An overview of postharvest biology of fresh produce
The trainees should be able to:
• Describe the physiological and environmental factors that affect the post harvest life of horticultural produce.
• Propose appropriate handling technology for a given produce taking into consideration the available technology of the locality or country.
• Connect Bhutanese fresh produce handling to the developing trends worldwide.
Harvested fresh produce is ‘living’ and continues to perform its metabolic functions in the post-harvest state. These metabolic functions influence greatly on the quality and shelf life of fresh produce.
A basic understanding of post-harvest physiological processes and mechanisms for their control is critical for effective quality maintenance throughout horticultural supply chains to reduce losses as well as for quality product. This component describes the physiological factors that impact on the quality of horticultural produce.
(Click Figure 1 below to access illustration)
Source: Kader and Rolle, 2004
Figure 1. Fresh produce physiology
(Click Figure 2 below to access illustration)
Figure 2. Development phase
Physiological processes of fresh produce
Harvested produce is alive, which means that it is constantly respiring. Respiration involves the breakdown of carbohydrate (example sugars) and other food reserves (organic and fatty acids) in the plant or in harvested produce and results in the production of carbon dioxide, water and heat. Respiration occurs both pre- and post-harvest.
C6H12O6 + 6 O2 6 CO2 + 6H2O + Heat (2830 kJ)
In the post-harvest phase, respiration is supported by carbohydrate reserves of the produce; this leads to a net loss in its dry weight or negative growth. The more rapid the respiration rate, the faster the produce will consume its carbohydrate reserves, the greater will be the heat produced and the shorter will be the post-harvest life of the fruit or vegetable.
Carbohydrate breakdown during respiration leads to losses in food value, flavour, texture and weight, and thus to overall quality loss. Loss in weight, in particular, results in economic loss to the producer. Every effort must, therefore, be made to slow down the respiration rate of produce in order to minimise quality losses, extend shelf life and minimise economic losses to the producer.
Figure 3. Respiration rate and perishability of fresh produce
Figure 4. Respiration rates of fruits and vegetables
Factors that impact on respiration rates:
• Atmospheric composition
• Physical stress
Temperature has a significant influence on the respiration rate of harvested produce and without doubt has the greatest impact on the deterioration of produce post-harvest. The higher the storage temperature of fresh produce, the greater is its rate of respiration. The rate of deterioration of horticultural produce increases two to three-fold with every 10°C increase in temperature (see Table 1. below).
Table 1. Effect of temperature on the deterioration rate of a non-chilling- sensitive commodity
Respiration rates can be slowed by storing produce at a low temperature that does not cause physiological damage to the produce. Temperature management is pivotal to controlling respiration and to maintaining quality.
Figure 5. Temperature effect on respiration
Adequate levels of O2 are required to support the process of aerobic respiration in harvested produce. The exact level of O2 required to reduce respiration rates, while at the same time allowing aerobic respiration, varies in accordance with the commodity concerned. An O2 level of around 2 to 3 per cent generally produces a beneficial reduction in respiration rates and in other metabolic reactions of fresh produce. Lower O2 levels could lead to anaerobic respiration and off-flavour development as a result of alcohol formation.
C6H12O6 2 CO2 + 2C2H5OH + Heat (118 kJ)
Post-harvest handling treatments such as waxing, coating, film wrapping and controlled atmosphere packaging can be used to regulate the availability of oxygen to harvested produce and so to reduce respiration rates.
Mild physical stress can perturb the respiration rates of produce. Bruising can, for example, result in substantial increases in the respiration rate of harvested produce. The avoidance of mechanical injury through proper packaging and handling is critical to assuring produce quality.
Transpiration or water loss
Fresh produce contains between 70 and 95 % water and is losing water constantly to the environment in the form of water vapour. The rate of water loss varies in accordance with morphological characteristics (such as tissue structure, dimensions and number of stomata and the presence of a waxy layer) of the epidermis (skin) of the produce item, the exposed surface area of the produce and the vapour pressure deficit (VPD) between the produce and its environment. The VPD bears an inverse relationship to the relative humidity of the environment. Under conditions of low relative humidity the VPD is high and water is lost rapidly. The rate of water loss increases exponentially with increasing temperature and linearly under conditions of low relative humidity.
