ReviewChanges in biochemistry of fresh produce in response to ozone postharvest treatment
Introduction
Consuming fruits and vegetables is recently getting more attention due to the awareness of humans about dietary and health benefits of consuming fresh produce (Gibson et al., 1998; Baranowski et al., 2003; Shahidi and Ambigaipalan, 2015, 2018). Fresh produce has a short shelf life, mainly because it continues with metabolic processes even after harvest (Lee et al., 1995; Kader, 2002). The major threat to postharvest quality is a loss of moisture through transpiration. Moisture loss is a major quality determinant of fresh fruit and vegetables, mainly because it determines the economic value of a produce, since fresh horticultural products are sold on the mass basis (Tesfay et al., 2017). The loss of moisture is a huge financial loss, it must, therefore, be controlled before the quality or appearance of produce is jeopardised (Kerbel, 2004). Several postharvest treatments have been used to control such losses, including chemical treatments, controlled atmosphere (CA), modified atmosphere packaging (MAP), and coatings (Thompson et al., 2018; Ghidelli and Pérez-Gago, 2018). All properties of these postharvest management techniques can be linked to the reduced rate of respiration and transpiration, resulting in a slower moisture loss leading to sustaining quality of fresh produce. Moisture loss increases the susceptibility of fruit and vegetables to postharvest physiological disorders (Kader, 2002; Hernández-Muñoz et al., 2008; Magwaza et al., 2014).
The physiological changes occurring in fruits and vegetables as a result of moisture loss can even result in the loss of produce quality in a form of dietary and nutritional quality, thus reducing the health benefits of consuming that fresh produce (Rico et al., 2007). Fresh produce are well-known components of human diet because of the dietary benefits, which are associated with a reduction of cancer, heart failure and other chronic diseases (Jacobs et al., 2009; Tilman and Clark, 2014). This is because fruits and vegetables contain relatively higher concentrations of antioxidants, mainly phenolic compounds, carotenoids, and ascorbic acid (Cartea et al., 2011; Fu et al., 2011; Haminiuk et al., 2012). Recently, there is a high demand for fruits and vegetables due to consumer awareness regarding their benefits, however, these horticultural products inherently have a shorter shelf life (Miller et al., 2013). In developing countries, approximately 50 % of harvested fresh horticultural produce is not consumed due to postharvest losses (Lundqvist et al., 2008).
Several postharvest treatments have been tested to prolong quality including chemical disinfectants such as chlorinated and ozonized water which are mainly effective for reducing microbial load and suppressing fungal growth (Hassenberg et al., 2008; Akata et al., 2015). Chlorinated water treatment is becoming less important due to consumers awareness about food safety and the danger of using chemical treatments in the fresh produce industry (Lynch et al., 2009). Ozone can be the solution in this regard, mainly because it is a well-known powerful disinfectant with a higher oxidation potential. The disinfection potential of ozone is about 150 % stronger than the well-known commercial disinfectant, chlorine (Kim et al., 2003; Suslow, 2004). Furthermore, ozone has been generally recognized as a safe disinfectant of foods in United States (Suslow, 2004). Most reviews on ozone are discussing its mode of action with reference to antimicrobial properties of ozone (Ong et al., 2013; Akata et al., 2015).
Several studies have been conducted to establish the effect ozone have on the biochemistry of fruits and vegetables (De Alencar et al., 2013; Tran et al., 2013). However, little has been discussed in the literature about the biochemical mode of action of ozone that contributes to its ability to preserve quality of fresh horticultural produce. Furthermore, there is no detailed literature on the direct effect of ozone on antioxidants which serve as a defense mechanism of horticultural crops. Therefore, this review aims to fill that gap by discussing the mode of action of ozone as a postharvest treatment of horticultural crops, with more focus on antioxidants as major components of human diet. Many reviews have been made on ozone as a postharvest treatment of fruits and vegetables (Table 1). Therefore, this document aims to review the mode of action of ozone treatment on postharvest quality of fruits and vegetables with a focus on biochemistry and antioxidants.
Section snippets
An overview of ozone
Ozone is a triatomic molecule of oxygen (Fig. 1). It is a highly unstable molecule with a very short half-life of approximately 22 min on average (Kim et al., 2003). After it has been released for more than 22 min it prefers to revert back to its more stable state, which is diatomic oxygen. In order to revert to the diatomic state, ozone releases a single atom of oxygen. Chemists define an easy ability of ozone to release a single molecule of oxygen as an oxidizing power. Oxidizing power is the
Forms of ozone
Both gaseous and aqueous forms of ozone are effective postharvest treatments for fruits and vegetables, however, various factors affect their efficacy. For example, the efficacy is affected by a type of produce treated, stage of development (age) of a produce, duration of treatment, method of treatment, namely, continuous or intermittent, storage temperature and relative humidity. Out of these two forms of ozone, gaseous ozone is the most effective form to be used as a postharvest treatment of
Ozone as a postharvest treatment for horticultural crops
Reduction of postharvest losses and maintaining fresh produce quality are the key aspects of the sustainable food system (Opara, 2013). The high occurrence of postharvest losses and food wastage have caused a serious problem of food insecurity worldwide. Approximately 20−50% of harvested fresh produce does not reach a consumer because of postharvest losses between harvesting and consumption (Kader, 2004; Hodges et al., 2011; Aulakh et al., 2013). Postharvest losses can be controlled by using
Biochemical mechanism of ozone as a postharvest treatment
The mode of action of ozone in prolonging fruit quality is associated with its capacity to inhibit metabolic processes, taking place during postharvest. Ethylene production is one of the prominent metabolic processes that lead to quality deterioration (Huyskens-Keil et al., 2012). Therefore, understanding the process of ethylene biosynthesis and factors that affect it can give an answer. Under normal conditions, methionine is dissociated to form S-Adenosyl Methionine (S-AdoMet.). This reaction
Ozone on antioxidants
Among the antioxidants compounds available in fruit and vegetables, phenolic acids, flavonoids, anthocyanins and tannins are the most frequently researched and found to possess more health benefits (Ignat et al., 2011; Fu et al., 2011). This attracted the interest of several researchers to pay more attention to these compounds. The most common characteristic of these compounds is that they are highly reactive, particularly with the reactive oxygen species (ROS), therefore they form a backbone
Antioxidant enzyme activities
The antioxidant catalysing enzymes, including superoxide (SOD), catalase (CAT) and peroxidase (POD), have their special role to metabolize oxidative toxic intermediates (Zhang et al., 2011). The major role of SOD is to catalyse the dismutation reaction of superoxide radical anions into hydrogen peroxide (H2O2) and molecular oxygen. POD catalyse the oxidoreduction between H2O2 and various reductants. CAT catalyse the decomposition of H2O2 into water and oxygen (Neill et al., 2002). POD and CAT
Conclusion and future prospects
A lot of research has been done on evaluating the efficacy of ozone is being used as a postharvest treatment or prolonging shelf life of fruit and vegetables. However, in most studies, the focus was on the microbial or disinfecting properties of ozone, which is a good aspect and feature that makes ozone a functional postharvest treatment. But ozone has additional features related to the manipulation of atmospheric composition within the produce surroundings, which plays an important role in
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The College of Agriculture, Engineering and Science of the University of KwaZulu-Natal (UKZN), the National Research Foundation (NRF) of South Africa and Moses Kotane Institute are acknowledged for financial support of this study.
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