure requirements often receive the most attention. However, one of the most influential factors affecting pump performance is frequently underestimated: viscosity.

Viscosity directly impacts how a product flows through piping systems, how efficiently a pump operates, how much energy is required, and how gently the product is handled during processing. Many pump performance issues can ultimately be traced back to viscosity assumptions that do not accurately reflect real operating conditions.

For hygienic processors handling products ranging from water-like beverages to thick creams, yogurt, sauces, peanut butter, or pharmaceutical formulations, understanding viscosity is essential for achieving reliable and efficient system performance.

What Is Viscosity?
Viscosity is a fluid’s resistance to flow. In simple terms, it describes how “thick” or “thin” a product behaves during processing.

Low-viscosity fluids such as water, alcohol, or CIP solutions flow easily with minimal resistance, while high-viscosity products such as syrups, yogurt, creams, gels, and concentrates require significantly more energy to transfer.

In hygienic processing environments, viscosity can vary dramatically depending on product composition, temperature, solids concentration, entrained air, and mixing conditions. As a result, a product’s viscosity may change substantially throughout the process.

Viscosity is commonly measured using viscometers or rheometers and is typically expressed in centipoise (cP).

To put viscosity into perspective, water at room temperature measures approximately 1 cP. Milk may range from 2–5 cP, while ketchup can exceed 50,000 cP depending on formulation and temperature. Peanut butter and similar products may reach several hundred thousand cP.

Because viscosity can change significantly with temperature and shear conditions, processors should evaluate viscosity under actual operating conditions whenever possible.

Why Viscosity Matters in Pump Selection
Viscosity influences nearly every aspect of pump performance.

As viscosity increases, fluid friction inside piping systems also increases. This creates higher pressure losses, affects flow characteristics, and changes how pumps behave hydraulically.

For centrifugal pumps, increasing viscosity generally reduces flow rate and efficiency while increasing power consumption and heat generation. Higher viscosity can also create more challenging suction conditions, particularly if the system was originally designed for lower-viscosity products.

Positive displacement pump technologies are often better suited for higher-viscosity applications because they can maintain stable flow across a broader operating range while typically providing gentler product handling.

Selecting the wrong pump technology for a given viscosity range can result in unstable flow, excessive shear, increased energy usage, premature wear, and inconsistent process performance.

The Impact of Temperature on Viscosity
One of the most important considerations in hygienic processing is that viscosity is rarely constant.

Many products become dramatically thinner as temperature increases. Chocolate, syrups, sauces, creams, and personal care products may flow very differently at processing temperature compared to ambient conditions.

Conversely, products may thicken significantly during cooling cycles, startup conditions, or intermittent production stoppages.

This creates an important challenge during equipment selection: which viscosity should the pump actually be sized for?

In many applications, pumps must be capable of handling both startup and operating viscosities to ensure reliable performance throughout the process.

Shear Sensitivity and Apparent Viscosity
Some hygienic products behave differently under movement or agitation.

Products such as yogurt, cultured dairy products, gels, sauces, creams, and certain pharmaceutical formulations may temporarily become thinner when exposed to shear forces during pumping or mixing. This behavior is known as shear thinning.

Other products may become thicker when agitated, while some fluids exhibit viscosity changes over time under constant shear. These non-Newtonian behaviors can complicate pump selection because the product’s apparent viscosity changes dynamically during operation.

Understanding how a product responds to shear is especially important when handling delicate or texture-sensitive products where maintaining consistency and product integrity is critical.

Viscosity and Product Integrity
Highly viscous products often require more energy to move through a system. If the wrong pump technology is selected, this additional energy can generate excessive shear, heat, or pressure fluctuations that negatively impact product quality.

Excessive shear may damage particulates, alter texture, reduce protein integrity, destabilize emulsions, or affect final product consistency.

In many hygienic applications, maintaining product integrity is just as important as achieving target flow rates.

Common Viscosity-Related Pumping Challenges
When viscosity is not fully considered during pump selection, processors may encounter several common issues.

High-viscosity products create significantly greater resistance in piping systems, often requiring more pressure than originally anticipated. This can lead to undersized motors, poor flow performance, or excessive energy consumption.

Viscous products may also create difficult suction conditions, particularly when pumps are undersized or installed with long suction piping runs. In severe cases, this can contribute to cavitation or unstable product flow.
Additional friction generated while pumping viscous products can increase product temperature, potentially affecting sensitive formulations, emulsions, or texture-sensitive products.

Cleaning performance is another important consideration. Highly viscous products are often more difficult to remove during CIP cycles, increasing the importance of hygienic pump design, proper line velocities, and effective cleaning procedures.

In applications involving changing viscosity conditions, processors may also experience inconsistent flow behavior if the pump was not properly selected for the full operating range.

Twin screw pumps have become increasingly popular in hygienic applications because they can handle a broad range of viscosities while supporting both product transfer and CIP operations within a single pump.

Looking Beyond Published Viscosity Numbers
One of the most common mistakes in hygienic pump selection is relying solely on a single published viscosity value.

Processors should evaluate viscosity across the full operating window, including startup conditions, temperature changes, shear sensitivity, CIP requirements, batch-to-batch variation, and future product flexibility.

Understanding the complete operating environment helps ensure reliable long-term pump performance under real-world production conditions.

Final Thoughts
Viscosity plays a central role in hygienic pump performance, influencing everything from flow stability and energy consumption to product integrity and cleaning effectiveness.

Because viscosity can change throughout the process — sometimes dramatically — successful pump selection requires a broader understanding of both the product and the operating environment.

By evaluating viscosity alongside shear sensitivity, temperature, CIP requirements, and overall system design, processors can improve reliability, maintain product quality, and reduce long-term operating costs.

To learn more about selecting the right hygienic pump technology for viscous or shear-sensitive applications, contact your local authorized Fristam distributor or connect with one of our application experts.