Post Harvest Technology of Fruits and Vegetables

By Mrs. Farzana Panhwar
May 2006

The Author is President of the Sindh Rural Women´s Up-lift Group in Hyderabad, Sindh - Pakistan. She also manages the Panhwar Fruit Farm, where some 25 new fruit crops have been successfully introduced to Sindh. → See also:

Introduction

Worldwide postharvest fruit and vegetables losses are as high as 30 to 40% and even much higher in some developing countries. Reducing postharvest losses is very important; ensuring that sufficient food, both in quantity and in quality is available to every inhabitant in our planet. The prospects are also that the world population will grown from 5.7 billion inhabitants in 1995 to 8.3 billion in 2025. World production of vegetables amounted to 486 million ton, while that of fruits reached 392 million ton. Reduction of post-harvest losses reduces cost of production, trade and distribution, lowers the price for the consumer and increases the farmers income.

Proper postharvest processing and handling is an important part of modern agricultural production. Postharvest processes include the integrated functions of harvesting, cleaning, grading, cooling, storing, packaging, transporting and marketing. The technology of postharvest handling bridges the gap between the producer and the consumer - a gap often of time and distance. Postharvest handling involves the practical application of engineering principles and knowledge of fruit and vegetable physiology to solve problems.

Utilizing improved postharvest practices often results in reduced food losses, improved overall quality and food safety, and higher profits for growers and marketers. It is estimated that 9 to 16 percent of the product is lost due to postharvest problems during shipment and handling. Mechanical injury is a major cause of losses. Many of these injuries cannot be seen at the time that the product is packed and shipped, such as internal bruising in tomatoes. Other sources of loss include over-ripening, senescence, the growth of pathogens and the development of latent mechanical injuries.

Many factors contribute to postharvest losses in fresh fruits and vegetables. These include environmental conditions such as heat or drouth, mechanical damage during harvesting and handling, improper postharvest sanitation, and poor cooling and environmental control. Efforts to control these factors are often very successful in reducing the incidence of disease. For example, reducing mechanical damage during grading and packing greatly decreases the likelihood of postharvest disease because many disease-causing organisms (pathogens) must enter through wounds.

Chemicals have been widely used to reduce the incidence of postharvest disease. Although effective, many of these materials have been removed from the market in recent years because of economic, environmental, or health concerns.
Increased interest in the proper postharvest handling of fresh fruits and vegetables has prompted the widespread use of flumes, water dump tanks, spray washers, and hydrocoolers. To conserve water and energy, most postharvest processes that wet the produce recirculate the water after it has passed over the produce. This recirculated water picks up dirt, trash, and disease-causing organisms. If steps are not taken to prevent their spread, these organisms can infect all the produce that is subsequently processed. In the past, various fungicides and bactericides have been used (alone or in combination with chlorination) to prevent the transmission of diseases. These materials have often been favored over chlorination because they provide some residual protection after treatment.

Chlorination

At present, chlorination is one of the few chemical options available to help manage postharvest diseases. When used in connection with other proper postharvest handling practices, chlorination is effective and relatively inexpensive. It poses little threat to health or the environment The primary objective was to reduce post harvest losses of fruits and vegetables during harvesting, farm handling, storage, marketing, transportation and processing.

The Impact of Postharvest Losses

Too much of the world's food harvest is lost to spoilage and infestations on its journey to the consumer. In developing countries, where tropical weather and poorly developed infrastructure contribute to the problem, losses are sometimes of staggering proportions. Losses occur in all operations from harvesting through handling, storage, processing and marketing. They vary according to the influence of factors such as the perishability of the commodity; ambient temperature and relative humidity which determine the natural course of decay; fungal and bacterial decay; damage by pests -insects, rodents and birds; the length of time between harvesting and consumption; and practices of postharvest handling, storage and processing.

