🍶 Fruit and Vegetable Preservation — Principles and Methods
Complete guide to post-harvest preservation covering canning, pasteurization, freezing, cryopreservation, drying, chemical preservatives, waxing, maturity indices, and blanching for competitive exams.
- Highest food processing industries is in Andhra Pradesh. This makes the state a significant hub for food processing and value addition in India.
Post-Harvest Losses
Total post-harvest losses in fruits and vegetables range from 20-40%. In India, post-harvest losses are estimated at 14-36% in fruits and 10-25% in vegetables.
| Fruit/Vegetable | Post-Harvest Losses (%) |
|---|---|
| Mango | 17-37 |
| Banana | 12-14 |
| Papaya | 90-100 |
| Citrus | 8-31 |
| Apple | 10-25 |
| Grapes | 23-30 |
| Cauliflower | 10-15 |
| Onion | 15-30 |
| Potato | 15-20 |
| Tomato | 10-20 |
WARNING
Papaya has the highest post-harvest losses (90-100%) among all fruits and vegetables. This is a very frequently asked fact.
Post-Harvest Practices
- Washing: Chlorine used as disinfectant to wash fruits & vegetables
- Curing: Hardening of epidermal layer of bulb & root crops by exposing to high RH & temperature. E.g.: Onion, Garlic, Sweet Potato, Tapioca
- Degreening: Degradation of chlorophyll in mango, banana, tomato, citrus by artificial application of ethylene
- Best degreening temp: 27°C & 85-90% RH
- Most widely used growth regulator for degreening: Ethrel
- Artificial ripening: Ethylene/Ethrel (Mango, Banana)
- Synthetic ripening: Calcium Carbide produces acetylene gas (CaC₂) — Banned
- Ethylene absorber/scrubber: KMnO₄
- Irradiation: Exposing food either packaged for gamma rays for a specific time
- Waxing: Application of fungicides or growth substance & other chemicals along with edible wax for increased shelf life. E.g.: Apple, Pomegranate
- Pre-Cooling: Rapid removal of field heat from harvested vegetables & fruits. E.g.: Okra, Garden Pea
- Pre-cooling temperature is slightly higher than the freezing injury temperature
- Hydrocooling (cold water) is a rapid cooling method
- Bio-Preservation: Naturally increases shelf life. E.g.: Curd
- Bio Preservatives: Lactic acid bacteria — Nicin, Pimaricin etc.
- Sulphuring: Exposure of whole fruit, slice/pieces into burning sulphur fumes in sulphur box. Prevents browning @ 30-60 minutes
Maturity Index of Important Horticulture Crops
| Horticultural Crop | Maturity Index |
|---|---|
| Mango | Tapka |
| Banana | Finger filling/Angularity |
| Jack Fruit & Watermelon | Tapping |
| Muskmelon | Netting or full slip stage |
| Onion & Garlic | Neck fall (50%) |
| Citrus | Juice Content (50%) |
| Avocado | Oil Content |
| Apple | T-Stage |
| Pineapple | Flattening of eyes |
| Papaya | Colour break stage |
| Guava | Green firm stage |
| Peach | Calendar date, DFFB, freeness of Pit |
| Strawberry | 1/2 to 3/4 of skin develops colour |
| Walnut | PTB stage (Packing tissue turn brown) |
| Fig | Opening of ostiole & disappearance of milky latex |
TIP
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- Highest food processing industries is in Andhra Pradesh. This makes the state a significant hub for food processing and value addition in India.
Post-Harvest Losses
Total post-harvest losses in fruits and vegetables range from 20-40%. In India, post-harvest losses are estimated at 14-36% in fruits and 10-25% in vegetables.
| Fruit/Vegetable | Post-Harvest Losses (%) |
|---|---|
| Mango | 17-37 |
| Banana | 12-14 |
| Papaya | 90-100 |
| Citrus | 8-31 |
| Apple | 10-25 |
| Grapes | 23-30 |
| Cauliflower | 10-15 |
| Onion | 15-30 |
| Potato | 15-20 |
| Tomato | 10-20 |
WARNING
Papaya has the highest post-harvest losses (90-100%) among all fruits and vegetables. This is a very frequently asked fact.
