Litchi (Litchi chinensis) is a subtropical to tropical fruit with an attractive red appearance, great taste, and rich nutritive value that has been highly enjoyed by consumers worldwide for many years (Ali et al., 2016; Gong et al., 2014). However, postharvest litchi is very fragile, which is mainly indicated by its susceptibility to mechanical injury (Chen et al., 2014) and high decay rate (Zhang and Quantick, 1997). The fragility of postharvest litchi has been given much attention by researchers in past decades, but there are still many problems that require further research.
Mechanical injury primarily occurs during harvest and the transportation process, which has been given less attention. Due to the thin pericarp, thick flesh, and high water content of litchi fruit, cell rupture, cell disruption, and cell separation are easily caused by collision and extrusion (Chen et al., 2013a). Mechanical injury also opens a channel for pathogenic bacteria to enter more easily, which increases the decay rate of fruit. Litchi fruit with serious decay due to mechanical injury loses its commercial value and should be removed so they do not infect the surrounding litchis with their pathogenic bacteria. The influence of mechanical injury on litchi pericarp (Chen et al., 2013a, 2013b) has been analyzed, but how mechanical injury affects the storage quality of litchi is still unknown. Accurate detection of the initial mechanical injury to litchi is crucial for postharvest litchi management; however, it has not yet been reported.
In addition to mechanical injury, the natural decay rate of litchi fruit is incredibly fast after harvesting (Dharini et al., 2008). The red color of postharvest litchi pericarp rapidly fades and turns fully brown within a few days if stored at room temperature due to the degradation of anthocyanin in its pericarp (Hu et al., 2004). Although the current preservation technology can slow the decay rate of litchi to some extent (Khan et al., 2012), the reality of the fast decay of postharvest litchi fruit is still a problem. Fully brown litchi has worse resistance to pathogenic bacteria and almost zero commercial value. Therefore, a rapid and accurate litchi storage quality detection method should be developed for sellers to handle litchi fruit storage timely and accurately.
Fresh food with less storage time has an increasingly important role in consumer habits because of better standards of living (Elshiekh and Habiba, 1996). Because cold storage extends the storage life of litchi fruit, the outward appearance changes less, especially during the initial stage. Many consumers want to know the storage time of litchi.
At present, there are two traditional methods of litchi quality detection: the sensory detection method (Alves et al., 2011) and the physicochemical detection method (Huang et al., 2016). The sensor detection method evaluates the qualities of litchi fruit such as the pericarp color, flavor, and fragrance based on multiple human perceptive organs. The physicochemical detection method detects the total soluble solid content, titratable acidity, and weight using chemical analysis or physical measurements. The sensor detection method provides direct evaluation results from humans but is flawed because it is time-consuming, labor-intensive, and easily affected by human subjectivity. The physiochemical detection method is objective and accurate, but it is destructive, complicated, and time-consuming. Therefore, the traditional ways cannot meet the requirements of the progressing litchi industry. Even though machine vision (Xiong et al., 2011) and spectrum technologies (Xiong et al., 2018) have allowed intelligent and fast detection of many agricultural products, they are unsuitable for stored litchi quality detection because litchi fruits cover each other during storage.
The electronic nose (E-nose), also known as a bionic olfaction instrument, acquires sample information by mimicking the human olfactory system (Röck et al., 2008). An E-nose is usually composed of a sampling and cleaning channel, gas-sensitive sensor array, and pattern recognition subsystem. Furthermore, the E-nose has a sensor array that contains several gas sensors that are sensitive to different substances, which gives the entire sensor array the ability to detect simple and complex odors (Pearce et al., 2006). The E-nose is a portable tool that can detect sample quality easily, quickly, and intelligently. Compared with sensory and physicochemical detection methods, the E-nose can overcome the flaws associated with time and labor requirements, destruction, complications, and human subjectivity. Compared with the E-tongue, the E-nose can nondestructively detect characteristics of samples (Zhang and Tong, 2005). Compared with other machine detection methods like machine vision and spectrum, the E-nose can overcome the limit of the visual angle. Therefore, the E-nose is more suitable than other detection methods for litchi quality supervision.
Accordingly, this study applied an E-nose to detect the quality of litchis with mechanical injuries and normal litchis after harvesting to determine a feasible method of expanding litchi quality supervision during the postharvest circulation process. After E-nose sampling, browning indexes, total soluble solid content, and titrable acidity were recorded by sensory detection, a soluble solids refractometer, and acidity titration, respectively. The objectives of this research were to 1) to test the impact of mechanical injury and storage time on the quality of postharvest litchi; 2) to test the feasibility of using the E-nose to detect mechanical injury of litchi; and 3) to find an efficient way to detect the quality of litchi fruit during storage.
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