Tomato is second to potato among the world’s most cultivated vegetables   . Currently, tomato is utilized as a commercial source of vitamins A and C, and antioxidants such as lycopene . However, due to the perishability of tomatoes, lack of awareness and knowledge of postharvest handling techniques, and poor packaging, farmers encounter 20% - 50% postharvest losses  . Physical damages to tomatoes when transported to markets and distribution centers from farmers are the common post-harvest concern. Therefore, to obtain optimal shelf-life, it is essential to minimize physical damage to fresh produce. Reducing such post-harvest losses of vegetables can contribute substantially to improving food security of developing countries .
Packaging contributes to the vegetable’s production value chain through providing protection, containment, marketing platform, convenience, and traceability . All over the world, plastics usage in food packaging increases every day because of their diverse availability at relatively low costs. Also, the functional characteristics of plastic such as good optical properties, good tensile and tear strength, good permeability properties to oxygen and aroma compounds and heat seal ability favor its continual use   . The continuous plastic use, however, raises environmental and public health concerns since they are non-biodegradable  .
Plastics have been used for decades as packaging materials. However, scientists are making attempts to replace them with materials which are environmentally friendly    . Biodegradable packaging materials which are environmentally friendly are generated from naturally occurring organic matter and can be generated from agricultural wastes contributing to bettering waste management  . Packaging materials are colorless and ﬂavorless by design, not to interfere with food chemical properties (sensory or nutritional appeal) . Studies show that organic materials like papaya, mango, guava can be utilized to produce edible films that could be substituted as packaging material    . According to , papaya puree has high pectin content which serves as a matrix to produce environmentally edible films. This paper therefore purposely critically explores the potential of using papaya as a packaging material for tomato.
2. Tomato Production
Solanum lycopersicum L., commonly known as cultivated tomato, is believed to originate from South American countries of Peru, Ecuador, and other parts of South America including the Galapagos Islands. Mexico is the center for domestication and scientific modification  , of the second most important, widely grown and consumed vegetable crop after potato  . Due its numerous inherent nutrients, it contributes heavily to the human health . It is a source of vitamin A, C, E, and lycopene, a red pigment serving as a natural antioxidant   , calcium, water, and niacin, which are essential for metabolism.
Worldwide, tomato is grown for use locally or as an export commodity. Between 2010-2018, the Nigeria has been the leading producer of tomatoes on average annually in Sub-Saharan Africa (Table 1). In Uganda, production was on the rise from 2010 to 2017 but experienced a slight drop in 2018. Studies conducted on the tomato crop along different stages of production have improved its economic value    . However, study efforts have been rooted
Table 1. Tomato production in Sub-Saharan Africa.
mainly concentrated on on-farm production related issues like pests and diseases leaving a gap on the tail end of the value chain, post-harvest making production vulnerable to losses . Despite the high yields from the harvests, farmers encounter losses during post-harvest operations starting at harvest, post-harvest handling, packaging, transportation, and distribution. Post-harvest losses for fruits and vegetables range between 30% to 80% , the highest reaching 70% during storage and handling in the market places . Losses increase during transportation of the produce from points of production to market centers .
3. Packaging of Tomatoes
Packaging simplifies the handling, transport, and distribution of products to the final consumers. Packaging of products provides a series of functions which include: protection/preservation, containment and waste reduction, marketing and information, traceability, convenience, tamper indication and many others  . Therefore, employing appropriate packages, post-harvest losses in fruits and vegetables can be reduced. Packaging protects food like tomatoes from damages which are incurred from different sources (physical, chemical or biological) . However, using unsuitable packaging normally results to fruit damage translating into losses along the food chain. Various packaging materials are used for commercial purpose for the sale of fresh produce at both retail and wholesale level. During storage and transport of fresh produce, the quality and shelf life is normally determined by the type and quality of packaging material.
3.1. Traditional Baskets
Traditional woven baskets made out of palm fronds and bamboo continue to dominate the handling of tomatoes within most developing countries’ farmers  . However, the use of the traditional baskets have cost both the small and large scale farmers at the local markets post-harvest losses ranging between 30% - 50% . The losses result from excessive pressure to the fruits at the lower section resulting from packing   . Farmers minimize the physical damages on to the produce through making smooth lining of the basket’s inner lining and lining up faster a cushioning layer of dry grass. However, the grass instead tends to interrupt with air movements hence increasing temperature which severally affects the tomatoes. Farmers are then recommended to use plastic crates which has holes for proper aeration.