Post-Harvest Practices
- Washing: Chlorine used as disinfectant to wash fruits & vegetables
- Curing: Hardening of epidermal layer of bulb & root crops by exposing to high RH & temperature. E.g.: Onion, Garlic, Sweet Potato, Tapioca
- Degreening: Degradation of chlorophyll in mango, banana, tomato, citrus by artificial application of ethylene
- Best degreening temp: 27°C & 85-90% RH
- Most widely used growth regulator for degreening: Ethrel
- Artificial ripening: Ethylene/Ethrel (Mango, Banana)
- Synthetic ripening: Calcium Carbide produces acetylene gas (CaC₂) — Banned
- Ethylene absorber/scrubber: KMnO₄
- Irradiation: Exposing food either packaged for gamma rays for a specific time
- Waxing: Application of fungicides or growth substance & other chemicals along with edible wax for increased shelf life. E.g.: Apple, Pomegranate
- Pre-Cooling: Rapid removal of field heat from harvested vegetables & fruits. E.g.: Okra, Garden Pea
- Pre-cooling temperature is slightly higher than the freezing injury temperature
- Hydrocooling (cold water) is a rapid cooling method
- Bio-Preservation: Naturally increases shelf life. E.g.: Curd
- Bio Preservatives: Lactic acid bacteria — Nicin, Pimaricin etc.
- Sulphuring: Exposure of whole fruit, slice/pieces into burning sulphur fumes in sulphur box. Prevents browning @ 30-60 minutes
Maturity Index of Important Horticulture Crops
| Horticultural Crop | Maturity Index |
|---|---|
| Mango | Tapka |
| Banana | Finger filling/Angularity |
| Jack Fruit & Watermelon | Tapping |
| Muskmelon | Netting or full slip stage |
| Onion & Garlic | Neck fall (50%) |
| Citrus | Juice Content (50%) |
| Avocado | Oil Content |
| Apple | T-Stage |
| Pineapple | Flattening of eyes |
| Papaya | Colour break stage |
| Guava | Green firm stage |
| Peach | Calendar date, DFFB, freeness of Pit |
| Strawberry | 1/2 to 3/4 of skin develops colour |
| Walnut | PTB stage (Packing tissue turn brown) |
| Fig | Opening of ostiole & disappearance of milky latex |
TIP
Mango = Tapka, Onion = Neck fall (50%), Banana = Finger filling. These three maturity indices are the most commonly asked.
Fruit Preservation
- The art and science of keeping fruits for longer time without deterioration in quality. Preservation techniques aim to prevent microbial spoilage, enzymatic degradation, and chemical deterioration so that fruits retain their nutritional value, taste, and appearance over extended periods.
NOTE
Preservation works by targeting three main causes of spoilage: microorganisms, enzymes, and chemical reactions (like oxidation). Every method below addresses one or more of these.
Principle of Preservation
Canning
- It is method of food preservation in which food is processed and sealed in an airtight container (jars like mason jars and steel and tin cans). The airtight seal prevents the entry of new microorganisms and air, which are the primary causes of food spoilage.
- Packed in vacuum — removing air from the container before sealing helps prevent oxidation and inhibits the growth of aerobic microorganisms.
- Fruits can be processed at a temperature of 100 °C. Since fruits are naturally acidic (pH below 4.5), a lower processing temperature is sufficient to destroy spoilage-causing organisms.
- Vegetables can be processed at temperature of 115-121 °C. Vegetables are low-acid foods (pH above 4.5) and require higher temperatures, typically achieved using a pressure cooker or retort, to eliminate heat-resistant spores like Clostridium botulinum.
IMPORTANT
Fruits = 100 °C (acidic, pH < 4.5) | Vegetables = 115-121 °C (low-acid, pH > 4.5). The pH determines the required processing temperature.
- Lidding or Clinching
- Cans after being filled, are covered loosely with lid and passed through the exhaust box. The purpose of exhausting is to remove air from the headspace to create a vacuum inside the sealed can.
- Lidding is now replaced by clinching in which the lid is partially seamed to the can by a single first roller action of double seamer. This allows steam to escape during processing while keeping the lid in place.
Pasteurization
- Heating of fruits and vegetable juice at 85-90 °C for 30 minutes. This is a mild heat treatment designed to extend shelf life while preserving the flavor and nutritional quality of the product.
- It kills only harmful microbes. Unlike sterilization, pasteurization does not eliminate all microorganisms — it targets pathogens and spoilage organisms while leaving some heat-resistant, non-harmful bacteria alive. This is why pasteurized products still need refrigeration.
Sterilization
- Heating of fruits and vegetables above 100 °C. This is a more intense heat treatment compared to pasteurization.
- It kills both beneficial and harmful bacteria. Because all microorganisms are destroyed, sterilized products can be stored at room temperature for extended periods. However, the higher temperature may cause some loss of nutrients, color, and flavor.
