The total amount of horticultural crops (including fruits, vegetables, treenuts, root and tubers) produced in 2019 was estimated to be approximately 3.05 M tonnes, of which fruits comprised >34% (FAO, 2021). Huge efforts and natural resources are being used for the production of these crops. Thus, each and every one of these products is very important both for ensuring sustainability in horticultural production and for human consumption. However, several studies have estimated that around 30–50% of horticultural products never reach the final consumers due to postharvest losses, which is highly dependent on products’ physiology and can be lower or higher (Gunders, 2017; Kahramanoğlu, 2017). Prevention or lowering of these losses would result in a reduction in the need for horticultural production, which then reduces the pressure on the natural resources. This is crucial for ensuring sustainability in horticultural production, where the availability of natural resources have been decreasing while the human population is continuously increasing (Cole et al., 2018; Kang et al., 2009).
Freshly harvested fruits and vegetables are alive and they continue to undergo respiration and transpiration, which are the main causes of senescence and deterioration of the fruits (Kahramanoğlu, 2017). The main means of deterioration in fruits include physiological changes/losses (ageing, colour changes, chilling injury, freezing injury etc.); physical losses (mass loss, texture loss and mechanical damages); biochemical changes (in: soluble solids content, titratable acidity, vitamin C, phytochemicals etc.); and changes in enzymatic activities and pathological deterioration (diseases) (Yahia et al., 2011; Kahramanoğlu, 2017). Postharvest diseases are among the main causes of postharvest losses. For example, losses caused by
The most important alternatives to fungicides include hot water treatments (48–55°C for 2–5 min) (Kahramanoğlu et al., 2020), light irradiation (Papoutsis et al., 2019), protein hydrolysates (Lachhab et al., 2015), modified atmosphere packaging (Kahramanoğlu and Wan, 2020) and edible films (EF)/edible coatings (EC) with plant natural products (Riva et al., 2020). Ferreira et al. (2016) reported that over 50% of the food in European countries is packed in plastics, which are dangerous for the environment due to their intrinsic long-lasting characteristics. On the other hand, EF and coating derived from plant-based natural products, including natural polymers (proteins, polysaccharides and lipids) and plant-derived secondary metabolites (i.e. essential oils), have been reported to have a very important role in fruit storage (Abdel-Rahim and Abo-Elyousr, 2017; Chen et al., 2019; Ju et al., 2019; Riva et al., 2020). These materials are environmentally friendly with high biodegradability (Nor and Ding, 2020) and correct use of them may have no negative impacts on human health. Harvested fruits are alive and they continue to respire and transpire during storage. Both of these metabolic activities cause fruits to deteriorate. Most of the tropical and subtropical fruits contain superficial cuticle, which helps fruits to regulate respiration and transpiration and protects them against microbial decay. However, the waxy cuticle generally gets removed or damaged during washing or other handling practices. Therefore, the application of protective coatings (including wax) has been used in the fruit industry since the twelfth century, not only against microbial decay but also to prevent physiological changes, physical losses and biochemical changes (Yahia et al., 2011). Thus, the application of EF and other protective coatings serves the same role as the application of waxes and helps to reduce postharvest losses. The potential impact of the EC/EF derived from natural plant-based products is significantly related to their ability to form a barrier over the fruit surface, and thus their usefulness in reducing the movement of atmospheric gases and water vapour (Ncama et al., 2018). Besides, natural plant-based products may have the ability to retard ethylene production, reduce the activity of free radicals and oxidation, and prevent enzymatic degradation and prevent/induce physiological changes (Ali et al., 2019; Maringgal et al., 2020). Moreover, another important characteristic of natural plant-based products is their hydrophobic potential, which provides antimicrobial activity. Besides, the antimicrobial activity of natural plant-based products can be explained with two separate mechanisms as follows: (a) direct prevention of the disease by inducing some biochemical reactions in pathogens and (b) indirect prevention of diseases by improving the product's tolerance to pathogens (Kahramanoğlu et al., 2020). There are numerous research and review articles available concerning the potential effects of natural plant-based products on the physical, physiological, biochemical and mechanical quality of stored fruits (Ncama et al., 2018; Parreidt et al., 2018; Kubheka et al., 2020; Kahramanoğlu et al., 2020; Riva et al., 2020); however, review studies with postharvest diseases are limited. Therefore, it was aimed in this review paper to summarise and discuss the potential effects of natural plant-based products on the postharvest disease in fruits.
Numerous phytopathogens cause damages to fruits after harvest. A few of them are very important and have a wide host range. The most important postharvest fungi are given in Table 1 and discussed in this section.