Postharvest disorders or losses

Postharvest disorders or losses in quality have economic impacts vastly greater than the actual losses caused by frequency and intensity of their occurrence. For example there are direct financial losses incurred by the grower from batches of fruit expressing the disorder. Direct losses Horticultural produce is alive and has to stay alive long after harvest. Like other living material it uses up oxygen and gives out carbon dioxide. It also means that it has to receive intensive care. For a plant, harvesting is a kind of amputation. In the field it is connected to roots that give it water and leaves which provide it with the food energy it needs to live. Once harvested and separated from its sources of water and nourishment it must inevitably die. The role of postharvest handling is to delay that death for as long as possible.

Effect of temperature on postharvest

The ideal temperature often depends on the geographic origin of the product. Tropical plants have evolved in warmer climates and therefore cannot tolerate low temperatures during storage. Plants from tropical origins must be stored above 12°C. This is in contrast to plants which have evolved in temperate, cooler climates which can be stored at 0°C. Table gives a few examples of products and their recommended storage temperature.

Recommended storage temperature a selection of fruits and vegetables
1 - 4 °C 5 - 9 °C > 10 °C
Apple Avocado (temperate origin) Avocado (sub-tropical)
Asparagus Zucchini Pawpaw
Berry fruits French Bean Grape fruit and Lemon
Broccoli Passion fruit Mangoes
Peach and Plum Egg plant Banana
Cherry Capsicum Pineapple
Grapes Cucumber Sweet potato
Lettuce Mandarin orange Tomato
Mushroom Potato Pumpkin
Carnation Protea Ginger

The rapid cooling after harvest is so important. If the temperature is lowered and the harvested products are put in refrigerated storage, water and quality loss can be reduced. Fresh produce is alive, living and breathing. The general term for all the processes going on inside a living organism is called metabolism. Temperature has a big effect on the rate of metabolism of the product. When the temperature of the product rises, so too does the rate of metabolism. One of the main processes of metabolism is respiration which is the process of Temperature management for fresh produce is the key to quality. Lowering the temperature as quickly as possible after harvest will slow the rate of metabolism and therefore extend the product’s shelf life. For some flowers, sugar can be added (always with a biocide) to the vase solution to provide carbohydrate, or food for metabolism. As a result, for these types of produce respiration is not limited and the vase life is extended. At extreme of temperature product are damaged. Some suffer chilling injury, some suffer damage at very high temperatures and all products are damaged if they freeze.

Temperature is the most important influence on the rate of deterioration in the quality of produce. High temperatures accelerate ripening and the speed at which rots develop. A 10°C increase in temperature will cause fruit and vegetables to deteriorate twice as fast, as well as encouraging disease organisms to grow twice as fast as well. This is why it is important to remove field heat from the produce as quickly as possible after harvest. Important as it regulates the quantity of refrigerant entering the evaporator coils. When the liquid moves into the evaporator coils the liquid changes into a gas because it absorbs heat energy from the surrounding air (latent heat of vaporisation). As a result the air in the cool room is cooled.

Postharvest Losses

Most losses of fresh produce occur between leaving the farm and reaching the consumer. Losses during this period have been estimated to be about 20% of the total crop. These losses may be caused by complete wastage of the product or by lower prices due to a reduction in quality. The cost of these losses is also important as the value of the product increases several fold from the farm gate to the final consumer, so in dollar terms postharvest losses are even more significant.

There are generally three main causes of postharvest losses:

  1. Disease caused by fungi and/or bacteria
  2. Physical injuries due to insects, mechanical force, chemicals, heat or freezing
  3. Non-disease disorders resulting from storage conditions that upset normal metabolism

when the product is rejected further down the marketing chain. Other factors such as cultivar, weather and crop management also influence the development of disease. Some cultivars are more susceptible to diseases than others and wet weather can promote the development of disease and hamper efforts to control outbreaks.

Essential oils for the control of postharvest

These plant extracts are generally assumed to be more acceptable and less hazardous than synthetic compounds. This means that essential oils that are registered food grade materials, could be used as alternative anti-fungal and anti-bacterial treatments for fresh produce. The potential for these types of plant extracts is considerable. It is a resource that has not been fully explored.

