FINITE ELEMENT ANALYSIS AND EXPERIMENTAL EVALUATION OF BIOYIELD PROBES FOR MEASURING APPLE FRUIT FIRMNESS

Abstract

The bioyield phenomenon occurs during compression of an apple fruit by a mechanical probe, which results in cell failure without tissue rupture. Because of its minimal damage to the fruit, the bioyield phenomenon can be used for nondestructive or minimally destructive evaluation of fruit firmness. The objective of this research was to develop a probe for better measurement of the bioyield point for apple fruit to assess their firmness. The research was based on the premise that the soft-tipped probe would produce constant contact and a uniform pressure distribution over the contact area of the fruit and thus enhance the detection of the bioyield point. The finite element method was used to analyze the contact stress distribution in the apple fruit resulting from the compression of soft bioyield probes of different sizes, thicknesses, and elastic moduli. The modeling results showed that to achieve uniform contact pressure, the bioyield probe should have a soft tip with an elastic modulus comparable to or less than that of the fruit and a thickness greater than 2 mm. Six bioyield probes, along with the standard destructive Magness-Taylor (MT) firmness tester, were tested on four apple cultivars (‘Gala,’ ‘Golden Delicious,’ ‘Fuji,’ and ‘Red Delicious’). Probe size (between 6.4 mm and 11.1 mm) did not affect the correlation between force at the bioyield point and MT firmness. The probe with a 1.6 mm thick tip, which was undesirable based on the finite element modeling, had a high missing rate (13%) of detecting the bioyield point, compared to that (2%) for the other five probes with thicker soft tips. The 6.4 mm probe with a 3.2 mm thick rubber tip and an elastic modulus of 3.27 MPa had a good correlation (r = 0.828) with MT firmness measurement. Since the smaller probe causes minimal damage to the fruit, it is a better choice for the bioyield measurement.