Fruits and vegetables are living tissues that remain alive to maintain their keeping qualities. Like all living entities, they respire, consuming oxygen and liberating carbon dioxide. The primary task of the respiratory process is to produce energy needed for various metabolic activities. They rely on organic matter, generally replenished during growth.
Upon removal from the mother plant, fruits and vegetables are cut off from their normal supplies of water, minerals, and organic matter provided by other plant parts. The produce, however, remains capable of continuing a wide range of metabolic activities, breaking down stored organic matter to meet energy requirements.
Some of these physiological activities lead to deterioration of the produce. Biological factors include the rate of respiration, sprouting, rooting, changes in color, flavor, texture, nutrition, ethylene production, water loss, and pathological deterioration.
This article discusses post-harvest changes in foods, with particular reference to fruits and vegetables. Previous studies on food deterioration and spoilage noted that chemical changes within a food over time can render it unsafe for consumption.
Fruits and vegetables begin to deteriorate as soon as they are harvested and should be handled with care to reduce such deteriorative changes. This article explores the factors causing these changes, the classification of fruits based on respiration rate, and methods to control post-harvest changes.
Read Also: Sheep and Goat Housing Complete Guide
Physiological Changes in Post-Harvest Produce
Some physiological activities are highly desirable for achieving optimal eating qualities, such as the ripening of fruits like banana, mango, papaya, and pineapple, and the maintenance of tissue vigor, which provides a defense against spoilage organisms. The key physiological changes associated with post-harvest include:
1. Respiration in Post-Harvest Produce
Respiration is the most critical physiological activity and has a direct bearing on produce quality. It involves the complex oxidation of organic matter (starch, sugars, acids, fats, proteins, etc.) into simpler molecules, such as CO2 and H2O, with the concurrent production of energy and intermediate products.
i. Undesirable Consequences of Respiration
- Loss of food value (stored organic matter is degraded).
- Hastening of senescence (the aging process).
- Loss of saleable weight.
- Reduced quality (usually after the ripening process for fruits).
ii. Desirable Effects of Respiration
- Provides energy for numerous metabolic processes.
- Supplies valuable intermediates.
- Maintains tissue vigor.
iii. Classification of Fruits Based on Respiration Rate
Fruits are classified as climacteric or non-climacteric based on their respiration and ethylene production rates during development, maturity, ripening, and senescence.
a. Climacteric Fruits: These exhibit a large and sudden increase in respiration and ethylene production rates, almost coincident with ripening. Examples include avocado, banana, apple, guava, jackfruit, mango, papaya, passion fruit, peach, plantain, plum, soursop, tomato, and watermelon. Ethylene evolution in climacteric fruits can reach 30–500 ppm/kg/h during ripening at 20–25°C.
b. Non-Climacteric Fruits: These do not show a sudden increase and emit considerably lower ethylene levels (0.1–0.5 ppm/kg/h during ripening at 20–25°C). Examples include oranges, lemons, strawberries, grapes, pineapples, and cucumbers.
Climacteric fruits respond to external ethylene treatment with early induction and increased levels of ethylene and CO2, accelerating ripening in a concentration-dependent manner. Non-climacteric fruits show increased respiration and ethylene production in response to external ethylene but do not accelerate ripening time.
Read Also: Guide On How To Increase Goats Milk
2. Transpiration in Post-Harvest Produce
Transpiration is the second most significant factor affecting quality loss during storage and transportation. It refers to the loss of water from produce due to evaporation. Any moist material loses moisture when exposed to air that is not saturated.
Moisture loss occurs through evaporation and vapor diffusion. When attached to the plant, lost water is replenished; however, post-harvest, no replacement occurs, resulting in a loss of saleable weight.
A moisture loss of 3–4% can render produce unsellable. The acceptable moisture loss varies: 3% for leafy tissues, 5% for common vegetables, and up to 10% for onions.
