|Papaya ripening on the plant. |
Here are some of the basic topics included whenever we talk about postharvest handling.
The Basics of Postharvest Technology
The three main objectives of applying postharvest technology to harvested fruits and vegetables are:
- to maintain quality (appearance, texture, flavor and nutritive value)
- to protect food safety, and
- to reduce losses (both physical and in market value) between harvest and consumption.
Effective management during the postharvest period, rather than the level of sophistication of any given technology, is the key in reaching the desired objectives. While large scale operations may benefit from investing in costly handling machinery and high-tech postharvest treatments, often these options are not practical for small-scale handlers. Instead, simple, low cost technologies often can be more appropriate for small volume, limited resource commercial operations, farmers involved in direct marketing, as well as for suppliers to exporters in developing countries.
Many recent innovations in postharvest technology in developed countries have been in response to the desire to avoid the use of costly labor and the desire for cosmetically "perfect" produce. These methods may not be sustainable over the long term, due to socioeconomic, cultural and/or environmental concerns. For example, the use of postharvest pesticides may reduce the incidence of surface defects but can be costly both in terms of money and environmental consequences. In addition, the growing demand for organically produced fruits and vegetables offers new opportunities for small-scale producers and marketers.
Local conditions for small-scale handlers may include labor surpluses, lack of credit for investments in postharvest technology, unreliable electric power supply, lack of transport options, storage facilities and/or packaging materials, as well as a host of other constraints. Fortunately, there is a wide range of simple postharvest technologies from which to choose, and many practices have the potential of meeting the special needs of small-scale food handlers and marketers. Many simple practices have successfully been used to reduce losses and maintain produce quality of horticultural crops in various parts of the world for many years.
There are many interacting steps involved in any postharvest system. Produce is often handled by many different people, transported and stored repeatedly between harvest and consumption. While particular practices and the sequence of operations will vary for each crop, there is a general series of steps in postharvest handling systems that are often followed.
- Harvesting and preparation for market
- Curing root, tuber and bulb crops
- Packinghouse operations
- Packing and packaging materials
- Decay and insect control
- Temperature and relative humidity control
- Storage of horticultural crops
- Transportation of horticultural crops
- Handling at destination
If you have questions or comments on any of these topics, visit our Postharvest BLOG to submit your idea or question. A practically oriented extension manual authored by Lisa Kitinoja and Adel Kader is available for free download in 10 languages from the UC Davis Postharvest Technology Center online library.
Websites with more postharvest resources for smallholder users:
Packing and Packaging Practices
|This container is too large to protect produce. The tomatoes at the bottom are squashed, and the handler is overloaded with a much too heavy container. Ghana-- 2002 |
A recent study by WFLO (funded by the Bill & Melinda Gates Foundation) reported on many issues related to the use of poor quality packages in Africa and India. The scientists field tested a variety of improved packages, including plastic crates, liners for rough containers, and smaller sized sacks, and found them all to be simple to use and cost effective.
Full Report " Appropriate Postharvest Technologies for SSA and South Asia" Here are some more ideas for improved packing practices and packaging materials that can reduce postharvest losses and improve incomes for small-scale produce farmers, handlers and marketers.
Packing Practices and Packaging Materials
Throughout the entire handling system, packaging can be both an aid and a hindrance to obtaining maximum storage life and quality. Packages need to be vented yet be sturdy enough to prevent collapse.
If produce is packed for ease of handling, waxed cartons, wooden crates or rigid plastic containers are preferable to bags or open baskets, since bags and baskets provide no protection to the produce when stacked. Sometimes locally constructed containers can be strengthened or lined to provide added protection to produce.
Waxed cartons, wooden crates and plastic containers, while more expensive, are cost effective when used for the domestic market. These containers are reusable and can stand up well to the high relative humidity found in the storage environment.
Adding a simple cardboard liner to a crate will make it less likely to cause abrasion to produce.
Containers should not be filled either too loosely or too tightly for best results. Loose products may vibrate against others and cause bruising, while over-packing results in compression bruising. Shredded newspaper is inexpensive and a lightweight filler for shipping containers (if the ink used for newspaper print is non-toxic).
For small-scale handlers interested in constructing their own cartons from corrugated fiberboard, Broustead and New (1986) provide detailed information. Many types of agricultural fibers are suitable for paper making and handlers may find it economically sensible to include these operations in their postharvest system. Corrugated fiberboard is manufactured in four flute types - type B (1/8 inch in height, 47 to 53 flutes per inch; with a basis weight of 26 lb per 1,000 ft2) is the most commonly used for handling perishables.
