Advanced Studies on the Quality Control and Metabolism of Bioactive Compounds in Postharvest Horticultural Crops

Fruits and vegetables are rich in nutrients such as vitamins C and A, minerals, electrolytes, and dietary fiber [1], which are important to people’s lives as they reduce the incidence of chronic diseases [2], cardiovascular diseases, obesity, and cancer [3]. With the elucidation of the concept of a nutritional diet, people’s demand for high-quality fruits and vegetables is increasing. At the same time, there are higher requirements for the quality of storage and preservation of fruits and vegetables.

Fresh fruits and vegetables have high water contents and continue to undergo respiration, water evaporation, and microbial activity after harvesting [4]. These processes can negatively impact their quality during transportation and storage. Consequently, unfavorable conditions can lead to deterioration and decay, resulting in a significant loss of nutritional value. It is estimated that 20–25% of the losses are caused by pathogen-induced postharvest fruit and vegetable decay [5]. Fruits and vegetables are seasonal, leading to potential supply shortages influenced by factors such as production, demand, and storage. Consequently, extending the shelf life of these perishable items is crucial. Addressing this issue can help alleviate the ongoing imbalance between the supply and demand for fruits and vegetables.

Currently, three primary methods are employed for the preservation of fruits and vegetables: physical, chemical, and biological techniques. Physical preservation methods include low temperatures, air conditioning, ultrasound, photocatalysis, heat shock, and radiation. Chemical preservation typically involves the application of chemical preservatives and plant growth regulators, which can be soaked, sprayed, or coated onto the produce. Biological preservation methods utilize coatings, natural extracts, and genetic engineering to enhance the shelf life of fruits and vegetables. While traditional chemical preservatives are easier to use, they often leave chemical residues and raise food safety concerns [6]. Given the limitations of conventional chemical methods, there is a pressing need to focus on advanced and environmentally friendly preservation technologies. These innovations should aim to maintain the quality of postharvest fruits and vegetables while facilitating the extraction and transformation of active substances.

Low-temperature storage can slow down the metabolism of most cells, the degradation of cell wall pectin [7], and membrane lipid peroxidation [8] and maintain the phenolic and ascorbic acid content [9], thereby extending the freshness of fruits and vegetables. While low temperatures are generally effective for preserving most fruits and vegetables, they are not universally applicable. For instance, low temperatures can increase kiwifruit’s sensitivity to ethylene, which accelerates its ripening process [10]. Furthermore, tropical and subtropical crops are vulnerable to cold injury when subjected to inappropriate cold storage. This damage can lead to a decline in quality. Heat treatment increases the content of unsaturated fatty acids and the expression of heat shock protein genes and boosts antioxidant capacity. It also modulates enzyme activity and promotes sugar metabolism [11]. These mechanisms collectively help mitigate the adverse effects of cold damage. Modified atmosphere preservation (MAP) inhibits cellular respiration and slows metabolism by regulating the ratios of gas components, temperature, humidity, and air pressure. MAP can inhibit the activity of polyphenol oxidase and peroxidase [12] and can delay pathological diseases associated with endosperm, black sigatoka, and acidification [13]. Ultrasonic treatment effectively removes pesticide residues, reduces microbial decay, inhibits cell wall-degrading enzymes, and preserves the stability of cell wall polysaccharides and the content of chlorophyll, total flavonoids, and reducing sugars, thereby enhancing the quality attributes of fruits and vegetables [14]. While MAP and ultrasonic preservation technologies provide a fresher environment that prevents fruits and vegetables from producing harmful substances, their practical application involves greater equipment, technology, and cost requirements, making large-scale use inconvenient [15]. Light irradiation is an innovative method for preserving fruits and vegetables. This technique is characterized by its non-thermal and non-toxic properties. Common types of light irradiation include light-emitting diodes [16], ultraviolet light [17], pulsed light [18], and fluorescent light [19]. The primary purpose of light irradiation is to extend the shelf life of produce. It achieves this by inhibiting microbial growth and reproduction [20] and enhancing antioxidant capacity [21].

Chemical preservatives can be categorized into two main types: plant growth regulators and chemical fungicides. Plant growth regulators are physiologically active substances that influence various physiological processes in plants. They play a crucial role in helping plants defend against biotic and abiotic stresses. Postharvest treatment with gibberellic acid can effectively inhibit pectin degradation and regulate cell wall metabolism, thereby slowing down the decline in quality and softening of okra [22]. However, the misuse of these regulators can lead to adverse effects on human health. Fungicides are mainly used to control fruit and vegetable spoilage caused by fungi [23]. Prolonged use of chemical fungicides may lead to pathogen resistance and impact human and environmental health [24]. For example, ozone rapidly decomposes into oxygen under controlled conditions, leaving minimal to no residue on food. However, at high concentrations, ozone is an irritating and toxic gas [25].

