Review about different natural organic acids in postharvest

Organic acids (OAs) are organic compounds that are acidic in nature.The most common OAs is a carboxylic acid (R-COOH), whose acidity originates from the carboxyl group (-COOH). The carboxyl group is the functional group of carboxylic acid. Except for formic acid (H-COOH), carboxylic acid can be regarded as a derivative of a hydrogen atom in a hydrocarbon molecule after being replaced by carboxyl group. It can be expressed by the general formula (Ar) R-COOH (Xin et al., 2021). OAs include natural OAs and synthetic OAs. Natural OAs are mainly OAs with a certain physiological activity obtained from plants or agricultural by-products in nature, while synthetic OAs are OAs obtained through chemical synthesis, enzymatic catalysis and microbial fermentation. In addition to carboxyl groups, natural OAs also contain some other functional groups, including sulfonic acid (RSO3H), sulfinic acid (RSOOH), sulfuric acid (RCOSH), and phenolic hydroxyl groups. Different chemical groups usually give OAs different characteristics. For example, phenolic acid (such as caffeic acid, chlorogenic acid, gallic acid, protocatechuic acid, and ferulic acid), OAs with the phenol rings, its structure gives phenolic acid significant antioxidant activity. Although amino acids also have carboxyl groups, their characteristic structures do not conform to OAs and therefore do not belong to natural OAs (Xin et al., 2021).?? Natural OAs are widely distributed in fruit and medicinal plant, and their antioxidant, antibacterial and anti-inflammatory properties in human health have been well-documented (O?arowski & Karpi?ski, 2019). OAs commonly found in fruit are mainly divided into aliphatic OAs, including citric acid (CA), oxalic acid, tartaric acid, malic acid, butyric acid and ascorbic acid (ASA); and aromatic OAs, including salicylic acid (SA) and caffeic acid, etc. These OAs are the main flavor nutrients in fruit, and most of them also have positive effects on human health (Liu et al., 2016).?? Fresh fruit not only have a unique and delicious flavor, but also have many essential nutrients, including vitamins, polyphenols, dietary fiber and minerals, which are essential components of people?s daily diets (Zhang & Jiang, 2019). The consumption of fresh fruit is known to prevent a variety of diseases, including stomach cancer, colon cancer, heart disease and diabetes (Clifton et al., 2014; Lunet et al., 2005). However, postharvest fruit are still metabolically active organisms with rapid senescence, and as senescence proceeds, numerous quality deteriorations occur, including the degradation of nutrients. In addition, fresh fruit and vegetables are highly susceptible to microbial infestation due to their fragility and high water content, resulting in the occurrence of decay, which can cause substantial losses in the postharvest stage (Zhang et al., 2021). Therefore, in recent years scientists have worked to find effective preservation techniques to reduce postharvest fruit losses, such as 1-MCP and SO2. However, improper application of 1-MCP may cause abnormal ripening of climacteric fruits, which is not conducive to postharvest quality (Song et al., 2020). Considering the safety issues caused by residues of some chemical preservatives, such as SO2, finding natural postharvest fruit preservatives is the primary goal. The OAs metabolism is an important metabolic pathway for plant growth and development, and it also plays an important regulatory role during fruit ripening and senescence (Liu et al., 2016). It has been reported that intracellular OAs accumulation could enhance plant stress resistance and affect the synthesis of other plant hormones (Lv et al., 2021). OAs, as natural preservatives, have great potential in improving the quality of postharvest fruit products. In recent years, some exogenous natural OAs could be used to manage the quality of postharvest fruit and extend the shelf life of postharvest fruit (Huang et al., 2013). Common natural OAs that have the ability to preserve postharvest fruit include oxalic acid (Huang et al., 2013), CA (Liu et al., 2016), ASA (Sogvar et al., 2016) and SA (Moosa et al., 2021). In addition, the application of some other natural OAs has also improved the quality of postharvest fruit, including phenolic acid (Su et al., 2019), malic acid (Huang et al., 2016), and ursolic acid (Shu et al., 2019). Among those that have received more attention from scientists in recent years are exogenous salicylic and oxalic acid applications to alleviate chilling injury, increase disease resistance and delay senescence of postharvest fruit (Huang et al., 2013; Moosa et al., 2021). It is worth noting that the application of different exogenous natural OAs manages the quality of postharvest fruit through different biochemical mechanisms.? Moreover, in recent years, the application form of natural OAs has changed from a single application to a diversified form, for example in the form of an edible coating, combined with ultrasound and UV-C treatment and other chemical substances such as methyl jasmonate (Chen et al., 2016; Khademi et al., 2019; Siboza et al., 2017; Sogvar et al., 2016).?? Although many studies have revealed the beneficial effects of exogenous natural OAs in postharvest fruit quality management, the current relevant information is relatively scattered, and there is no relevant review work. Therefore, this work summarizes the role of common natural OAs in improving the quality of postharvest fruit, and discusses the biochemical mechanisms that may be involved, providing a reference for the application of natural OAs in postharvest fruit.? Contents (in the original paper, access below) 2 Oxalica acid 2.1 Delaying the senescence 2.2 Inhibiting postharvest disease 2.3 Alleviating chilling injury and browning 3 SA 3.1 Delaying the senescence 3.2 Inhibiting postharvest disease 3.3 Alleviating chilling injury 4 ASA 5 CA, malic acid, and acetic acid 6 Phenolic acid 6.1 Chlorogenic acid 6.2 Other phenolic acid 7 Terpenoid acids? 