Horticultural produce plays a significant role in human nutrition by supplying vitamins, minerals, fiber and anti-oxidants to the diet. Fruits and vegetables contribute approximately 91 per cent of vitamin C, 48 per cent of vitamin A, 27 per cent of vitamin B6, 17 per cent of niacin, 16 per cent of magnesium, 19 per cent of iron and 9 per cent of calories to the human diet. Other important nutrients supplied by fruits and vegetables, include riboflavin, zinc, calcium, potassium, phosphorus etc. The consumption of fruit and vegetable has increased in response to growing health consciousness. Their consumption has been strongly linked to reduced risk of some forms of cancer, heart disease, stroke and other chronic diseases. Fruits and vegetables are sources of antioxidants which modify the metabolic activation and detoxification or disposal of carcinogens, or even influence processes that alter the course of tumor cell growth.

Both quantitative and qualitative losses occur in horticultural commodities between harvest and consumption. The quality and safety of horticultural produce reaching the consumer hinges upon pre-harvest factors as well as proper post-harvest management practices throughout the chain, from field to consumer. Each stakeholder along the post-harvest chain i.e. those involved in harvesting, handling and marketing of fresh produce has a role to play in assuring safety and quality of fresh produce. In order to get good economic returns, fresh horticultural produce is required to be marketed at far off places from the point of production. To achieve this, we need to stretch the post-harvest life of these perishables commodities to its limit. Disease and disorder can occur in these living commodities due to various reasons till it reaches the consumer, thereby, greatly hampering shelf life thus, the profitability of enterprise. In addition, a number of factors threaten the safety of fruits and vegetables. These include naturally-occurring toxicants, such as glycoalkaloids in potatoes; natural contaminants, such as fungal toxins (mycotoxins) and bacterial toxins, and heavy metals (cadmium, lead, mercury); environmental pollutants; pesticide residues; and microbial contamination. The health authorities and scientists regard microbial contamination as the number one safety concern. The post harvest handling along with physical and physiological condition determines the loss due to rot. Generally debilitating environments are to be avoided and physical injuries to be prevented. In large part, post-harvest environments are designed to reduce the rate of respiration to the minimum required to maintain vital processes. The stored reserves are thereby, conserved and post-harvest life of the fruit is extended to a maximum. The rise in respiration roughly coincides with a striking reduction in fruit’s resistance to certain pathogens.

Figure 1: Current practices of post harvest handling

Low temperature is the most effective means to accomplish this task of extending the shelf life while keeping many of rotting fungi at bay. In fact, temperature management is so critical to post-harvest disease control that all other control measures act as supplement to it (table 1).

Low-tech Cooling Technologies

Shade: Covering fresh produce and protecting it from direct sunlight is a low-cost way to reduce heat gain. Using roofing or cloth tenting for providing deep shade over all assembly points and working areas is recommended. A deep overhanging roof extension (at least one meter) can provide shade for windows or doorways and a light colored or reflective roof can reduce surface temperatures and temperatures under the shelter by upto 20C.

Painting storage buildings white or silver: This will reflect sunlight, reducing surface temperature and thus, reducing the heat transmitted to the cold room through exterior walls.

Outsulation combined with thick high thermal capacity walls: Highly reflective insulating materials on the outside of the building will permit the inside walls to remain cool, especially, if they are fairly massive with a high thermal capacity. Cooling inside of a building and inside walls at night significantly reduces the amount of energy required for refrigeration, and the thick walls (e.g., concrete block) act as a thermal flywheel, with the external highly reflective insulation significantly limiting solar heating of the walls.

High altitude storage: Typically, air temperatures decrease by 10C (18F) for every one-kilometer increase in altitude. If handlers have an option to pack and store commodities at higher altitude, costs could be reduced. Cooling and storage facilities operated at high altitudes require less energy than those at sea level to achieve the same results. As a rule, night ventilation effectively maintains product temperature when the outside air temperature is below the desired product temperature for 5 to 7 hours per night.