TIP
Pasteurization = kills only harmful microbes (needs refrigeration) | Sterilization = kills all microbes (room temperature storage). This is a classic comparison question.
Freezing
- Cooler storage: 15 °C — used for temporary storage of fruits and vegetables that do not require very low temperatures. It slows down respiration and ripening.
- Refrigeration or chilling: 0-5 °C — this range significantly retards microbial growth and enzymatic reactions, extending the shelf life of perishable produce for days to weeks.
Cryopreservation
- Preserve in liquid nitrogen at -196 °C. At this extremely low temperature, all biological activity — including microbial growth and enzymatic reactions — is effectively halted. Cryopreservation is widely used for preserving germplasm, cell cultures, and certain high-value food products.
Lyophilization or Cryodesiccation
- Freeze drying, also known as lyophilization or cryodesiccation, is a low temperature dehydration process that involves freezing the product, lowering pressure, then removing the ice by sublimation (direct conversion of ice to vapor without passing through the liquid phase).
- This contrasts with dehydration by most conventional methods that evaporate water using heat. The major advantage of freeze drying is that it preserves the shape, color, flavor, and nutritional value of the product far better than conventional drying, making the product easy to rehydrate later.
Drying
- Removal of moisture by applying heat is called drying. By reducing the moisture content below a critical level (typically below 14% for most products), microbial growth is inhibited because microorganisms need water to survive and multiply.
- E.g. Raisins (Kismis) — grapes dried in the sun or in mechanical dryers to produce a long-lasting, shelf-stable product.
Preservation through Osmosis
- High concentration of sugar draws water out of microbial cells through osmotic pressure, effectively dehydrating and killing them. This principle is called osmotic dehydration.
- E.g. Jam (68% sugar), Agari ka petha. The high sugar content creates an environment where microorganisms cannot survive, thus preserving the product for months.
Salt Preservation
- Salt concentration of 10-25% is sufficient (in pickles: 15%). Similar to sugar, salt creates high osmotic pressure that draws water out of microbial cells, inhibiting their growth and multiplication. Salt also interferes with enzyme activity in microorganisms.
Preservation by Chemicals
-
KMS (Potassium Meta bi Sulphite): @ 500 ppm / 0.05%.
- Sulphur dioxide is responsible for preservation. When KMS dissolves in the fruit juice, it releases SO₂ gas, which acts as a powerful antimicrobial and antioxidant agent.
- It is used against colourless fruit juices/pulp. SO₂ can cause bleaching, so it is preferred for products where color loss is not a concern.
-
Sodium benzoate: @ 700 ppm / 0.07%.
- Benzoic acid is responsible for preservation. It works by lowering the internal pH of microbial cells and inhibiting glycolysis (the process by which microbes convert glucose to energy).
- Used in colored fruit (only in non-acid fruits). Since sodium benzoate does not bleach the product, it is ideal for preserving the natural color of colored fruit juices and pulps.
IMPORTANT
KMS (500 ppm) → colourless juices (SO₂ bleaches) | Sodium Benzoate (700 ppm) → colored juices (no bleaching). This pairing is a guaranteed exam question.
Fermentation
- Grape wine (alcohol 7-20%) is the oldest example of fermented beverage. During fermentation, yeast converts sugars into ethanol and carbon dioxide under anaerobic conditions. The alcohol produced acts as a natural preservative by inhibiting the growth of spoilage-causing microorganisms.
Asepsis
- Prevent entry of microbes. Asepsis refers to maintaining a sterile or microbe-free environment during food processing and packaging. This is achieved through proper sanitation, use of sterilized equipment, and maintaining hygienic conditions throughout the processing chain.
Oxidation
- Oxidation can be checked by antioxidants (Ascorbic acid / Vit. C). Oxidative deterioration causes browning, off-flavors, and loss of nutritional value in preserved foods. Antioxidants like ascorbic acid work by donating electrons to free radicals, thereby neutralizing them and preventing the chain reaction of oxidation.
Enzymes
- Checking of enzymatic spoiling: Blanching in cauliflower. Blanching involves briefly immersing the produce in boiling water or steam to inactivate enzymes (such as polyphenol oxidase and peroxidase) that cause browning, off-flavors, and texture changes during storage.
- Clostridium pasteurianum is a beneficial bacteria for fruit preservation industry. UPPSC 2021 This bacterium plays a useful role in certain fermentation processes in the food industry.
Waxing
- Wax coating treatment enhances the shelf life of fruit by inhibiting respiration. A thin layer of edible wax (such as paraffin, beeswax, or carnauba wax) is applied to the fruit surface, which reduces moisture loss, slows down gas exchange, and gives the fruit an attractive glossy appearance for better marketability.