Main postharvest fungi in fruits.
Common names | Latin names | Main host crops | Main damages/symptoms | References |
---|---|---|---|---|
Blue mould | Citrus species, guava, mango, pome fruits, stone fruits, grapes and berries | Blue or green mould; fruit decay; softens fruits and causes rapid senescence | (Palou, 2014; CABI, 2021a, 2021b Palou, 2014) | |
Green mould | ||||
Grey mould | Pome fruits, stone fruits, grapes and berries | Soft rotting; prolific grey conidiophores with collapsed and water-soaked parenchyma tissues occur | (Abdel-Rahim and Abo-Elyousr, 2017; Williamson et al., 2007) | |
Alternaria rot | Pome fruits and stone fruits | Grey, green and/or black spore colonies and spots on fruits; sunken lesions; over-ripe and softer fruits | (Aschehoug et al., 2012; Nowicki et al., 2012) | |
Anthracnose | Apples, avocados, bananas, mangoes and temperate fruits | Brown lesions; microbial decay | (Thomidis, 2014; Shivas et al., 2016) | |
Brown rot | Apricots, apples, pears, quinces, peaches, nectarines, other stone and pome fruits | Brown rot; blossom blight; cankers; fruit rots | (Martini and Mari, 2014; Uysal-Morca and Kinay-Teksür, 2019) |
The genus name
Both of the fungi may enter into the fruits from the mechanical damages during harvest or handling and can spread from fruit to fruit. On the other hand, the stem end of the fruits is an important entry point for all
This is a necrotrophic (kills host cells and uses the nutrients for its own growth) and airborne fungus which is reported to affect many plant species, mostly pome fruits, stone fruits, grapes and berries, and attacking more than 200 crops (Williamson et al., 2007). It can cause soft rotting not only on the fruits, but also on the aerial plant parts, including vegetables. It mostly causes damage on the mature tissues of plants, but its entry into the host takes place during the early states when the tissues are young. Thus, the main damages caused by
Another important fungal pathogen which causes brown rot, especially in the stone fruits, is
EC are thin layers of coating materials derived from the edible biomaterials on fruits, vegetables or other food products, whereas EF are thin layers of the same materials (McHugh, 2000). The applications of these two also differ from each other and can be the main difference among them. EC are applied in liquid form by dipping the fruits into the solutions, whereas EF are produced from biomaterials such as solid sheets, and the fruits are covered/packed/wrapped with these films. The most important benefit of these materials is that they create a modified atmosphere around the fruits and prevent the transfer of gases and water vapour, resulting in a reduction in the respiration and transpiration rates (Falguera et al., 2011). Currently, EF and coatings derived from natural plant-based products are highly used in the food packaging industry. These materials provide several benefits in postharvest fruit storage, including prevention of microbial decay, reducing mass loss, improving appearance and texture, protection of the phytochemicals and biochemical quality (Ncama et al., 2018; Kahramanoğlu et al., 2020). In this section, the three most important groups (proteins, polysaccharides and lipids) and the secondary metabolites of plant-based products used for the production of EC and EF are discussed.
Proteins are biomaterials formed by amino acids. In total, 20 amino acids have been identified till today, and numerous of them are attached to each other in long chains and form as proteins. Hydrolysis (cleavage of a covalent bond) can be used to release amino acids from the proteins. Proteins have been used to produce EF or EC to preserve fruits and vegetables (Baldwin et al., 2011).
Wheat gluten is a food produced from the main protein (gluten) of wheat. It is composed of charged amino acids (lysine, histidine and arginine) in a lesser proportion, while more of it is composed of non-polar amino acids. They come together for hydrophobic interactions. According to the solubility of the proteins, they can be grouped into four primary fractions: water soluble (albumins), soluble in dilute salt solutions (globulins), soluble in ethanol (gliadins) and insoluble (glutenin) (Chism and Haard, 1996). Wheat gluten has moderate elasticity and cohesiveness abilities but plasticisers with glycerin have been reported to be used for enhancing the flexibility of the coatings or films (Gennadios et al., 1994).
Compared to other proteins, corn zein protein has some special characteristics. Zein has higher percentages of non-polar amino acids and low percentages of other elements. The most abundant amino acids of corn zein protein are glutamine, leucine and proline. The corn zein is thus insoluble in water. Among the constituents of corn zein, the alpha-zein fractions are soluble in 95% ethanol, whereas the beta-zein fractions are soluble in 60% ethanol (Shukla and Cheryan, 2001). Corn zein is of hydrophobic nature and has a good film-forming characteristic (Gennadios et al., 1994). It has a very good ability to block the transport of moisture and prevent transpiration (Hassan et al., 2018).