Postharvest Laboratory looking at the use of essential oils for the control of postharvest pathogens. Essential oils are made up of many different volatile compounds and the make up of the oil quite often varies between species. It seems that the anti-fungal and anti-microbial effects are the result of many compounds acting synergistically. These means that the individual components by themselves are not as effective. Quite a lot of preliminary work has been done to demonstrate the potential of essential oils for use against postharvest pathogens. One work showed that essential oils from red thyme (Thymus zygis), clove buds (Eugenia caryophyllata) and cinnamon leaf (Cinnamomum zeylanicum) prevented the growth of Botrytis cinerea. Other researchers have shown that the essential oil of Monarda citrodora and Melaleuca alternifolia also exhibit antifungal activity against a wide range of common postharvest pathogens.

We have looked at the effect of tea tree oil as a vapour on the growth of Botrytis from grapes Tea tree oil has antibacterial and antifungal properties that have secured it a place in the commercial pharmaceutical market. We wanted to determine if these properties may also be useful for the postharvest control of fungi on grapes. Our work showed that concentrations of between 100 and 500 ppm were able to prevent the growth of this fungi when it was grown in the laboratory.

Role of ethylene in the postharvest life

Ethylene plays a role in the postharvest life of many horticultural crops. The important role of ethylene as a plant growth regulator has only been established over the last 50 years but its effects have been known for centuries. The most commonly know use of ethylene is to trigger ripening is some crops, such as bananas and avocados. The application of ethylene at a controlled rate means that these products can be presented to the customer as „ready to eat”. The concentration of ethylene required for the ripening of different products varies. The concentration applied is within the range of 1 and 100 ppm. The time and temperature of treatment also influences the rate of ripening with fruit being ripened at temperatures between 15 to 21°C and relative humidity of 85 – 90 %. Although controlled ripening is the major postharvest use of ethylene it can also be applied pre-harvest to promote postharvest benefits. The chemical Ethephon produces ethylene and is applied in the field. Ethephon can promote several benefits such as fruit thinning (apples, cherries), fruit loosening prior to harvest (nuts), colour development (apples), degreening (citrus), flower induction (pineapples).

Other factors of postharvest losses

In addition to genetic traits, environmental factors such as soil type, temperature, wind during fruit set, frost, and rainy weather at harvest can have adverse effects on storage life, suitability for shipping, and quality. Cultural practices may have dramatic impacts on postharvest quality. Good Agricultural Practices during harvest operations and any subsequent postharvest handling, minimal or fresh-cut processing, and distribution to consumers must be developed.

Common Postharvest Diseases

Commodity and Disease – Pathogena*

Apples
Blue mold Penicillium expansum (f)
Gray mold Botrytis cinerea (f)
Black rot Physalospora obtusa (f)
Bitter rot Glomerella cingulata (f)
Grapes and small fruit
Blue mold Penicillium sp. (f)
Gray mold Botrytis cinerea (f)
Rhizopus rot Rhizopus stolonifer (f)
Potatoes
Fusarium tuber rot Fusarium spp. (f)
Wet rot Pythium sp. (f)
Bacterial soft rot Erwinia spp. (b)
Slimy soft rot Clostridium spp. (b)
Peaches and plums
Brown rot Monilinia fructicola (f)
Rhizopus rot Rhizopus stolonifer (f)
Gray mold Botrytis cinerea (f)
Blue mold Penicillium sp. (f)
Alternaria rot Alternaria sp. (f)
Gilbertella rot Gilbertella persicaria (f)
Sweet Potatoes
Bacterial soft rot Erwinia chrysanthemi (b)
Black rot Ceratocystis fimbriata (f)
Ring rot Pythium spp. (f)
Java black rot Diplodia gossypina (f)
Fusarium surface rot Fusarium oxysporum (f)
Fusarium root and stem rot Fusarium solani (f)
Rhizopus soft rot Rhizopus nigricans (f)
Charcoal rot Marcrophomina sp. (f)
Tomatoes and peppers
Alternaria rot Alternaria alternata (f)
Buckeye rot Phytophthora sp. (f)
Gray mold Botrytis cinerea (f)
Soft rot Rhizopus stolonifer (f)
Sour rot Geotrichum candidum (f)
Bacterial soft rot Erwinia spp. (b) or
Pseudomonas spp. (b)
Ripe rot Colletotrichum sp. (b)
Vegetables in general
Watery soft rot Sclerotinia sp. (f)
Cottony leak Pythium butleri (f)
Fusarium rot Fusarium sp. (f)
Bacterial soft rot Erwinia sp. (b) or
Pseudomonas spp. (b)

