Current Status and Future Perspective of Spectroscopy and Imaging Technique Applications in Bruise Detection of Fruits and Vegetables: A Review

Fruits and vegetables are subjected to different degrees of squeezing, collision or friction during harvesting, transportation, storage, sorting, packaging and marketing, resulting in external bruising like crushing, cracking and abrasions, as well as internal bruising like black core, water core, browning rot, mildly heart disease. The initial characteristics of bruising in fruits and vegetables are not obvious, and the appearance is the same as that of normal fruit. However, the bruised tissue deteriorates and spreads with time, which will eventually cause others and the whole box of fruits and vegetables to rot and deteriorate, leading to huge economic losses in the fruits and vegetables industry. There are diverse methods for postharvest bruising detection of fruits and vegetables. Among them, manual detection is the simplest and most commonly used. However, this method is not only time-consuming, labor-intensive and wrong mistakes, but also can not realize the bruising beneath the peel or internal bruising detection that is invisible to the naked eye. With the rapid development of computing technology, more and more non-destructive inspecting techniques have been widely used for bruising detection of fruits and vegetables. Among them, spectroscopy and imaging techniques are the most popular. Spectral imaging techniques usually achieve the goal of bruising detection by using the signal difference (i.e., spectroscopy or image) of the bruised and non-bruised fruits and vegetables with the procedure of image processing, spectral analysis, chemometrics, statistical analysis and other methods. These techniques are non-destructive and fast and can overcome the shortcoming of manual detection (i.e., time-consuming, labor-intensive and low accuracy). This review mainly summarizes the research progress of eight kinds of spectroscopy and imaging techniques (near-infrared spectroscopy, Raman spectroscopy, fluorescence spectroscopy, hyper-spectral imaging, spatial-frequency domain imaging, nuclear magnetic imaging, X-ray imaging and thermal imaging) in bruising detection of fruits and vegetables. Working principles and main technical features of these techniques were described, followed by their applications in detecting bruising of fruits and vegetables. Finally, a discussion on the future perspectives was given. We hope to provide references for non-destructive detection of bruising in fruits and vegetables.