Translate
🍶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
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
Freezedrying, 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
colourlessfruit 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.
Quick Revision: Preservation Methods at a Glance
| Method | Key Parameter | Principle |
|---|---|---|
| Canning | 100 °C (fruits), 115-121 °C (vegetables) | Airtight seal + heat |
| Pasteurization | 85-90 °C, 30 min | Kills harmful microbes only |
| Sterilization | Above 100 °C | Kills all microbes |
| Freezing | 0-5 °C (chilling), 15 °C (cooler) | Retards microbial growth |
| Cryopreservation | -196 °C (liquid N₂) | Halts all biological activity |
| Freeze Drying | Low temp + low pressure | Sublimation of ice |
| KMS | 500 ppm | SO₂ — colourless juices |
| Sodium Benzoate | 700 ppm | Benzoic acid — colored juices |
| Sugar Preservation | 68% (jam) | Osmotic dehydration |
| Salt Preservation | 10-25% (pickles: 15%) | Osmotic pressure |
Methods of Preservation — Detailed
1. Preservation by High Temperature
- Asepsis: Absence of infection — keeping out micro-organisms
- Filtration: Removal of micro-organisms
| Pasteurization | Sterilization |
|---|---|
| Partial destruction of microbes | Complete destruction of microbes |
| Commonly used for fruit juices | Commonly used for canning of vegetables |
| Heating of fruit juices @ 85-90°C for 30 min | Aseptic canning/Martin aseptic canning/UHT sterilization: 149°C in 1-2 sec |
| Temperature: <100°C | Temperature: >100°C |
NOTE
Flash pasteurization: Fruit juice is heated rapidly about 5.5°C higher than the pasteurization temperature. Fruit and tomato products should be heated at 100°C for 30 min to kill the spore-forming bacteria.
2. Preservation by Low Temperature
- Best way of preserving pure fruit juice: Freezing
- Frozen foods should always be kept at below: -5°C
- Cellar storage: 15°C (Root crops, potatoes, onions and apples are most suitable)
- Refrigeration: 0-5°C
- Freezing method: -18 to -40°C
3. Preservation by Chemical Preservatives
- Preservative: Any substance capable of inhibiting, retarding the process of fermentation or decomposition of food
- pH level of majority fruit juices: 3.5 to 4.0
- General pH in citrus squashes and cordials: 2.5 to 3.5
- pH range for growth: Moulds (1.5-8.5), Yeasts (2.5-8.0), Bacteria (4.0-7.5)
- Class-I Preservatives: Sugar, Salt, Spices
- Class-II Preservatives: Sodium benzoate & Potassium metabisulphite
- Salt as preservative: 15-25%
- Sugar as preservative: 62-65%
- Alcohol as preservative: 14%
- Vinegar contains acetic acid @ 5%
- Permitted preservatives in all countries: Sulphur dioxide and Benzoic acid
4. Preservation by Fermentation
- Acetic acid fermentation — Vinegar
- Alcoholic fermentation — Wine
- Lactic acid fermentation — Pickles & Sauerkraut (Cabbage fermentation)
5. Preservation by Carbonation
- Fruit juice beverages are bottled with CO₂ @ 1-8 g/Litre
- Required CO₂ for complete inhibition of microbial activity: 14.6 g/litre
6. Preservation by Irradiation
- Also known as cold sterilization — food is free of micro-organisms without high temperature treatment
- WHO and IABA recommended: radiation dose up to 1 Mrad is not hazardous
7. Preservation by Antibiotics
- Nisin — commonly used for canning mushrooms, tomatoes & milk products
- Pimaricin — fruits & fruit juices
- Subtilin — Asparagus, Corn & Peas
Canning — Detailed Process
Father of Canning: Nicolas Appert. Canning is also known as Appertization. Canning of fruits introduced in USA on commercial scale by William Underwood (1817). 1g of soil contains 10¹² spores of microorganisms.