Water lost due to transpiration in harvested produce cannot be replaced, thus resulting in wilting, shrivelling, loss of firmness, crispiness, succulence and overall loss of freshness. These undesirable changes in appearance, texture and flavour, coupled with weight loss, greatly reduce the economic value of horticultural produce. Wilted leafy vegetables may, for example, require excessive trimming to make them marketable.
Water loss can be controlled through temperature management, packaging and adjustment of the relative humidity of the storage environment of the produce. However, care must be taken to avoid condensation of moisture on the surface of the produce, since this could contribute to the development of decay.
Figure 6. Nature of produce affecting transpiration
Ethylene (C2H4) is a naturally occurring organic molecule that is a colourless gas at biological temperatures. Ethylene is synthesised in small quantities by plants and appears to co-ordinate their growth and development. It is also associated with the decomposition of wounded produce. Given its gaseous nature, ethylene readily diffuses from the sites where it is produced. Continuous synthesis is, therefore, needed for maintenance of biologically active levels of ethylene in plant tissues.
Ethylene is also an environmental pollutant, being produced by internal combustion engines, propane powered equipment, cigarette smoke and rubber materials exposed to ultra violet light.
Fresh produce can be categorised as being either climacteric or non-climacteric (see Table 2. below) on the basis of its ability to produce ethylene during the ripening process.
• Climacteric produce – produces a burst of ethylene and shows an increase in respiration on ripening. Ripening of climacteric fruit after harvest typically involves softening and a change in colour and taste in terms of sweetness.
• Non-climacteric produce – does not show increased ethylene production on ripening, with relatively little quality change after harvest. Non-climacteric produce undergoes slight softening with a loss in green colour after harvest, with relatively little change in eating quality. Examples of climacteric and non-climacteric fruit are shown in Table 2.
Table 2. Examples of climacteric and non-climacteric fresh produce
Ethylene has both beneficial and harmful effects on the quality of horticultural produce. Ethylene enhances produce quality by promoting desirable colour development and stimulating the ripening of climacteric fruit. However, its undesirable effects include accelerated ripening and softening of fruits, accelerated senescence and loss of green colour in leafy, floral and immature fruit/vegetables, russet spotting on lettuce and the abscission of leaves.
Because of these diverse and often opposite effects of ethylene, controlling its action in fresh produce is of great economic importance to producers, wholesalers, retailers and consumers of fresh fruits and vegetables.
The deleterious effects of ethylene can be overcome through low temperature storage, controlled or modified atmosphere storage, ventilation of ripening rooms, segregation of ethylene producing commodities from ethylene sensitive ones, the use of ethylene absorbers such as potassium permanganate (KMnO4) in cold rooms as well as the scrubbing of ethylene from cold rooms. The ethylene inhibitor 1-methylcyclopropene, currently sold under the trade name ‘Smart Fresh’ is currently approved for use on selected produce items in the United States.
Figure 7. Adverse effects of ethylene on fresh produce
Figure 8. Categorization of horticultural produce based on ethylene production
Many changes in pigmentstake place during development and maturation of commodity on plant; some may continue after harvest and can be desirable or desirable.
Loss of chlorophyll (green colour) is desirable in fruits but not in vegetables. Development of carotenoids (yellow and orange colours) is desirable in fruits and apricots, peaches and citrus. Red colour development in tomatoes and pink grapefruits is due to a specific carotenoid (lycopene); beta-carotene is provitamin A and thus is important in nutritional quality.
Changes in carbohydrates include starch to sugar conversion (undesirable in potatoes, desirable in apple, banana and other fruits) sugar to starch conversion (undesirable in peas and sweet corn; desirable in potatoes) and conversion of starch and sugars to carbon dioxide and water through respiration. Breakdown of pectins and other polysaccharides results in softening of fruits and a consequent increase in susceptibility to mechanical injuries. Increased lignin content is responsible for toughening of asparagus spears and root vegetables.
Changes in organic acids, proteins, amino acids, and lipids can influence flavour quality of commodity. Loss in vitamin content, especially ascorbic acid (vitamin C) is detrimental to nutritional quality; production of flavour volatiles associated with ripening of fruits is very important to their eating quality.