Maturity Indices of Fruits
👉🏻 Symptoms which indicate that fruits are ready to harvest: These indices are essential for determining the optimum harvest time — harvesting too early results in poor quality and flavor, while harvesting too late leads to over-ripening and reduced shelf life.
| Fruit | Maturity Indices |
|---|---|
| Mango | Specific gravity of fruits ranges between 1.01 to 1.02. TSS reaches 11-15 Degree Brix. Days taken from shooting: i.e. 3-0.3.5 months. |
| Banana | Pulp peel ratio reaches 1.2 - 1.6. |
| Grape | Grape is harvested when they reach a TSS of 16 to 24% depending on variety. |
| Papaya | For long distance transport: When the skin colour changes from green to yellow to the extent of 6%. For local market: When 25% of surface changes to yellow colour. For long distance: When all the tires are still green and have no trace of yellow colour (75 - 80% maturity). |
| Jackfruit | A dull, hollow sound is produced when the fruit is tapped by the finger, aromatic odour |
| Pomegranate | The fruits are ready for harvest between 135-170 days after anthesis. |
| Citrus | The International Standards Organization has set in minimum juice content of citrus as follows: Washington navel oranges 30%, Other orange varieties 35%, Grapefruit 35%, Mandarin orange 33%, Lemons and limes 25% |
Post Harvest Handling
Degreening of fruit is a post-harvest process by which the green color is removed from the peel by treating the fruit with ethylene gas to degrade chlorophyll and reveal yellow and orange colors for commercial and marketing purposes. This is commonly practiced in citrus fruits like oranges and lemons, where the fruit may be physiologically mature but still has a green peel that consumers associate with unripe fruit.
Summary Cheat Sheet
| Concept / Topic | Key Details / Explanation |
|---|---|
| Post-harvest loss facts | Total post-harvest loss in fruits and vegetables is about 20-40%, Andhra Pradesh is noted for the highest food processing industries, and papaya is the highest-loss crop in the table at 90-100%. |
| Basic post-harvest practices | Important practices listed are washing with chlorine, curing, degreening, irradiation, waxing, pre-cooling, sulphuring, and bio-preservation. |
| Degreening and ripening | Degreening is done with ethylene or Ethrel, best condition is about 27°C and 85-90% RH, calcium carbide gives banned acetylene-based ripening, and KMnO4 is the ethylene scrubber. |
| Key maturity indices | Frequently tested indices are mango = Tapka, banana = finger filling or angularity, onion and garlic = 50% neck fall, citrus = 50% juice content, apple = T-stage, pineapple = flattening of eyes, papaya = colour break, walnut = PTB, and fig = opening of ostiole with loss of milky latex. |
| Preservation concept | Preservation means extending shelf life without quality loss by controlling microorganisms, enzymes, and chemical reactions such as oxidation. |
| Canning | Canning uses airtight vacuum-sealed containers; fruits are processed at about 100°C because they are acidic, while vegetables need about 115-121°C because they are low-acid foods. |
| Pasteurization and sterilization | Pasteurization is around 85-90°C for 30 minutes and kills harmful microbes only, while sterilization is above 100°C and destroys all microbes. |
Summary Continued
| Concept / Topic | Key Details / Explanation |
|---|---|
| Cooling and freeze-based preservation | Cooler storage is around 15°C, refrigeration or chilling is 0-5°C, cryopreservation uses liquid nitrogen at -196°C, and lyophilization means freeze drying by sublimation. |
| Drying, sugar, and salt preservation | Drying reduces moisture, raisins are the main example, osmotic preservation works through high sugar as in jam (68%) and petha, and salt preservation generally uses 10-25% salt with 15% common in pickles. |
| Chemical preservatives | KMS is used at 500 ppm or 0.05% for colourless juices because released SO2 may bleach colour, while sodium benzoate is used at 700 ppm or 0.07% for coloured products. |
| Fermentation, asepsis, and oxidation | Grape wine is the classic fermented product, asepsis prevents entry of microbes, and oxidation is checked using antioxidants such as ascorbic acid. |
| Enzymes and waxing | Blanching checks enzymatic spoilage, Clostridium pasteurianum is mentioned as a beneficial bacterium, and waxing extends shelf life by reducing respiration and moisture loss. |
| Extra maturity notes | The lesson also notes fruit-specific maturity signs such as grape TSS 16-24%, papaya skin colour change for transport or local market, jackfruit tapping plus aroma, pomegranate harvest at 135-170 days after anthesis, and citrus minimum juice standards by fruit type. |