Recent Advancements in Postharvest Fruit Quality and Physiological Mechanism

1. Introduction

Fresh fruits can provide people with various nutrients, including carbohydrates, vitamins, minerals, phenols, and other bioactive compounds, and they are an indispensable component of the human diet. Fruit development and ripening is a genetically programmed series of coordinated processes, with various physiological changes occurring, such as a progressive loss of firmness, the release of volatile compounds, and the accumulation of soluble sugars and a wide range of secondary metabolites [1,2]. These physiological and biochemical changes make horticultural fruit more palatable and desirable to consumers. Appropriate postharvest commercialization techniques are particularly crucial for maintaining or improving fruit quality. This Special Issue concerns the biological and technological postharvest research of horticultural fruit crops and aims to enhance readers’ in-depth understanding of the advanced technologies and regulatory mechanisms for maintaining postharvest fruit quality. A total of twelve articles (two review papers and ten research papers) are rigorously reviewed and included in this Special Issue, which determines and discusses the quality maintenance and physiological regulation mechanisms of postharvest fruit and fresh-cut products, as well as different fruit quality testing technologies.

2. Special Issue Overview

2.1. Preharvest Treatments Affect Postharvest Fruit Quality

The effects of preharvest applications on postharvest behavior are determinative in terms of fruit quality and storability [3]. The preharvest application of chemical elicitors, plant growth regulators, or controlled environmental factors can maintain postharvest quality and extend the storage period, especially for fruits with poor storability and a short shelf life [4,5]. Retamal-Salgado et al. (contribution 1) evaluated the effect of three preharvest applications of oxalic acid (OA) and salicylic acid (SA) on fruit firmness and phenolic compounds in blueberry (‘Stella Blue’ and ‘Kirra’). The results showed that preharvest applications of OA and SA can improve fruit firmness by up to 20% at different harvest times. In addition, 2 mM SA generated a 100% increase in polyphenolic content and antioxidant capacity in ‘Stella Blue’, while 4 mM OA increased total anthocyanin and antioxidant capacity by 100% and 20%, respectively. The preharvest application of gibberellic acid (GA₃) on plum also led to a decrease in weight loss, with a corresponding increase in fruit firmness, total soluble solids (TSSs), and titratable acidity (TA); however, the total phenolic content values during storage did not exhibit any significant changes. According to these results, producers can be advised to apply 50 ppm GA₃ in the preharvest stage of plum [3]. Taking into consideration the potential side effects of preharvest applications on ecological conditions and cultural practice, it is advised that tests are repeated for different locations and/or fruit varieties to determine the site- and variety-specific responses before large-scale promotion and application.

2.2. Postharvest Treatments Affect Fresh Fruit Quality

Sufficient evidence suggests that cell wall disassembly is responsible for a major portion of fruit softening, and this is initiated by a complex interplay between hormonal cues, epigenome changes, and the tightly regulated expression of numerous transcription factors (TFs) and downstream genes [6]. The postharvest handling of horticultural fruit is an effective means of maintaining or improving product quality and extending storage and shelf life, thus increasing the value of fruit after harvesting. 1-Methylcyclopropene (1-MCP) is an inhibitor of ethylene perception that is widely used to maintain the quality of climacteric fruits during storage [7]. Rivera-Ponce et al. (contribution 2) evaluated the diversity of postharvest characteristics in 10 chayote groups of varieties that allow them to be used in different ways including as a fresh fruit, in agroindustrial transformation, or in mixing with other vegetables. Moreover, 600 nL L⁻¹ 1-MCP sealed treatment reduced the chilling injury, weight loss, and evident dehydration for five chayote varieties during cold storage. Shu et al. (contribution 3) found that a chitosan coating (2%, w/v, 310–375 kDa) containing 0.2% (v/v) carvacrol maintained the postharvest quality of guava compared to chitosan alone, with higher firmness, TSS, TA, and total phenol content and lower weight loss and pericarp browning. Mi et al. (contribution 4) showed that LED white light treatment (LWT) preserved the postharvest quality of ‘Zaosu’ pear fruit by inhibiting respiration and ethylene production, reducing weight loss and ascorbic acid degradation, and promoting the ratio of sugar and organic acid. In addition, LWT retarded the decrease in chlorophyll content of pear fruit by increasing the activities of chlorophyll synthase-associated enzymes, and suppressing the chlorophyll degradation-related enzymes and their gene expressions in pear peel. Zhao et al. (contribution 5) demonstrated that low-concentration NAA treatment on peach reduced the transcription level of PpPG, Ppb-GAL, and PpACS1 genes. Furthermore, an interaction between the auxin receptor PpTIR1 (Transport Inhibitor Response 1) and PpIAA1/3/5/9/27 proteins was unveiled, and the results show that the PpTIR1-Aux/IAA module has a possible regulatory effect on fruit ripening and softening. Zhang et al. (contribution 6) reviewed the important role of gibberellins (GAs) in the physiological regulation of postharvest fruits, which includes improving the intrinsic and extrinsic quality, enhancing postharvest biotic and abiotic stress resistance, and effectively controlling some postharvest fruit diseases. The authors also suggested that GAs have important application prospects in postharvest fruits.

