Postharvest Technology of Fruit Crops
Unit 1: Postharvest Physiology and Handling
1. Maturity Indices
Definition: Maturity is the stage of development at which the crop meets the quality standards for the consumer or processor.
- Physiological Maturity: The stage when a fruit has reached maximum growth and will ripen properly even if harvested (e.g., a green mango that will turn yellow later).
- Horticultural/Commercial Maturity: The stage when the plant part possesses the necessary characteristics for use by consumers (e.g., a green cucumber is commercially mature, but physiologically immature).
Types of Maturity Indices
Reliable indices are vital to ensure good quality and shelf life.
A. Computational (Chronological) Methods
- Days from Full Bloom (DFFB): Counting the number of days from the peak flowering date to harvest.
Example: Mango (Dashehari) takes approx. 85-90 days; Apple (Red Delicious) takes 135-145 days. - Heat Units (Degree Days): The accumulation of mean daily temperatures above a base temperature (usually 10°C) required for ripening. Accurate but varies with weather.
B. Physical Methods
- Skin Color: The most common index. Change from green to yellow/red due to chlorophyll degradation (breakdown of green pigment) and synthesis of carotenoids/anthocyanins.
- Example: Papaya (color break from green to yellow traces), Tomato.
- Size and Shape:
- Fullness of cheeks: In Mango, "fullness of shoulders" (the top part near the stem rises) indicates maturity.
- Angularity: Bananas change from angular (sharp edges) to round when mature.
- Firmness: Measured using a Penetrometer (an instrument that measures pressure required to puncture fruit flesh).
- Principle: As fruit matures, protopectin (hard glue between cells) converts to soluble pectin, making it softer.
- Example: Apple (measured in lb/sq inch or Newtons).
- Specific Gravity (Float Test): Used widely for Mango.
- Mature mangoes sink in water (Sp. Gravity > 1.0) due to starch accumulation.
- Immature mangoes float (Sp. Gravity < 1.0).
C. Chemical Methods
- Total Soluble Solids (TSS): Measured using a Refractometer (measures light bending in juice). Expressed in °Brix.
Trend: TSS increases as fruit matures (starch converts to sugar).
Standard: Grapes (16-18 °Brix), Mango (10-12 °Brix at harvest). - Titratable Acidity: Measured by titration against an alkali (NaOH).
Trend: Acidity generally decreases during maturation (organic acids are used up in respiration).
Example: Citrus (Citric acid), Grapes (Tartaric acid). - TSS: Acid Ratio: The most reliable index for citrus and grapes. It balances sweetness with sourness.
Example: In Mandarins, a ratio of 12:1 or 14:1 is considered mature. - Starch-Iodine Test: Used for Apples/Pears.
Cut fruit is dipped in iodine solution. Dark blue = High starch (Immature). Light/No color = Starch has turned to sugar (Mature).
D. Physiological Indices
- Respiration Rate: Measuring CO2 evolution. Low rate indicates harvest maturity for storage.
- Ethylene Production: Measuring internal ethylene concentration (ppm).
Maturity Standards Summary Table
| Fruit | Index | Standard for Harvest |
|---|---|---|
| Mango | Sp. Gravity / Shape | Sinks in water (>1.01); Shoulders squared |
| Banana | Shape / Age | Fingers become rounded (disappearance of angles) |
| Citrus | Juice / TSS:Acid | >40% juice content; Ratio 8-10:1 |
| Grapes | TSS | Minimum 14-16° Brix (Does not ripen after harvest) |
| Apple | Firmness / Starch | 6.5-8.0 kg/cm2; Starch clearing at core |
| Avocado | Oil Content | Minimum 8% oil content |
| Papaya | Color | Trace of yellow at apical end |
2. Harvesting Practices for Specific Market Requirements
Harvesting must be done carefully to avoid mechanical injury (cuts/bruises), which becomes an entry point for pathogens (germs).
Harvesting Methods
- Hand Harvesting: Most fresh fruits are hand-picked.
- Tools: Ladders, picking bags, poles with catchers (for mango).
- Technique: Fruits should be "snapped" or twisted, or clipped with shears (e.g., Citrus, Grapes) to leave the "button" (calyx) intact, preventing rot.
- Lift and Twist: Used for Apple/Pear (preserves spur).
- Clippers/Secateurs: Used for Mango/Citrus/Grapes to retain a short stem (prevents rot).
- Pros: Selective, less damage. Cons: Labor intensive, slow.
- Mechanical Harvesting: Used mostly for processing crops (nuts, berries for juice). Uses tree shakers. (High damage rate, not suitable for fresh market).
- Pros: Fast, low labor cost. Cons: High damage (bruising), strictly for processing industry.
Requirements Based on Market Destination
| Market | Maturity Requirement | Handling | Reason |
|---|---|---|---|
| Local (Short <100km) | Near Ripe: Full color, high flavor. | Simple crates, minimal cooling. | Fruit has maximum flavor, sugar, and size. Since transport time is short (1-2 days), softness is acceptable. |
| Distant (Long >500km) | Mature Green/Breaker: Firm, less color. Ripens in transit. | Pre-cooling, cushioning, CFB boxes. | Fruit must be firm to withstand handling and transport. It will ripen during transit or storage. |
| Export (International) | 75-80% Mature: Specific size/weight, zero defects. | Cold chain (Reefer), strict quarantine. | Fruit must be firm to withstand handling and transport. It will ripen during transit or storage. |
| Processing (Factory) | Fully Ripe: Max sugar/juice, visual defects allowed. | Bulk bins, handled quickly. | High TSS (sugar) and intense color are required for jams/juices. Visual appearance (shape/spots) matters less than internal quality. |
Best Time to Harvest:
Cool hours of the day (Early morning or late evening).