Peeling Methods
| Hand Peeling | Steam Peeling | Mechanical Peeling | Lye Peeling | Flame Peeling |
|---|---|---|---|---|
| Mango, Papaya & Irregular fruits | Potato, Tomato, Free Stone & Clingstone Peaches | Apple, Peach, Pineapple, Cherries & Root Crops | Peach, Apricot, Sweet Orange, Mandarin & Vegetables | Garlic & Onion |
- Lye peeling is done by 1-2% NaOH
Blanching
- Also known as scalding, parboiling or pre-cooking
- Inactivates natural enzymes & microbes in fruits & vegetables
- Temperature: 82.2° - 93.3°C @ 2-5 min
Syruping & Brining
- Syruping: A solution of sugar in water. Practised only in fruits. Filled at 79 to 82°C, leaving head space of 0.3 to 0.5 cm
- Brining: A solution of salt in water. Practised only in vegetables. Concentration: 1-3%. Filled at 79 to 82°C
- Lacquering: Coating the inside of can with lacquer (Golden coloured enamel) to prevent discolouration
Preservation by Dehydration
Dehydration refers to the process of removal of moisture by application of artificial heat under controlled conditions of temperature, relative humidity & air flow.
- Initial temp: 43°C
- Dehydration temperature for vegetables: 60-66°C
- Dehydration temperature for fruits: 66-71°C
- Common vegetables suitable for dehydration: Onion, Peas, Tomato, Okra
- Best example of dehydration: Raisins (<17% moisture)
Freezing — Detailed
- Commercial & household refrigerators run at 4.4°C - 7.2°C (40-45°F)
- Pure water freezes at 0°C (32°F)
- Good frozen storage generally: -18°C (0°F) or below
- Most food spoilage micro-organisms grow rapidly at above 10°C (50°F)
- No significant growth of micro-organisms in food below: -9.4°C (15°F)
Important Chemical Preservatives — Comparison
| Sulphur Dioxide | Benzoic Acid |
|---|---|
| Salt: Potassium meta bisulphite | Salt: Sodium benzoate |
| Most effective against Bacteria & Moulds (Fungus) | Most effective against Yeast |
| Used to preserve most of the juice/pulp | Used for coloured juice/pulp along with citric acid |
Explore More
Summary Cheat Sheet
| Fact | Answer |
|---|---|
| Highest food processing industries state | Andhra Pradesh |
| Post-harvest losses in fruits & vegetables | 20–40% |
| Highest post-harvest loss crop | Papaya (90–100%) |
| Degreening best temperature | 27°C & 85–90% RH |
| Most used growth regulator for degreening | Ethrel |
| Ethylene absorber | KMnO₄ |
| Banned ripening agent | Calcium Carbide (CaC₂) — produces acetylene |
| Mango maturity index | Tapka |
| Onion & Garlic maturity index | Neck fall (50%) |
| Canning temperature — fruits | 100°C (acidic, pH < 4.5) |
| Canning temperature — vegetables | 115–121°C (low-acid, pH > 4.5) |
| Father of Canning | Nicolas Appert (Appertization) |
| Pasteurization | 85–90°C for 30 min; kills only harmful microbes |
| Cryopreservation | Liquid nitrogen at −196°C |
| KMS preservative dose | 500 ppm (0.05%) — colourless juices |
| Sodium benzoate dose | 700 ppm (0.07%) — coloured juices |
| Blanching temperature | 82.2–93.3°C for 2–5 min |
| Lye peeling concentration | 1–2% NaOH |
| Dehydration — best example | Raisins (< 17% moisture) |
| Frozen storage temperature | −18°C (0°F) or below |
🔐
Pro Content Locked
Upgrade to Pro to access this lesson and all other premium content.
Pro Popular
₹ 199 /mo
₹2388 billed yearly
- All Agriculture & Banking Courses
- AI Lesson Questions (100/day)
- AI Doubt Solver (50/day)
- Glows & Grows Feedback (30/day)
- AI Section Quiz (20/day)
- 22-Language Translation (30/day)
- Recall Questions (20/day)
- AI Quiz (15/day)
- AI Quiz Paper Analysis
- AI Step-by-Step Explanations
- Spaced Repetition Recall (FSRS)
- AI Tutor
- Immersive Text Questions
- Audio Lessons — Hindi & English
- Mock Tests & Previous Year Papers
- Summary & Mind Maps
- XP, Levels, Leaderboard & Badges
- Generate New Classrooms
- Voice AI Teacher (AgriDots Live)
- AI Revision Assistant
- Knowledge Gap Analysis
- Interactive Revision (LangGraph)
🔒 Secure via Razorpay · Cancel anytime · No hidden fees
- 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
Freezedrying, 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
colourlessfruit 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.