Cold water (from deep wells or mountain streams): Well water is often much cooler than air temperature in most regions of the world. Water from deep wells and mountain streams typically will be measured to be at a temperature that is the average annual air temperature for the area. Well water can be used for hydro-cooling and as a spray or mist to maintain high relative humidity in the storage environment. Water from streams, however, is often contaminated and is not suitable for contact with food items.

Passive evaporative cooling: Wetting the walls of a packing house or using porous materials on one end (such as found in large greenhouses) can provide passive cooling via water evaporation from the wall when air is pulled through the wet pad by ventilation fans.

Night air ventilation: If the outside air is cooler than the product being stored, natural convection, using manually operated vents, will work well and require no power. If it is feasible, the storage room should be opened only at night when air temperatures are lowest. Simple cool storage facilities can be operated manually by opening the vents at night and closing them just before sunrise. A series of vents should be spaced around the perimeter of a building near ground level with a similar area of vents near the highest part of the storage building. This vent placement allows the warmer air in the top of the storage to exit the building via natural convection and draw in cool air from near ground level. If natural convection is not sufficient, a small fan (60 to 100 watts) can be used to help move warm air out of the building via a roof vent. A fan placed near the peak or gable of a storage building should be operated only during the cooler hours of the night-time, allowing cool air to be pulled into the building to replace the warmer daytime air.

Zero-energy cool chamber: A specially designed, low-cost brick and sand unit can maintain inside air temperature of 15C to 18C and a relative humidity of 95 per cent when outside air temperatures are over 30C. These chambers work best under dry conditions, such as during the dry season or in arid or semi-arid environments, and the small sized units (holding 100 to 200 kg of produce) require no electricity or fuel. Larger- sized cool chambers are constructed as a round walk-in room with a slatted floor and a small ventilation fan (60 to 100 W) added to the roof.

Figure 2a: Shade to reduce wall heating
Figure 2b: Night air ventilation

Electric Powered Technologies

Evaporatively cooled storage rooms: Evaporative coolers, sometimes called ‘swamp coolers’ or ‘desert coolers,’ use the evaporation of water to cool a storage room. Evaporative coolers have a low initial cost, and use much less electricity than conventional air conditioners. In a direct evaporative cooler, a blower forces air through a permeable, water-soaked pad. The pads can be made of straw, wood shavings or other materials that absorb and hold moisture while resisting mildew. Aspen wood pads, also called excelsior, need to be replaced every season or two. As the air passes through the pad, it is filtered, cooled, and humidified. Evaporative coolers should be sized based on cubic feet per minute of airflow. Improper sized evaporative coolers will waste water and energy and may cause excess humidity. Two-speed coolers are available that can handle varying cooling loads. Evaporatively cooled storage rooms require fans with a capacity of 0.3 m3/second per MT of fresh produce (64CFM/MT). Assuming the fan operates against a static pressure of 0.6 cm of water column and has 50 per cent efficiency, the system will require 0.09 kWh per MT of product storage capacity for one day of operation. The fan will operate continuously when the outside air temperature is greater than the desired storage temperature. The fan should have the capacity to exchange the air in the room completely once every two minutes.

Room cooling: Room cooling is a simple but slow method of reducing the temperature of produce prior to cold storage, where packages of produce are placed inside a cold room and allowed to slowly cool down. Room cooling commonly requires 24 to 48 hours or more, and is not recommended for highly perishable crops. Fans should be capable of providing 90 CFM/MT during initial cooling. The fan speed can be reduced to provide air flow at 18 to 25 CFM/MT once target temperatures have been achieved.

Ice: A central ice-making plant and ice distribution system allow produce cooling in locations where electricity and mechanical refrigeration are not available. This was the original basis for the development of the long-distance perishables business in the United States. However, cooling using ice is relatively inefficient because only about half the cooling effect is actually used to cool the produce. The rest is lost to heat exchange with the warm environment (Thompson and Chen 1989). In addition, there can be significant loss of ice as it melts in transit from the central refrigeration plant to the cooling facility. Unless the packages and ice can be used within a well insulated environment (such as in an ice chest-style container), at least 50 per cent of the original ice will be lost before it can be used. As an ice machine has several electricity-driven components, making ice is an energy-intensive process and can be expensive. Most ice machines produce between 5 and 12.5 kg of pure ice per kWh. One of the most energy efficient ice makers available, produces 18.5 kg of pure ice per kWh and has a capacity of 146 kg of pure ice per hour. Since 330 kg of ice would be required to cool one MT of fresh produce by 28C, requiring upto 66 kWh, the cost of making ice for this purpose can often be prohibitive. Melting one kg of ice has a cooling effect of approximately 316 BTU. One kg of ice will lower the temperature of fresh produce or water weighing 3 kg by about 28C.

