APPARENT MODULUS OF ELASTICITY FROM FIRMTECH 2 FIRMNESS MEASUREMENTS OF BLUEBERRIES

The firmness of blueberries and other small fruits can be measured rapidly with a FirmTech 2 Fruit Firmness Tester. The output of the instrument is the slope of a chord joining selected minimum and maximum forces on a force/deformation curve. The overall objective of this study was to improve the firmness measurements obtained with the Firm Tech 2. In this study, both simulations and tests with rubber balls showed that firmness measurements of a single ball increased as minimum or maximum force was increased. If repeatable results are to be obtained from the FirmTech instrument, standard values for the force settings must be established. Equations from ASABE Standard S368.4 were adapted for calculating the apparent modulus of elasticity (Ea) for both parallel plate and single plate compression using FirmTech 2 output. Ea values for a simulated force/deformation curve with deformation taken to the 3/2 power were the same when calculated using the ASABE equations and when calculated using the equations developed, regardless of which combination of minimum and maximum force was selected. When rubber balls with three diameters were compressed with a universal testing machine, the Ea values calculated from the ASABE equations were similar to those calculated using the equations developed in this study. The firmness of blueberries was measured using both the FirmTech 2 and the universal testing machine. The Ea values were calculated using the equations developed in this article and compared to values calculated from the universal testing machine curves using the ASABE equations. Results indicated the loading applied by the FirmTech 2 support plate was somewhere between a parallel plate and a single plate compression test. Additional research could show benefits from using the equations developed in this study for calculating Ea from loading curves obtained using universal testing machines.