3.2. Wooden and Plastic Crates
Wooden and plastic crates are the other materials dominating packaging and transport amongst farmers in developing countries, because they are cheap in making since they can be constructed from locally available materials  . However, tomatoes are often damaged due to the size and inner rough surface of crates and the difficulty in handling . To reduce the damages resulting from the impact in packaging crates during handling and transportation, packaging liners are used to serve as shock/impact absorbers. But, even with shock absorbers, the fruits damaged fruits remain high, ranging from 20% - 50% . The damage can relatively be reduced to below 40% with plastic-lined baskets; however, the cost of buying plastics is high, limiting their use. In Nigeria, a participatory development approach on addressing post-harvest losses along the value chain of tomatoes was conducted in 2018 with local small-scale farmers. At the end of the case study, 89% of participants bought plastic crates to replace traditional baskets for transporting tomatoes after discovering that the latter causes higher losses . In South Africa plastic crates and cartons are adopted for use in open-air markets for marketing tomatoes by small-scale growers and retailers .
3.3. Plastic Films
Food packaging for market sales is an important aspect of food distribution. The material type selected to design a package normally determines market value quality of tomato . Polyethylene (PE) and polypropylene (PP), because of their low water permeability properties, are highly preferred plastic films for packaging   . In the plastic film application study, fully ripe tomato fruits were stored for 8, 15, and 22 days at temperatures of 20˚C and 95% relative humidity using polyethylene film, perforated polyethylene film, cellophane, and perforated cellophane. The packaging did not affect the color and overall acceptability, however, there was a significant weight loss (>0.1 g/day) for perforated film packaged tomatoes because of the undisturbed evaporation process . The physiological weight loss for non-perforated polyethylene bags was 0.29% at three days and 1.72% at 6 days of storage. After the storage period, 24.57% of the perforated polythene bags experienced physiological weight loss. Non-perforated polyethylene had a 60% decay loss of tomato fruits higher than the perforated ones after the storage period.
Plastic films are used in the Modified Atmosphere Packaging (MAP) process. MAP is technique using the predetermined composition of respiratory gases (oxygen (O2) and carbon dioxide (CO2)) gases appropriate to preserve the produce for a specified period of time . MAP packaging materials primarily function to create a permeation barrier for the gases until a stable equilibrium is reached between the external gases and those inside the package . The MAP packaging materials commonly used include; polyethylene terephthalate (PET), low-density polyethylene (LDPE), high-density polyethylene (HDPE), polyvinyl chloride (PVC), polypropylene, polystyrene  , and some chemically modified derivatives.
 compared different plastic packaging materials for their effects on the quality attributes of tomatoes during cold storage and after a shelf life period. Perforated (0.4%) low-density polyethylene (LDPE), sealed low-density polyethylene (LDPE) and sealed stretch bags were used for storing tomatoes for five weeks at 10˚C ± 1˚C and 90% - 95% relative humidity. During both cold storage and shelf-life periods, all the packaging materials performed better in preserving the fruit’s firmness and vitamin C. However, fruit weight losses, fruits decay, Total Soluble Solids (TSS), and lycopene during both storage periods were significantly affected.
Tomatoes are pretreated with chemicals for preventing deterioration during transportation and storage periods.  used 70% ethanol and 0.2% benomyl for pretreating tomatoes before packaging in low-density polyethylene, high-density polyethylene, and raffia palm basket stored at 27˚C - 32˚C for up to 16 days. Benomyl-treated fruits in low-density polyethylene retained the highest total soluble solids (TSS) compared to other treatments. Fruits treated with benomyl had the lowest microbial load, however, fruit spoilage increased with storage time for all treatments. Hot water when used as a pretreatment helped in maintaining color, firmness, total soluble solids and beta-carotene .
3.4. Edible Coatings and Films
Edible ﬁlms and coatings are thin layers of material with thickness generally less than 0.3 mm casted on food products to substitute or/and fortify the food’s outer layer for which can be eaten as a part of the product . They are applied to foods either by first producing them separately and utilized as a wrap or immersion of the food product in to the suspension or spraying/drenching (coating) of the suspension to the food produce  . Edible films and coatings serve as a diffusion barrier which selectively allows exchange of moisture, carbon dioxide, oxygen, and ethylene. The two terms are used synonymously, however, a coating is applied in a liquid form, usually by dipping the product in solution of the edible material, while the film is pre-manufactured before application  .
Way back in the 12th centuries, Chinese applied wax applied on oranges and lemons as edible films and coating for preservation of moisture and aesthetics during transportation . The Japanese boiled and air-dried soymilk protein to produce “Yuba” film as their first edible film during the 15th century. Fats were used in prevent loss of moisture from meat in the process called “larding” during 16th century, in the United Kingdom . The increasing cost and competition for petroleum, the structural materials of the synthetic packaging materials has promoted the utilization of the cheaply available materials . The increasing demands for fresh and fresh foods also attract the promotion of investing in biodegradable packages.