Growth and development
Sprouting of potatoes, onions, garlic and root crops greatly reduces their food value and accelerates deterioration. Rooting of onions and root crops is also undesirable. Asparagus spears continue to grow after harvest; elongation and curvature (if the spears are held horizontal) are accompanied by increased toughness and decreased palatability. Similar geotropic responses occur in cut gladiolus and snap dragon flowers stored horizontally. Seed germination inside fruits such as tomatoes, peppers and lemons is an undesirable change.
Exposure of commodity to undesirable temperatures can result in physiological disorders:
Freezing injury results when commodities are held below their freezing temperatures. Disruption caused by freezing usually results in immediate collapse of the tissues and total loss of the commodity.
Chilling injury occurs in some commodities (mainly those of tropical and subtropical origin) held at temperatures above their freezing point and below 5oC to 15oC depending on the commodity. Chilling injury symptoms become more noticeable upon transfer to higher (non-chilling) temperatures. The most common symptoms are surface and internal discoloration ( browning) , pitting, water soaked areas, uneven ripening or failure to ripen, off-flavour development, and accelerated incidence of surface moulds and decay ( especially the incidence of organisms not usually found growing on healthy tissue.
Heat injury is induced by exposure to direct sunlight or excessively high temperatures. Its symptoms include bleaching, surface burning or scalding, uneven ripening, excessive softening and desiccation.
Certain types of physiological disorders originate from preharvest nutritional imbalances. For example, blossom end rot in tomatoes and bitter pit in apples results from calcium deficiency. Increasing calcium content by pre-harvest or post harvest treatments can reduce the susceptibility to physiological disorders. Calcium content also influences the textural quality and senescence rate of fruits and vegetables; increased calcium content has been associated with improved firmness retention, reduced CO2 and C2H4 production rates and decreased decay incidence.
Very low O2 (<1%) and high CO2 (>20%) atmospheres can cause physiological breakdown of most fresh horticultural commodities and C2H4 can induce physiological disorders in certain commodities. The interaction amon O2, CO2 and C2H4 concentrations, temperature and duration of storage influence the incidence and severity of physiological disorders related to atmospheric composition.
Various types of physical damage (surface injuries, impact bruising, and vibration bruising and so on) are major contributors to deterioration. Browning of damaged tissues results from membrane disruption, which exposes phenolic compounds to the polyphenol oxidase enzyme. Mechanical injuries not only are unsightly but also accelerate water loss, provide sites for fungal infection and stimulate CO2 and C2H4 production by the commodity.
One of the most common and obvious symptoms of deterioration results from the activity of bacteria and fungi. Attack by most organisms follows physical injury or physiological breakdown of commodity. In a few cases, pathogens can infect apparently healthy tissues and become the primary cause of deterioration. In general, fruits and vegetables exhibit considerable resistance to potential pathogens during most of their post harvest life. The onset of ripening in fruits and senescence in all commodities renders them susceptible to infection by pathogens. Stresses such as mechanical injuries, chilling and sunscald lower the resistance to pathogens.
Postharvest technology procedures
Temperature management procedures
Temperature management is the most effective tool for extending the shelf life of fresh horticultural commodities. It begins with rapid removal of field heat by following one of the cooling methods: hydro-cooling, in-package icing, top icing, evaporative cooling, room cooling, forced air cooling, vacuum cooling or hydro-vacuum cooling. Cold storage facilities should be well engineered and adequately equipped. They should have good construction and insulation, including a complete vapour barrier on the warm side of the insulation; strong floors; adequate and well-positioned doors for loading and unloading; effective distribution of refrigerated air, sensitive and properly located controls; enough refrigerated coil surface to minimize the difference between the coil and air temperatures. Commodities should be stacked in cold rooms with spaces between pallets and room walls to ensure good air circulation. Storage rooms should not be loaded beyond their limit for proper cooling. In monitoring temperature, commodity temperature rather than room temperature should be measured.
Control relative humidity (RH)
Relative humidity can influence water loss, decay development, incidence of some physiological disorders and uniformity of fruit ripening. Condensation of moisture on the commodity ( sweating over long periods of time is probably more important in enhancing decay than is the RH of ambient air.proper relative humidity is 85-95% for fruits and 90-98% for vegetables except dry onions and pumpkins (70-75%). Some root vegetables can be best held at 95-100% RH. RH can be controlled by one or more of the following procedures:
• Adding moisture (water mist or spray steam,) to air by humidifiers
• Regulating air movement and ventilation in relation to the produce load in the cold storage room.
