تخزين المحاصيل الزراعية

Post-Harvest Physiological Disorders in Horticultural Crops

• These are not caused by living organisms, but by variations in the environment surrounding the fruits in the field and after harvest.
• Poor nutrition and agricultural practices during growth, variations in atmospheric components, high ethylene levels around the fruit after harvest, and other non-disease factors contribute to these disorders.
• Attention should be paid to the crop from the field, before harvest, during harvest, and ending with optimal storage conditions.

Temperature Injuries

• Freezing Injury: Occurs when the crop is exposed to temperatures below its freezing point due to ice crystal formation within the intercellular spaces. This increased volume presses against cell walls causing damage, rapid deterioration, and tissue breakdown.
• It also involves the loss of several compounds and can cause an unpleasant odour, as well as fungal diseases after the fruit is removed from storage.
• Damaged fruit becomes very sensitive to mechanical damage during handling, transportation, and storage.
• Symptoms include watery spots on the fruit or plant tissue, collapse, and damage to the outer and inner layers.
• Vegetables vary in their sensitivity to freezing damage:
  • Highly sensitive to freezing.
  • Moderately sensitive to freezing.
  • Less sensitive to freezing.
  • Examples of crops:
    • Leafy greens (e.g., lettuce, spinach).
    • Root vegetables (e.g., carrots, beets).
    • Bulbs, tubers, etc.
    • Some fruits (e.g., apples, pears, stone fruits).
  • The sensitive crops will have obvious damage with a single freezing event.
  • Crops that can tolerate it have variable cold tolerance and can withstand multiple freezing events without damage.
• To prevent freezing damage, store produce at a temperature above its freezing point by 0.5°C to 1°C. The storage temperature should always be above the crop's freezing point.

Chilling Injury

• It occurs in cold-sensitive crops, particularly those of tropical or subtropical origin.
• The condition manifests when placed at temperatures above the freezing point of water (0°C) and below the threshold temperature.
• This threshold temperature is the temperature at which the crop should be stored to avoid cold damage, regardless of the storage period.
• Cold damage is a physiological disorder with visible surface and internal symptoms, that do not show up during refrigerated storage, but develop quickly and become apparent shortly after refrigerated storage for a few hours to a couple of days, coinciding with market display. This leads to a decline in quality and decreased prices.
• The following table provides safe storage temperatures for some cold-sensitive crops.

Crop Factors Affecting Cold Sensitivity

• Genetic factors: Legumes are generally sensitive to cold storage, while lettuce is an exception.
• Maturity at harvest: The susceptibility of fruits to cold damage decreases as they approach maturity.
• Fruit position on the plant: Fruits harvested from the outer periphery of trees are more susceptible than those from the interior.

Storage Factors Affecting Cold Sensitivity

• Temperature: Cold damage symptoms do not emerge as long as the storage temperature is above the threshold temperature.
• Reheating and acclimation: Repeated warming of fruits during refrigerated storage can significantly reduce their sensitivity to cold damage.
• Relative humidity in storage: The sensitivity of fruits to cold damage decreases with increased relative humidity.
• Modified or controlled atmosphere storage: Fruit sensitivity to cold damage decreases with high levels of carbon dioxide (in permissible limits).
• Hot water treatment: Fruit treatment with hot water at temperatures from 45°C to 60°C for 3-5 minutes can reduce the sensitivity to cold damage.

Crop Postharvest Handling

• Pre-storage calcium dip: Dipping fruits in calcium solution before storage helps reduce their sensitivity to cold damage.
• Post-harvest preparation: Processes like dry cleaning, washing, or adding chemicals to the wash water can weaken fruit skins and increase cold-damage vulnerability.
• Packaging and wax coatings: Both packaging and wax coatings can reduce fruit susceptibility.
• Artificial ripening: Treatment with ethylene or ethrel reduces the sensitivity to cold damage.
• Exposure to higher-than-critical temperature: Pre-cooling fruits to a temperature above the threshold for a few days can reduce sensitivity (e.g., exposing melons to 20-25°C for a few days before refrigeration at 5°C).

