APPARENT MODULUS OF ELASTICITY FROM FIRMTECH 2 FIRMNESS MEASUREMENTS OF BLUEBERRIES

Abstract

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