Microstructural understanding of chilling injury in mango in relation to fruit quality

Mango is considered the king of tropical fruit due to its appealing color and flavor. It is also considered as the most important fruit of Asia. The fruit currently ranks fifth in the world production with the Philippines included among the ranks of top mango producers. Mango is recommended to be included in the daily diet of consumers due to its richness in antioxidants. Mango, however, has a short shelf life, which requires cold storage to preserve quality and extend shelf stability. In addition, application of cold storage is also very limited as storage below optimal temperature for a sufficient time induces chilling injury (CI). This condition limits the postharvest preservation and marketing potential of mango, which consequently affects the export potential of mango to distant markets.
CI is classified as a physiological disorder, which is characterized by grayish scald like discoloration and pitting in the peel, followed by uneven ripening and poor flavor and color development. CI, like other physiological disorders, is the result of imbalances in metabolism which are greatly affected by respiration and gas transport. This underlines the importance of three-dimensional (3-D) microstructure and pore network in the development of CI. The absence of a technique to completely control CI reflects the lack of sufficient understanding on the changes prior to the visual appearance of the symptoms and the need to unravel the mechanism of tissue breakdown due to CI. Previous studies analyzed only 2-D images, not based on measurements of the whole volume and cannot be used for determining the changes in the 3-D microstructure.
To this end, X-ray microtomography (μCT) protocols and image processing tools were developed to characterize the microstructure of mango. The protocols and tools were applied to obtain 3-D images of mango tissues at different stages during ripening of the fruit, and via 3-D image processing and multivariate statistics to investigate the changes in 3-D microstructure of a healthy fruit. This provided the basis for comparing the 3-D microstructural changes in the fruit during CI. 3-D analysis of structural parameters revealed changes in microstructure along the radial axis of the fruit and clear changes in the microstructure during ripening. Multivariate statistical analysis unveiled that ripening was associated with a decrease in pore size, and increase in pore fragmentation and pore specific surface area.
An integrated understanding on the effect of these microstructural changes in the postharvest ripening of the fruit was presented by investigating the observed microstructural changes in relation to the evolution of quality during ripening. This was accomplished by monitoring the changes in 3-D microstructure at different stages of ripening and observing the concurrent changes in quality. Quality changes are known to be influenced by the postharvest metabolism, which is directly affected by the changes in microstructure. The changes in quality indices such as firmness, color, taste and aroma greatly affect price competitiveness and consumer acceptability, which are essential to the marketability of the fruit. Microstructural changes associated with cell leakage were supported by the increase in the electrolyte leakage measurement of the peel. In addition, possible effects of microstructure on respiration, color development, firmness and flavor development were identified. Multivariate statistics identified structural parameters, aroma volatiles and other quality parameters that are important to ripening.
Next, we investigated the 3-D microstructural changes during CI. This was accomplished through the use of X-ray μCT of tissue obtained at different stages during cold storage, three-dimensional image processing and multivariate statistics in relation to changes in CI indicators. Microstructural changes observed during CI included a decrease in porosity, pore size and pore connectivity due to cell leakage and tissue breakdown, which was followed by an increase in pore size due to cavity formation. These changes contribute significantly to CI by drastically changing tissue aeration and water movement. Multivariate statistics identified pore connectivity and Euler number as the most important parameters in relation to CI. These changes were found to adversely affect the quality resulting in the expression of CI symptoms, increase of electrolyte leakage, limited respiration, less color development, impaired citric acid metabolism and aroma volatile production. Multivariate statistics identified structural parameters, aroma volatiles and other quality parameters that are important to CI.
In conclusion, the study showed the feasibility of using μCT to characterize the microstructure of mango fruit. Automatic segmentation and analysis allowed quantification of individual cells and tissue breakdown due to CI. Microstructure analysis allowed identification of the role of intracellular water leakage in tissue breakdown, cavity formation and peel and lenticel discoloration. Microstructural changes during CI support the hypothesis that changes in tissue and pore structure in mango fruit contribute significantly to the development of postharvest tissue disorders by drastically affecting tissue aeration and water movement, identical to what has been observed in other fruit species. This study confirmed the effect of microstructure on aroma volatiles, due to its adverse effects on gas exchange properties, limiting the respiration and shifting the metabolism of the fruit. Multivariate statistics allowed identification of structural and quality parameters that are important to ripening and CI. It served as a tool to formulate a conceptual model to describe CI, revealing a complex and in-depth relation underlying the effect of CI on microstructure and quality.