Friday, February 23, 2024
Crops

Storage Life of Harvested Crop Materials

Harvested crop materials or 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. 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.

The Postharvest quality of a product develops during growing of produce and is maintained, quality cannot be improved after harvest.

Production of high quality produce 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.

Maximum postharvest quality for any cultivar can be achieved only by understanding and managing the various roles that pre-harvest factors play in postharvest quality.

Physiological Processes of Fresh Produce

Storage Life of Harvested Crop Materials

1. Respiration

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.

2. Aerobic respiration

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.

Breaking down of glucose in the presence of oxygen leading to formation of carbon dioxide, water and energy( heat) C6H12O6 + 6 O2 → 6 CO2 + 6H2O + Heat (2830 kJ)

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 minimize quality losses, extend shelf life and minimize economic losses to the producer.

Factors that impact the respiration rates include the following:

Storage Life of Harvested Crop Materials
  • Temperature
  • Atmospheric composition
  • Physical stress

a. Temperature

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 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.

b. Atmospheric composition

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.

Breaking down of glucose in the absence of oxygen (anaerobic respiration) leading to the formation of carbon dioxide and energy (heat).

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.

c. Physical stress

Mild physical stress can perturb the respiration rates of the harvested crop materials. 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.

3. 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, shriveling, 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.

Ethylene Production

Ethylene (C2H4) is a naturally occurring organic molecule that is a colorless gas at biological temperatures. Ethylene is synthesized 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 categorized as being either climacteric or non-climacteric on the basis of its ability to produce ethylene during the ripening process.

Climacteric crops produce a burst of ethylene and show 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.

4. Mechanical injuries

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.

5. Pathological breakdown

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.

Read Also : Maturity Indices of Crop Maturity and Crop Products

Pre-Harvest Factors on Post-Harvest Life

Storage Life of Harvested Crop Materials

The postharvest 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.

Pre-harvest 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 pre-harvest factors to post harvest quality preclude generalizations about pre-harvest 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 pre-harvest factors play in postharvest quality.

1. 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.

2. Mineral nutrition

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 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 commodities.

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 pre-harvest control practice.

3. Irrigation

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 management 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 evapo-transpirative 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.

Storage Life of Harvested Crop Materials

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.

4. Crop rotations

Crop rotation may be an effective management practice for minimizing postharvest losses by reducing decay inoculum in a production field. Because soil-borne 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 non-cucurbit 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.

In summary, a basic understanding of post-harvest physiological processes i.e. respiration, transpiration, ethylene production, pathological break down, and mechanical injuries 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.

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.

Read Also : Different Methods of Processing Crop Products

The Postharvest quality of a product develops during growing of produce and is maintained, quality cannot be improved after harvest.

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.

These metabolic functions (i.e. respiration, transpiration and ethylene production) influence greatly 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.

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 quality.

Maximum postharvest quality for any cultivar can be achieved only by understanding and managing the various roles that pre-harvest factors play in postharvest quality.

Read Also: Types of Wastes: Solid Waste, Liquid Waste and Gaseous Waste

Agric4Profits

Benadine Nonye is an agricultural consultant and a writer with over 12 years of professional experience in the agriculture industry. - National Diploma in Agricultural Technology - Bachelor's Degree in Agricultural Science - Master's Degree in Science Education... Visit My Websites On: 1. Agric4Profits.com - Your Comprehensive Practical Agricultural Knowledge and Farmer’s Guide Website! 2. WealthinWastes.com - For Effective Environmental Management through Proper Waste Management and Recycling Practices! Join Me On: Twitter: @benadinenonye - Instagram: benadinenonye - LinkedIn: benadinenonye - YouTube: Agric4Profits TV and Agric4Kids TV - Pinterest: BenadineNonye4u - Facebook: BenadineNonye

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