Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/17/2005
Publication Date: 2/22/2006
Citation: Lu, R., Srivastava, A.J., Ababneh, H.A. 2006. Finite element analysis and experimental evaluation of bioyield probes for measuring apple fruit firmness. Transactions of the ASABE. 49(1):123-131. Interpretive Summary: The decision on when to harvest fruit and how to handle fruit after harvest will have a significant impact on their quality and shelf life, and this often requires accurate determination of fruit condition and quality such as firmness. Firmness is an important parameter in determining harvest time and the postharvest quality grade of fresh apples. The current fruit firmness measurement technique destroys the fruit during the testing process, and hence it cannot be used to monitor, sort or grade fruit. This research investigated a new mechanical probe for determining fruit firmness, which does not degrade or damage the fruit. The probe detects force at the bioyield point as a measure of fruit firmness. Bioyield is a phenomenon occurring when the fruit is subjected to external force loading, resulting in tissue damage at the cell level but without macroscopic failure, i.e., tissue rupture. Using computer simulations, we analyzed different design parameters for the mechanical probe including its size and material, from which an optimal probe design was determined. We further tested six probe designs on apple fruit and compared them with the Magness-Taylor (MT) firmness tester, an industry standard destructive technique. Experiments confirmed that a small probe with soft tip resulted in good correlation with MT firmness (r=0.828). This new bioyield probe will be useful for measuring the firmness of fruit during the pre-harvest season and can also be used to monitor fruit quality during postharvest handling and storage.
Technical Abstract: The objective of this research was to develop a minimally destructive or nondestructive mechanical sensor to measure or estimate fruit firmness. Bioyield force (or BY = force to cause bioyield by a soft-tip probe) was used as a measure of fruit firmness. A new design of using soft tip probe was proposed to enhance the detection of the bioyield point. Finite element (FE) simulations were performed to analyze stress-strain distributions within the contact area of the fruit with different designs of mechanical probes (i.e., elasticity and thickness of soft tip and probe size). FE simulations showed that the use of soft material with an elastic modulus of no greater than two times that of apples would produce quasi-uniform stress distributions in the contact region, thus enhancing the detection of BY. The soft tip should be 2 mm or more in thickness for producing quasi-uniform stress distributions. Based on FE simulation results, six bioyield probes were designed and tested on apple fruit. Experiments confirmed that the probe of 6.4 mm diameter and 3 mm thick soft tip had good correlation with Magness-Taylor firmness (r=0.828). The bioyield probe does not degrade fruit and thus provides a nondestructive means for estimating apple fruit firmness.