Imagine if you purchased a spray cheese that wouldn’t come through the nozzle, or if you went to spray your pan with the PAM (or other brand you use) and it wouldn’t spray. That would be frustrating and would negatively impact your experience as a consumer. It is the job of the rheologists and formulations scientists developing the food products we consume to ensure this doesn’t happen.
Food and beverage production is a complex rheological field to work with because of the wide range of sample types and behaviors encountered. Understanding the viscosity of food and beverage products helps in both development (texture, shelf stability, spreadability, squeezability, etc) and processability (can the products be processed on your existing lines).
When working in food and beverage production you encounter both Newtonian fluids (viscosity is independent of the shear rate) such as water, milk and coffee and non-Newtonian fluids (viscosity is dependent on shear rate) such as salad dressings, corn starch, and cheese sauce. These non-Newtonian fluids can exhibit a wide array of behaviors:
- Pseudoplastic or shear thinning: viscosity decreases as the shear rate increases
- Salad dressings, mayonnaise, process cheese, cream cheese
- Dilatant or shear thickening fluids: viscosity of the fluid increases as the shear rate increases
- Concentrated corn starch
- Thixotropic: time-dependent shear thinning property in which fluids are thick (or viscous) under static conditions, but will flow over time when shaken or agitated and then return to a more viscous state after a fixed amount of time
- Ketchup, cheese sauce
- Rheopectic: a rare behavior in which the longer a fluid undergoes shearing force, the higher its viscosity will be
- Maize corn starch
When producing food products, one important characteristic to consider in formulation is shelf stability. Most food products are emulsion based and having physical stability is essential for the shelf life of your product. Understanding fluid behavior at different temperatures and conditions allows formulation optimization for shelf stability and product shelf life. Predicting shelf stability at room temperature vs. refrigeration temperatures can also help to determine storage condition requirements and recommendations of food products.
We’ve all heard the saying “the customer is always right” and this is ever so applicable when it comes to the foods and beverages we consume. When formulating food and beverage products, experts must not only consider the rheological properties, but must also consider and formulate to optimize for consumer approval of flavor, texture, consistency, etc. Zero shear viscosity is important to mouth feel and texture (thickness, creaminess, etc.) of food products whereas yield stress is the stress at which your products start shear thinning, which is very important for squeezability, processability, smoothness, chewiness, flavor release, etc. Viscosity measured across a range of shear rates allows formulation scientists to develop food products that have the right strength and capabilities to withstand the high shear rates it can happen during processing and cooking. Different ingredients can also have powerful impact on more than just the taste of food products, such as the spreadability, texture, and shelf stability. Viscosity measurements allow you to measure, analyze and optimize formulation for all these properties
Regardless of fluid behavior, most food materials will follow the cross model where zero-shear viscosity is approached at very low shear rates, while an infinite shear viscosity is approached at very high shear rates.
There are many different methods and instruments used for measuring the viscosity of food and beverage products:
- Rotational viscometer: measures the torque required to rotate the spindle or fixed geometry at a fixed speed estimate viscosity
- This method is used for quality control purposes as it allows detection of change in viscosity due to any perturbation during processing
- Vibrational viscometer: viscosity is measured from the dampening of electromagnetic resonator immersed in a sample fluid
- This method is for measuring gelation of pudding and Jell-O
- Capillary tube viscometer: gravitational flow of a fluid through a fixed distance is used to estimate viscosity.
- This is suitable for low viscosity fluids such as oils or tomato serums, but is not a realistic measurement technique for higher viscosity fluids
- Rheometer: used for viscoelastic characterization of fluids under a known flow field. This tool provides a wide array of rheological characterization.
- With the attachment of tribology tools rheometers can also provide tribo-rheometry measurements, characterizing frictional properties of food products which determine the friction food applies on the tongue (and also helps to characterize taste, texture and overall experience of the consumer for some food products)
- Rectangular slit viscometer: intrinsic viscosity measurement to characterize molecular weight distribution of biopolymers in food products, allowing evaluation of their texturizing ability which contributes to overall taste, texture and consumer experience of your food products
Viscosity measurements are essential in food processing to ensure processability, products with the right texture and stability, and optimum consumer liking. Want to learn more about measuring viscosity of food and beverage products? Download our Virtual Viscosity Summit presentation from Kraft Heinz Senior Principal Scientist, Shamsheer Mahammad.
Written by: Eden Reid, RheoSense Senior Marketing Associate