Citrus fruits (Citrus sp) are one of the most important horticultural fruit in the world, as they are characterised they pleasant flavour and aroma as well as high antioxidants for health benefits to humans against various diseases (Juaimi,2014). They are classified as non-climacteric fruit, which are fruits that are harvested when they are ready to be eaten and have minor ethylene effect and respiration rate. The genus Citrus falls under the family Rutaceae which consists many different fruit that that include sweet oranges (Citrus sinensis L.), grapefruit (Citrus paradise Macf L.), lemons (Citrus limon), limes (Citrus aurantifolia), satsumas (Citrus unshiu Marcow) and mandarins (Citrus reticulata). Citrus is believed to have originated in South East Asia and it is currently grown worldwide under optimum conditions of sub-tropical to tropical in both humid and arid regions (Seymour, Taylor and Tucker, 1983).
The quality of fruits is produced in the orchard and only maintained at postharvest(Kader,2002). Maintaining the quality of stored fruit and improving the shelf life of the fruit is achieved by reducing moisture loss and monitoring the internal concentrations of CO2 and O2 by applying proper postharvest management (Machado et al.2002). The issue concerning inappropriate techniques to prolong storage of fruit during transport ad market storage results to fruit quality losses. The inappropriate postharvest management tends to reduce fruit shelf life and the commodity rice, which is a worldwide challenge on citrus and other fruits industry (Mditshwa,2012).
The Packhouse must be able to put citrus fruit of the right type, quality and size, that has been treated in the right way, into packaging materials and carton, and ensure that fruit get sent to the market that is destined for. This review will be looking at the commercial flow processes for postharvest handling of citrus from orchard to shipping
Maturity index and quality characteristics
The principles dictating at which stage of maturity a fruit or vegetable should be harvested are crucial to its subsequent storage and marketable life and quality (FAO,2003). Postharvest physiologists distinguish three stages in the lifespan of fruits and vegetables: maturation, ripening and senescence. The citrus fruit crop is represented by different species and varieties that display unique aesthetic, organoleptic and nutritional characteristics. However, fruits of different varieties share common mechanisms and many biochemical pathways during growth, development and ripening, that are critical factors for determining the maturation indexes and harvest time for each variety. Harvest time determination is challenging and is dependent on citrus growing areas and market requirements, Different market destination influences harvest time, therefore the decision of fruit harvesting to provide highly quality fruits into the markets is a compromise between the climatic conditions, agricultural and demanding countries or consumers (Lado et al,.2014). Rind colour, total soluble solids, titratable acids and juice content are relevant parameters considered for harvest decision fresh citrus fruit main. In citrus fruits, internal quality is often expressed as TSS: TA ratio. For navel oranges a minimum TSS: TA ratio of 6:5:1 is required (OECD,2010). Mostly citrus fruits including oranges, mandarins and grapefruit the minimum accepted TSS 7.5.
Receiving and drenching
After fruit being harvested from the orchard, they are transported to the Packhouse for further treatment. In some countries citrus fruits are cured for oleocellosis or brown spots. Fruits are exposed to certain temperature for few hours depending on the type of cultivar to remove alight moisture from the peel so that it becomes suitable for mechanical handling. Water content of fruit peel is an important factor, determining turgidity of rind tissues that turn determines the extent of injuries and disorders after harvest. When these fruits are harvested early in the morning or wet weather, they tend to develop oleocellosis or brown spots on the rind with slight pressure or bruises. Lemons are generally cured at 150C to 170C for 1 -2 days so that the rind physiological disorders are minimized (Ashebre, 2015). While South Africa citrus fruits are stored at low temperature (-0.60C) to reduce the rate of fruit respiration, decay development, water loss as well as other associated physiological processes (Hordijk, 2013). It is also used to sterilize insect larvae of major pests in citrus fruits during shipment to ensure good quality maintenance and to comply with the market. Fruits are drenched to remove the field heat, to kill fungal spores and leave fungicide residues on the fruit to protect it during de-greening.