2.3. Testing Technology for Fruit Internal Quality

In recent years, the development of non-destructive sensing techniques for measuring fruit internal quality has been a challenge. The rapid identification of defective fruits is beneficial for reducing unnecessary storage and avoiding quality degradation [8]. Tang et al. (contribution 7) built internal quality prediction models for Korla fragrant pears. After determination, it showed that the model based on partial least-squares regression (PLSR) and using the dielectric constant as a variable predicted hardness the most accurately, while the model based on PLSR using the dielectric loss factor as a variable was the best for predicting SSC. Therefore, the study provides a new method for the non-destructive online testing of the internal quality of pear. Ni et al. (contribution 8) indicated that there would continue to be active research and publications on non-destructive testing technology for fruit quality. China and the USA are the major contributors to research in this field. The detection technologies mainly include electronic nose (E-nose) technology, machine vision technology, and spectral detection technology. In the future, technological developments in artificial intelligence and deep learning will further promote the maturation and application of non-destructive testing technologies for fruit quality. Firmness is a crucial feature that significantly influences the postharvest preservation of fruit and its acceptability by customers. The methods for measuring fruit firmness are mostly destructive, using a texture analyzer or firmness tester. Karageorgiadou et al. (contribution 9) studied the impact of fruit physiology under various and fixed distances for the firmness evaluation of sweet cherry fruit. The results suggested that a fixed distance of 0.16 mm and a minimal 1% deformation force possess the potential to be employed and implemented for monitoring the firmness of sweet cherries using the texture analyzer during postharvest preservation.

2.4. Quality Maintenance and Regulation of Fresh-Cut Fruit Products

Fresh-cut fruit products are in considerable demand owing to the convenience of buying and cooking. However, the shelf-life of fresh-cut fruit products is short due to their physiological changes and maturation [9]. Yuan et al. (contribution 10) found that 0.4 g/L of nisin reduced the weight loss rate and whitening rate, inhibited the respiration rate, and maintained the firmness of fresh-cut pumpkins. Meanwhile, 0.4 g/L of nisin increased antioxidant-metabolism-related enzyme activities and prevented the rapid increase in reactive oxygen species (ROS), as well as maintaining higher contents of ascorbate and glutathione. Guan et al. (contribution 11) explored the effect of cutting methods on the quality of fresh-cut cucumbers, and found that the vitamin C content was gradually reduced in the sliced, pieced, and stripped cucumbers, while the glutathione content increased significantly compared with whole cucumbers. Furthermore, the fresh-cutting operation enhanced the total phenol content, but also induced the production of ROS (O₂^−· and H₂O₂). In general, the degree of quality indexes was sliced > pieced > stripped. This revealed that JA biosynthesis was activated by mechanical damage, and the up-regulation of phenylalanine metabolism and phenylalanine, tyrosine, and tryptophan metabolism affected phenylpropanoid biosynthesis, which may promote lignin synthesis. The lignin produced during secondary metabolism lignifies fresh-cut cucumber, which seriously affects the taste and appearance. Through transcriptome analysis, Wang et al. (contribution 12) demonstrated that pathways of amino acid metabolism, lipid metabolism, and secondary metabolism were affected by mechanical damage. Furthermore, the up-regulation of phenylalanine metabolism and phenylalanine, tyrosine, and tryptophan metabolism affected phenylpropanoid biosynthesis, which may promote lignin synthesis.

3. Conclusions

The twelve articles in this Special Issue were selected from a large number of submissions through rigorous evaluation, and they are of a high standard. This Special Issue aims to enable researchers to understand the cutting-edge technologies and methods related to maintaining postharvest fruit quality, while also providing new thoughts for in-depth research on the regulatory mechanisms related to postharvest fruit quality control. At present, consumers have increasingly high requirements for fruit quality, so it is necessary to continuously develop new technologies to maintain the postharvest quality of fruits. In addition, researchers need to strengthen their study on the mechanisms involved in the maintenance of fruit quality.