Soy protein is isolated from soybeans. Beta-conglycinin and glycinin compromise approximately 37% and 41% of the soy proteins (Kunte et al., 1997). Glycinin has a film-forming ability, which is used as an emulsifier or gelling agent (Subirade et al., 1998). The film formation of soy protein is significantly affected by alkaline conditions and heat. Beta-conglycinin is less heat stable than glycinin (Renkema and Van Vliet, 2002). In a recent study, it was noted that the combination of soybean protein isolate with cinnamaldehyde provided higher efficacy for improving the storability of banana fruits, while it reduced the fungal decay of the fruits (Li et al., 2019).
Lipids are biomolecules that include hydrocarbons and make up the building blocks of the structure and function of living cells. They include hydrophobic (water-repellent) compounds which are insoluble in water and soluble in non-polar solvents. Fats, oils, waxes and some vitamins (i.e. A, D, E and K) are some examples of lipids (Baldwin et al., 2011). The fragile characteristic of the lipids makes it necessary for them to be commonly incorporated with other compounds, such as polysaccharides (Donhowe, 1994). Oils and waxes are the most important lipids used in food preservation. Besides, dispersion of oils or waxes in water or other hydrophilic solutions is also known as emulsion coating (Baldwin et al., 2011).
Oils are made up of carbon (C), hydrogen (H) and oxygen (O) and are mainly composed of unsaturated (double or triple bonds among carbon-to-carbon) fatty acids which makes them liquid at room temperature. Several different types of oils, including essential oils, mineral oils, rapeseed oil and vegetable oils (peanut, corn and soy) have been used in food packaging (Baldwin et al., 2011). The most widely known fatty acids are palmitic acid (16-carbon fatty acids) and stearic acid (16-carbon fatty acids). Stearic acid is very common in vegetable oils, including cocoa oil. Other important fatty acids are palmitoleic acid, oleic acid and linoleic acid (Srivastava, 2002). In a most recent study, Bahadırlı et al. (2020) recommended that the essential oils of myrtle leaves (with 39.38% of eucalyptol) have the ability to prevent decay incidence in loquat fruits. In a different study, Jahani et al. (2020) confirmed that the application of eucalyptus, galbanum and clove oil provides a higher ability to control
Plant waxes are among the most common natural plant-based products used in postharvest handling practices, which are commonly found as granular or continuous layers that form outside or within the cuticle of many above-ground plant parts (Lan, 2019). They generally have waterproofing ability in the amorphous layer of the plant outer surface. Most common plant waxes are candelilla (
Polysaccharides obtained from animals (chitin and chitosan) or plants (cellulose, pectin, gum polymers, starch,
Cellulose is found in the cell walls of plants and so is among the most abundant polymers in the biosphere. It has the ability to form fibres with strong hydrogen bonds, which makes it a suitable material for papers and corrugated packages (Babu et al., 2013). Cellulose is commonly produced from sugarcane, cotton fibres and wood. Besides its biodegradability, it has low density, high mechanical strength and film-forming characteristics (Credou and Berthelot, 2014). Cellulose-derived EC and EF are transparent, hydrophilic and odourless and they have a moderate ability for oxygen and gaseous transfer (Hassan et al., 2018).
Most plants (such as wheat, corn, rice, potatoes, manioc, amaranth and cashew) store energy as starch, which is a type of carbohydrate, consisting of carbon, hydrogen and oxygen atoms (Babu et al., 2013). Starch is composed of amylose (linear) and amylopectin (branched) polymers and is insoluble in cold. The most important characteristic of starch polymers is its ability of forming coatings with low oxygen permeability. It also has high hydrophilic ability and low flexibility (Ortega-Toro et al., 2015). To improve its flexibility, plasticisers (such as glycerol and sorbitol) are commonly used (Müller et al., 2008). Studies with EC developed from rice starch with sucrose esters showed that coatings are highly effective in delaying ethylene biosynthesis and senescence (Thakur et al., 2019). Although no studies were tested on diseases in the current research, it is well known that delaying the senescence improves the fruits’ resistance against phytopathogens. The EC produced from the kernel starch of mango fruits was reported to have the ability to enhance the shelf-life of tomato fruits and prevent fungal decay at the same time (Nawab et al., 2017).
