An Exploration Into the Measurement and Reporting of Food Firmness and Hardness.

Hardness is a commonly reported food property, measured by compression at a high strain where the food structure breaks. Hardness should not be reported for foods that deform without breaking. Firmness is an intermediate level of hardness associated with nondestructive compression, at strains typically around 0.1. Sensory perception enables accurate classifications of hardness. Conversely (and perhaps counterintuitively), instrumental measurements of hardness while often precise are not necessarily accurate. The outcome depending on the test protocol, whereby the geometry of the test apparatus, the speed of the test and the degree of deformation all influence the result. Ambiguity occurs in how instrumental measurements of hardness are reported, with some authors using stress, while others use force.