• Adding polythene liners in containers and plastic films for packaging.
• Wetting floors in storage rooms
• Adding crushed ice in retail displays for commodities that are not injured by ice.
• Sprinkling produce with water during retail marketing( use on leafy vegetables, cool season root vegetables, immature fruit vegetables such as snap beans, peas, sweet corn, summer squash.
Recent trends in perishanble handling
Selection of cultivars
For many commodities, producers are using cultivars with superior quality and /or long post harvest life, long shelf life tomatoes, super sweet sweet corn, sweeter melons etc. Plant geneticists in public and private institutions are using molecular biology methods along with plant breeding procedures to produce new genotypes that taste better, maintain firmness better, are more is disease resistant, have less browning potential and have other desirable characteristics.
Packing and packaging
The produce industry is increasingly using plastic containers that can be reused and recycled in order to reduce waste disposal problems. For example, stacking returnable plstic crates are becoming widely used. There is continued use of modified atmosphere and controlled atmosphere [packaging (MAP and CAP) system at the pallet, shipping container (fibreboard box liner), and consumer package levels. In addition, the use of absorbers of C2H4, CO2, O2 and /or water vapour as part of MAP and CAP is increasing.
Cooling and storage
The current trend is towards increased precision in temperature and relative humidity management to provide the optimal environment for fresh fruits and vegetables during cooling and storage. Forced air cooling continuous to be the predominant cooling method for horticultural perishables.
Post harvest Integrated pest management
Biological control agents are being used alone or in combination with reduced concentration of post harvest fungicides, heat treatments, and/or fungistatic CA for control of post harvest diseases.Chemical fumigants especially methyl bromide is still the primary method sued for insect control in harvested fruits when such treatment is required by quarantine authorities of importing countries. Many stiuides are underway to develop alternative methods of insect control that are effective and not phytotoxic to the fruits and present no health hazard to the consumer. These alternatives include cold treatments, hot water or air treatments, ionizing radiation and exposure to reduced (0.5%) O2 or elevated CO2 (40-60%) atmospheres. This is a high-priority research and development area because of the possible loss of methyl bromide as an option for insect control.
Improvements are continually being made in attaining and maintaining the optimal environmental conditions( temperature, RH and concentrations of O2, CO2 and C2H4) in transport vehicles. Produce is commonly cooled before loading and is loaded with an air space between the palletized produce and the walls of the transport vehicles to improve temperature maintenance. In some cases vehicle and produce temperatures data are transmitted by satellite to control centre, allowing all the shipments to be continuously monitored. Some new trucks have air ride suspension, which eliminate vibration damage. As the industry realises the importance of air ride, its popularity will increase.
Handling at wholesale and retail market
Wholesale and retail markets have been increasingly using automated ripening in which the gas composition of the ripening atmosphere, the room temperature and the fruit colour are continuously monitored and modulated to meet desired ripening characteristics. Improved ripening systems will lead to greater use of ripening technology to deliver products that are ripened to the ideal eating stage.Better refrigerated display units, with improved temperature and RH monitoring and control systems are being used in the retail markets especially for fresh cut fruits and vegetables. Many retail nad food service operators are using Hazard Analysis Critical Control Points (HACCP) programs to assure consumers that the food products are safe.
Food Safety Assurance
During the past few years, food safety became and continuous to be the number one concern for the fresh produce industry. US Food and Drug Administration have published guidesto minimize microbial food safety hazards for fresh fruits and vegetables. This guide is based on the following basic principles and practices and is practical in any developed or developing country.
Principle 1. Prevention of microbial contamination is favoured over reliance on corrective actions once contamination has occurred.
Principle 2. To minimize microbial food safety hazards in fresh produce, growers, packers or shippers should use good agricultural and management practices in those areas over which they have control.
Principle 3. Fresh produce can become microbiologically contaminated at any point along the farm to table food chain. The major source of microbial contamination of fresh produce is associated with human or animal faeces.
Principle 4. Whenever water comes in contact with produce, the quality of the water dictates the potential for contamination. Minimize the potential of microbial contamination from water used with fresh fruits and vegetables.
Principle 5. Practices using animal manure or municipal biosolid wastes should be managed closely to minimize the potential for microbial contamination of fresh produce.
