An introduction to the rheological study of food sauces

Food sauces are one of the most important condiments in the human diet and gastronomic culture around the world. The almost endless combinations of flavors, textures, and colors, as well as the sensorial characteristics that consumer feels savoring food sauces, make these products one of the most consumed and important for the economy in food industry. The food sauces market generated US$120 billion in 2016, with a trend to increase in the next years.

Products for human consumption need a careful evaluation of all physico-chemical properties and sensorial characteristics.

Rheology is the study of deformation and flow of matter under well-defined conditions and is an important tool to characterize fundamental material properties of food systems. Rheology applied to food processing and product design is an area with a huge impact due to the important information obtained. Food rheology is essential for:

• quality and stability control
• design flow and handling systems
• sensory evaluation of food

Rheological and mechanical properties like consistency, degree of fluidity and flow, are the most important parameters to be determined in food sauces and will be important to determine how stable the final product will be and how long it can be stored. Rheological studies of fluid foods are so important that developments in the fields of process engineering application, equipment design and transport system are based in these results.

Rheological properties of food sauces

Food sauces are a complex mixture constituted essentially by water or oil. Hydrocolloids have an essential role in food sauces formulation once they will be responsible for viscosity control and stability of the formulation, prolonging shelf-life. Sauces are semi-solid foods, behaving as soft solids or non-Newtonian fluids and exhibiting pseudoplastic flow with a distinct yield stress and thixotropic (time-dependent) behavior.

Rheological characterization is essential to quantify the functional relationships between deformation, stresses, and the resulting rheological properties such as viscosity, elasticity or viscoelasticity. Viscoelastic properties play dominant roles in the handling and quality of the sauce. Such properties are not only dependent on time but also on processing temperature, solid contents and other ingredients used in many types of formulations.

Usually food sauces are shear-thinning (pseudoplastic) fluids and exhibit a yield stress, defined as the stress that must be exceeded for flow to occur. The consistency index, apparent viscosity, yield stress are important product properties. Thus, the rheological parameters are a useful tool in understanding changes in food structure during processing, to control the quality of the product and to consumer handling with the product. Within rheology, the oscillatory rheological method is the most popular method to characterize viscoelasticity, since relative contributions of viscous and elastic response of materials can be measured.

Main rheological parameters to determine in the quality control of food sauces

• Viscosity

Viscosity is an important property of fluid foods. It is defined as the internal friction of a liquid or its ability to resist flow. As viscosity changes the flow properties of a liquid food changes and influences the appearance and consistency of a product. Viscosity of a food sauce is affected by factors like temperature, concentration of solute, molecules and suspended matter; it can be used as an indicator of quality by the final consumer, as in most cases a thicker food sauce is thought to have superior quality when compared to a thinner product.

Like most fluid foodstuffs, sauces show complex rheological behavior. Sauces like mayonnaise and ketchup are sold in different containers including the traditional glass bottle or plastic squeeze dispensers, so controlling the different dispensing properties is necessary. When used in a glass bottle, the sauce is not expected to flow freely until the bottle is agitated. In the plastic bottle, on the other hand, the sauce should flow when a gentle squeeze (pressure) is applied for several seconds. In general, mayonnaise and ketchup exhibit pseudoplastic behavior with a flow threshold, presenting thixotropy (shear stress dependence on the time of shear); also, the decrease in the oil content of mayonnaise normally lowers the viscosity of the final product. In Figure 1 is depicted the dependence of viscosity on shear rate, or applied motion – a higher shear rate will promote a lower viscosity and flow.

• Yield stress

Yield stress is a physical and rheological property for liquid and solid materials. Yield stress determination is an essential property of food sauces and it is a measure of the strength of the material structure, being defined as the minimum stress required to make a material flow. Pseudoplastic materials behave as solids under small applied stresses, and as liquids at high stresses. Figure 2 relates stress with strain, highlighting the yield stress as the moment when the material cannot hold more elastic deformation and flow begins.

In practice, the yield stress of products such as mayonnaise, ketchup, salad dressings, etc, represents the force that has to be applied by the user in order to break down its structure and initiate its dispensing. Since no more relevant stress is applied after reaching the yield point, and the material is left to rest over time, its structure rebuilds, becoming solid again.

Yield stress is also a key product characteristic which determines its texture, and thus the consumer sensory perception during use and application.

• Polymer behavior, stability and shelf-life

Oscillatory time sweeps are important when testing materials, such as dispersions and polymers, that may undergo macro or micro structural rearrangements with time. These rearrangements directly influence rheological behavior. Oscillatory time sweeps directly provide the necessary information about how a material changes with time.

By oscillatory time sweeps of food sauces, it is possible to determine possible polymer degradation, solvent evaporation, dispersion settling, cure information and/or time dependent thixotropy of formulations. Oscillatory tests give important information about food sauces stability and shelf-life, once the evaluation of the material’s behavior with time can be monitored directly with variation of frequency and the temperature of interest.

By oscillatory time sweeps one may measure two parameters: G’, referent to the “storage” or “elastic” modulus and G’’ referent to the “loss” or “plastic” modulus.

A stable formulation with no structural modifications is represented in a plot as shown in Figure 3, where the G’ and G’’ are independent (the formulation maintains its fluid characteristics). On the other hand, if the food sauce formulation is not stable, and its identity varies with variation of frequency or temperature, the G’ and G’’ will cross.

Instruments for rheological measurements

The increasing social and economic importance of food production, together with the complexity of production technology, processing, handling and acceptance of these highly perishable and fragile food materials require a more extensive knowledge of their physical properties.  There are numerous instruments available to the food industry to measure viscosity for quality control and thus ensure that products made are of consistent quality. These commonly-used viscometers are capable of measuring Newtonian and non-Newtonian fluids in a wide viscosity range. The consistometer determines the consistency of a food by measuring the distance it flows under its own weight. Rotational viscometers measure the torque required to turn a spindle in a sample of fluid at a known speed. Capillary viscometers, also known as U-Tube or Ostwald, measure the time for a fluid to pass between two points of a capillary tube under the force of gravity.

Once food sauces are subjected to variations in their temperature during production, transport, storage, preparation and consumption, and taking into account the influence of the overall properties of the final product, e.g., taste, appearance, texture and stability, rheological characterization is essential  to optimize processing conditions and improve product quality.