Moisture loss adversely affects appearance, texture, and flavor, leading to wilting, shriveling, shrinkage, drying, dehydration, or desiccation.
i. Factors Affecting Transpiration Rate
Environmental Factors
1. Temperature: Higher temperatures increase the transpiration rate.
2. Relative Humidity: Lower relative humidity (a measure of water vapor in the air) increases water loss from produce.
3. Wind Speed: Higher wind speeds increase evaporation from the surface of fruits or vegetables.
Biological Factors
1. Fruit Size: Transpiration decreases with increasing fruit size.
2. Surface Area/Weight Ratio: Transpiration rate is directly proportional to surface area. For commodities of similar shape, the surface area/weight ratio decreases as size increases.
3. Stage of Development/Maturity: The maturity stage influences transpiration rates, affected by changes in size, surface area/weight ratio, and other factors.
4. Injuries, Wounds, and Cracks: Tissue wounds from disease or mechanical injury increase transpiration and may allow penetration of pathogenic microorganisms.
5. Presence of Leaves, Stems, Flowers, or Calyx: Transpiration increases when commodities are attached to plant parts with high transpiration rates.
6. Cultivars: Transpiration rates vary among cultivars due to differences in epidermal permeance or surface area/weight ratio resulting from size or shape variations.
3. Ripening and Senescence in Fruits
Ripening, a term reserved for fruit, begins during the later stages of maturation and marks the first stage of senescence. Senescence is the period when anabolic (synthetic) biochemical processes give way to catabolic (degradative) processes, leading to aging and eventual tissue death.
Fruit ripening involves complex developmental processes with profound physiological and biochemical transformations. During early growth, fleshy fruits are green, accumulate water and nutrients, and are covered by thick epidermal layers for seed protection.
After development, ripening involves transformations in color, texture, aroma, and nutrients, making the fruit attractive for seed dispersal by predators and nutritious for human consumption. Different structural, physiological, and biochemical mechanisms operate during ripening in various fruit types.
a. Changes During Ripening
i. Color: Degradation of green chlorophyll and an increase in carotenoid content, mainly lycopene, cause fruits to change from green to red or yellow upon ripening.
ii. Softening: Pectinases, activated during ripening, degrade pectin, softening fruits and enhancing their appeal for seed dispersal. However, oversoftening reduces transportability, storage time, and post-harvest shelf life. Pectin provides mechanical rigidity in fruits and vegetables.
iii. Flavor and Aroma: Aroma, crucial for optimal eating quality, develops due to volatile organic compounds synthesized during ripening.
iv. Organic Acids: Organic acids decline during ripening as they are used in respiration or converted to sugars, making ripe fruits less tart than unripe ones.
v. Carbohydrate Metabolism: The most significant change is the breakdown of carbohydrate polymers, such as the near-total conversion of starch to sugars, altering taste and texture.
Increased sugar content makes the fruit sweeter and more palatable. The breakdown of pectic substances and hemicelluloses weakens cell walls and cell cohesion, initially improving texture but eventually leading to structural disintegration.
b. Control of Ripening
i. Modified Atmosphere Packaging (MAP): MAP involves enclosing food products in barrier films with a modified gaseous environment to slow respiration rates, reduce microbiological growth, and retard enzymatic spoilage, extending shelf life.
ii. Good Harvesting Practices: Fruits should be harvested at peak maturity, avoiding bruises and maintaining good hygiene during and after harvest.
Post-harvest changes are physiological processes that begin as soon as a fruit or vegetable is harvested, potentially leading to deteriorative changes. These must be controlled to extend the commodity’s shelf life.
Key physiological activities include respiration, transpiration, and ripening, which may cause sprouting, rooting, changes in color, flavor, texture, nutrition, ethylene production, water loss, and pathological deterioration. Fruits are classified as climacteric (e.g., mango, banana) or non-climacteric (e.g., orange, lemon, cucumber) based on respiration and ethylene production rates.
Climacteric fruits show a sudden increase in these rates during ripening, while non-climacteric fruits do not. Ripening can be managed through modified atmosphere packaging and proper harvesting practices to minimize deterioration and maintain quality.
Do you have any questions, suggestions, or contributions? If so, please feel free to use the comment box below to share your thoughts. We also encourage you to kindly share this information with others who might benefit from it. Since we can’t reach everyone at once, we truly appreciate your help in spreading the word. Thank you so much for your support and for sharing!