Postharvest Disorders

Many types of postharvest disorders and infectious diseases affect fresh fruits and vegetables (Table). Disorders are the results of stresses related to excessive heat, cold, or improper mixtures of environmental gases such as oxygen, carbon dioxide, and ethylene. Some disorders may be caused by mechanical damage, but all are abiotic in origin (not caused by disease organisms) and cannot be controlled by chlorination or most other postharvest chemicals. However, abiotic disorders often weaken the natural defenses of fresh produce, making it more susceptible to biotic diseases those that are caused by disease organisms. Further, in many cases injuries caused by chilling, bruising, sunburn, senescence, poor nutrition, and other factors can mimic biotic diseases.

The control of biotic postharvest diseases depends on understanding the nature of disease organisms, the conditions that promote their occurrence, and the factors that affect their capacity to cause losses. Postharvest diseases may be caused by either fungi or bacteria, although fungi are more common than bacteria in both fruits and vegetables. Postharvest diseases caused by bacteria are rare in fruits and berries but somewhat more common in vegetables. Viruses seldom cause postharvest diseases, although they, like postharvest disorders, may weaken the produce.

Chilling injury

Tropical fruits are susceptible to many postharvest diseases. They are sensitive to chilling injury, and cannot be kept at low temperatures. It is very important to harvest tropical fruits at the right stage of maturity. Treatments with sulfur dioxide, fungicides and antioxidants help to reduce losses of tropical fruits from postharvest diseases, as does careful attention to humidity and temperature. Most important is careful handling to minimize injuries. Bruised fruit, or fruit with a damaged skin, is more vulnerable to diseases, spoils more quickly and sells more slowly.

Development of the postharvest sector commonly requires inputs such as pesticides, cement, galvanized sheeting, wire netting, adequate packaging for the safe handling and transportation of perishables, sprays, boxes, and simple machines. Practical technical interventions are researched and promoted to prevent losses. For example, storage bins for grain must be cleaned out completely between seasons and disinfected before re-use; shade must be provided for holding perishables together with appropriate containers for their transportation and marketing. Improved technologies for drying fruits, vegetables and root crops have also been introduced to reduce losses arising from seasonal gluts.

Biological control

Biological control of postharvest diseases (BCPD) has emerged as an effective alternative. Because wound-invading necrotrophic pathogens are vulnerable to biocontrol, antagonists can be applied directly to the targeted area (fruit wounds), and a single application using existing delivery systems (drenches, line sprayers, on-line dips) can significantly reduce fruit decays. The pioneering biocontrol products BioSave and Aspire were registered by EPA in 1995 for control of postharvest rots of pome and citrus fruit, respectively, and are commercially available. The limitations of these biocontrol products can be addressed by enhancing biocontrol through manipulation of the environment, using mixtures of beneficial organisms, physiological and genetic enhancement of the biocontrol mechanisms, manipulation of formulations, and integration of biocontrol with other alternative methods that alone do not provide adequate protection but in combination with biocontrol provide additive or synergistic effects.

Another critical factor for reducing food losses is quality standards and incentives for delivery of better quality produce through the introduction of a fair and practical grading system. The operation of such a system often calls for training and extension to improve handling, storage, packing, sorting and grading practices.

