Precooling is another important parameter to define the quality of fresh fruits. This is a necessary step in postharvest treatment that is done for almost all perishable items. Precooling is the removal of field temperature rapidly from the freshly harvested fruits before transportation, storage, and marketing. The main aim of this step is rapid cooling of fresh produce to retards the growth of microorganisms that causes spoilage of fruits, reduces respiratory rate, and loss of moisture, enzymatic activity and all reactions related to biological process from newly harvested fruits. Therefore, proper precooling inhibits spoilage and retards loss of quality and freslmess during preharvest (Becker and Fricke, 2002). It is also important to increase the shelf life of fresh produce. During postharvest the shelf life of stored fruits is extended by storing at low temperature with reduced respiratory rate and thus senescence is delayed (Becker and Fricke, 1996). Generally, important perishable fruits need precooling immediately such as all berries except cranberries, apricots, tart cherries, plums, prunes, and avocados. Fruits of tropical and subtropical regions, like mango, papayas, pineapple, are prone to chilling injury and, therefore, it must be cooled according to their individual temperature requirements. Some other fruits like grapes, citrus fruits, pears, and sweet cherries have maximum postharvest life, but still, immediate cooling is necessary during holding to maintain quality. There are four principal methods of precooling-hydrocooling, forced air cooling, vacuum cooling, and package icing. For each crop, specific methods were developed.
In hydrocooling fruits are cooled by spraying chilled water as cold water is used as cooling medium. It is the most effective method used for cooling a wide range of fruits before packing. This method is in direct contact with the fresh produce. The rate at which the internal cooling of fruits is done is limited due to the rate of heat transfer from the internal structure to the surface and depends mainly on the fruit volume in relation to its thermal properties and surface area (Stewart and Lipton, 1960). Here the water is mostly cooled by refrigeration process, and then this chilled water is circulated around the fresh produce either by spraying, like shower system, or by immersing it directly in cold water. Hydro coolers are generally of two types: shower type hydrocooler and immersion hydrocooler. In shower hydrocooler the fruits are passed under a shower of chilled water that can be build up by a perforated pan flooded with chilled water. With the help of gravitational force, the water comes out through perforated pan and flows over the fruits. This type of hydrocooler may be batch or continuous operated mode. Here the water flows typically at a range between 0.17 and 0.33 gal/s/f2 of cooling area (Boyette et al., 1992).
Immersion hydrocoolers contains shallow huge tanks consisting of agitated chilled water. At one end of the tank, fruit crates are loaded onto the conveyor and is immersed or submerged into the tank and at the opposite end of the tank the produce is removed (Fig. 2.1). This type of hydrocooler is suitable for fruits like apples that sink in water (Thompson et al., 1998).
FIGURE 2.1 Immersion hydrocooling.
Depending on the operation size the size of hydrocooler varies, but huge quantities of ice or refrigerated water are needed to maintain the water temperature of 33-36°F. In this system the cooling coils are kept directly inside the tank in which water is circulated rapidly. Fruits are either dumped in the water bath or are immersed in plastic crates. In addition, another factor is the quality of water. Hydrocooling water should be clean and properly sanitized to prevent the microbial infection of fruits. Improper hygienic conditions would lead to an increase in microbial load.
Hydrocoolers cool fruits much faster than forced air cooler because water is the best heat transfer medium rather than air. Different size of fruits requires a different temperature to cool the produce. Smaller diameter fruit, like cherries, cool in less time of around 10 min but large diameter fruits, like melons, cool in 45-60 min (Thompson et al., 1998).
FORCED AIR COOLING
Forced air cooling is also known as pressure cooling. In this type of hydrocooler cool air is passed with high speed over a product in order to remove the field temperature. Here refrigerated air is used as a cooling medium (Fig. 2.2). Inside this system, fan pulls out hot air from the fruit crates and back to the cooling chamber, this process is repeated till the desired temperature is attained.
FIGURE 2.2 Forced air-cooling system for large volumes of packed cartons.
Tunnel cooler is the most common type of forced air-cooling system. Fruits are cooled in batches and the time required for cooling with air requires a much longer time than cooling with vacuum or water. This method is best for suited for fruits, like stone fruits, berries, and mushrooms.