Whenever packages are handled in a high humidity environment, much of their strength is lost. Collapsed packages provide little or no protection, requiring the commodity inside to support all of the weight of the overhead load. Packing is meant to protect the commodity by immobilizing and cushioning it, but temperature management can be made more difficult if packing materials block ventilation holes.
Packing materials can act as vapor barriers and can help maintain higher relative humidity within the package. In addition to protection, packaging allows quick handling throughout distribution and marketing and can minimize impacts of rough handling.
Produce can be hand-packed to create an attractive pack, often using a fixed count of uniformly sized units. Packaging materials such as trays, cups, wraps, liners and pads may be added to help immobilize the produce. Simple mechanical packing systems often use the volume-fill method or tight-fill method, in which sorted produce is delivered into boxes, then vibration settled. Most volume-fillers are designed to use weight as an estimate of volume, and final adjustments are done by hand.
Ethylene absorber sachets placed into containers with ethylene sensitive produce can reduce the rate of ripening of fruits, de-greening of vegetables or floral wilting. Sachets can be purchased from internet based companies.
Broustead, P.J. and New, J.H 1986. Packaging of fruit and vegetables: a study of models for the manufacture of corrugated fibreboard boxes in developing countries. London: TDRI. (for information contact NRI, Central Avenue, Chatham Maritime, Kent, ME4 4TB, United Kingdom).
Some illustrations of improved practices:
|Insulated produce truck in Indonesia 2000|
Training Assistance in Temperature Management
Dr. Lisa Kitinoja is available via the blog at postharvest.tumblr.com to serve as a mentor or advisor to young professionals, extension workers, farm advisors, and private consultants around the world who are involved in activities related to postharvest handling, packaging, cooling, shipping and storage of fresh fruits and vegetables.
Several articles on pre-cooling and cooling practices for small-scale farmers and marketers can be downloaded from the UC Davis postharvest website. Check the links provided at the left side of this page.
Temperature and Relative Humidity Control
Throughout the period between harvest and consumption, temperature control has been found to be the most important factor in maintaining product quality. Fruits, vegetables and cut flowers are living, respiring tissues separated from their parent plant. Keeping products at their lowest safe temperature (0 °C or 32 °F for temperate crops or 10-12 °C or 50-54 °F for chilling sensitive crops) will increase storage life by lowering respiration rate, decreasing sensitivity to ethylene gas and reducing water loss. Reducing the rate of water loss slows the rate of shriveling and wilting, causes of serious postharvest losses.
Keeping products too cool can also be a serious problem. It is important to avoid chilling injury, since symptoms include failure to ripen (bananas and tomatoes), development of pits or sunken areas (oranges, melons and cucumbers), brown discoloration (avocados, cherimoyas, eggplant), increased susceptibility to decay (cucumbers and beans), and development of off-flavors (tomatoes) (Shewfelt, 1990). Cooling involves heat transfer from produce to a cooling medium such as a source of refrigeration. Heat transfer processes include conduction, convection, radiation and evaporation.
If a ready supply of electricity is available, mechanical refrigeration systems provide the most reliable source of cold. Methods include room cooling, forced-air cooling and evaporative cooling. A variety of portable forced-air coolers have been designed for use by small-scale growers and handlers (Talbot and Fletcher, 1993; Rij et al, 1979; Parsons and Kasmire, 1974). However, a variety of simple methods exist for cooling produce where electricity is unavailable or too expensive. Some examples of alternative systems (from Thompson in Kader, 1992) include night air ventilation, radiant cooling, evaporative cooling, the use of ice and underground (root cellars, field clamps, caves) or high altitude storage. Ice can be manufactured using simple solar cooling systems, where flat plate solar collectors are used to generate power to make ice, which is then used to cool produce (Umar, 1998). Ice can be used either directly as package ice, to cool water for use in a hydro-cooler, or as an ice bank for a small forced air or room cooling system.
Several simple practices are useful for cooling and enhancing storage system efficiency wherever they are used, and especially in developing countries, where energy availability may be limited and any savings may be critical. Shade should be provided over harvested produce, packing areas, for buildings used for cooling and storage and for transport vehicles. Using shade wherever possible will help to reduce the temperatures of incoming produce and will reduce subsequent cooling costs. Trees are a fine source of shade and can reduce ambient temperatures around packinghouses and storage areas. Light colors on buildings will reflect light (and heat) and reduce heat load. Sometimes spending money will save money, as when purchasing lighting equipment. High pressure sodium lights produce less heat and use less energy than incandescent bulbs.
