Characteristics of Perishable Crops Affecting their Postharvest life

1. Requires energy – occur since horticultural produce are living plant materials
2. Constant state of change- deterioration process cannot be stopped but can be slowed down within certain limits.
3. High in water content
4. React to adverse environmental conditions.
5. Subject to attack by pathogens and insects.

Physiological processes carry out by perishable crops

1. Respiration
2. Transpiration
3. Senescence

RESPIRATION – the process of converting sugars, fats, proteins and other food reserves into metabolic energy which is needed to keep tissues alive and functioning.

Implication: Reducing 02 or increasing C02 level (e.g. modified atmosphere packaging,

A high respiration rate leads to:

• • • • Rapid metabolite depletion
      • Increased heat generation

Factors Affecting Respiration

1. Internal Factors

• Tissue type (genetic make-up)
• Physiological status or stage of development

Classification of commodities according to rate of respiration:

a. Commodities with high respiration rates
Actively growing points/ immature tissues
Examples: okra, young cob corn, string beans, cauliflower
Fruits starting to ripen or senescence

b. Commodities with low respiration rate
Storage organs
Examples: potato, gabi, cassava, yam
Dormant tissues
Examples: cured onion and garlic

c. Commodities with intermediate respiration rate
Crops harvested at their physiological maturity

2. External ( environmental) factors

a. Temperature – the single most important factor in maintaining quality after harvest.

b. Composition of the Atmosphere ( CO₂,O₂, C₂H₄ levels)

c. Causes of stress or injury.
– Dropped or bruised produce respire faster
– Handle perishables with tender loving care.

Ways to avoid excessively high respiration rates in perishables

• Temperature management
  • Keep produce cool
  • Avoid temperatures below 12.5°C, the usual threshold temperature for chilling injury in tropical produce.
• Modified or controlled atmospheres (MA or CA)
  • Store at optimal O2 (reduced) and/or recommended CO2 (elevated) levels at optimal temperatures.
• Harvesting at recommended commercial / physiological maturity.
• Protection against ethylene exposure and stress.
• Handling with tender loving care.

TRANSPIRATION – Water evaporation process from the plant organ

• if not excessive

1. Helps remove heat of respiration

• if excessive, leads to following undesirable effects:

1. Loss in salable weight
2. Wilting and shriveling (wilting can be reversed by rehydration in water)
3. Toughening, discoloration and flavor changes
4. Loss in nutrients (proteins and Vitamin C)
5. Accelerated senescence (aging)

Ways to protect perishables against excessive transpiration

• Temperature management
• Control of relative humidity
  • • Hold produce at recommended RH
    • Avoid exposing produce to wind
• Harvesting at recommended harvest maturity.
• Handling with TLC!

SENESCENCE

• • Precedes end of postharvest life
  • Occurs before or coincides with deterioration

Ethylene, the stress hormone, is also called the senescence or ripening hormone. This makes the fruits ripen faster; flowers fade, wither, and shrivel when exposed to ethylene.

Ethylene “producers” should not be stored with fruits, vegetables, or flowers that are sensitive to it. The result could be loss of quality, reduced shelf life, and specific symptoms of injury.

Factors Affecting Disease Development

1. VARIETY – ‘Saba’ less susceptible to postharvest diseases

2. OPENINGS in dermal system – stomates, lenticels

3. STAGE OF DEVELOPMENT

• Immature tissues
• Ripening produce
• Injury/cuts

4. ENVIRONMENTAL FACTORS

• Temperature – if high, microbial growth is favored; chill- injured produce less resistant.
• Relative Humidity – High RH favors disease. Free moisture increases RH and disease.
• Gas Composition – O2, CO2, ethylene, Carbon monoxide and sulfur dioxide inhibit fungal growth.

Factors Affecting Vegetable Quality

1. Physiological factors

• Harvested vegetables are composed of living cells and tissues.
• Since they are cut off from the growing medium, harvested produce has to live on stored reserves (carbohydrate and water). Depletion of carbohydrates through respiration and loss of water through transpiration lead to quality loss (Figure 3).

Figure 3 Basic characteristics of fresh vegetables.

• A basic understanding of physiological processes is important for effective maintenance of product quality.

a. Respiration
b. Transpiration
c. Ethylene production
 
2. Mechanical factors
Mechanical damage or physical injury increases respiration, water loss,
and ethylene production and susceptibility to microbial infection.
 
3. Microbiological factors
Microorganisms cause spoilage of vegetables. Major postharvest diseases
of vegetables are caused by bacteria and fungi.
 
4. Entomological factors
Insect pests can seriously disrupt trade among countries.
 
5. Environmental factors
Temperature, humidity, atmosphere or even wind movement affect
product quality through their effects on respiration, water loss and ethylene metabolism as well as disease and insect pest attack.
 
6. Production factors

• Crop variety

7. Climatic factors

• Temperature
• Light
• Rainfall

8. Cultural factors

• Seed material
• Mineral nutrition
• Irrigation