General Symptoms

• Discoloration and ripening failure: Impairment of fruit ripening and discoloration of fruit in certain fruits like bananas, tomatoes, mangoes, and citrus fruits, due to reduced ethylene production or enzyme inhibition.
• Pitting and spotting: Pits or spots appear near or under the fruit surface after refrigeration, may develop into watery areas prone to fungal infections. Common in stone fruit, cucumbers, zucchini, and bell peppers.
• Internal breakdown: Affects fruit like citrus fruits (e.g., plums and apples), causing tissue discoloration, dryness, sourness, and water-soaked areas (particularly near the seed).
• Watery breakdown: A condition affecting fruit like oranges, where the skin becomes spongy and watery, or in certain apples and Bartlett pears.
• Increased susceptibility to microorganisms: Fruits often lose their ability to resist pathogenic microorganisms and become susceptible to fungal and bacterial infections.
• Reduced palatability and nutritional value: A clear decrease in the flavour, taste, and nutritional quality of fruits.

Symptoms on Certain Fruits

• Banana finger browning
• Woolly appearance of some susceptible plum fruits
• Vascular tissue malformation in custard apples
• Pitting and pitting in peppers and cucumbers, and loss of shine in eggplants.

Causes of Chilling Damage

• Reduced catalase activity (an antioxidant)
• Respiratory abnormalities
• Accumulation of toxic respiratory byproducts
• Membrane damage
• Reduced energy production and enzyme activity (especially respiratory enzymes).

Chilling Damage in Specific Crops

• This section lists various crops and their associated chilling damage symptoms. Details are provided on each crop's specific reactions.

Heat Injuries

• Exposure to high temperatures due to direct sunlight or elevated storage temperatures leads to significant quality and quantity losses.
• Symptoms such as increased metabolic rate and reduced storage time, inhibition and failure of ripening (ripening failure), and poor fruit discoloration is evident in fruits like bananas and tomatoes.
• Temperatures exceeding 40°C cause cell death.
• Fruits lose firmness, deteriorate quickly, their respiration rates increase, stored compounds are lost, and they age prematurely.

Sunburns

• Sun scald is the death, hardening, and whitening or yellowing of a part of the fruit exposed to high temperatures (42°C or more).
• This leads to protein accumulation and cell death in the exposed part.
• Fruit shrivel and wilt as a result of significant moisture loss due to high temperatures and low relative humidity.
• Fruits lose their characteristic aromas due to the cessation of volatile gas production.
• Fruits exhibit a cooked appearance, with browning of some skin areas.

Atmospheric Gases Physiological Disorders

• Black heart in potatoes: Occurs due to low oxygen levels in storage.
• Physiological damage from ethylene gas (previously discussed).

Nutritional Deficiencies (Nutritional Factors)

• Calcium deficiency:
  • Core rot in apples
  • Brown speckles in pears
  • Blossom-end rot (occurs with low transpiration rates and reduced calcium uptake; prevent with appropriate watering).
  • Dark heart in cabbage
  • Scorched tips in lettuce
• Nitrogen deficiency/excess:
  • Increased incidence of brown rot
  • Increased moisture in onions and garlic with high nitrogen levels, delaying ripening and reducing storage life.
  • Hollow heart and cracking in broccoli and cauliflower
  • Increased incidence of soft rot in tomatoes.
• Boron deficiency:
  • Hollow stems in cauliflower.
  • Other symptoms related to boron deficiency.

Environmental and Agricultural Practices

• A list of environmental and agricultural practices related to physiological damage is provided. This includes sunburn, cracked fruit, "cat's eye" fruit, hollow fruit, irregular fruit coloration, secondary growth or cracking, greening, fruit deformities, cucumber and bean pod twisting, thick onion necks, planting methods, and others.