Avoid: Harvesting during rain (wet fruit rots 2x faster) or peak afternoon heat (high field heat increases respiration).
3. Influence of Pre-harvest Practices
Quality is "grown in the field." Postharvest technology can only maintain quality; it cannot improve it.
- Genetic Factors (Variety):
Some varieties naturally have thicker skins or lower respiration rates.
Example: 'Newtown' apples store better than 'Delicious' apples. - Environmental Factors:
- Temperature: High temp during fruit growth leads to early maturity but poor color.
- Light: Essential for Anthocyanin (red pigment) formation in apples and strawberries. Pruning is done to let light inside the canopy.
- Management/Cultural Practices:
- Mineral Nutrition (Fertilizers):
Nitrogen (N): Excess N makes fruit large but soft, poorly colored, and creates low storage potential.
Calcium (Ca): The most critical nutrient for storage. Deficiency causes disorders like "Bitter Pit" in apples and "Spongy Tissue" in mango. Ca strengthens cell walls.
Potassium (K): Improves acid-sugar balance and fruit color. - Irrigation (Watering):
Moisture Stress: Lack of water can produce small fruits with high TSS.
Excess Water: Just before harvest, too much water dilutes sugars and causes cracking (e.g., in Pomegranate, Cherry). - Canopy Management:
Thinning: Removing excess fruits leads to larger size and better sweetness in remaining fruits. - Growth Regulators:
GA3 (Gibberellic Acid): Delays ripening (extends harvest season). Used in Citrus to keep rind green/firm.
Ethrel (Ethephon): Promotes early ripening and uniform color.
4. Enzymatic and Textural Changes
As fruit ripens, the texture changes from hard/crisp to soft/juicy.
A. Textural Changes (Softening)
- Cell Wall Degradation: The plant cell wall is rigid. During ripening, enzymes break it down.
- Key Mechanism:
Protopectin: In immature fruit, pectin exists as "Protopectin," which is insoluble and acts like cement holding cells together (Firm texture).
Soluble Pectin: As fruit ripens, enzymes break Protopectin into soluble pectin (Soft texture). - Key Enzymes involved:
Polygalacturonase (PG): Cuts the pectin chains (depolymerization).
Pectin Methylesterase (PME): Removes methyl groups, making pectin accessible to PG.
B. Enzymatic Browning
This is a negative change (spoilage). When fruit is cut or bruised, it turns brown.
Reaction: Phenols (natural compounds) + Oxygen →(PPO Enzyme) Quinones (Brown Pigment/Melanin).
Enzyme: Polyphenol Oxidase (PPO).
Prevention: removing oxygen (vacuum pack), lowering pH (adding lemon juice/citric acid), or blanching (heat).
5. Respiration
Definition: Respiration is the oxidative breakdown of complex molecules (Starch/Sugars) into simpler molecules (CO2, water), releasing Energy and Heat.
Significance:
Respiration = Aging.
Higher Respiration Rate = Shorter Shelf Life.
The heat released is called "Vital Heat" and must be removed by cooling to prevent rotting.
Patterns of Respiration: Climacteric vs. Non-Climacteric
This is the most important classification in postharvest physiology.
- Climacteric Fruits:
Show a sudden, sharp rise in respiration (and ethylene production) during ripening.
Can be harvested mature green and will ripen off the tree.
Examples: Mango, Banana, Apple, Papaya, Guava, Fig, Tomato - Non-Climacteric Fruits:
Do not show a rise in respiration. The rate steadily declines after harvest.
Will not ripen (get sweeter) if harvested green. Must be harvested only when fully ripe.
Examples: Citrus (Orange/Lemon), Grape, Pineapple, Litchi, Pomegranate, Strawberry.
6. Transpiration (Water Loss)
Definition: The physical loss of water in the form of vapor from the fruit to the surrounding air.
Consequences:
- Weight Loss: Direct economic loss (fruit is sold by weight).
- Shriveling/Wilting: Loss of gloss and crispness. usually, 5-10% water loss makes fruit unmarketable.
Mechanism:
Water moves from High Water Potential (inside the fruit, approx 100% humidity) to Low Water Potential (dry air outside).
Vapour Pressure Deficit (VPD): The difference between humidity inside the fruit and the air. High VPD = High water loss.
Factors Influencing Transpiration:
- Surface Area to Volume Ratio:
Small fruits (Grapes/Berries) have a large surface area relative to their weight → Dry out faster.
Large fruits (Melons) dry out slower. - Surface Structure:
Cuticle/Wax: Thick natural wax reduces water loss (e.g., Apple).
Stomata/Lenticels: Natural openings on skin allow water to escape.
Hairs: Pubescence (hairs) can trap moisture (e.g., Peach), reducing loss, but sometimes increase surface area. - Physical Injuries: Cuts and bruises break the natural wax barrier, accelerating water loss.
Control Measures:
- High Relative Humidity (RH): Store leafy greens/thin-skinned fruits at 90-95% RH.
- Waxing: Applying artificial wax (e.g., Carnauba wax) to seal pores.
- Packaging: Polyethylene bags create a high humidity micro-climate around the fruit.
📚 References & Suggested Readings
Bhutani, R.C. (2003). Fruit and Vegetable Preservation. Biotech Books.
Wills, R., McGlasson, W.B., Graham, D., & Joyce, D. (1998). Post Harvest: An Introduction to the Physiology and Handling of Fruits, Vegetables and Ornamentals. CABI.
Sudheer, K.P. & Indira, V. (2007).29 Post Harvest Technology of Horticultural Crops. New India Publ. Agency.
Kader, A.A. (2002).30 Postharvest Technology of Horticultural Crops. University of California.
Mitra, S.K. (1997). Post Harvest Physiology and Storage of Tropical and Sub-tropical Fruits. CABI.