A consumer panel complains about the difficulty of spreading your company's new cheese blend after being left out in an open container. Your plant engineer isn't happy, because seals blow out on a pump when it's restarted for packaging honey. The quality manager can't find a test that clearly distinguishes between batches of chocolate syrup that have a slimy or a rich taste.

These are examples of the many problems in the food products industry that can be solved by measuring flow behavior (rheology) at well-chosen conditions. The solution involves the following steps:

1. Identify the approximate flow forces or rates of flow, both before and during the critical behavior;
2. Design a rheometric test to simulate these conditions, for materials with good and poor performance;
3. Relate differences in rheometric results to performance differences; and
4. Recommend solutions—either by modifying the formulation, or its conditions of processing or application.

Impact Analytical has the expertise and facilities to help you in this process. Our rheometers apply constant, ramped or oscillating forces or flow rates over wide ranges of magnitude, time rates, and temperatures. Viscosities from 10 to 10¹² times that of water (0.01 to 10⁹ Pa s) can be measured. Excellent reproducibility and accuracy of the viscosity or viscoelastic results are obtained as these conditions are scanned.

A purchased sample of clover honey was tested using a cone/plate rheometer from TA Instruments. The results discussed below illustrate how rheometry may be applied to many other flowable or spreadable food products.

Flow of honey during processing, dispensing and spreading

In Figure 1, the viscosity of freshly dispensed honey at 25° C is shown as a function of shear rate over the range of interest for the behaviors discussed below. The three curves show results as the shear rate was ramped from 0.01 to 300 radians/second within 90 seconds, cycled back down, and up again. Viscosity, as the honey first flows at the lowest shear rate (first cycle), is more than ten times greater than when it is returned to this low rate after being highly sheared (second cycle). When immediately cycled again, intermediate viscosity values are seen. This dependence on the shearing history, called thixotropy, was not seen at shear rates greater than 10 rad/s, where most processing occurs.

Figure 1

The high initial viscosity shows why it is relatively difficult to start pumping the honey after process flow is interrupted for a time. It also shows why the honey is not too runny when spooned without earlier shearing. Also, the spreadability is easier, because of the much lower viscosity at the higher shear rates of spreading (about 1-10 rad/s). Resistance to runniness after spreading is indicated by the recovery, after rest, of higher viscosities (third cycle). Finally, steeper slopes in Figure 1 have been correlated with a good, non-slimy "mouthfeel" of other food products by taste panels [Szczesniak and Farkas (1962). J. Food Sci. 27, 381].

Thickening after Aging and Drying of Honey

Our sample in the rheometer was exposed to air for nearly three days and cycled twice to 50° C to promote drying and granulation at its exposed edge. Repeating the shear rate cycling described above gave about eight times higher initial viscosity for the aged sample. (Results are not shown here.) Subsequent results throughout the three cycles showed similar curves to Figure 1. and a narrowing of the differences, but the aged honey remained two to three times more viscous.

This quantitates the difficulty in dispensing or initial spreading of air-aged honey. Less effect of aging on spreading is expected after some working, but the substrate (e.g. bread) may be torn in the process.

Thinning of Aged Honey at Higher Temperatures

Figure 2 depicts the rapid loss of dynamic viscosity of the air-aged honey from 25° to 50° C, and its reversible nature during the return cycle. The ten-fold decrease of dynamic viscosity corresponds to about 10 % decrease for each degree C. [In this test, the sample was oscillated at fixed frequency and peak shear strain while temperature was changed at 2.5 degrees per minute. The sample had been previously cycled once to 50° C. Dynamic viscosity is the ratio of peak stress to peak strain rate.] This indicates the strong effect of heating on flow in processing, dispensing, and spreading. A temperature increase of about 10° C should offset the aging effects noted above. The reversible curves indicate the stability of the honey's structure after this pre-conditioning. Also, a viscoelastic analysis of the dynamic viscosity shows that its elastic (energy storage) component is greater that its dissipative (energy loss) component from 25° to 45° C. Thus, most of the resistance to flow is due to recoverable distortion of the structures, rather than to their purely viscous slippage past each other. At 50° C, relaxations are faster, and the components are equal at this frequency.

Figure 2

Applications to other food products:

Rheometry has been used to characterize foods that range in consistency from milk, gelatin, and bread to thick dough, and solids such as cheese, chocolate, ice cream, or raw fruits and vegetables. Results are correlated to

• Process flow
• Firmness, resilience or "body"
• Customer perceptions of appearance and taste, and
• Stability in various aging environments.

Impact Analytical combines rheometry and thermal/mechanical analysis methods to cover these broad ranges of products.