Quick Revision: Preservation Methods at a Glance
| Method | Key Parameter | Principle |
|---|---|---|
| Canning | 100 °C (fruits), 115-121 °C (vegetables) | Airtight seal + heat |
| Pasteurization | 85-90 °C, 30 min | Kills harmful microbes only |
| Sterilization | Above 100 °C | Kills all microbes |
| Freezing | 0-5 °C (chilling), 15 °C (cooler) | Retards microbial growth |
| Cryopreservation | -196 °C (liquid N₂) | Halts all biological activity |
| Freeze Drying | Low temp + low pressure | Sublimation of ice |
| KMS | 500 ppm | SO₂ — colourless juices |
| Sodium Benzoate | 700 ppm | Benzoic acid — colored juices |
| Sugar Preservation | 68% (jam) | Osmotic dehydration |
| Salt Preservation | 10-25% (pickles: 15%) | Osmotic pressure |
Methods of Preservation — Detailed
1. Preservation by High Temperature
- Asepsis: Absence of infection — keeping out micro-organisms
- Filtration: Removal of micro-organisms
| Pasteurization | Sterilization |
|---|---|
| Partial destruction of microbes | Complete destruction of microbes |
| Commonly used for fruit juices | Commonly used for canning of vegetables |
| Heating of fruit juices @ 85-90°C for 30 min | Aseptic canning/Martin aseptic canning/UHT sterilization: 149°C in 1-2 sec |
| Temperature: <100°C | Temperature: >100°C |
NOTE
Flash pasteurization: Fruit juice is heated rapidly about 5.5°C higher than the pasteurization temperature. Fruit and tomato products should be heated at 100°C for 30 min to kill the spore-forming bacteria.
2. Preservation by Low Temperature
- Best way of preserving pure fruit juice: Freezing
- Frozen foods should always be kept at below: -5°C
- Cellar storage: 15°C (Root crops, potatoes, onions and apples are most suitable)
- Refrigeration: 0-5°C
- Freezing method: -18 to -40°C
3. Preservation by Chemical Preservatives
- Preservative: Any substance capable of inhibiting, retarding the process of fermentation or decomposition of food
- pH level of majority fruit juices: 3.5 to 4.0
- General pH in citrus squashes and cordials: 2.5 to 3.5
- pH range for growth: Moulds (1.5-8.5), Yeasts (2.5-8.0), Bacteria (4.0-7.5)
- Class-I Preservatives: Sugar, Salt, Spices
- Class-II Preservatives: Sodium benzoate & Potassium metabisulphite
- Salt as preservative: 15-25%
- Sugar as preservative: 62-65%
- Alcohol as preservative: 14%
- Vinegar contains acetic acid @ 5%
- Permitted preservatives in all countries: Sulphur dioxide and Benzoic acid
4. Preservation by Fermentation
- Acetic acid fermentation — Vinegar
- Alcoholic fermentation — Wine
- Lactic acid fermentation — Pickles & Sauerkraut (Cabbage fermentation)
5. Preservation by Carbonation
- Fruit juice beverages are bottled with CO₂ @ 1-8 g/Litre
- Required CO₂ for complete inhibition of microbial activity: 14.6 g/litre
6. Preservation by Irradiation
- Also known as cold sterilization — food is free of micro-organisms without high temperature treatment
- WHO and IABA recommended: radiation dose up to 1 Mrad is not hazardous
7. Preservation by Antibiotics
- Nisin — commonly used for canning mushrooms, tomatoes & milk products
- Pimaricin — fruits & fruit juices
- Subtilin — Asparagus, Corn & Peas
Canning — Detailed Process
Father of Canning: Nicolas Appert. Canning is also known as Appertization. Canning of fruits introduced in USA on commercial scale by William Underwood (1817). 1g of soil contains 10¹² spores of microorganisms.