Fresh citrus fruits often require artificial degreening since internal has been reached without adequate rind colour (Mditshwa et al.,2017), but not all fruits are degreened. Fruits are taken out from cold room after drenching or steri-preconditioning room to degreening rooms where they are exposed to ethylene gas at specific concentration for certain period of time until colour change to a certain point. Ethylene degreening is one of the most common practice in citrus postharvest management that stimulate one of the most common practice in citrus postharvest management that stimulate ripening process, as the destruction of chlorophyll pigments and formation of ripening of carotenoids in the peel tissue (Rodrigo and Zacarias, 2007). Rind colour influences the consumers perception. Different effect or severe physiological and biochemical, that can even involve alterations in the volatile composition of the aromatic pattern and the development of off-flavours in relation to the accumulation of ethanol (Cupane et al.,2015). If stored at 00C, fruit became more susceptible to Cl when exposed to ethylene, even at very low ethylene concentration concentrations compared to fruit where pre-treated before storage with 0.1 or 10µ-1 ethylene for ethylene for 4 days, the incidence of this disorder was reduced during storage (Ehlers,2016).
Dumping and washing
Citrus fruits are brought into the packhouse process of dumping and washing either directly from the orchard or after has been degreened, where fruits are ether subjected to wet or dry dumping. Mostly fresh citrus fruits are dumped into clean water and washed gently by hands. Most South African citrus producers use chlorine to disinfect water, chlorine dioxide and hydrogen peroxide are also suitable. Chlorine removes electrons from organic substances such as dust, bacteria and fungi, destroying the cell walls of the microorganisms. Chlorine is three times more effective at pH 7 and pH 8, at which the level of available chlorine drops to only 25%. For best results. Chlorine should be used according to the instructions at a pH between 6.5 and 7.5 and concentration of 150ppm to 200ppm, while Americans recommends 100 to 150ppm (Kitinoja and Kader,2002). Chlorine activity is influenced by water pH, chlorine concentration, exposure time, amount of organic matter in the water, water temperature and type and growth stage of pathogens (Ritenour et al.,2002). Disinfection of water used in washers of for general cleaning purpose can be achieved by ozone or UV treatment (Ashebre,2015). Wet dumping decreases bruising and abrasions. Dry dumping the fruit moves over a set of brushes under nozzles that spray water on them and decrease injuries to produce.
Pre-sorting and fungicide treatment
Pre-sorting citrus fruits or any fresh produce is usually done to eliminate injured, decayed or otherwise defective produce (culls) before cooling or additional handling. Removing decaying produce items will limit the spread of infection to other fruits, especially if postharvest pesticides are not being used. Fruits that will not be sold for fresh market are used for citrus processing industry for producing juice and oil products. Citrus fruits are very susceptible to pathogens when fruit are high in nutrients and have a low pH and high moisture content such as at time of harvest maturity (Ehlers,2016). Therefore, citrus fruits commercially are treated with fungicide bath to reduce postharvest decay and reduce resistance to Cl. Thiabendazole (TBZ) is a fungicide known to control a wide range of fungal diseases of citrus fruits and it is also reduce Cl (Pranamornkith,2009). The uptake of TBZ is also affected by pre and post-harvest factors, dosage, method of application, storage conditions and cultivar (Cabras et al,.1999). The extensive use of TBZ in packhouses has led to development of some resistant strains of Penicillium spp. That causes blue and green mould. To combat Penicillium spp. Resistance, TBZ is used in combination with other chemicals such as Imazalil (IMZ) and Guazatine (Dodd et al,. 2010). The efficacy of TBZ on fungal diseases and Cl increases when it is in combination with hot water. However, recently the use of TBZ is questioned due to chemical residue level and harmful effect as different markets have their limited residue levels. Therefore, more researchers are interested in identification of edible coatings to replace fungicides.
Other treatment, ionizing radiation has long been known to be effective in sterilizing and killing pest in fruit. It is shown that at these doses, irradiation is not an option as a stand-alone treatment as it causes prohibitive external quality losses. Irradiation is not distributed evenly through a pallet fruit, some of the fruit is exposed to doses twice or three times the minimum required dosage. According to United States Animal and Plant Health (APHIS) approved irradiation as a quarantine treatment for fruit and vegetables at a generic dose of 150Gy for fruit fly (Tephritidae) and 400Gy for all other pest.
