Internal bruising prediction in watermelon compression using nonlinear models H. Sadrnia a,b, * , A. Rajabipour b , A. Jafari b , A. Javadi c , Y. Mostofi d , J. Kafashan a,c , E. Dintwa a , J. De Baerdemaeker a a Division of Mechatronics, Biostatistics and Sensors, Faculty of Bioscience Engineering, Katholieke Universiteit Leuven, Kasteelpark Arenberg 30, B-3001 Leuven, Belgium b Faculty of Biosystem Engineering, College of Agriculture, University of Tehran, Karaj, Iran c Agriculture Engineering Research Institute, Karaj, Iran d Faculty of Horticulture Science and Plant Protection, College of Agriculture, University of Tehran, Karaj, Iran Received 19 March 2007; received in revised form 2 October 2007; accepted 5 October 2007 Available online 22 October 2007 Abstract Fruit bruising could occur during handling, transportation and storage. It is invisible in many cases of fruits such as watermelon. In this study, nonlinear finite element analysis (FEA) was used to investigate internal bruising in watermelon compressed in longitudinal and transverse directions by parallel plates. The applied forces on models were equal to 10% of breaking force or 152 N and 92 N, respec- tively for Crimson sweet and Charleston gray varieties in the longitudinal direction. The simulation results show that equivalent stresses in the red flesh are higher than the failure stress in both varieties (27 kPa and 37 kPa for Crimson sweet and Charleston gray, respec- tively). However, the maximum equivalent stresses located near the surface of watermelon are well below the failure stresses of rind (1.2 MPa and 1.1 MPa for Crimson sweet and Charleston gray, respectively). This indicates that the bruise of the red flesh is the primary form of mechanical damage of watermelons under compression in both directions. The nonlinear FEA data confirmed well experimental results and has the ability to predict bruising in watermelons under different load conditions. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Mechanical properties; Bruise; Quality; Watermelon; Nonlinear models; Finite element analysis 1. Introduction Bruise is visible on the surface of most fruits (e.g. apple, pear, potato), while skin thickness in watermelon makes it difficult to detect flesh bruising (Diezma-Iglesias et al., 2004). Also, watermelons are mostly loaded in bulk ship- ments. Bulk loading and storage requires protection of watermelons from load shifting injury (Boyhan et al., 2000). To avoid mechanical damage, the magnitude of the applied stress is required to be less than the minimum stresses which cause internal bruise in watermelon. It is extremely difficult to measure internal stresses caused by the applied compressive forces. An alternative approach is to estimate the stresses using finite element analysis (FEA). With the finite element method (FEM), objects of irregular shapes and non-homogeneous material properties (e.g. modulus of elasticity) can be modeled. FEA has the potential to solve nonlinear problems such as those involv- ing contact and large deformations. Studies relating the use of FEA applications for post- harvest modeling and estimation of stress distributions in fruits and vegetables are abundant in the literature. Carde- nas-Weber et al. (1991) applied FEA to estimate both deformations and stresses with application to robot gripping. They concluded that FEA can predict the stresses 0260-8774/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2007.10.007 * Corresponding author. Address: Division of Mechatronics, Biostatis- tics and Sensors, Faculty of Bioscience Engineering, Katholieke Univer- siteit Leuven, Kasteelpark Arenberg 30, B-3001 Leuven, Belgium. Tel.: +32 016 321470; fax: +32 016 328590. E-mail address: hassan.sadrnia@yahoo.com (H. Sadrnia). www.elsevier.com/locate/jfoodeng Available online at www.sciencedirect.com Journal of Food Engineering 86 (2008) 272–280