Principle 6. Worker hygiene and sanitation practices during production, harvesting, sorting, packing and transport play a critical role in minimizing the potential for microbial contamination of fresh produce.
Principle 7. Follow all applicable local, state and country laws and regulations or corresponding or similar laws, regulations or standards for operations.
Knowing the physiological processes in fresh produce and factors that influence them is important in designing measures to maintain or improve quality and reduce postharvest losses.
Pre-harvest factors on post-harvest life
The trainees should be able to:
• Describe the importance of input quality and cultural factors on the quality and postharvest life of horticultural crops.
The Post harvest quality of a product develops during growing of produce and is maintained, quality cannot be improved after harvest. Therefore, the goal of a producer is to supply safe, high-quality produce, which confirms to consumer and market requirements. This objective hinges on good quality inputs, good cultural practices (planting, weeding, fertilizer application etc.) as well as good hygiene management during production so as to minimize microbial and/or chemical contamination of produce.
Preharvest factors often interact in complex ways that depend on specific cultivar characteristics and growth or development age sensitivities. The tremendous diversity of fruits and vegetables that are produced commercially and the general lack of research relating preharvest factors to postharvest quality preclude generalizations about preharvest influences that uniformly apply to all fruits and vegetables. Maximum postharvest quality for any cultivar can be achieved only by understanding and managing the various roles that preharvest factors play in postharvest quality.
Cultivar and rootstock genotype
Cultivar and rootstock genotype have an important role in determining the taste, quality, yield, nutrient composition, and postharvest life of fruits and vegetables. The incidence and severity of decay, insect damage, and physiological disorders can be reduced by choosing the correct genotype for given environmental conditions. Breeding programs are constantly creating new cultivars and rootstocks with improved quality and better adaptability to various environmental and crop pest conditions.
Some experts consider the most important cultivar characteristic for fruits and vegetables to be disease resistance, including resistance to diseases that diminish postharvest quality. Control of some postharvest diseases may include breeding for resistance to the vector (e.g., aphid, nematode, leafhopper, or mite), rather than just for the pathogen.
Nutritional quality may also vary greatly according to cultivar. L-ascorbic acid levels in different pepper types also vary considerably. For example, in jalapeno peppers, the highest ascorbic acid levels were in Jaloro (131 mg100g-1) and the lowest were in Mitla (49 mg 100g-1). Wide variation in beta-carotene content of several cultivars of sweet potato has similarly been reported; Georgia Jet, suggested for processing, contained low concentrations of beta-carotene (6.9 mg 100g-l). There is a need to identify and develop cultivars that are suitable for processing and high in antioxidant vitamin content.
Genetic engineering can be a successful tool in altering the quality and yield of certain vegetables, but its commercial application will depend largely on consumer acceptance and food safety issues. Future advances will depend on successful team efforts between plant breeders, plant pathologists, molecular geneticists, and consumer education programs.
Nutritional status is an important factor in quality at harvest and postharvest life of various fruits and vegetables. Deficiencies, excesses, or imbalances of various nutrients are known to result in disorders that can limit the storage life of many fruits and vegetables. Fertilizer application rates vary widely among growers and generally depend upon soil type, cropping history; and soil test results, which help indicate nitrogen (N), phosphorous (P), and potassium (K) requirements. To date, fertilization recommendations for fruits and vegetables have been established primarily for productivity goals, not as diagnostics for good flavor quality and optimal postharvest life.
The nutrient with the single greatest effect on fruit quality is nitrogen. Response of peach and nectarine trees to nitrogen fertilization is dramatic. High nitrogen levels stimulate vigorous vegetative growth, causing shading and death of lower fruiting wood. Although high-nitrogen trees may look healthy and lush, excess nitrogen does not increase fruit size, production, or soluble solids content (SSC). Furthermore, excessive nitrogen delays stone fruit maturity, induces poor red colour development, and inhibits ground colour change from green to yellow. However, nitrogen deficiency leads to small fruit with poor flavor and unproductive trees.