8 Conclusion and outlookIn conclusion, this work indicates that numerous natural OAs could be used not only as food preservatives, but also for postharvest fruit quality management. Numerous studies in recent years have shown that natural OAs can be used as postharvest fruit preservatives to improve postharvest fruit quality, including delaying senescence, controlling disease, alleviating chilling injury and inhibiting browning.? Different natural OAs have different roles in postharvest fruit quality regulation; for example, oxalic acid can be used in postharvest fruit as an ethylene inhibitor to delay postharvest fruit senescence and as a plant elicitor to enhance postharvest fruit resistance to diseases and low temperatures to inhibit the occurrence of diseases and chilling injury. In addition, oxalic acid can also be used as an antibrowning agent to inhibit the occurrence of postharvest fruit browning. SA and other phenolic acids are mainly used as plant elicitor to activate the postharvest fruit defense system and reduce the occurrence of postharvest fruit diseases and enrichment of active nutrient substances. It is worth noting that some natural OAs are simultaneously involved in postharvest fruit quality management in multiple roles; for example, some OAs with antifungal activity can be involved in postharvest fruit disease control through their direct antifungal action, induction of postharvest fruit resistance and as compensators to enhance the efficiency of antagonistic microorganisms. Therefore, numerous small molecule natural OAs are promising preservatives. And the key about the application of OAs in postharvest fruit should be noted in addition to the dose effect, the chemical modification of natural organic counts may make them better for postharvest fruit preservation efficiency. However, the current limitations of various natural OAs for large scale industrial applications are mainly the processing methods and effectiveness.? Future research should focus on the development of some new natural OAs, such as those found in terpenoids. In addition, there is limited information on the contribution of natural OAs to postharvest fruit flavor, and some natural OAs, such as CA, are key substrates in the metabolism of postharvest fruit flavor substances and therefore are likely to affect postharvest fruit flavor formation. Since the lack of postharvest fruit flavor is currently a major concern for consumers, there seems to be potential to use OA treatments to improve postharvest fruit flavor. Although some natural OAs are effective for postharvest fruit quality improvement, they still do not achieve the desired effect with alone application, so future research should focus on the combined application of other postharvest treatments such as ultrasound, cold plasma and pulsed light, among other emerging technologies. AbstractFresh fruits have good flavor and high nutritional value, but in the postharvest stage they will age and decay rapidly, so we need to find green postharvest fruit preservatives. And natural organic acids are promising natural fruit preservatives due to their safety and effectiveness. The present work reviews recent applications of natural organic acids in postharvest fruit quality management and discussed its potential biochemical mechanisms. This work indicates that numerous natural organic acids are effective postharvest fruit preservatives, such as oxalic acid, salicylic acid, citric acid, ascorbic acid, phenolic acid and terpenic acid. Natural organic acid treatments can improve postharvest fruit quality, as reflected by delaying senescence, alleviating chilling injury, controlling disease, and inhibiting browning. And natural organic acid can have multiple benefits on postharvest fruit. Natural organic acids also play a variety of roles in postharvest fruit disease control. In addition, chemical modification and cotreatment of natural organic acids can be performed to improve the efficiency of postharvest fruit preservation applications. The work provides an important reference for postharvest fruit quality management and the development of natural OAs fruit.? Picture is Fig. 1 of the orignal paper, FIGURE 1 Different roles of oxalic acid in postharvest fruit quality management.SourceThe role of different natural organic acids in postharvest fruit quality management and its mechanismWanli Zhang, Yueming Jiang & Zhengke ZhangFood Frontiers, April 2023DOI: 10.1002/fft2.245https://onlinelibrary.wiley.com/doi/pdf/10.1002/fft2.245?