Analysis of texture characteristics and microstructure of passion fruit pericarp

Abstract: 【Objective】In this study, we compared the pericarp texture characteristics of passion fruit by means of the texture profile analysis (TPA) and whole fruit puncture test to screen out suitable test meth-ods for the texture characteristics of passion fruit. Combined with the observation results of pericarp mi-crostructure, the correlation was comprehensively evaluated between the pericarp texture characteristics and the pericarp microstructure, so as to provide reference for improving the storage resistance and germplasm resources of passion fruit.【Methods】In this study, the mature fruits of Tainong 1 (TN) and five passion fruit varieties (lines) coded as MB, H2, DH and ZS were used as experimental materials. Three kinds of pericarp texture analysis test modes were set up in the experiment: TPA, whole fruit  puncture (puncture depth of 5 mm) and whole fruit puncture (puncture depth of 10 mm). Among them, TPA test required sample pre-treatment, and pericarps of 1.5 cm×1.5 cm square size on both sides of the central line were cut for testing. The puncture test samples were complete fruits, and the puncture site was along the central line of the horizontal axis of the fruit. In addition, we made paraffin sections of the pericarp and observed the microstructure of the longitudinal section of the pericarp. We also ana-lyzed the correlation between pericarp texture characteristics and microstructure parameters.【Results】(1) The pericarp hardness trend of TPA test was not consistent with that of puncture test, while puncture 5 mm test and puncture 10 mm test had many consistent results. In addition, correlation analysis showed that there were no significant correlations in texture indexes between TPA test and puncture test. We speculated that the consistency of sample size could not be accurately controlled when the fruit pericarp samples were cut, and it was found in the test that the passion fruit pericarps were not com-pletely flat and fitted to the test table, so the TPA test results would be affected. Therefore, we judged that puncture test was more suitable for texture analysis of passion fruit pericarp. (2) In puncture test mode (puncture depth of 10 mm), the results showed that there were significant correlations among tex-ture parameters: pericarp hardness was significantly and extremely significantly positively correlated with deformation and pericarp puncture work; deformation was significantly and positively correlated with pericarp puncture work; adhesive force was significantly and positively correlated with adhesive-ness. Therefore, puncture hardness, deformation, pericarp puncture work, adhesive force and adhesive-ness can all be used as the parameters to reflect the texture of passion fruit pericarp. (3) The pericarp of passion fruit was mainly composed of exocarp and mesocarp. The exocarp was composed of cuticle, epidermal cells and inner epidermal cells from outside to inside. The thickness of the outermost cuticle was about 2.5-5.0 μm. There were 1-4 layers of epidermal cells, which were closely arranged rectangu-lar cells with the ratio of length to diameter by 1.15-1.65. There were 1-4 layers of inner epidermal cells, which were colorless and transparent, and were larger than the epidermal cells. Mesocarp cells gradually increased in shape from outside to inside, and the cell transition was from tight to sparse ar-rangement. Mesocarp was the most important part of passion fruit pericarp structure. (4) The correlation analysis of texture parameters and microstructure parameters of pericarp showed that there were significant and positive correlations between pericarp thickness and TPA hardness, gumminess, chewiness and puncture hardness. The thickness of cuticle was positively correlated with TPA hardness and gummi-ness, but negatively correlated with cohesiveness. Furthermore, the long and short diameters of meso-carp cells were 400-1000 μm and 1500-2300 μm away from exocarp, the short diameter of epidermal cells was significantly and negatively correlated with deformation, but significantly and positively corre-lated with adhesive force and adhesiveness. These results suggested that the relatively elongated epider-mal cell morphology and the mesocele structure with compact arrangement of small cells were more conducive to improving the deformation and reducing the adhesive force and adhesiveness. (5) Among the 5 passion fruit varieties (lines), TN had the smallest cuticle and pericarp thickness, and its mesocarp cells were large and sparse, resulting in the lowest pericarp hardness. In contrast, the mesocarp cells of H2 were small and compact, giving it greater pericarp hardness. Besides, DH and ZS had the thickest pericarp, and ZS had the thickest cuticle and was more resistant to storage and transportation, while MB had the thinnest exocarp, and its thickness of cuticle, epidermal and pericarp was small.【Conclusion】Compared with TPA test, whole fruit puncture test was more suitable for analyzing the texture of pas-sion fruit pericarp, the puncture depth can be set to 10 mm. There was a significant correlation between the texture and microstructure of passion fruit pericarp. According to this study, it can be concluded that the hard pericarp of passion fruit is characterized as large thickness of cuticle, epidermis and pericarp,and small and tight mesocarp cells.