Artifacts and errors in the measurement of the stickiness of liquid foods with tack tests.

We encounter stickiness in many areas of our daily life and as humans, we are able to discriminate different levels of stickiness. Yet attempts to measure stickiness with instruments have been challenging. One of the commonest approaches has been the "tack test" in which a probe is brought into contact with the sticky food and then pulled away while measuring the resisting force-various indices, such as the maximum force or the area below the force curve have been used to describe stickiness. This work rationalizes results from tack tests for liquid foods and helps us explain the influence of probe geometry. Photographic evidence of the way that the liquid adheres/detaches from the probe suggests that the terms "cohesive" and "adhesive" failure depend on the speed of the test. Application of a fixed deformation with time shows rapid loss of adhesive force suggesting that liquid samples flow from the probe. We propose that stickiness of liquid foods is entirely due to the liquid's viscosity and surface tension, and that measurements of tack for liquid foods-while highly reproducible-are entirely artifacts of the test method employed and are in effect snapshots in time of non-equilibrium processes.