Vacuum cooling is one of the most rapid cooling methods at very low atmospheric pressure in the artificial hermetically sealed chamber. The principle behind this cooling system is evaporation of water at a very low temperature and at reduced pressure. The pressure is reduced by reducing the atmospheric pressure. Water evaporates at 100°C at a normal pressure of 760 mmHg, but if pressure is reduced to 5 mmHg water starts evaporating even at 1°C. Vacuum cooling of fresh fruits is done by the evaporation process. Here the water is evaporated rapidly from fruits having high transpiration coefficient and a high ratio of surface area to volume. The thermodynamic process for vacuum cooling is divided into two phases. In the first phase, the fresh produce is loaded into the flash chamber for cooling at ambient temperature; the temperature remains constant until saturation pressure is attained. The second phase simultaneously begins and cooled product at saturation till the desired temperature is reached.
Icing method is an effective way of precooling fruit crates. In this system, ice is crushed finely and for later use, it is stored in an ice bunker. Then ice is kept directly into the shipping container (Fig. 2.3). This process takes less tune to cool the produce at maintained temperature.
Liquid ice known as pumping slush ice is placed into the crates and connected to the package with the help of special nozzle and hose for cooling some products. Ice can cool down entire pallet at the same time. Wooden crates replace the corrugated containers and top icing reduces the heat temperature. However, after packaging icing and hydrocooling are done for wax impregnated corrugated containers.
SORTING AND GRADING
After harvesting, grading, and sorting of fruits is a preliminary step in post- harvest operation. This method is basically done to check the quality parameters and to remove the spoiled or defective fruits from the whole batch.
Grading of fruits is due to different physical characteristics, like color, shape, size, weight, density difference, and extent of defect, etc. Grading can be done manually or mechanically. Fruits having round shape are easily graded mechanically. On the basis of size, the fruits are graded as small, medium, large, and extra-large. Maturity grading of fruits is graded as immature, properly mature, and overmature.
FIGURE 2.3 Process of icing.
The sorting process is used to remove the product that does not meet the quality parameters. In this process damaged or decayed fruits are eliminated. It also removes misshapen, overripe, and other defects related to fruit. Sorting is done manually and mechanically too. For manual it requires skillful manpower for eliminating diseased, defected fruits. Mechanically electronic color sorters are used in some types of fruits, like apple operation (Fig. 2.4).
FIGURE 2.4 Grading of fruits.
PACKAGING, PACKAGING MATERIAL, AND PALLATIZATION
During postharvest, quality maintenance of fruits is the most essential criteria and to retain this quality, packaging should be done in a proper way. Packaging begins with keeping the produce into the boxes. These boxes can be corrugated or noncorrugated, plastic, fiberboard, wood, or bioplastics that can easily decompose. Polyethylene terephthalate is the most common plastic packaging material. Mechanical or vacuum forming material is also used to wrap fruits (Figs. 2.5 and 2.6).
Another packaging material is wooden boxes, that are commonly wire bound and is known for the traditional form of produce packaging but their use is diminished with the time as these boxes are quite heavy, costly, and gives damage to fresh fruits. It is essential to choose the standard size of packaging material during postharvest process so that farmers can easily estimate the total weight of harvest, count, and volume and, therefore, easy way to communicate to their buyers regarding the source and type of fruit, the net weight of package, product’s unit size, and other information according to government regulations (Daniels and Slama, 2010). Packed fruits for shipment are to be palletized first and then transported like in muskmelon, cantaloupe, etc. In addition to it the good package protects the produce against the harsh environmental factors, such as ingress of moisture, dust, bruising, and injuries caused by friction that arises during handling and transportation (Fig. 2.7).
FIGURE 2.5 Thermoformed produce insert thermoformed tray for fruits.
FIGURE 2.6 Polyethylene terephthalate (PET) containers for strawberries.
FIGURE 2.7 Wooden wire bound produce containers.
USE OF CUSHIONING MATERIAL
Cushioning material is used to protect the product from being getting damaged and has a unique property of shock absorbance. The produce is placed into the plastic crates or any other rigid boxes. To fix the product inside the packages cushioning material is used to prevent the fruits from getting mixed to each other (Fig. 2.8).
FIGURE 2.8 Foam protection.
Product can be damaged at any stage during handling or distribution due to dropping, vibration bruising as roads are not in good condition, also compression like stacking, commonly known as touching marks. These bruises are not immediately visible but later on after certain days blackening or browning color appears and finally the fruit starts rotting. So, in order to reduce the deformation and rotting of product, the harvested produce is first placed into the plastic containers. These boxes are then transported directly from field to packhouse and finally to the market. Some commonly used cushioning materials are newspaper sheet, molded plastic trays, rice straw, foam plastic sheet, finely shredded wood, gunny bags, plastic bubble pads, and plastic film bags. These materials are often used to protect the quality of fresh fruits during postharvest (Fig. 2.9).
FIGURE 2.9 Strong transparent air bag.