HORMA - 303
Postharvest Technology For Fruit Crops
Postharvest Technology For Fruit Crops
UNIT II: Physiology of Ripening, Ethylene & Pre-cooling
📚 Contents
1. Physiology and Biochemistry of Fruit Ripening
Definition: Ripening is the final stage of maturation where the fruit undergoes complex physiological and biochemical changes to become edible (soft, sweet, and aromatic). It is a genetically programmed irreversible process.
Biochemical Changes During Ripening
These changes transform the fruit from an inedible plant organ to an attractive food source.
A. Change in Color (Pigmentation)
- Chlorophyll Degradation: The green pigment (chlorophyll) breaks down due to the enzyme Chlorophyllase and changes in pH.
- Carotenoid Synthesis: Unmasking or new synthesis of yellow/orange pigments.
Example: Beta-carotene in Mango; Lycopene in Tomato. - Anthocyanin Synthesis: Formation of red/purple pigments in presence of light and sugar.
Example: Red Delicious Apple, Grapes, Strawberry.
B. Change in Texture (Softening)
- Solubilization of Pectin: Insoluble protopectin (which makes fruit hard) converts to soluble pectin.
Enzymes involved:
Polygalacturonase (PG): Breaks pectin chains.
Pectinesterase (PE): Prepares pectin for breakdown. - Starch Hydrolysis: In starchy fruits (Banana/Mango), starch granules break down, contributing to softening.
C. Change in Carbohydrates (Sweetness)
- Starch to Sugar Conversion: The most dramatic change in climacteric fruits.
Example: In Banana, starch drops from ~20% (green) to ~1% (ripe), while sugars rise from 1% to ~20%.
Enzymes: Amylase (breaks starch) and Invertase (converts sucrose to glucose + fructose). - Note: In non-climacteric fruits (Grape/Citrus), sugar does not increase after harvest; they must be harvested sweet.
D. Change in Organic Acids (Sourness)
- Decline in Acidity: Organic acids (Citric, Malic, Tartaric) decrease because they are used as substrates (fuel) for respiration during ripening.
Result: The fruit becomes less sour and the Sugar:Acid ratio increases, improving taste.
E. Aroma Volatiles Production
- Specific volatile compounds are synthesized, giving fruit its characteristic smell.
Banana: Isoamyl acetate.
Apple: Ethyl-2-methyl butyrate.
Orange: Limonene.
2. Ethylene Evolution and Management
Definition: Ethylene (C2H4) is a gaseous plant hormone known as the "Ripening Hormone." It triggers ripening, senescence (aging), and abscission (shedding) even in minute quantities (<0 .1 ppm>
A. Biosynthesis of Ethylene (The Yang Cycle)
It is synthesized from the amino acid Methionine.
Pathway:
Methionine
SAM Synthase
SAM
ACC Synthase
ACC
ACC Oxidase
Ethylene
Methionine →(SAM Synthase) SAM →(ACC Synthase) ACC →(ACC Oxidase) Ethylene
SAM: S-adenosyl-methionine.
ACC: 1-aminocyclopropane-1-carboxylic acid (The direct precursor).
Key Rate-Limiting Enzyme: ACC Synthase.
B. Classification based on Ethylene Response
- Climacteric Fruits: Produce a burst of ethylene (Auto-catalytic production) during ripening. Treating them with ethylene triggers rapid ripening (e.g., Banana, Mango, Tomato).
- Non-Climacteric Fruits: Produce very low ethylene. Treating them with ethylene only degrades chlorophyll (degreening) but does not improve sugar/taste (e.g., Citrus, Grapes).
C. Ethylene Management
To extend shelf life, we must manage ethylene levels (either promote it for ripening or remove it for storage).
1. Removing/Inhibiting Ethylene (For Storage)
- Ventilation: Simple air exchange to flush out ethylene.
- Chemical Scavengers (Scrubbers): Potassium Permanganate (KMnO4): Impregnated in alumina pellets/blocks. It oxidizes ethylene into CO2 and water. The purple color turns brown when exhausted.
- Inhibitors of Action (Blocking the Receptor):
1-MCP (1-Methylcyclopropene): A gas that binds to ethylene receptors in the fruit, "locking" them so the fruit cannot sense ethylene. Sold commercially as "SmartFresh."
Silver Thiosulfate (STS): Used in cut flowers to block ethylene.