Since the advent of edible films and coatings, several studies have been carried out.  evaluated the quality attributes of cherry tomatoes packaged with edible films produced from varying ratios of yam starch and glycerol. The sanitized cleaned tomatoes were immersed in suspensions of yam starch and glycerol and kept in a controlled environment of 25˚C and 70% relative humidity, for 18 days. A composite coating of 7.5% yam starch and 30% glycerol proportions retained higher stability for the loss in mass, antioxidant activity, and lycopene content relative to the freshly harvested fruit. Chitosan films were used by  to study the storage behaviors of tomatoes. Respiration studies showed that 1% of chitosan treatments created a more balanced environment in terms of respiratory gases, CO2 and O2. Corn-zein film coatings are also other protein-based edible packaging film. Corn-zein coatings with 5 and 15 μm thickness delayed the ripening of tomatoes by 6-days without adverse effects and the 66 μm coating distinctly delayed color development and due to anaerobic fermentation, there was high weight loss . A composite edible coating consisting of Soy Protein Isolate (90% protein), Glycerol (plasticizer) and Carboxymethyl Cellulose, Oleic Acid, Sodium Benzoate and Ascorbic Acid resulted in to a significant effect on titratable acidity, vitamin C content, TSS and total and reducing sugars after the storage of nine days, except the pH for coated samples as compared to the control sample after nine days of storage .
3.5. Papaya as a Packaging Material
Biodegradable/edible coatings and films are categorized according to their structural building material. Proteins, lipids, and polysaccharides are the major structural components however, due to natural abundance and low toxicity, polysaccharides are the highly utilized   . Starch, cellulose, pectin, chitosan, alginate, agar, and carrageenan are the main polysaccharide materials utilized in edible packaging materials    . Research is currently concentrated on developing composite or multicomponent films and coatings to help improve the functional characteristics of individual components contributing to the composite product   .
Papaya is among the widely grown fruit in Uganda. In 2017, the fruit contribution to exports in terms of mass was about 5200 tones . It was observed that, the fruit experiences about 75% losses after harvesting , however, the waste can be utilized  . Papaya consists of various biopolymer components which can be used to develop composite films and coatings. Cellulose is a linear, high molecular weight polymer and a biodegradable material can be extracted from papaya peels for making edible packages . It has strong inter and intra molecular hydrogen bonds rendering it high strength making them hard to melt and dissolving in common solvents . Therefore, utilizing cellulose in food applications, it is converted first into its derivatives. Carboxymethyl cellulose, the most common derivative is a linear, long-chain, water-soluble, anionic polysaccharide . When purified, it has a white to cream color, tasteless, odorless, and free flowing. It is reported to be applied in biodegradable films  .
Papaya is also a potential source of pectin, another material used in the manufacture of edible films . Pectin is a gelatin-like polymer with high molecular weight found in the middle lamella of plant cells, contributing to their structure  . Pectin extracted from papaya puree blended with gelatin, and defatted soy protein produced packaging films/coatings which exhibited good mechanical, barrier, and optical properties . Biodegradable films based on gelatin and papaya peel showed high antioxidant activity which is a key attribute for packaging .
Papaya exhibits excellent packaging attributes compared to other fruit packaging materials. Papaya has high tensile strength ranging between 20 - 30 MPa when hydroxypropyl methylcellulose is used as a binding agent . Apple follows with 9 - 20 MPa using carboxymethylcellulos, gellatin, methylcellulose and poly lactic acid as binding agents . Also, tomato packaging materials with high methoxyl pectin binding agent exhibit 8.9 - 14.8 MPa of tensile strength . In terms of water vapour permeability, papaya has low values (2.15 - 3.16 m2/h/kPa) . However, they are improved (5.55 - 8.45 m2/h/kPa) when blended with starch, soya proteins and gellatin . Compared with others, apples have good water vapour permeability (5.84 - 13.57 m2/h/kPa)  . Papaya puree films have low oxygen permeability (7.5 cm3 μm/m2/d/kPa)  compared to apple puree films (83.6 cm3 μm/m2/d/kPa) . However, all these values are remarkably smaller than conventional films from HDPE (427 cm3 μm/m2/d/kPa) and LDPE (1870 cm3 μm/m2/d/kPa)—measured at 23˚C and 50% RH .
Papaya also has a wide range of medicinal and nutritional properties . Cancer cells are significantly reduced when pectin collected at different stages of papaya ripening are applied . Therefore, incorporating papaya in the composite biodegradable edible composite packaging films and coatings is a great prospect both to the environment and human life.
This study reviewed the different means for packaging tomatoes and the prospects of using papaya-based materials. Synthetics have been used to produce tomato packaging materials; however, due to environmental and health concerns they need a substitute. Biodegradable edible packaging films and coatings have the potential for replacing conventional synthetic materials. However, using single structural components for developing these materials results in some properties not appropriate. Therefore, the use of different sources of structural material to form composite films and coating is needed. Papaya has good potential if blended with other biopolymers to edible films and coatings. Papaya has both high percentages of pectin and cellulose which are the major building materials for biodegradable packaging materials.
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