Short communication Mechanical properties of fruit-cuticular membranes isolated from 27 cultivars of Diospyros kaki Thunb. Shuntaro Tsubaki a,⇑ , Yoshihiko Ozaki b , Keizo Yonemori c , Jun-ichi Azuma a a Division of Environmental Science and Technology, Graduate School of Agriculture, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan b National Institute of Fruit Tree Science, National Agriculture and Food Research Organization, 2-1 Fujimoto, Tsukuba, Ibaraki 305-8605, Japan c Division of Agronomy and Horticultural Science, Graduate School of Agriculture, Kyoto University, Kitashirakawa Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan article info Article history: Received 15 August 2011 Received in revised form 25 November 2011 Accepted 13 December 2011 Available online 20 December 2011 Keywords: Cuticular membrane Diospyros kaki Wax Cutin abstract Fruit cuticular membranes (CMs) were isolated from 27 cultivars of Diospyros kaki Thunb., and relation- ships among their chemical compositions, densities and mechanical properties were analysed. Wide cul- tivar dependency was observed in both chemical compositions and mechanical properties, however, astringency did not show clear correlation with them. Densities of the CMs showed significant positive correlation with maximum stress and elastic modulus, but negative correlation with maximum strain. Positive correlations were also observed in elastic modulus-polysaccharide content and maximum strain-polysaccharide content, and negative correlation in elastic modulus-cutin content on a weight over unit area (lg/cm 2 ) basis. The integration of densities of fruit CMs and their chemical constituents may explain the mechanical properties which are specific to each cultivar of D. kaki fruit. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Fruits, leaves and peels of Japanese persimmon, Diospyros kaki Thunb., have been appreciated for their medicinal effects (Yokozawa, Kim, Kim, Lee, & Nonaka, 2007). The uniformity and thickness of fruit peels of D. kaki make their cuticular membranes (CMs) attractive for analysing relationships between their chemical and mechanical characteristics in relation to quality of the fruits. CM is an integrated epidermal layer of green plants, composed of cutin, wax, polysaccharides and cutan. CM is a defending barrier against pathogen and physical stresses (Kolattukudy, 1980). In the cases of merchandised crops, especially in fruits, mechanical sup- port by their CMs plays a decisive role for their transportation, storage and maintenance of their retail quality on shelves. Mechanical properties of tomato fruit CMs were exclusively analysed by using tensile test or load-creep test (Domínguez, Cuartero, & Heredia, 2011; Domínguez, Heredia-Guerrero, & Heredia, 2011). However, no research has been reported on fruit CMs of D. kaki. Understanding the properties of their CMs could lead to new approaches in fruit management and improvement of fruit production of D. kaki. The purpose of this study was to characterise the chemical and mechanical properties of CMs of Japanese persimmon fruits and to measure the variability among 27 cultivars. Statistical analysis of the relationships among chemical and physical properties of the CMs was also performed, to investigate the factors which deter- mine the mechanical properties of the CMs. 2. Materials and methods 2.1. Isolation of fruit cuticular membranes of D. kaki Fruits of D. kaki were harvested at the Experimental Fruit Garden, Graduate School of Agriculture, Kyoto University in Sep- tember 2008 and 2010. Manually stripped peels of the D. kaki fruits were incubated in a mixed solution of pectinase (2.0% w/v, Asper- gillus niger, Sigma–Aldrich, St. Louis, MO) and a commercial cellu- lase preparation (0.5% w/v, Meicelase CEP 16710, Trichoderma viride, carboxymethyl cellulase activity 2920 U, Meiji Seika Kaisha Ltd., Tokyo, Japan) in sodium acetate buffer (5 mM, pH 5.0) for 1–7 days at 37 °C for isolation of the CMs. The surface areas of iso- lated CMs were calculated by Photoshop CS5 (Adobe Systems Inc., San Jose, CA). 2.2. Chemical composition analysis of fruit cuticular membranes of D. kaki Chemical compositions of CMs were gravimetrically determined by the modified method of Domínguez, Espana, López-Casado, 0308-8146/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodchem.2011.12.039 ⇑ Corresponding author. Present address: Oceanography Section, Science Re- search Center, Kochi University, Akebono-cho 2-5-1, Kochi-shi, Kochi 780-8520, Japan. Tel./fax: +81 88 844 8927. E-mail addresses: stsubaki@kochi-u.ac.jp, shuntaro.tsubaki@gmail.com (S. Tsubaki). Food Chemistry 132 (2012) 2135–2139 Contents lists available at SciVerse ScienceDirect Food Chemistry journal homepage: www.elsevier.com/locate/foodchem