Peeling Methods
| Hand Peeling | Steam Peeling | Mechanical Peeling | Lye Peeling | Flame Peeling |
|---|---|---|---|---|
| Mango, Papaya & Irregular fruits | Potato, Tomato, Free Stone & Clingstone Peaches | Apple, Peach, Pineapple, Cherries & Root Crops | Peach, Apricot, Sweet Orange, Mandarin & Vegetables | Garlic & Onion |
- Lye peeling is done by 1-2% NaOH
Blanching
- Also known as scalding, parboiling or pre-cooking
- Inactivates natural enzymes & microbes in fruits & vegetables
- Temperature: 82.2° - 93.3°C @ 2-5 min
Syruping & Brining
- Syruping: A solution of sugar in water. Practised only in fruits. Filled at 79 to 82°C, leaving head space of 0.3 to 0.5 cm
- Brining: A solution of salt in water. Practised only in vegetables. Concentration: 1-3%. Filled at 79 to 82°C
- Lacquering: Coating the inside of can with lacquer (Golden coloured enamel) to prevent discolouration
Preservation by Dehydration
Dehydration refers to the process of removal of moisture by application of artificial heat under controlled conditions of temperature, relative humidity & air flow.
- Initial temp: 43°C
- Dehydration temperature for vegetables: 60-66°C
- Dehydration temperature for fruits: 66-71°C
- Common vegetables suitable for dehydration: Onion, Peas, Tomato, Okra
- Best example of dehydration: Raisins (<17% moisture)
Freezing — Detailed
- Commercial & household refrigerators run at 4.4°C - 7.2°C (40-45°F)
- Pure water freezes at 0°C (32°F)
- Good frozen storage generally: -18°C (0°F) or below
- Most food spoilage micro-organisms grow rapidly at above 10°C (50°F)
- No significant growth of micro-organisms in food below: -9.4°C (15°F)
Important Chemical Preservatives — Comparison
| Sulphur Dioxide | Benzoic Acid |
|---|---|
| Salt: Potassium meta bisulphite | Salt: Sodium benzoate |
| Most effective against Bacteria & Moulds (Fungus) | Most effective against Yeast |
| Used to preserve most of the juice/pulp | Used for coloured juice/pulp along with citric acid |
Explore More
Summary Cheat Sheet
| Fact | Answer |
|---|---|
| Highest food processing industries state | Andhra Pradesh |
| Post-harvest losses in fruits & vegetables | 20–40% |
| Highest post-harvest loss crop | Papaya (90–100%) |
| Degreening best temperature | 27°C & 85–90% RH |
| Most used growth regulator for degreening | Ethrel |
| Ethylene absorber | KMnO₄ |
| Banned ripening agent | Calcium Carbide (CaC₂) — produces acetylene |
| Mango maturity index | Tapka |
| Onion & Garlic maturity index | Neck fall (50%) |
| Canning temperature — fruits | 100°C (acidic, pH < 4.5) |
| Canning temperature — vegetables | 115–121°C (low-acid, pH > 4.5) |
| Father of Canning | Nicolas Appert (Appertization) |
| Pasteurization | 85–90°C for 30 min; kills only harmful microbes |
| Cryopreservation | Liquid nitrogen at −196°C |
| KMS preservative dose | 500 ppm (0.05%) — colourless juices |
| Sodium benzoate dose | 700 ppm (0.07%) — coloured juices |
| Blanching temperature | 82.2–93.3°C for 2–5 min |
| Lye peeling concentration | 1–2% NaOH |
| Dehydration — best example | Raisins (< 17% moisture) |
| Frozen storage temperature | −18°C (0°F) or below |
Knowledge Check
Take a dynamically generated quiz based on the material you just read to test your understanding and get personalized feedback.
Lesson Doubts
Ask questions, get expert answers
Lesson Doubts is a Pro feature.Upgrade