In vegetable crops, excessive nitrogen levels induce delayed maturity and increase several disorders that diminish postharvest quality. Disorders such as grey wall or internal browning in tomato, hollow stem of broccoli, lower soluble solids concentration in potato, fruit spot in peppers, and growth cracks and hollow heart in broccoli and cauliflower have been associated with high nitrogen. High nitrogen has also been associated with increased weight loss during storage of sweet potatoes and soft rot in tomatoes. Excessive soil nitrogen can negatively impact vegetable quality in several ways. High nitrogen can result in composition changes such as reduced ascorbic acid (vitamin C) content, lower sugar content, lower acidity, and altered ratios of essential amino acids. In leafy green vegetables grown under low light, it can result in the accumulation of nitrates in plant tissues to unhealthy levels. High nitrogen fertilization can lead to reduced volatile production and changes in the characteristic flavor of celery.
Although calcium (Ca) is classified as a secondary nutrient, it is involved in numerous biochemical and morphological processes in plants and has been implicated in many disorders of considerable economic importance to the production and postharvest quality of fruits and vegetables. Bitter pit in apple, corkspot in pear, blackheart in celery, blossom end rot in tomato, cavity spot and cracking in carrot, and tipburn of lettuce are calcium deficiency disorders that reduce the quality and marketability of these commodies. Certain calcium deficiency disorders, such as bitter pit in apples and blossom end rot in tomatoes, may be lessened through proper irrigation, fertilizer management, and supplemental fertilization. However, for tip bum of lettuce, a physiological disorder caused by the lack of mobility of calcium in the heads during warm weather and rapid growing conditions, there is currently no preharvest control practice.
Despite the important role of water in fruit growth and development, few studies have been done on the influence of the amount and the timing of water applications on fruit and vegetable quality at harvest and during postharvest.
Water management as a direct determinant of postharvest quality has also been investigated for a number of vegetables produced in semiarid irrigated regions such as California and Israel. Except for a few studies, however, which have comprehensively tested a broad range of water management practices and conditions and their impacts on postharvest quality, it is often difficult to generalize about the effects of water manage ment from the site-specific irrigation regimes that have been reported.
There is considerable evidence that water stress at the end of the season, which may be achieved by irrigation cutoff or deficit irrigation relative to evapotranspirative demand for generally more than 20 days prior to harvest, may markedly improve SSC in tomatoes. Irrigation cutoffs may also facilitate harvests and minimize soil compaction from mechanical harvest operations. Late-season irrigations with saline water have also been shown to increase tomato SSC. Although a higher SSC may result in premiums paid to producers, because of the link between applied water and yield, irrigation practices typically aim at the best overall economic balance between productivity and quality.
Melon postharvest quality is also quite sensitive to water management. Overirrigation can result not only in low SSC in melons but also unsightly ground spots and fruit rots (and measles in honeydews). Rapid growth resulting from irrigations following extended periods of soil water deficits may result in growth cracks in carrots, potatoes, tomatoes, and several other vegetable crops. Uneven irrigation management may also increase the incidence of "spindle"- or "dumb-bell"shaped potatoes, depending on the growth stage during which soil Water was limited. Postharvest losses due to storage diseases such as neck rot, black rot, basal rot, and bacterial rot of onions can be influenced by irrigation management. Selecting the proper irrigation system relative to the crop stage of growth, reducing the number of irrigations applied, and assuring that onions cure adequately prior to harvest can help prevent storage losses.
Management of water frequently poses a dilemma between yield and postharvest quality. A deficiency or excess of water may influence postharvest quality of berry crops. Extreme water stress reduces yield and quality; mild water stress reduces crop yield but may improve some quality attributes in the fruit; and no water stress increases yield but may reduce postharvest quality. In strawberries, reduction of water stress by natural rainfall or irrigation during maturation and ripening decreases firmness and sugar con- tent and provides more favorable conditions for mechanical fruit injury and rot. If straw- berry plants are overirrigated, especially at harvest, the fruit is softer and more susceptible to bruising and decay.
Crop rotation may be an effective management practice for minimizing postharvest losses by reducing decay inoculum in a production field. Because soilborne fungi, bacteria, and nematodes can build up to damaging levels with repeated cropping of a single vegetable crop, rotations out of certain vegetables are commonly recommended in intensive vegetable production regions. Four-year rotations with noncucurbit crops are routinely recommended for cucurbit disease management, as are 4-year rotations for garlic to decrease postharvest disease incidence. There is also evidence that the use of plastic mulches can increase postharvest losses from decay in vegetables such as tomatoes.