Figure 3: Zero energy cool chamber for horticultural produce

Evaporative forced air cooling: Using an electric fan and a wet pad to move cool air through containers of fresh produce will speed the cooling process. Produce temperatures can be reduced using evaporative cooling to a few degrees above the ambient dew point temperature (the temperature at which moisture begins to form on a slick surface, indicating 100 per cent saturation of the air with moisture). The fan must be able to provide airflow of 1 L/ sec/kg against a wide range of static pressures. Doubling the airflow will speed cooling somewhat but the cost will rise considerably because the fan would need to have approximately four times greater horsepower to accomplish the same work.

Figure 5: Forced air cooling

Forced air pre-cooling inside a cold room: Forced air (FA) cooling can speed the cooling of a batch of packaged produce stacked inside a cold room from two or more days to less than 8 hours. If a cold room with adequate refrigeration capacity is available, adding a portable forced air-cooling tunnel that can cool four pallets at a time will increase the fan’s power use by only 800 to 1,500 watts per hour. A cold room with 5 tons of refrigeration can cool 3 MT of horticultural produce from an initial temperature of 27C to a target temperature of 2C in 6 to 8 hours. The area of the vents on the sides of produce containers should be at least 5 per cent of the container surface area in order to accommodate airflow without excessive pressure drop across the box. Fans for FA coolers usually operate within a typical range of 0.5 to 2.0 L/kg/sec (1 L/kg/sec equals about 1 CFM per lb). Doubling the airflow rate will speed cooling somewhat (perhaps by 40 per cent) but the energy cost will rise considerably because the fan would need to use 5 or 6 times as much power. For example, airflow for 3MT at 1 L/kg/sec and 1.3 cm w.c. (water column pressure) requires 1.12 HP (0.85 kW). If airflow is doubled, the fan size will need to increase to about 7 HP. Centrifugal fans with forwarded blades are suited for most small-scale cooling applications. Commonly available industrial propeller fans are more suited for applications with low air pressures. In the US prices typically range from USD 1,000 for a ½ HP fan to USD 1,600 for a 1 HP fan.

Hydro-cooler: Water used for cooling must be kept very cold using ice or mechanical refrigeration. Water is a far better heat-transfer medium than air, so hydro-coolers cool produce much more quickly than forced-air coolers. In well designed shower type hydro-coolers, small diameter produces such as cherries will cool in less than 10 minutes. Large diameter products such as melons will cool in 45 to 60 minutes.

Batch-style hydro-coolers will hold one or more pallets of produce and shower cold water over the tops of the stacked containers, allowing the water to filter down through the containers and contact the produce, removing heat as it passes down through the load.

Immersion hydro-coolers are large, shallow, rectangular tanks that hold moving chilled water. Crates or boxes of warm produce are loaded into one end. Crushed ice or a mechanical refrigeration system keeps the water cold, and a pump keeps the water in motion. The duration of time the produce remains in the water varies with the initial conditions and desired ending temperature. Immersion-type hydro-coolers have longer cooling times than shower coolers because the water moves past the produce at a slower speed, but cooling speed can be improved if the water is properly agitated.

Mechanically refrigerated cold rooms: Cold rooms are a very common feature of horticultural operations, and come in many sizes and types. Capital costs and energy use estimates for small-scale cold rooms vary considerably. The new prefabricated cold rooms and used refrigerated highway vans are the most expensive on an area basis. The least expensive options are used prefabricated cold rooms,if they are available locally, and owner-built facilities. Purchase costs for pre-fabricated cold rooms increase considerably for floor areas under the 40 m2 used as a baseline floor area in large facilities with hundreds of square meters of floor area cost about the same as the new prefabricated rooms.