Another aspect to consider when handling fruits and vegetables is the relative humidity of the storage environment. Loss of water from produce is often associated with a loss of quality, as visual changes such as wilting or shriveling and textural changes can take place. If using mechanical refrigeration for cooling, the larger the area of the refrigerator coils, the higher the relative humidity in the cold room will remain. It pays however, to remember that water loss may not always be undesirable, for example if produce is destined for dehydration or canning.
For fresh market produce, any method of increasing the relative humidity of the storage environment (or decreasing the vapor pressure deficit (VPD) between the commodity and its environment) will slow the rate of water loss. The best method of increasing relative humidity is to reduce temperature. Another method is to add moisture to the air around the commodity as mists, sprays, or, at last resort, by wetting the store room floor. Another way is to use vapor barriers such as waxes, polyethylene liners in boxes, coated boxes or a variety of inexpensive and recyclable packaging materials. Any added packaging materials will increase the difficulty of efficient cooling, so vented liners (about 5 percent of the total area of the liner) are recommended. The liner vents must line up with the package vents to facilitate cooling of the produce inside. Vented liners will decrease VPD without seriously interfering with oxygen, carbon dioxide and ethylene movement.
Storage of horticultural crops
If produce is to be stored, it is important to begin with a high quality product. The lot of produce must not contain damaged or diseased units, and containers must be well ventilated and strong enough to withstand stacking. In general proper storage practices include temperature control, relative humidity control, air circulation and maintenance of space between containers for adequate ventilation, and avoiding incompatible product mixes.
Commodities stored together should be capable of tolerating the same temperature, relative humidity and level of ethylene in the storage environment. High ethylene producers (such as ripe bananas, apples, cantaloupe) can stimulate physiological changes in ethylene sensitive commodities (such as lettuce, cucumbers, carrots, potatoes, sweet potatoes) leading to often undesirable color, flavor and texture changes.
Temperature management during storage can be aided by constructing square rather than rectangular buildings. Rectangular buildings have more wall area per square feet of storage space, so more heat is conducted across the walls, making them more expensive to cool. Temperature management can also be aided by shading buildings, painting storehouses white or silver to help reflect the sun's rays, or by using sprinkler systems on the roof of a building for evaporative cooling. The United Nations' Food and Agriculture Organization (FAO) recommends the use of ferrocement for the construction of storage structures in tropical regions, with thick walls to provide insulation. Facilities located at higher altitudes can be effective, since air temperature decreases as altitude increases. Increased altitude therefore can make evaporative cooling, night cooling and radiant cooling more feasible. Underground storage for citrus crops is common in Southern China, while in Northwest China, apples are stored in caves (Liu, 1988). This system was widely used in the U.S. during the early pert of this century.
Certain commodities, such as onions and garlic, store better in lower relative humidity environments. Curing these crops and allowing the external layers of tissue to dry out prior to handling and storage helps to protect them from further water loss.
The air composition in the storage environment can be manipulated by increasing or decreasing the rate of ventilation (introduction of fresh air) or by using gas absorbers such as potassium permanganate or activated charcoal. Large-scale controlled or modified atmosphere storage requires complex technology and management skills, however, some simple methods are available for handling small volumes of produce.
Cool Storage Ideas
Here is a report on the construction of a simple Charcoal Cool Storage Room that works via the principle of passive evaporative cooling. No electricity is required, but a small fan can help improve effectiveness by moving air more quickly through the cool room. A list of materials is provided, and ideas for testing its effectiveness for lowering temperature are provided. This kind of cool room works best when relative humidity is low, such as in drier climates or during the dry season. You can download the report by clicking on this link: CHARCOAL COOL ROOM REPORT
Here is a presentaion on the Zero Energy Cool Chamber (ZECC). It is designed by Dr. S.K. Roy in India in the 1980s and has been field tested and redesigned over the past few years to improve its function. The ZECC can be constructed in various sizes (from 100 kg to 1 MT capacity) depending upon the volume of fresh fruits or vegetables that you want to store. It is designed as a temporary on-farm evaporative type cool storage chamber, and can be self-constructed of bricks and sand. Here is the link to the presentation prepared by Dr. S.K. Roy in 2009:
ZERO ENERGY COOL CHAMBER PPT
Field clamp for root and tuber crops
(developed by CIP). Illustration of a Field Clamp
Before storing root and tuber crops, sorting out diseased units and curing the crop will heal harvest wounds and allow the periderm to thicken and provide protection from water loss and diseases. Here is an illustration of a simple curing practice that can be used for field curing for 2 to 5 days: Illustration of Field Curing
TRAINING IN POSTHARVEST TECHNOLOGY