2. Application of Ethylene (For Uniform Ripening)
- Ethrel / Ethephon: A liquid chemical that releases ethylene gas when it enters plant tissues (pH > 4). Used for uniform ripening of Mangoes and Bananas before sale.
- Catalytic Generators: Machines that convert ethanol into ethylene gas in ripening chambers.
3. Factors Leading to Post-Harvest Loss
Post-harvest losses in developing countries range from 20% to 40% for fruit crops. Factors are broadly classified into Primary and Secondary.
A. Primary Factors (Direct Causes)
- 1. Biological/Physiological:
Respiration: High rate leads to rapid nutrient loss and shriveling.
Ethylene: Premature ripening in storage.
Sprouting/Rooting: (More common in onions/potatoes, but seeds inside papaya/citrus can germinate). - 2. Microbiological (Pathological):
Fungi & Bacteria: Attack fruit through cuts/wounds.
Example: Anthracnose in Mango, Blue mold in Apple.
High moisture and temperature favor microbial growth. - 3. Environmental (Physical):
Temperature:
High Temp: Increases respiration and water loss.
Low Temp: Causes Chilling Injury in tropical fruits (e.g., Banana turns black below 12°C).
Humidity: Low RH causes shriveling; High RH encourages mold. - 4. Mechanical Injury: Cuts, bruises, and abrasions during harvesting, packing, or transport. These break the skin (natural barrier) and allow entry of germs.
B. Secondary Factors (Indirect/Socio-Economic)
- Lack of Infrastructure: No cold storage or packhouses near the farm.
- Poor Transportation: Use of open trucks on rough roads instead of refrigerated vans (Reefer vans).
- Poor Harvesting Techniques: Shaking trees instead of hand-picking.
- Lack of Market Info: Farmers harvesting when there is a glut (excess supply) in the market, leading to dumping.
4. Pre-cooling
Definition: Pre-cooling is the rapid removal of Field Heat from freshly harvested produce before it is transported or stored.
Field Heat: The heat the fruit holds from the sun and ambient temperature at harvest time.
Field Heat: The heat the fruit holds from the sun and ambient temperature at harvest time.
Importance:
It is the first step in the cold chain.
Reduces metabolic rate (respiration) immediately.
Reduces the cooling load on the refrigerated transport/storage unit later.
7/8th Cooling Rule: The target is usually to reduce the temperature of the fruit by 7/8th of the difference between field temperature and cooling medium temperature.
Methods of Pre-cooling
- 1. Room Cooling (Slowest):
Placing produce in a cold storage room.
Mechanism: Cold air circulates naturally or via fans.
Disadvantage: Very slow; heat transfer is inefficient. Suitable only for crops with low respiration (e.g., Apple, Potato). - 2. Forced Air Cooling (Most Common):
Mechanism: Cold air is sucked or blown through the boxes/crates using high-pressure fans.
Efficiency: 4 to 10 times faster than room cooling.
Suitability: Best for most fruits (Grapes, Strawberries, Mangoes). - 3. Hydro-cooling (Fastest):
Mechanism: Showering or immersing fruit in ice-cold water (0-1°C). Water removes heat 15x faster than air.
Suitability: Commodities that tolerate wetting (Mango, Peach, Melon).
Precaution: Water must be chlorinated (sanitized) to prevent spreading disease. Not for berries/grapes (they spoil if wet). - 4. Vacuum Cooling:
Mechanism: Reducing pressure in a sealed chamber. Water evaporates from the produce surface, taking heat (Latent heat of vaporization) with it.
Suitability: Mostly for leafy vegetables (Lettuce). rarely used for fruits as it causes water loss. - 5. Package Icing:
Placing crushed ice or ice slurry directly on top of the produce in the box.
Suitability: For high-value, high-respiration crops (Broccoli, some Berries, Cantaloupes).
📚 References
Kader, A.A. (2002). Postharvest Technology of Horticultural Crops. University of California Agriculture and Natural Resources.
Wills, R., et al. (1998). Post Harvest: An Introduction to the Physiology and Handling of Fruits, Vegetables and Ornamentals. CABI.
Sudheer, K.P. & Indira, V. (2007). Post Harvest Technology of Horticultural Crops. New India Publ. Agency.
Salunkhe, D.K. & Desai, B.B. (1984). Postharvest Biotechnology of Fruits. CRC Press.