Pectin (heteropolysaccharide) is a type of starch which is naturally produced in the cell walls of fruits and vegetables; it is mostly included also in the peels of citrus fruits or apple pomace (Pérez et al., 2003). Pectin has a gelling characteristic with the ability to maintain low moisture (Liu et al., 2007) and control of phytopathogens. Valdes et al. (2015) recommended that pectin films are now used for the storage of several fruits including peach, guava, apples, apricot and berries. Coatings prepared with a combination of pectin (0.16% w/v), candelilla wax, glycerol and
Plant gums are complex polymers of carbohydrates which may include proteins or other components. Gummosis is the release process of plant gums mainly in response to environmental stress conditions, including mechanical injuries. They are very common in commercial horticultural crops, and are most commonly found in citrus crops and
Besides the above-described primary products (metabolites) of plants, numerous studies have suggested that phytochemicals (secondary metabolites) isolated from plants can be used to control many phytopathogens, including thymol (Pérez-Alfonso et al., 2012), pinocembrin (Chen et al., 2020), citral (Wei et al., 2021) and alkaloids (Musto et al., 2014) isolated from plant sources, extracts or essential oils (Moraes Bazioli et al., 2019). In a very recent study, Yang et al. (2021) noted that the cinnamaldehyde, eugenol and carvacrol nanoemulsion have a high ability to reduce the decay of citrus fruits caused by
Discussion of the existing literature has suggested that protein-, lipid- and polysaccharide-based EC and EF, and other secondary plant metabolites, have important abilities to control postharvest phytopathogens together with the improved storability of fruits (Table 2). These naturally occur ring biomaterials are healthy and biodegradable. In addition, they are edible, including some essential nutrients having an important role in the human diet. The main advantages of these natural plant-based products are that they provide a barrier at the outer surface of fruits against water vapour and gases, and result in reduced respiration and transpiration. Moreover, the fruit senescence is delayed and the healthy fruits are more resistant to postharvest diseases. Besides, some of these natural plant-based products, especially oils and secondary plant metabolites, have the ability to directly control some phytopathogens or indirectly induce fruit resistance against diseases by activating fruits’ defence mechanisms (Kahramanoğlu et al., 2020). Although there are some commercial EC and EF available in the market, it is still necessary to conduct more studies, mainly about the costs of their bulk production and use. The commercial development of EC and EF, and increasing their use in postharvest fruit preservation, will provide significant benefits not only in the field of fruit storage but also for sustainable production and environmental protection.
Natural plant-based products for controlling postharvest phytopathogens.
Main groups | Sub-groups | Important features and main advantages in postharvest handling | References |
---|---|---|---|
Proteins | Wheat gluten | Insoluble in neutral solvents. Has moderate elasticity and cohesiveness abilities. Provides barrier to water vapour. | (Gennadios et al., 1994) |
Corn zein | Insoluble in water. Has a hydrophobic nature and exhibits a good film-forming characteristic. Blocks the transport of moisture and prevents transpiration. | (Hassan et al., 2018) | |
Soy protein | Insoluble in water. Has film-forming ability for blocking moisture transfer. | (Renkema and Van Vliet, 2002) | |
Lipids | Oils | Insoluble in water. Provides barrier against atmospheric gases and water vapour. Also has direct and indirect impact on fungi. | (Bahadırlı et al., 2020; Mossa et al., 2021) |
Waxes | Most common products in postharvest handling. Has high waterproofing ability. Prevents respiration, transpiration and fungal infections. | (Castro et al., 2012; Lan, 2019) | |
Polysaccharides | Cellulose | Has low density, high mechanical strength and film-forming characteristics. It has moderate ability for oxygen and gaseous transfer. | (Credou and Berthelot, 2014; Hassan et al., 2018) |
Starch | Insoluble in cold water. Has low oxygen permeability but low flexibility. Delays ethylene biosynthesis and the fruit senescence. | (Ortega-Toro et al., 2015; Thakur et al., 2019) | |
Pectin | Has a gelling characteristic with ability to maintain low moisture and controls phytopathogens. | (Liu et al., 2007; Aguirre-Joya et al., 2019) | |
Has high film-forming ability and high antifungal characteristics. | (Navarro et al., 2011; Kahramanoğlu et al., 2019) | ||
Plant gums | They have the ability to reduce the respiration rate and ethylene production, improve the resistance of products to phytopathogens and delay the fruit senescence. | (Mahfoudhi and Hamdi, 2015) | |
Secondary metabolites | Citral, eugenol, thymol etc. | They may have direct or indirect antifungal activity (by improving products’ tolerance or damaging fungi). | (Wei et al., 2021; Yang et al., 2021) |