Irradiation

In both domestic and international agricultural markets, expanding the use of irradiation can help to reduce the need for methyl bromide for the post-harvest control of insect pests. Currently, irradiation treatments have been approved for a variety of food use applications by the U.S. Food and Drug Administration (FDA). The United States Department of Agriculture (USDA) Animal and Plant Health Inspection Service (APHIS) Plant Protection and Quarantine Service (PPQ) has outlined policy positions regarding the development and use of irradiation treatments for quarantine pest control, and is actively seeking ways to incorporate additional irradiation uses into their plant protection program (USDA 1995, USDA 1996a, 1996b, 1996c).
Food irradiation is a process by which products are exposed to ionizing radiation to sterilize or kill insects and microbial pests by damaging their DNA. The FDA permits three types of ionizing radiation to be used on foods: gamma rays from radioactive cobalt-60 and cesium-137, high energy electrons, and x-rays. Although all three have similar effects, gamma rays are most commonly used in food irradiation because of their ability to deeply penetrate pallet loads of food (Forsythe and Evangelou 1993, Morrison 1989). Gamma irradiation equipment irradiates packaged or bulk commodities by exposing the product to gamma energy from cobalt-60 in closed chambers, which range in size from single modular pallet irradiators to large research or contract irradiation facilities. Absorbed dose is measured as the quantity of radiation imparted per unit of mass of a specified material. The unit of absorbed dose is the gray (Gy) where 1 gray is equivalent to 1 joule per kilogram (ICGFI 1991, NAPPO 1996).

In addition, by interfering with cell division, irradiation inhibits sprouting in tubers, bulbs, and root vegetables (potatoes, onions) and can delay ripening of some tropical fruits, resulting in an extended shelf life for many foods. In turn, longer shelf lives will enhance trade opportunities between nations by extending time constraints under which fresh produce must be delivered to more distant geographic markets or by allowing the use of slower and less expensive modes of transportation (Kader 1986, Moy 1991, OTA 1985).

Irradiation is used as a pest control tool in over 40 countries, including the United States, Russia, Great Britian and Brazil (Nordion 1995). The disinfestation of grain as it enters the Soviet Union at the Black Sea Port of Odessa, estimated at over 500,000 metric tons per year, is one of the largest documented commercial industrial applications (Giddings 1991). In the United States, the FDA approved low-doses irradiation for wheat, wheat flour, and potatoes in the early 1960s. In 1984 and 1985, the FDA approved irradiation of spices and pork, and in the following year, approved low-dose irradiation (up to 1 kGy) to control insects in foods and extend the shelf life of fresh fruits and vegetables (Kader 1986, Morrison 1989). Irradiation has also been used to sterilize food for U.S. hospital patients and astronauts (Morrison 1992). Further, irradiation disinfestation has been found to be effective for treatment of dried fruits, spices, nuts, cut flowers, lumber, and wood chips (ICGFI 1994, Marcotte 1992, Morrison 1989, OTA 1985). At doses below 1 kGy, irradiation is an effective treatment against various species of fruit flies, mango seed weevils, naval orange worms, potato tuber moths, codling moths, and other insect species of significance to quarantine situations (Kader 1986). For irradiation to be approved as a quarantine treatment in the United States, either as a single treatment, or as part of a combined approach (e.g., systems approach), USDA/APHIS/PPQ will require that the level of efficacy be scientifically demonstrated, and that efficacy be demonstrated under commercial settings (USDA 1996).

Factors influencing the response of fresh fruits and vegetables to irradiation include the type of commodity and cultivar, production area and season, maturity at harvest, initial quality, and post harvest handling procedures. Similarly, environmental conditions during irradiation (temperature and atmospheric composition), and dose rates are also influencing factors (ICGFI 1994, Kader 1986, OTA 1985, Morrison 1992).