Drying and waxing
Then, after the fungicide bath, the fruit must be dried properly before waxing. Any wet fruit going into wax application system could cause the wax to break down resulting in erratic wax coverage on the fruit. Waxes are used to replace some of the natural waxes removed in washing and cleaning operations and can help reduce water loss during handling and marketing (Kitinoja and Kader, 20002). Waxes and coatings for polishing and improving shininess of citrus fruits, and waxes are available in various forms such as solvent waxes, aqueous emulsions, and resin solutions. Most South African citrus producers use citrashine natural wax with IMZ (500ppm in water bath), and chitosan and polyethylene-shellac treatment of citrus fruits are some of the commonly used postharvest coatings. Hydroxypropyl methylcellulose (HPMC) incorporated with natural wax founded to extend the shelf life of citrus fruits and improve quality (Armon et al., 2015). Synthetic and non-edible coatings and waxes known to be harmful to research. As fresh produce is waxed, the wax coating must be allowed to dry thoroughly before further handling.
Grading and sizing
Fruits are now ready for sizing and grading after being waxed. The fruits are separated into different sizes and grades, or classes. The local market fruit is taken out and sent on a different packline that leads to where fruit are put into pockets, and exports fruit is graded into number of different classes. Grading occur after the fruit has been treated with fungicides and waxed and usually before the fruit is sized. In some packhouses, an optic sizer is used that can also detect fruit shape, colour and blemishes, but in most cases, sizers only separate the fruit into size categories, and it is up to graders to manually separate the fruit into classes (Kitinoja and Kader, 2002). South Africa producers categorize fruits into two major export classes, being class 1 and class 2. Fruit that has below class 1 are exported as industrial fruit or sold locally as fresh fruit and sent for processing. It is important for a grader to the minimum standards for all citrus types, because different markets have their own standards. The recommendation is that the grading table should be 0.9 metres high above the tables. Exports fruit is normally sized mechanically after being graded.
Labelling and grading
After fruit has been graded and sized properly, fruits are now ready for labelling. Markets require that fruits are labelled. In most packhouses, mechanical fruit labelling is used, but some cases packers stick the labels on by hand. Export fruit are packed in the right carton, in the packing pattern that the market requires. The fruit may also be wrapped, if that is what the market wants. In some packhouses automatic packing machines are used. Cartons are the labelled, indicating the variety, grade, size, packing date, pack-line number, production unit code and packhouse code. It is important to label cartons proper for good traceability. Sealed package significantly reduces incidence of Cl, decay and weight loss in lemon and grapefruit while TSS or TA did not get affected (Ismail and Menshawy, 1997) and therefore modifying the atmosphere around the fruit will retain its quality.
Waxing and palletising
Cartons are weighed before they are stacked on pallets to make sure that they conform to the minimum weighed requirements. This information is also used to make sure that trucks are not overloading. The packed cartons are stacked neatly on pallets, with the stacking pattern depending on the type of carton. Corner pieces are put in place and strapping is used to stabilise and secure pallets. For open-top cartons, securing sheets and pallet caps are also used.
Inspection, storing and dispatching
Inspectors will now inspect the packed and palletised frit to make sure that it complies with the minimum requirements for export. In South Africa, the fruit inspection is done by Perishable Products Export Control Board (PPECB. After being palletised and inspected, the fruit is stored either in a cold room, or in separate area in the pack house. In the case of postharvest storage, mandarins are kept at 5-80C while oranges are kept at 4-50C with relative humidity of 90-95% (Kader, 2002). Prompt pre-cooling after harvest helps to prevent pitting and other peel disorders, such as stem-end rind breakdown and blossom-end clearing, helps to prevent the development of decay during storage, and slows respiration and water loss. At some packhouses, the pallets of fruit are packed directly into shipping containers to save harbour handling costs. Pallets are loaded onto trucks for dispatch to the harbour. Alternatively, the pallets are loaded into containers, which are either transported on trucks or by rail.
The majority of citrus fruits are consumed as fresh product, and this clearly indicates the importance to preserve the natural qualities of fresh citrus after harvesting. All packing house operations until the arrival of the products to the final market play a very important role in maintaining the quality characteristics of the fruits.