Common bio-preservation materials include natural extracts, essential oils, and polysaccharide-based preservatives, which are non-toxic, free from side effects, and biodegradable. Natural extracts effectively remove pesticide residues [26] and serve as alternatives to insecticides [27]. Similarly, essential oils with bacteriostatic activity can reduce postharvest rot caused by microorganisms [28]. In addition, essential oils have a protective effect on the proteins, fibers [29] and ascorbic acid [30] in fruit and vegetables. Chitosan coatings effectively inhibit bacterial growth, decrease ethylene release, and limit gas transfer to the fruit surface, thereby preserving nutritional quality [31].

The five papers in this Special Issue explore various physical, chemical, and biological preservation techniques. Smrke et al. (contribution 1) investigated the impact of air conditioning on the short-term storage of blueberries. Their study demonstrates that atmospheres with normal air, 10% CO₂, and 100% CO₂ all effectively maintain the firmness of blueberries. Although these atmospheres result in differences in the parameters of blueberry skin color, these changes are not perceptible to the naked eye. Furthermore, all three atmospheric conditions successfully preserve the appropriate sugar/organic acid ratios in the fruit when stored at 2 °C for 48 h. Notably, the study finds that air temperature exerts a more significant influence on the total phenolic content of blueberries than the atmospheric composition. Consequently, a 10% CO₂ atmosphere is more advantageous for extending the shelf life of blueberries compared to a 100% CO₂ atmosphere, particularly concerning the preservation of total phenolic content.

Montesdeoca-Flores et al. (contribution 2) evaluated the antifungal capacity of four Streptomyces strains and examined the impact of extracts from the most effective strains on the storage quality of cherry tomatoes. The results showed that the extract of a single strain of S. netropsis greatly inhibited the sporulation of F. oxysporum and the growth of A. alternata and B. cinerea. The extract reduced fruit infections in tomatoes by half. It is worth noting that the extract of S. mauvecolor was able to completely inhibit F. oxysporum. Extracts of S. pratensis and S. ardesiacus also showed some antibacterial effects. The extracts of these new strains of Streptomyces could provide new ideas and methods for environmental antimicrobial control.

Geng et al. (contribution 3) conducted a study to examine the effects of combining preharvest foliar fertilizers with postharvest salicylic acid treatments on the quality and softening of panzao during storage. The results demonstrated that all treatment groups effectively inhibited the reddening of panzao during storage, preserved the levels of total soluble solids, ascorbic acid, and phenolic contents, and decelerated the processes of fruit oxidation and aging. Notably, the combination of preharvest foliar fertilizer application and postharvest salicylic acid treatment exhibited synergistic effects. The study further revealed that salicylic acid plays a crucial role in regulating the activity of enzymes responsible for starch and cell wall degradation. This regulation delays the reduction of antioxidant substances and the degradation of macromolecules such as starch and pectin, thereby mitigating the decline in quality and softening of the fruit.

Silva et al. (contribution 4) conducted a study to evaluate the effects of chitosan and gelatin coatings on the quality of papaya. The research involved analyzing the infrared spectra of the films used as coatings, as well as various quality indicators of the fruit. The findings demonstrated that the coatings had a beneficial impact on papaya quality. Specifically, the treatment effectively preserved the freshness, firmness, color, and ascorbic acid content of the papayas. Additionally, the coating treatment enhanced cell protection, delaying the ripening process. Consequently, the application of these coatings extended the shelf life of papayas by at least 8 days.

Niu et al. (contribution 5) investigated the impact of chitosan composite coating on citrus. The study utilized an edible coating composed of chitosan and carboxymethyl cellulose, which significantly enhanced the morphology and microstructure of the citrus. As a result, the treated citrus exhibited increased surface brightness, hardness, weight, and ascorbic acid content. Overall, the application of this coating effectively preserved the quality of postharvest citrus.

Research on the preservation of fruits and vegetables has been extensively conducted both domestically and internationally. These studies have focused on inhibiting postharvest respiration and microbial reproduction to enhance preservation. However, traditional physical and chemical methods present significant drawbacks, including complex technology, high equipment costs, and the potential for chemical residues. Consequently, there is a growing interest in combining advanced green preservation technologies, such as ozone treatment, cold plasma, chitosan coatings, and natural extracts. The ongoing advancement of these preservation techniques is essential to meet the increasing demand for high-quality produce and improved quality of life.