Table 1   Relative Tolerance of Fresh Fruits and Vegetables to Irradiation below 1 kGy
High Apple, cherry, date, guava, longan, muskmelon, nectarine, papaya, peach, rumbutan, raspberry, strawberry, tamarillo, tomato
Medium Apricot, banana, cherimoya, fig, grapefruit, kumquat, loquat, lychee, orange, passion fruit, pear, pineapple, plum, tangelo, tangerine
Low Avocado, cucumber, grape, green bean, lemon, lime, olive, pepper, sapodilla, soursop, summer squash, leafy vegetables, broccoli, cauliflower

Source: Kader 1986

Table 2   Comparison of Estimated Post-Harvest Treatment Costs for Selected Crops
Crop Methyl Bromide
(cents per pound)
Irradiation
(cents per pound)
Strawberries 0.88 to 0.94 2.5 to 8.1
Papaya 0.88 to 0.94 0.9 to 4.2
Mango 0.88 to 0.94 Data not available

Sources: Forsythe and Evalgelou 1993 and 1994, Morrison 1989

Fumigation with methyl bromide

The fumigant methyl bromide is generally preferred for the disinfection of both durable and perishable agricultural commodities. This is because of its low phytotoxicity and its wide-ranging action against a large number of pests. Furthermore, large, bulky shipments can be rapidly and easily treated. However, methyl bromide has now been identified as one of the chemicals that are damaging the world's ozone layer. Efforts are being made to develop alternatives, and treatment systems which recycle methyl bromide.

VHT (Vapor heat treatment)

Fruitfly is a major problem in the import and export of tropical fruits and vegetables. VHT is an effective, non-chemical method of treatment, widely used to treat mangoes and other tropical fruits grown for export. VHT treatment maintains produce at a certain temperature for a fixed period of time, using a hot water/steam vapor system. This destroys any larvae of the fruitfly which might be present inside the fruit, as well as the eggs and pupae.

Packaging

The packaging of fruits and vegetables should protect them from injury and water loss, and be convenient for handling and marketing. Packages should also provide information about the product, including the grade, handling instructions, and appropriate storage temperatures when the product is on display. The cost of the packaging is important, including whether the container can be recycled or reused.

Transport

Fresh horticultural products should be cooled after harvest and during transport. It is very important that the cold chain is continuous. Once fruit and vegetables have been cooled, they must stay cool. Trucks for road transport may be refrigerated, or may sometimes just be insulated. It is difficult to control the temperature of air shipments, but produce shipped by air should be covered and precooled.

The transportation and storage of fresh fruit and vegetables is an international operation for which the available technology must be used to ensure that produce reaches the consumer in the best possible condition. The use of controlled atmospheric conditions, as a way of reducing the use of chemical preservatives and pesticides, has great potential for the reduction of postharvest losses and the maintenance of nutritive value and organoleptic characteristics. The proper application of controlled atmosphere storage is likely to have as great an impact as the introduction of refrigeration technology a century earlier, yet its potential is only just becoming appreciated, despite its use for apples for many years.

Quality Considerations

Several factors are important in determining the quality of fruits and vegetables: the appearance, the flavor, the texture, the nutritional value and the safety. Only the first three can be easily identified by consumers.

Successful postharvest handling depends partly on the initial quality of the crop at harvest, including the degree of maturity. It also depends on careful handling to minimize mechanical damage, proper management of the environmental conditions, and good sanitation.

Most often, postharvest losses are a symptom rather than the problem. Knowledge of their cause is, therefore, essential for deciding measures to prevent them. Such measures may have to be taken by the small farmer, the private trader, a cooperative, the marketing board or other operator, handlers and transporters, wholesale and retail markets, etc.

Future post-harvest research priorities

Major Research Programmes

Research in horticulture focuses in the areas of:

Conclusion

Proper evaluation of postharvest technologies includes technical, economic and social components. It involves beneficiary participation throughout. Decisions to adopt new or improved technologies are made by individual farmers but their decisions are often strongly influenced by incentives and credit schemes, access to reliable sources of inputs, extension advice, training opportunities, and market information all of which are generally the responsibility of national governments or their agents.

References

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